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Do all body organs grow in proportion during the period of physical development?

Do all body organs grow in proportion during the period of physical development?


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It is obvious that during childhood and puberty, the human body grows uniformly or proportionally so that a child's arm length, for example, is shorter than an adult's arm length but is proportional to a child's leg length.

What I'd like to know is whether the same is true for internal organs, or are there any organs (even external but perhaps less obvious) that do not grow in dimensions when the rest of the body grows and develops (I am talking about physical growth).


Physical Growth of Infants and Children

Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this rapid infant and early toddler growth, growth slows until the adolescent growth spurt. As growth slows, children need fewer calories and parents may notice a decrease in appetite. Two-year-old children can have very erratic eating habits that sometimes make parents anxious. Some children may seem to eat virtually nothing yet continue to grow and thrive. Actually, they usually eat little one day and then make up for it by eating more the next day.

During the preschool and school years, growth in height and weight is steady. Children tend to grow a similar amount each year until the next major growth spurt occurs in early adolescence.

Different organs grow at different rates. For example, the reproductive system has a brief growth spurt just after birth, then changes very little until just before sexual maturation (puberty). In contrast, the brain grows almost exclusively during the early years of life. The kidneys function at the adult level by the end of the first year.

Children who are beginning to walk have an endearing physique, with the belly sticking forward and the back curved. They may also appear to be quite bow-legged. By 3 years of age, muscle tone increases and the proportion of body fat decreases, so the body begins to look leaner and more muscular. Most children are physically able to control their bowels and bladder at this time.

Doctors report how children are growing in relation to other children their age and monitor the children's weight gain compared to their height. From birth until 2 years of age, doctors record all growth parameters in a chart by using standard growth charts from the World Health Organization (WHO). After age 2, doctors record growth parameters by using growth charts from the Centers for Disease Control and Prevention (CDC).


Fetal Development: Stages of Growth

The start of pregnancy is actually the first day of your last menstrual period. This is called the gestational age, or menstrual age. It’s about two weeks ahead of when conception actually occurs. Though it may seem strange, the date of the first day of your last period will be an important date when determining your baby’s due date. Your healthcare provider will ask you about this date and will use it to figure out how far along you are in your pregnancy.

How does conception work?

Each month, your body goes through a reproductive cycle that can end in one of two ways. You will either have a menstrual period or become pregnant. This cycle is continuously happening during your reproductive years—from puberty in your teen years to menopause around age 50.

In a cycle that ends with pregnancy, there are several steps. First, a group of eggs (called oocytes) gets ready to leave the ovary for ovulation (release of the egg). The eggs develop in small, fluid-filled cysts called follicles. Think of these follicles as small containers for each immature egg. Out of this group of eggs, one will become mature and continue on through the cycle. This follicle then suppresses all the other follicles in the group. The other follicles stop growing at this point.

The mature follicle now opens and releases the egg from the ovary. This is ovulation. Ovulation generally happens about two weeks before your next menstrual period begins. It’s generally in the middle of your cycle.

After ovulation, the opened (ruptured) follicle develops into a structure called the corpus luteum. This secretes (releases) the hormones progesterone and estrogen. The progesterone helps prepare the endometrium (lining of the uterus). This lining, is the place where a fertilized egg settles to develop. If you don’t become pregnant during a cycle, this lining is what is shed during your period.

On average, fertilization happens about two weeks after your last menstrual period. When the sperm penetrates the egg, changes occur in the protein coating of the egg to prevent other sperm from entering.

At the moment of fertilization, your baby’s genetic make-up is complete, including its sex. The gender of your baby depends on what sperm fertilizes the egg at the moment of conception. Generally, women have a genetic combination of XX and men have XY. As the mother, you provide each egg with an X. Each sperm can be either an X or a Y. If the fertilized egg and sperm is a combination of an X and Y, it’s a boy. If there are two Xs, it’s a girl.

What happens right after conception?

Within 24 hours after fertilization, the egg begins rapidly dividing into many cells. It remains in the fallopian tube for about three days after conception. Then the fertilized egg (now called a blastocyte) continues to divide as it passes slowly through the fallopian tube to the uterus. Once there, its next job is to attach to the endometrium. This is called implantation.

Before implantation though, the blastocyte breaks out of its protective covering. When the blastocyte makes contact with the endometrium, the two exchange hormones to help the blastocyte attach. Some women notice spotting (slight bleeding) during the one or two days when implantation happens. This is normal and isn’t something you should worry about. At this point, the endometrium becomes thicker and the cervix (the opening between your uterus and birth canal) is sealed by a plug of mucus.

Within three weeks, the blastocyte cells ultimately form a little ball, or an embryo. By this time, the baby’s first nerve cells have formed.

Your developing baby has already gone through a few name changes in the first few weeks of pregnancy. Generally, your baby will be called an embryo from conception until the eighth week of development. After the eighth week, the baby will be called a fetus until it’s born.

How early can I know I’m pregnant?

From the moment of conception, the hormone human chorionic gonadotrophin (hCG) will be present in your blood. This hormone is created by the cells that form the placenta (food source for your baby in the womb). It’s also the hormone detected in a pregnancy test. Even though this hormone is there from the beginning, it takes time for it to build within your body. It typically takes three to four weeks from the first day of your last period for the hCG to increase enough to be detected by pregnancy tests.

When should I reach out to my healthcare provider about a new pregnancy?

Most healthcare providers will have you wait to come in for an appointment until you have had a positive home pregnancy test. These tests are very accurate once you have enough hCG circulating throughout your body. This can be a few weeks after conception. It’s best to call your healthcare provider once you have a positive pregnancy test to schedule your first appointment.

When you call, your healthcare provider may ask you if you are taking a prenatal vitamin. These supplements contain something called folic acid. It’s important that you get at least 400mcg of folic acid each day during a pregnancy to make sure your baby’s neural tube (beginning of the baby’s brain and spine) develops correctly. Many healthcare providers suggest that you take prenatal vitamins with folic acid even when you aren’t pregnant. If you weren’t taking prenatal vitamins before your pregnancy, your provider may ask you to start as early as possible.

What’s the timeline for my baby’s development?

Your baby will change a lot throughout a typical pregnancy. This time is divided into three stages, called trimesters. Each trimester is a set of about three months. Your healthcare provider will probably talk to you about your baby’s development in terms of weeks. So, if you are three months pregnancy, you are about 12 weeks.

You will see distinct changes in your baby, and yourself, during each trimester.

Traditionally, we think of a pregnancy as a nine-month process. However, this isn’t always the case. A full-term pregnancy is 40 weeks, or 280 days. Depending on what months you are pregnant during (some are shorter and some longer) and what week you deliver, you could be pregnant for either nine months or 10 months. This is completely normal and healthy.

Once you get close to the end of your pregnancy, there are several category names you might hear regarding when you go into labor. These labels divide up the last few weeks of pregnancy. They’re also used to look out for certain complications in newborns. Babies that are born in the early term period or before may have a higher risk of breathing, hearing or learning issues than babies born a few weeks later in the full term time frame. When you’re looking at these labels, it’s important to know how they’re written. You may see the week first (38) and then you’ll see two numbers separated by a slash mark (6/7). This stands for how many days you currently are in the gestational week. So, if you see 38 6/7, it means that you are on day 6 of your 38th week.

The last few weeks of pregnancy are divided into the following groups:

  • Early term: 37 0/7 weeks through 38 6/7 weeks.
  • Full term: 39 0/7 weeks through 40 6/7 weeks.
  • Late term: 41 0/7 weeks through 41 6/7 weeks.
  • Post term: 42 0/7 weeks and on.

Talk to your healthcare provider about any questions you may have about your baby’s gestational age and due date.

Stages of Growth Month-by-Month in Pregnancy

First trimester

The first trimester will span from conception to 12 weeks. This is generally the first three months of pregnancy. During this trimester, your baby will change from a small grouping of cells to a fetus that is starting to have a baby’s features.

Month 1 (weeks 1 through 4)

As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid. This is called the amniotic sac, and it helps cushion the growing embryo.

During this time, the placenta also develops. The placenta is a round, flat organ that transfers nutrients from the mother to the baby, and transfers wastes from the baby. Think of the placenta as a food source for your baby throughout the pregnancy.

In these first few weeks, a primitive face will take form with large dark circles for eyes. The mouth, lower jaw and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny "heart" tube will beat 65 times a minute by the end of the fourth week.

By the end of the first month, your baby is about 1/4 inch long – smaller than a grain of rice.

Month 2 (weeks 5 through 8)

Your baby's facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes and eyes are also forming.

The neural tube (brain, spinal cord and other neural tissue of the central nervous system) is well formed now. The digestive tract and sensory organs begin to develop too. Bone starts to replace cartilage.

Your baby’s head is large in proportion to the rest of its body at this point. At about 6 weeks, your baby's heart beat can usually be detected.

After the 8th week, your baby is called a fetus instead of an embryo.

By the end of the second month, your baby is about 1 inch long and weighs about 1/30 of an ounce.

Month 3 (weeks 9 through 12)

Your baby's arms, hands, fingers, feet and toes are fully formed. At this stage, your baby is starting to explore a bit by doing things like opening and closing its fists and mouth. Fingernails and toenails are beginning to develop and the external ears are formed. The beginnings of teeth are forming under the gums. Your baby's reproductive organs also develop, but the baby's gender is difficult to distinguish on ultrasound.

By the end of the third month, your baby is fully formed. All the organs and limbs (extremities) are present and will continue to develop in order to become functional. The baby’s circulatory and urinary systems are also working and the liver produces bile.

At the end of the third month, your baby is about 4 inches long and weighs about 1 ounce.

Since your baby's most critical development has taken place, your chance of miscarriage drops considerably after three months.

Second trimester

This middle section of pregnancy is often thought of as the best part of the experience. By this time, any morning sickness is probably gone and the discomfort of early pregnancy has faded. The baby will start to develop facial features during this month. You may also start to feel movement as your baby flips and turns in the uterus. During this trimester, many people find out the sex of the baby. This is typically done during an anatomy scan (an ultrasound that checks your baby’s physical development) around 20 weeks.

Month 4 (weeks 13 through 16)

Your baby's heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails and hair are formed. Teeth and bones become denser. Your baby can even suck his or her thumb, yawn, stretch and make faces.

The nervous system is starting to function. The reproductive organs and genitalia are now fully developed, and your doctor can see on ultrasound if you are having a boy or a girl.

By the end of the fourth month, your baby is about 6 inches long and weighs about 4 ounces.

Month 5 (weeks 17 through 20)

At this stage, you may begin to feel your baby moving around. Your baby is developing muscles and exercising them. This first movement is called quickening and can feel like a flutter.

Hair begins to grow on baby's head. Your baby's shoulders, back and temples are covered by a soft fine hair called lanugo. This hair protects your baby and is usually shed at the end of the baby's first week of life.

The baby's skin is covered with a whitish coating called vernix caseosa. This "cheesy" substance is thought to protect your baby's skin from the long exposure to the amniotic fluid. This coating is shed just before birth.

By the end of the fifth month, your baby is about 10 inches long and weighs from 1/2 to 1 pound.

Month 6 (weeks 21 through 24)

If you could look inside the uterus at your baby right now, you would see that your baby's skin is reddish in color, wrinkled and veins are visible through the baby's translucent skin. Baby's finger and toe prints are visible. In this stage, the eyelids begin to part and the eyes open.

Baby responds to sounds by moving or increasing the pulse. You may notice jerking motions if baby hiccups.

If born prematurely, your baby may survive after the 23rd week with intensive care.

By the end of the sixth month, your baby is about 12 inches long and weighs about 2 pounds.

Month 7 (weeks 25 through 28)

Your baby will continue to mature and develop reserves of body fat. At this point, the baby's hearing is fully developed. The baby changes position frequently and responds to stimuli, including sound, pain and light. The amniotic fluid begins to diminish.

If born prematurely, your baby would be likely to survive after the seventh month.

At the end of the seventh month, your baby is about 14 inches long and weighs from 2 to 4 pounds.

Third trimester

This is the final part of your pregnancy. You may be tempted to start the countdown till your due date and hope that it would come early, but each week of this final stage of development helps your baby prepare for childbirth. Throughout the third trimester, your baby will gain weight quickly, adding body fat that will help after birth.

Remember, even though popular culture only mentions nine months of pregnancy, you may actually be pregnant for 10 months. The typical, full-term pregnancy is 40 weeks, which can take you into a tenth month. It’s also possible that you can go past your due date by a week or two (41 or 42 weeks). Your healthcare provider will monitor you closely as you approach your due date. If you pass your due date, and don’t go into spontaneous labor, your provider may induce you. This means that medications will be used to make you go into labor and have the baby. Make sure to talk to your healthcare provider during this trimester about your birth plan.

Month 8 (weeks 29 through 32)

Your baby will continue to mature and develop reserves of body fat. You may notice that your baby is kicking more. Baby's brain is developing rapidly at this time, and your baby can see and hear. Most internal systems are well developed, but the lungs may still be immature.

Your baby is about 18 inches long and weighs as much as 5 pounds.

Month 9 (weeks 33 through 36)

During this stage, your baby will continue to grow and mature. The lungs are close to being fully developed at this point.

Your baby's reflexes are coordinated so he or she can blink, close the eyes, turn the head, grasp firmly, and respond to sounds, light, and touch.

Your baby is about 17 to 19 inches long and weighs from 5 ½ pounds to 6 ½ pounds.

Month 10 (Weeks 37 through 40)

In this final month, you could go into labor at any time. You may notice that your baby moves less due to tight space. At this point, your baby’s position may have changed to prepare for birth. Ideally, the baby is head down in in your uterus. You may feel very uncomfortable in this final stretch of time as the baby drops down into your pelvis and prepares for birth.

Your baby is ready to meet the world at this point.

Your baby is about 18 to 20 inches long and weighs about 7 pounds.

Last reviewed by a Cleveland Clinic medical professional on 04/16/2020.

References

  • The American College of Obstetricians and Gynecologists. How your fetus grows during pregnancy. Accessed 4/17/2020.
  • American Pregnancy Association. Fetal Development. Accessed 4/17/2020.
  • Centers for Disease Control and Prevention. During Pregnancy. Accessed 4/17/2020.
  • US Department of Health and Human Services, Office on Women’s Health. Stages of pregnancy. Accessed 4/17/2020.

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PHYSICAL HEALTH

This section reviews what is known about the relationship between physical activity and (1) somatic growth, development, and function and (2) health- and performance-related fitness.

Somatic Growth, Development, and Function

Growth occurs through a complex, organized process characterized by predictable developmental stages and events. Although all individuals follow the same general course, growth and maturation rates vary widely among individuals. Just as it is unrealistic to expect all children at the same age to achieve the same academic level, it is unrealistic to expect children at the same age to have the same physical development, motor skills, and physical capacity. Regular physical activity does not alter the process of growth and development. Rather, developmental stage is a significant determinant of motor skills, physical capacity, and the adaptation to activity that is reasonable to expect (see Box 3-2).

BOX 3-2

Growth, Development, and Maturation. Growth is the normal process of increase in size as a result of accretion of tissues characteristic of the organism growth is the dominant biological activity for most of the first two decades of life. Changes in (more. )

Developmental Stages

Postnatal growth is commonly divided into three or four age periods. Infancy spans the first year of life. Childhood extends from the end of infancy to the start of adolescence and is often divided into early childhood, which includes the preschool years, and middle childhood, which includes the elementary school years, into the 5th or 6th grade. Adolescence is more difficult to define because of variation in its onset and termination, although it is commonly defined as between 10 and 18 years of age (WHO, 1986). The rapid growth and development of infancy continue during early childhood, although at a decelerating rate, whereas middle childhood is a period of slower, steady growth and maturation. Differences between boys and girls are relatively small until adolescence, which is marked by accelerated growth and attainment of sexual maturity (Tanner, 1962).

Across developmental stages, neurological development and control of movement advance in cephalocaudal and proximodistal directions that is, they advance “head to toe” (cephalocaudal) and “midline to periphery” (proximodistal), while predictable changes in body proportions also occur. For example, the head accounts for 25 percent of recumbent length in an infant and only 15 percent of adult height, while the legs account for 38 percent of recumbent length at birth and 50 percent of adult height. These changes in body proportions occur because body parts grow at different rates. From birth to adulthood, as the head doubles in size, the trunk triples in length, and arm and leg lengths quadruple.

Coincident with these changes in body proportions, and in part because of them, the capacity to perform various motor tasks develops in a predictable fashion. For example, running speed increases are consistent with the increase in leg length. Neurological development also determines skill progression. Young children, for example, when thrown a ball, catch it within the midline of the body and do not attempt to catch it outside the midline or to either side of the body. As proximodistal development proceeds, children are better able to perform tasks outside their midline, and by adolescence they are able to maneuver their bodies in a coordinated way to catch objects outside the midline with little effort.

Physically active and inactive children progress through identical stages. Providing opportunities for young children to be physically active is important not to affect the stages but to ensure adequate opportunity for skill development. Sound physical education curricula are based on an understanding of growth patterns and developmental stages and are critical to provide appropriate movement experiences that promote motor skill development (Clark, 2005). The mastery of fundamental motor skills is strongly related to physical activity in children and adolescents (Lubans et al., 2010) and in turn may contribute to physical, social, and cognitive development. Mastering fundamental motor skills also is critical to fostering physical activity because these skills serve as the foundation for more advanced and sport-specific movement (Clark and Metcalfe, 2002 Hands et al., 2009 Robinson and Goodway, 2009 Lubans et al., 2010). Physical activity programs, such as physical education, should be based on developmentally appropriate motor activities to foster self-efficacy and enjoyment and encourage ongoing participation in physical activity.

Biological Maturation

Maturation is the process of attaining the fully adult state. In growth studies, maturity is typically assessed as skeletal, somatic, or sexual. The same hormones regulate skeletal, somatic, and sexual maturation during adolescence, so it is reasonable to expect the effect of physical activity on these indicators of maturity to be similar. Skeletal maturity is typically assessed from radiographs of the bones in the hand and wrist it is not influenced by habitual physical activity. Similarly, age at peak height velocity (the most rapid change in height), an indicator of somatic maturity, is not affected by physical activity, nor is the magnitude of peak height velocity, which is well within the usual range in both active and inactive youth. Discussions of the effects of physical activity on sexual maturation more often focus on females than males and, in particular, on age at menarche (first menses). While some data suggest an association between later menarche and habitual physical activity (Merzenich et al., 1993), most of these data come from retrospective studies of athletes (Clapp and Little, 1995). Whether regular sports training at young ages before menarche �lays” menarche (later average age of menarche) remains unclear. While menarche occurs later in females who participate in some sports, the available data do not support a causal relationship between habitual physical activity and later menarche.

Puberty is the developmental period that represents the beginning of sexual maturation. It is marked by the appearance of secondary sex characteristics and their underlying hormonal changes, with accompanying sex differences in linear growth and body mass and composition. The timing of puberty varies, beginning as early as age 8 in girls and age 9 in boys in the United States and as late as ages 13-15 (NRC/IOM, 1999). Recent research suggests that the onset of puberty is occurring earlier in girls today compared with the previous generation, and there is speculation that increased adiposity may be a cause (Bau et al., 2009 Rosenfield et al., 2009). Conversely, some data suggest that excess adiposity in boys contributes to delayed sexual maturation (Lee et al., 2010). Pubescence, the earliest period of adolescence, generally occurs about 2 years in advance of sexual maturity. Typically, individuals are in the secondary school years during this period, which is a time of decline in habitual physical activity, especially in girls. Physical activity trends are influenced by the development of secondary sex characteristics and other physical changes that occur during the adolescent growth spurt, as well as by societal and cultural factors. Research suggests that physical inactivity during adolescence carries over into adulthood (Malina, 2001a,b CDC, 2006).

It is critical that adolescents be offered appropriate physical activity programs that take into account the physical and sociocultural changes they are experiencing so they will be inspired to engage in physical activity for a lifetime. As discussed below, adequate physical activity during puberty may be especially important for optimal bone development and prevention of excess adiposity, as puberty is a critical developmental period for both the skeleton and the adipose organ.

Adolescence is the transitional period between childhood and adulthood. The adolescent growth spurt, roughly 3 years of rapid growth, occurs early in this period. An accelerated increase in stature is a hallmark, with about 20 percent of adult stature being attained during this period. Along with the rapid increase in height, other changes in body proportions occur that have important implications for sports and other types of activities offered in physical education and physical activity programs. As boys and girls advance through puberty, for example, biacromial breadth (shoulder width) increases more in boys than in girls, while increases in bicristal breadth (hip width) are quite similar. Consequently, hip-shoulder width ratio, which is similar in boys and girls during childhood, decreases in adolescent boys while remaining relatively constant in girls (Malina et al., 2004). Ratios among leg length, trunk length, and stature also change during this period. Prior to adolescence, boys have longer trunks and shorter legs than girls (Haubenstricker and Sapp, 1980). In contrast, adolescent and adult females have shorter legs for the same height than males of equal stature. Body proportions, particularly skeletal dimensions, are unlikely to be influenced by physical activity rather, body proportions influence performance success, fitness evaluation, and the types of activities in which a person may wish to engage. For example, there is evidence that leg length influences upright balance and speed (Haubenstricker and Sapp, 1980). Individuals who have shorter legs and broader pelvises are better at balancing tasks than those with longer legs and narrower pelvises, and longer legs are associated with faster running times (Dintiman et al., 1997). Also, longer arms and wider shoulders are advantageous in throwing tasks (Haubenstricker and Sapp, 1980), as well as in other activities in which the arms are used as levers. According to Haubenstricker and Sapp (1980), approximately 25 percent of engagement in movement-related activities can be attributed to body size and structure.

Motor Development

Motor development depends on the interaction of experience (e.g., practice, instruction, appropriate equipment) with an individual's physical, cognitive, and psychosocial status and proceeds in a predictable fashion across developmental periods. Clark and Metcalfe (2002) provide an eloquent metaphor—“the mountain of motor development”—to aid in understanding the global changes seen in movement across the life span. Early movements, critical for an infant's survival, are reflexive and dominated by biology, although environment contributes and helps shape reflexes. This initial reflexive period is followed quickly by the preadapted period, which begins when an infant's movement behaviors are no longer reflexive and ends when the infant begins to apply basic movement skills (e.g., crawling, rolling, standing, and walking) that generally are accomplished before 12 months of age. The period of fundamental motor patterns occurs approximately between the ages of 1 and 7 years, when children begin to acquire basic fundamental movement skills (e.g., running, hopping, skipping, jumping, leaping, sliding, galloping, throwing, catching, kicking, dribbling, and striking). Practice and instruction are key to learning these skills, and a great deal of time in elementary school physical education is devoted to exploration of movement. Around age 7, during the so-called context-specific period of motor development, children begin to refine basic motor skills and combine them into more specific movement patterns, ultimately reaching what has been called skillfulness. Compensation, the final period of motor development, occurs at varying points across the life span when, as a result of aging, disease, injury, or other changes, it becomes necessary to modify movement.

While all children need not be 𠇎xpert” in all movement skills, those who do not acquire the fundamental motor skills will likely experience difficulty in transitioning their movement repertoire into specific contexts and engagement in physical activity (Fisher et al., 2005 Barnett et al., 2009 Cliff et al., 2009 Robinson et al., 2012). A full movement repertoire is needed to engage in physical activities within and outside of the school setting. Thus, beyond contributing to levels of physical activity, physical education programs should aim to teach basic fundamental motor skills and their application to games, sports, and other physical activities, especially during the elementary years (i.e., the fundamental motor patterns and context-specific periods). At the same time, it is important to be mindful of the wide interindividual variation in the rate at which children develop motor skills, which is determined by their biological makeup, their rate of physical maturation, the extent and quality of their movement experiences, and their family and community environment.

An increasing amount of evidence suggests that people who feel competent in performing physical skills remain more active throughout their lives (Lubans et al., 2010). Conversely, those who are less skilled may be hesitant to display what they perceive as a shortcoming and so may opt out of activities requiring higher levels of motor competence (Stodden et al., 2008). Children who are less physically skillful tend to be less active than their skillful counterparts (Wrotniak et al., 2006 Williams et al., 2008 Robinson et al., 2012) and thus have a greater risk of overweight and obesity (Graf et al., 2004). Fundamental skills are the building blocks of more complex actions that are completed in sports, physical activities, and exercise settings. For example, throwing is a fundamental skill that is incorporated into the context-specific throw used in activities such as handball, softball, and water polo. Fundamental skills are of primary interest to both physical education teachers and coaches, and physical education classes should be designed to challenge learners to develop their motor skills.

In 1998 the Centers for Disease Control and Prevention's (CDC's) Division of Nutrition and Physical Activity organized a workshop to determine future directions for research on physical activity. The workshop convened 21 experts from a wide range of academic disciplines. One recommendation resulting from the proceedings was for future research to describe the temporal relationship between motor development and physical activity (Fulton et al., 2001), signifying the importance of better understanding of the nature of the relationship between motor competence and physical activity. The assumption of this relationship is implied in multiple models of motor development (Seefeldt, 1980 Clark and Metcalfe, 2002 Stodden et al., 2008), which emphasize the importance of motor competence as a prerequisite for engagement in physical activity throughout the life span.

Two models that are commonly used to examine this relationship are Seefeldt's (1980) hierarchical order of motor skills development and the dynamic association model of Stodden and colleagues (2008). Seefeldt proposed a hierarchical order of motor skills development that includes four levels: reflexes, fundamental motor skills, transitional motor skills (i.e., fundamental motor skills that are performed in various combinations and with variations and that are required to participate in entry-level organized sports, such as throwing for distance, throwing for accuracy, and/or catching a ball while in motion), and specific sports skills and dances. With improved transitional motor skills, children are able to master complex motor skills (e.g., those required for playing more complex sports such as football or basketball). At the end of this developmental period, children's vision is fully mature. The progression through each level occurs through developmental stages as a combined result of growth, maturation, and experience. Seefeldt hypothesized the existence of a “proficiency barrier” between the fundamental and transitional levels of motor skills development. If children are able to achieve a level of competence above the proficiency barrier, they are more likely to continue to engage in physical activity throughout the life span that requires the use of fundamental motor skills. Conversely, less skilled children who do not exceed the proficiency barrier will be less likely to continue to engage in physical activity. Thus, it is assumed that 𠇊 confident and competent mover will be an active mover” (Clark, 2005, p. 44). For example, to engage successfully in a game of handball, baseball, cricket, or basketball at any age, it is important to reach a minimum level of competence in running, throwing, catching, and striking. The assumption of the existence of a relationship between motor competence and physical activity is at the “heart of our physical education programs” (Clark, 2005, p. 44). A thorough understanding of how this relationship changes across developmental stages is crucial for curriculum development and delivery and teaching practices.

Lubans and colleagues (2010) recently examined the relationship between motor competence and health outcomes. They reviewed 21 studies identifying relationships between fundamental motor skills and self-worth, perceived physical competence, muscular and cardiorespiratory fitness, weight status, flexibility, physical activity, and sedentary behavior. Overall, the studies found a positive association between fundamental motor skills and physical activity in children and adolescents, as well as a positive relationship between fundamental motor skills and cardiorespiratory fitness. Other research findings support the hypothesis that the most physically active preschool-age (Fisher et al., 2005 Williams et al., 2008 Robinson et al., 2012), elementary school𠄺ge (Bouffard et al., 1996 Graf et al., 2004 Wrotniak et al., 2006 Hume et al., 2008 Lopes et al., 2011), and adolescent (Okely et al., 2001) youth are also the most skilled.

An advantage of the “proficiency barrier” hypothesis proposed by Seefeldt (1980) is its recognition that the relationship between motor competence and physical activity may not be linear. Rather, the hypothesis suggests that physical activity is influenced when a certain level of motor competence is not achieved and acknowledges that below the proficiency barrier, there is bound to be substantial variation in children's motor competence and participation in physical activity. The proficiency barrier is located between the fundamental and transitional motor skills periods. The transition between these two levels of motor competence is expected to occur between the early and middle childhood years. Stodden and colleagues (2008) suggest that the relationship between motor competence and physical activity is dynamic and changes across time. In their model the �velopment of motor skill competence is a primary underlying mechanism that promotes engagement in physical activity” (p. 290).

The relationship between skills and physical activity is considered reciprocal. It is expected that as motor skills competence increases, physical activity participation also increases and that the increased participation feeds back into motor skills competence. The reciprocal relationship between motor skills competence and physical activity is weak during the early childhood years (ages 2-8) because of a variety of factors, including environmental conditions, parental influences, and previous experience in physical education programs (Stodden et al., 2008). Also, children at this age are less able to distinguish accurately between perceived physical competence and actual motor skills competence (Harter and Pike, 1984 Goodway and Rudisill, 1997 Robinson and Goodway, 2009 Robinson, 2011), and thus motor skills are not expected to strongly influence physical activity. The literature supports this hypothesis, as indicated by low to moderate correlations between motor skills competence and physical activity in preschool (Sääkslahti et al., 1999 Williams et al., 2008 Cliff et al., 2009 Robinson and Goodway, 2009 Robinson, 2011) and early elementary school𠄺ge (Raudsepp and Päll, 2006 Hume et al., 2008 Morgan et al., 2008 Houwen et al., 2009 Ziviani et al., 2009 Lopes et al., 2011) children.

In older children, perceived competence is more closely related to actual motor skills competence. Older, low-skilled children are aware of their skills level and are more likely to perceive physical activity as difficult and challenging. Older children who are not equipped with the necessary skills to engage in physical activity that requires high levels of motor skills competence may not want to display their low competence publicly. As children transition into adolescence and early adulthood, the relationship between motor skills competence and physical activity may strengthen (Stodden et al., 2008). Investigators report moderate correlations between motor skills competence and physical activity in middle school𠄺ge children (Reed et al., 2004 Jaakkola et al., 2009). Okely and colleagues (2001) found that motor skills competence was significantly associated with participation in organized physical activity (i.e., regular and structured experiences related to physical activity) as measured by self-reports. A strength of the model of Stodden and colleagues (2008) is the inclusion of factors related to psychosocial health and development that may influence the relationship between motor skills competence and physical activity, contributing to the development and maintenance of obesity. Other studies have found that perceived competence plays a role in engagement in physical activity (Ferrer-Caja and Weiss, 2000 Sollerhed et al., 2008).

Motor skills competence is an important factor however, it is only one of many factors that contribute to physical activity. For instance, three studies have reported negative correlations between girls' motor competence and physical activity (Reed et al., 2004 Cliff et al., 2009 Ziviani et al., 2009), suggesting that sex may be another determining factor. A possible explanation for these findings is that since girls tend to be less active than boys, it may be more difficult to detect differences in physical activity levels between high- and low-skilled girls. It is also possible that out-of-school opportunities for physical activity are more likely to meet the interests of boys, which may at least partially explain sex differences in physical activity levels (Le Masurier et al., 2005). Previous research suggests that in general boys are more motor competent than girls (Graf et al., 2004 Barnett et al., 2009 Lopes et al., 2011) and that this trend, which is less apparent in early childhood, increases through adolescence (Thomas and French, 1985 Thomas and Thomas, 1988 Thomas, 1994), although one study reports that girls are more motor competent than boys (Cliff et al., 2009).

One component of motor competence is the performance of gross motor skills, which are typically classified into object control and locomotor skills. Consistent evidence suggests that boys are more competent in object control skills, while girls are more competent in locomotor skills (McKenzie et al., 2004 Morgan et al., 2008 Barnett et al., 2009). In light of these sex differences, it is important to examine the relationships of object control and locomotor skills with physical activity separately for boys and girls. For boys, object control skills are more related to physical activity than are locomotor skills (Hume et al., 2008 Morgan et al., 2008 Williams et al., 2008 Cliff et al., 2009), whereas evidence suggests that the reverse is true for girls (McKenzie et al., 2002 Hume et al., 2008 Cliff et al., 2009 Jaakkola et al., 2009). Three studies report a significant relationship between balance and physical activity for girls but not boys (Reed et al., 2004 Ziviani et al., 2009). Cliff and colleagues (2009) suggest that object control and locomotor skills may be more related to boys' and girls' physical activity, respectively, because of the activity type in which each sex typically engages.

The relationship between motor competence and physical activity clearly is complex. It is quite likely that the relationship is dynamic and that motor competence increases the likelihood of participating in physical activity while at the same time engaging in physical activity provides opportunities to develop motor competence (Stodden et al., 2008). Despite some uncertainty, the literature does reinforce the important role of physical education in providing developmentally appropriate movement opportunities in the school environment. These opportunities are the only means of engaging a large population of children and youth and providing them with the tools and opportunities that foster health, development, and future physical activity.

Stature

Regular physical activity has no established effect on linear growth rate or ultimate height (Malina, 1994). Although some studies suggest small differences, factors other than physical activity, especially maturity, often are not well controlled. It is important to note that regular physical activity does not have a negative effect on stature, as has sometimes been suggested. Differences in height among children and adolescents participating in various sports are more likely due to the requirements of the sport, selection criteria, and interindividual variation in biological maturity than the effects of participation per se (Malina et al., 2004).

Body Weight

Although physical activity is inversely related to weight, correlations are generally low (

r𠄰.15), and differences in body weight between active and inactive boys and girls tend to be small (Mirwald and Bailey, 1986 Saris et al., 1986 Beunen et al., 1992 Lohman et al., 2006), except in very obese children and adolescents. Similarly, physique, as represented in somatotypes, does not appear to be significantly affected by physical activity during growth (Malina et al., 2004). In contrast, components of weight can be influenced by regular physical activity, especially when the mode and intensity of the activity are tailored to the desired outcome. Much of the available data in children and adolescents is based on BMI, a surrogate for composition, and indirect methods based on the two-compartment model of body composition in which body weight is divided into its fat-free and fat components (Going et al., 2012). While studies generally support that physical activity is associated with greater fat-free mass and lower body fat, distinguishing the effects of physical activity on fat-free mass from expected changes associated with growth and maturation is difficult, especially during adolescence, when both sexes have significant growth in fat-free mass. The application of methods based on the two-compartment model is fraught with errors, especially when the goal is to detect changes in fat-free mass, and no information is available from these methods regarding changes in the major tissue components of fat-free mass—muscle and skeletal tissue.

Muscle

Skeletal muscle is the largest tissue mass in the body. It is the main energy-consuming tissue and provides the propulsive force for movement. Muscle represents about 23-25 percent of body weight at birth and about 40 percent in adults, although there is a wide range of “normal” (Malina, 1986, 1996). Postnatal muscle growth is explained largely by increases in cell size (hypertrophy) driving an increase in overall muscle mass. The increase in muscle mass with age is fairly linear from young childhood until puberty, with boys having a small but consistent advantage (Malina, 1969, 1986). The sex difference becomes magnified during and after puberty, driven primarily by gender-related differences in sex steroids. Muscle, as a percentage of body mass, increases from about 42 percent to 54 percent in boys between ages 5 and 11, whereas in girls it increases from about 40 percent to 45 percent between ages 5 and 13 and thereafter declines (Malina et al., 2004). It should be noted that absolute mass does not decline rather, the relative decline reflects the increase in the percentage of weight that is fat in girls. At least part of the sex difference is due to differences in muscle development for different body regions (Tanner et al., 1981). The growth rate of arm muscle tissue during adolescence in males is approximately twice that in females, whereas the sex difference in the growth of muscle tissue in the leg is much smaller. The sex difference that develops during puberty persists into adulthood and is more apparent for the musculature of the upper extremities.

Sex-related differences in muscular development contribute to differences in physical performance. Muscle strength develops in proportion to the cross-sectional area of muscle, and growth curves for strength are essentially the same as those for muscle (Malina and Roche, 1983). Thus the sex difference in muscle strength is explained largely by differences in skeletal muscle mass rather than muscle quality or composition. Aerobic (endurance) exercise has little effect on enhancing muscle mass but does result in significant improvement in oxygen extraction and aerobic metabolism (Fournier et al., 1982). In contrast, numerous studies have shown that high-intensity resistance exercise induces muscle hypertrophy, with associated increases in muscle strength. In children and adolescents, strength training can increase muscle strength, power, and endurance. Multiple types of resistance training modalities have proven effective and safe (Bernhardt et al., 2001), and resistance exercise is now recommended for enhancing physical health and function (Behringer et al., 2010). These adaptations are due to muscle fiber hypertrophy and neural adaptations, with muscle hypertrophy playing a more important role in adolescents, especially in males. Prior to puberty, before the increase in anabolic sex steroid concentrations, neural adaptations explain much of the improvement in muscle function with exercise in both boys and girls.

Skeleton

The skeleton is the permanent supportive framework of the body. It provides protection for vital organs and is the main mineral reservoir. Bone tissue constitutes most of the skeleton, accounting for 14-17 percent of body weight across the life span (Trotter and Peterson, 1970 Trotter and Hixon, 1974). Skeletal strength, which dictates fracture risk, is determined by both the material and structural properties of bone, both of which are dependent on mineral accrual. The relative mineral content of bone does not differ much among infants, children, adolescents, and adults, making up 63-65 percent of the dry, fat-free weight of the skeleton (Malina, 1996). As a fraction of weight, bone mineral (the ash weight of bone) represents about 2 percent of body weight in infants and about 4-5 percent of body weight in adults (Malina, 1996). Bone mineral content increases fairly linearly with age, with no sex difference during childhood. Girls have, on average, a slightly greater bone mineral content than boys in early adolescence, reflecting their earlier adolescent growth spurt. Boys have their growth spurt later than girls, and their bone mineral content continues to increase through late adolescence, ending with greater skeletal dimensions and bone mineral content (Mølgaard et al., 1997). The increase in total body bone mineral is explained by both increases in skeletal length and width and a small increase in bone mineral density (Malina et al., 2004).

Many studies have shown a positive effect of physical activity on intermediate markers of bone health, such as bone mineral content and density. Active children and adolescents have greater bone mineral content and density than their less active peers, even after controlling for differences in height and muscle mass (Wang et al., 2004 Hind and Burrows, 2007 Tobias et al., 2007). Exercise interventions support the findings from observational studies showing beneficial effects on bone mineral content and density in exercise participants versus controls (Petit et al., 2002 Specker and Binkley, 2003), although the benefit is less than is suggested by cross-sectional studies comparing active versus inactive individuals (Bloomfield et al., 2004). The relationship between greater bone mineral density and bone strength is unclear, as bone strength cannot be measured directly in humans. Thus, whether the effects of physical activity on bone mineral density translate into similar benefits for fracture risk is uncertain (Karlsson, 2007). Animal studies have shown that loading causes small changes in bone mineral content and bone mineral density that result in large increases in bone strength, supporting the notion that physical activity probably affects the skeleton in a way that results in important gains in bone strength (Umemura et al., 1997). The relatively recent application of peripheral quantitative computed tomography for estimating bone strength in youth has also provided some results suggesting an increase in bone strength with greater than usual physical activity (Sardinha et al., 2008 Farr et al., 2011).

The intensity of exercise appears to be a key determinant of the osteogenic response (Turner and Robling, 2003). Bone tissue, like other tissues, accommodates to usual daily activities. Thus, activities such as walking have a modest effect at best, since even relatively inactive individuals take many steps (ϡ,000) per day. Activities generating greater muscle force on bone, such as resistance exercise, and “impact” activities with greater than ordinary ground reaction forces (e.g., hopping, skipping, jumping, gymnastics) promote increased mineralization and modeling (Bloomfield et al., 2004 Farr et al., 2011). Far fewer randomized controlled trials (RCTs) examining this relationship have been conducted in children than in adults, and there is little evidence on dose response to show how the type of exercise interacts with frequency, intensity, and duration. Taken together, however, the available evidence supports beneficial effects of physical activity in promoting bone development (Bailey et al., 1996 Modlesky and Lewis, 2002).

Physical activity may reduce osteoporosis-related fracture risk by increasing bone mineral accrual during development by enhancing bone strength and by reducing the risk of falls by improving muscle strength, flexibility, coordination, and balance (Bloomfield et al., 2004). Early puberty is a key developmental period. Approximately 26 percent of the mineral content in the adult skeleton is accrued during the 2 years around the time of peak height velocity (Bailey et al., 2000). This amount of mineral accrual represents approximately the same amount of bone mineral that most people will lose in their entire adult lives (Arlot et al., 1997). The increase in mineral contributes to increased bone strength. Mineral is accrued on the periosteal surface of bone, such that the bone grows wider. Increased bone width, independent of the increased mineral mass, also contributes to greater bone strength. Indeed, an increase of as little as 1 mm in the outer surface of bone increases strength substantially. Adding bone to the endosteal surface also increases strength (Parfitt, 1994 Wang et al., 2009). Increases in testosterone may be a greater stimulus of periosteal expansion than estrogen since testosterone contributes to wider and stronger bones in males compared with females. Retrospective studies in tennis players and gymnasts suggest structural adaptations may persist many years later in adulthood and are greatest when “impact” activity is initiated in childhood (Kannus et al., 1995 Bass et al., 1998). RCTs on this issue are few, although the available data are promising (McKay et al., 2000 Fuchs et al., 2001 MacKelvie et al., 2001, 2003 Lindén et al., 2006). Thus, impact exercise begun in childhood may result in lasting structural changes that may contribute to increased bone strength and decreased fracture risk later in life (Turner and Robling, 2003 Ferrari et al., 2006).

Adipose tissue

The adipose “organ” is composed of fat cells known as adipocytes (Ailhaud and Hauner, 1998). Adipocytes are distributed throughout the body in various organs and tissues, although they are largely clustered anatomically in structures called fat depots, which include a large number of adipocytes held together by a scaffold-like structure of collagen and other structural molecules. In the traditional view of the adipocyte, the cell provides a storage structure for fatty acids in the form of triacylglycerol molecules, with fatty acids being released when metabolic fuel is needed (Arner and Eckel, 1998). While adipocytes play this critical role, they are also involved in a number of endocrine, autocrine, and paracrine actions and play a key role in regulating other tissues and biological functions, for example, immunity and blood pressure, energy balance, glucose and lipid metabolism, and energy demands of exercise (Ailhaud and Hauner, 1998 Frühbeck et al., 2001). The role of adipocytes in regulation of energy balance and in carbohydrate and lipid metabolism and the potential effects of physical activity on adipocyte function are of particular interest here, given growing concerns related to pediatric and adult obesity (Ogden et al., 2012) and the associated risk of cardiometabolic disease (Weiss et al., 2004 Eisenmann, 2007a,b Steele et al., 2008). Metabolic differences among various fat depots are now well known (Frühbeck et al., 2001), and there is significant interest in the distribution of adipose tissue, the changes that occur during childhood and adolescence, and their clinical significance.

Adipocytes increase in size (hypertrophy) and number (hyperplasia) from birth through childhood and adolescence and into young adulthood to accommodate energy storage needs. The number of adipocytes has been estimated to increase from about 5 billion at birth to 30 billion to 50 billion in the nonobese adult, with an increase in average diameter from about 30-40 μm at birth to about 80-100 μm in the young adult (Knittle et al., 1979 Bonnet and Rocour-Brumioul, 1981 Chumlea et al., 1982). In total the adipose organ contains about 0.5 kg of adipocytes at birth in both males and females, increasing to approximately 10 kg in average-weight-for-height males and 14 kg in females (Malina et al., 2004). There is wide interindividual variation, however, and the difficulty of investigating changes in the number and size of adipocytes is obvious given the invasiveness of the required biopsy procedures understandably, then, data on these topics are scarce in children and adolescents. Also, since only subcutaneous depots are accessible, results must be extrapolated from a few sites.

Based on such information, the average size of adipocytes has been reported to increase two- to threefold in the first year of life, with little increase in nonobese boys and girls until puberty (Malina et al., 2004). A small increase in average adipocyte size at puberty is more obvious in girls than in boys. There is considerable variation in size across various subcutaneous sites and between subcutaneous and internal depots. The number of adipocytes is difficult to estimate. Available data suggest that the cellularity of adipose tissue does not increase significantly in early postnatal life (Malina et al., 2004). Thus, gain in fat mass is the result of an increase in the size of existing adipocytes. From about 1-2 years of age and continuing through early and middle childhood, the number of adipocytes increases gradually two- to threefold. With puberty the number practically doubles, followed by a plateau in late adolescence and early adulthood. The number of adipocytes is similar in boys and girls until puberty, when girls experience a greater increase than boys.

The increases in the number of adipocytes during infancy and puberty are considered critical for enlargement of the adipose tissue organ and for the risk of obesity. Since size and number are linked, the number of adipocytes can potentially increase at any age if fat storage mechanisms are stimulated by chronic energy surfeit (Hager, 1981 Chumlea et al., 1982). Energy expenditure through regular physical activity is a critical element in preventing energy surfeit and excess adiposity. While cellularity undoubtedly is strongly genetically determined, regular physical activity, through its contribution to energy expenditure, can contribute to less adipocyte hyperplasia by limiting hypertrophy.

Fat distribution

Fat distribution refers to the location of fat depots on the body. The metabolic activities of fat depots differ, and small variation can have a long-term impact on fat distribution. Differences in metabolic properties across depots also have clinical implications. Visceral adipose tissue in the abdominal cavity is more metabolically active (reflected by free fatty acid flux) than adipose tissue in other areas (Arner and Eckel, 1998), and higher amounts of visceral adipose tissue are associated with greater risk of metabolic complications, such as type 2 diabetes and cardiovascular disease (Daniels et al., 1999 He et al., 2007 Dencker et al., 2012). In contrast, subcutaneous fat, particularly in the gluteofemoral region, is generally associated with a lower risk of cardiometabolic disease. Age- and sex-associated variations in fat distribution contribute to age- and sex-associated differences in cardiometabolic disease prevalence. Girls have more subcutaneous fat than boys at all ages, although relative fat distribution is similar. After a rapid rise in subcutaneous fat in the first few months of life, both sexes experience a reduction through age 6 or 7 (Malina and Roche, 1983 Malina and Bouchard, 1988 Malina, 1996). Girls then show a linear increase in subcutaneous fat, whereas boys show a small increase between ages 7 and 12 or 13 and then an overall reduction during puberty. The thickness of subcutaneous fat on the trunk is approximately one-half that of subcutaneous fat on the extremities in both boys and girls during childhood. The ratio increases with age in males during adolescence but changes only slightly in girls. In males the increasing ratio of trunk to extremity subcutaneous fat is a consequence of slowly increasing trunk subcutaneous fat and a decrease in subcutaneous fat on the extremities. In girls, trunk and extremity subcutaneous fat increase at a similar rate thus the ratio is stable (Malina and Bouchard, 1988). As a consequence, the sex difference in the distribution of body fat develops during adolescence. It is important to note that changes in subcutaneous fat pattern do not necessarily represent changes in abdominal visceral adipose tissue.

Tracking of subcutaneous fat has been investigated based on skinfold thicknesses and radiographs of fat widths in males and females across a broad age range (Katzmarzyk et al., 1999 Campbell et al., 2012). Results indicate that subcutaneous fat is labile during early childhood. After age 7 to 8, correlations between subcutaneous fat in later childhood and adolescence and adult subcutaneous fat are significant and moderate. Longitudinal data on tracking of visceral adipose tissue are not available, but percent body fat does appear to track. Thus children and especially adolescents with higher levels of body fat have a higher risk of being overfat at subsequent examinations and in adulthood, although variation is considerable, with some individuals moving away from high fatness categories, while some lean children move into higher fatness categories.

In cross-sectional studies, active children and adolescents tend to have lower skinfold thicknesses and less overall body fat than their less active peers (Loftin et al., 1998 Rowlands et al., 2000 Stevens et al., 2004 Lohman et al., 2006), although the correlations are modest, reflecting variation in body composition at different levels of physical activity, as well as the difficulty of measuring physical activity. Longitudinal studies indicate small differences in fatness between active and inactive boys and girls. Although some school-based studies of the effects of physical activity on body composition have reported changes in BMI or skinfolds in the desired direction (Gortmaker et al., 1999 McMurray et al., 2002), most have not shown significant effects. High levels of physical activity are most likely needed to modify skinfold thicknesses and percent body fat. In adults, visceral adipose tissue declines with weight loss with exercise. In contrast, in a study of obese children aged 7-11, a 4-month physical activity program resulted in minimal change in abdominal visceral adipose tissue but a significant loss in abdominal subcutaneous adipose tissue (Gutin and Owens, 1999). In adults, decreases in fatness with exercise are due to a reduction in fat cell size, not number (You et al., 2006) whether this is true in children is not certain but appears likely. Given that adipocyte hypertrophy may trigger adipocyte hyperplasia (Ballor et al., 1998), energy expenditure through regular physical activity may be important in preventing excess adipose tissue cellularity. Regular physical activity also affects adipose tissue metabolism so that trained individuals have an increased ability to mobilize and oxidize fat, which is associated with increased levels of lipolysis, an increased respiratory quotient, and a lower risk of obesity (Depres and Lamarche, 2000).

Cardiorespiratory System

The ability to perform sustained activity under predominantly aerobic conditions depends on the capacity of the cardiovascular and pulmonary systems to deliver oxygenated blood to tissues and on the ability of tissues (primarily skeletal muscle) to extract oxygen and oxidize substrate. By age 2 the systems are fully functional, although young children lack the cardiorespiratory capacity of older children and adults because of their small size (Malina et al., 2004). Children's aerobic capacity and consequently their ability to exercise for longer periods of time increase as they grow. Maximal aerobic power (liters per minute) increases fairly linearly in boys until about age 16, whereas it increases in girls until about age 13 and then plateaus during adolescence (Malina et al., 2004 Eisenmann et al., 2011). Differences between boys and girls are small (

10 percent) during childhood and greater after the adolescent growth spurt, when girls have only about 70 percent of the mean value of boys. Changes with age and sex differences are explained largely by differences in the size of the relevant tissues. Dimensions of the heart and lungs enlarge with age in a manner consistent with the increase in body mass and stature (Malina et al., 2004). The increase in the size of the heart is associated with increases in stroke volume (blood pumped per beat) and cardiac output (product of stroke volume and heart rate, liters per minute), despite a decline in heart rate during growth. Similarly, increase in lung size (proportional to growth in height) results in greater lung volume and ventilation despite an age-associated decline in breathing frequency. From about age 6 to adulthood, maximal voluntary ventilation approximately doubles (50� L/min) (Malina et al., 2004). The general pattern of increase as a function of height is similar in boys and girls. In both, lung function tends to lag behind the increase in height during the adolescent growth spurt. As a result, peak gains in lung function occur about 2 years earlier in girls than in boys.

Blood volume is highly related to body mass and heart size in children and adolescents, and it is also well correlated with maximal oxygen uptake during childhood and adolescence (Malina et al., 2004). Blood volume increases from birth through adolescence, following the general pattern for changes in body mass. Both red blood cells and hemoglobin have a central role in transport of oxygen to tissues. Hematocrit, the percentage of blood volume explained by blood cells, increases progressively throughout childhood and adolescence in boys, but only through childhood in girls. Hemoglobin content, which is related to maximal oxygen uptake, heart volume, and body mass, increases progressively with age into late adolescence. Males have greater hemoglobin concentrations than females, especially relative to blood volume, which has functional implications for oxygen transport during intense exercise.

Growth in maximal aerobic power is influenced by growth in body size, so controlling for changes in body size during growth is essential. Although absolute (liters per minute) aerobic power increases into adolescence relative to body weight, there is a slight decline in both boys and girls, suggesting that body weight increases at a faster rate than maximal oxygen consumption, particularly during and after the adolescent growth spurt (Malina et al., 2004). Changes in maximal oxygen consumption during growth tend to be related more closely to fat-free mass than to body mass. Nevertheless, sex differences in maximal oxygen consumption per unit fat-free mass persist, and maximal oxygen consumption per unit fat-free mass declines with age.

Improvements in cardiorespiratory function—involving structural and functional adaptations in the lungs, heart, blood, and vascular system, as well as the oxidative capacity of skeletal muscle—occur with regular vigorous- and moderate-intensity physical activity (Malina et al., 2004). Concern about the application of invasive techniques limits the available data on adaptations in the oxygen transport system in children. Nevertheless, it is clear that aerobic capacity in youth increases with activity of sufficient intensity and that maximal stroke volume, blood volume, and oxidative enzymes improve after exercise training (Rowland, 1996). Training-induced changes in other components of the oxygen transport system remain to be determined.

Health- and Performance-Related Fitness

Physical fitness is a state of being that reflects a person's ability to perform specific exercises or functions and is related to present and future health outcomes. Historically, efforts to assess the physical fitness of youth focused on measures designed to evaluate the ability to carry out certain physical tasks or activities, often related to athletic performance. In more recent years, the focus has shifted to greater emphasis on evaluating health-related fitness (IOM, 2012a) and assessing concurrent or future health status. Health- and performance-related fitness, while overlapping, are different constructs. Age- and sex-related changes in the components of both are strongly linked to the developmental changes in tissues and systems that occur during childhood and adolescence. Although genetic factors ultimately limit capacity, environmental and behavioral factors, including physical activity, interact with genes to determine the degree to which an individual's full capacity is achieved.

Health-Related Fitness

Cardiorespiratory endurance, muscular strength and endurance, flexibility, and body composition are components of health-related fitness historically assessed in school-based fitness assessment programs (IOM, 2012a). These components of health-related fitness are considered important since they can be linked to the risk of cardiometabolic disease and musculoskeletal disability, chronic hypokinetic-related diseases.

Cardiorespiratory endurance

Cardiorespiratory (aerobic) endurance reflects the functioning of the pulmonary and cardiovascular systems to deliver oxygen and the ability of tissues (primarily skeletal muscle) to extract oxygen from the blood. Defined clinically as the maximum oxygen consumption during a maximal graded exercise test, in practice it is usually measured indirectly as performance on a field test of endurance, such as 1- or 2-mile run time (IOM, 2012a). During childhood, aerobic capacity approximately doubles in both boys and girls, although girls on average possess a lower capacity. Males continue to improve during adolescence, up to ages 17-18, while aerobic capacity plateaus around age 14 in females (Malina et al., 2004), resulting in an approximately 20 percent difference between males and females (Rowland, 2005).

Favorable associations have been found between aerobic endurance and high-density lipoproteins, systolic blood pressure, diastolic blood pressure, BMI, measures of fatness, arterial stiffness, and measures of insulin sensitivity (Boreham et al., 2004 Imperatore et al., 2006 Hussey et al., 2007 Ondrak et al., 2007). Some evidence suggests a decline in aerobic endurance among U.S. youth in recent decades (Eisenmann, 2003 Carnethon et al., 2005 Pate et al., 2006), coincident with increased sedentariness and obesity and a greater prevalence of metabolic syndrome in youth. Aerobic exercise has been shown to increase cardiorespiratory endurance by about 5-15 percent in youth (Malina et al., 2004 HHS, 2008). The programs that produce this benefit involve continuous vigorous- or moderate-intensity aerobic activity of various types for 30-45 minutes per session at least 3 days per week over a period of at least 1-3 months (Baquet et al., 2002) improvements are greater with more frequent exercise (Baquet et al., 2003).

Muscle strength and endurance

Muscle strength is defined as the highest force generated during a single maximum voluntary contraction, whereas muscle endurance is the ability to perform repeated muscular contraction and force development over a period of time. Muscle strength and endurance are correlated, especially at higher levels of force production. Muscle strength is proportional to the cross-sectional area of skeletal muscle consequently, strength growth curves parallel growth curves for body weight and skeletal muscle mass (Malina et al., 2004).

Both males and females show impressive increases in muscle strength from childhood to adolescence. Strength in children increases linearly, with boys having a slight advantage over girls. However, these sex differences are magnified during the adolescent years as a result of maturation (Malina and Roche, 1983). Differences in muscle strength between boys and girls become more apparent after puberty, primarily as a result of the production of sex steroid hormones. In boys the increase in strength during adolescence lags behind the growth spurt by at least a year (peak height velocity), which may explain why some boys experience a brief period of clumsiness or awkwardness during puberty, as they have not yet acquired the muscle strength necessary to handle the changes associated with their larger bodies. Muscle strength increases at its greatest rate approximately 1 year after peak height velocity in boys, whereas for girls the strength spurt generally occurs during the same year as peak height velocity (Bar-Or, 1983).

A compelling body of evidence indicates that with resistance training children and adolescents can significantly increase their strength above that expected as a result of normal growth and maturation, provided that the training program is of sufficient intensity, volume, and duration (Committee on Sports Medicine Fitness, 2001). Both boys and girls can benefit, and strength gains in children as young as 5-6 have been reported (Faigenbaum et al., 2009), although most studies are of older children and adolescents. Gains in muscle strength of about 30 percent are typical, although considerably larger gains have been reported. Adolescents make greater gains than preadolescents in absolute strength, whereas reported relative (percent above initial strength) gains in strength during preadolescence and adolescence are similar. A variety of programs and modalities have proved efficacious (Council on Sports Medicine Fitness, 2008), as long as load (

10-15 repetitions maximum) and duration (

8-20 weeks) are adequate. As in adults, training adaptations in youth are specific to the muscle action or muscle groups that are trained, and gains are transient if training is not maintained (Faigenbaum et al., 2009).

Youth resistance training, as with most physical activities, does carry some degree of risk of musculoskeletal injury, yet the risk is no greater than that associated with other sports and activities in which children and adolescents participate (Council on Sports Medicine Fitness, 2008 Faigenbaum et al., 2009) as long as age-appropriate training guidelines are followed. A traditional area of concern has been the potential for training-induced damage to growth cartilage, which could result in growth disturbances. However, a recent review found no reports of injury to growth cartilage in any prospective study of resistance training in youth and no evidence to suggest that resistance training negatively impacts growth and maturation during childhood and adolescence (Faigenbaum et al., 2009). Injuries typically occur in unsupervised settings and when inappropriate loads and progressions are imposed.

In addition to the obvious goal of gaining strength, resistance training may be undertaken to improve sports performance and prevent injuries, rehabilitate injuries, and enhance health. Appropriately supervised programs emphasizing strengthening of trunk muscles in children theoretically benefit sport-specific skill acquisition and postural control, although these benefits are difficult to study and thus are supported by little empirical evidence (Council on Sports Medicine Fitness, 2008). Similarly, results are inconsistent regarding the translation of increased strength to enhanced athletic performance in youth. Limited evidence suggests that strength-training programs that address common overuse injuries may help reduce injuries in adolescents, but whether the same is true in preadolescents is unclear (Council on Sports Medicine Fitness, 2008). Increasing evidence suggests that strength training, like other forms of physical activity, has a beneficial effect on measurable health indices in youth, such as cardiovascular fitness, body composition, blood lipid profiles and insulin sensitivity (Faigenbaum, 2007 Benson et al., 2008), bone mineral density and bone geometry (Morris et al., 1997 MacKelvie et al., 2004), and mental health (Holloway et al., 1988 Faigenbaum et al., 1997 Annesi et al., 2005 Faigenbaum, 2007). Some work has shown that muscle fitness, reflected in a composite index combining measures of muscle strength and endurance, and cardiorespiratory fitness are independently and negatively associated with clustered metabolic risk (Steene-Johannessen et al., 2009). Moreover, children with low muscle strength may be at increased risk of fracture with exercise (Clark et al., 2011). Finally, muscle hypertrophy, which adds to fat-free mass, contributes to resting metabolic rate and therefore total daily energy expenditure. Resistance training may be particularly useful for raising metabolic rate in overweight and obese children without the risk associated with higher-impact activities (Watts et al., 2005 Benson et al., 2007).

Flexibility

Flexibility has been operationally defined as “the intrinsic property of body tissues, including muscle and connective tissues, that determines the range of motion achievable without injury at a joint or group of joints” (IOM, 2012b, p. 190). At all ages, girls demonstrate greater flexibility than boys, and the difference is greatest during the adolescent growth spurt and sexual maturation. Perhaps the most common field measure of flexibility in children and youth is the sit-and-reach test (IOM, 2012b) of low-back flexibility. Low-back flexibility as measured by this test is stable in girls from age 5 to 11 and increases until late adolescence. In boys, low-back flexibility declines linearly starting at age 5, reaching its nadir at about age 12, and then increases into late adolescence. The unique pattern of age- and sex-associated variation is related to the growth of the lower extremities and the trunk during adolescence. In boys the nadir in low-back flexibility coincides with the adolescent growth spurt in leg length. In both boys and girls, the increase during adolescence coincides with the growth spurt in trunk length and arm length, which influences reach. Flexibility in both males and females tends to decline after age 17, in part as a result of a decline in physical activity and normal aging.

The principal health outcomes hypothesized to be associated with flexibility are prevention of and relief from low-back pain, prevention of musculoskeletal injury, and improved posture. These associations have been studied in adults, with equivocal results (Plowman, 1992). Although flexibility has long been included in national youth fitness tests, it has proven difficult to establish a link between flexibility and health (IOM, 2012a). In contrast to other fitness components that are general or systemic in nature, flexibility is highly specific to each joint of the body. Although appropriate stretching may increase flexibility, establishing a link to improved functional capacity and fitness is difficult. A few studies suggest that improvements in flexibility as measured by the sit-and-reach test may be related to less low-back pain (Jones et al., 2007 Ahlqwist et al., 2008), but the evidence is weak. Consequently, the Institute of Medicine (IOM) Committee on Fitness Measures and Health Outcomes in its recent report elected to forego recommending a flexibility test for a national youth fitness test battery pending further research to confirm the relationship between flexibility and health and to develop national normative data (IOM, 2012a).

Body composition

Body composition is the component of health-related fitness that relates to the relative amount of adipose tissue, muscle, bone, and other vital components (e.g., organs, connective tissues, fluid compartments) that make up body weight. Most feasible methods for assessing body composition are based on models that divide the body into fat and fat-free (all nonfat constituents) components (Going et al., 2012). Although fat mass and adipose tissue are not equivalent components, fat mass is easier to estimate than adipose tissue, and it is correlated with performance and disease risk. In settings in which estimation of body fat is difficult, weight-for-height ratios often are used as surrogates for body composition. Indeed, definitions of pediatric overweight and obesity have been based on BMI, calculated as weight in kilograms divided by height squared. Child and adolescent obesity defined by BMI remains at all-time highs. Population surveys indicate that approximately 33 percent of all boys and girls are overweight, and nearly one in five are obese (Ogden and Flegal, 2011). The tendency for excess fatness to persist from childhood and adolescence into adulthood (Daniels et al., 2005), coupled with the strong association between obesity and chronic disease (Weiss and Caprio, 2005 Barlow, 2007), has caused great concern for future obesity levels and the health of youth and adults alike (IOM, 2005, 2012b).

The increase in prevalence of obesity is undoubtedly due to a mismatch between energy intake and expenditure. Population surveys have shown that few children and youth meet recommended levels of daily physical activity (see Chapter 2). Prospective studies have shown a significant and inverse relationship between habitual physical activity and weight gain (Berkey et al., 2003), and in some studies physical activity is a better predictor of weight gain than estimates of calorie or fat intake (Berkey et al., 2000 Janssen et al., 2005). These relationships are better established in adults than in children and youth, although even in preschool children, low levels of physical activity, estimated from doubly labeled water, were found to be indicative of higher body fat content (Davies et al., 1995). While studies of exercise without caloric restriction generally show only small effects on body weight, significant albeit moderate reductions of body fat are generally reported (Eisenmann, 2003). Moreover, even in the absence of significant weight loss, exercise has beneficial effects on risk factors for cardiometabolic disease (Ross and Bradshaw, 2009 Gutin and Owens, 2011).

Body mass index

Changes in weight for height with growth and maturation for U.S. boys and girls are described in CDC growth curves (Kuczmarski et al., 2000). Current growth curves were derived from U.S. population surveys conducted before the increase in weight for height that defines today's pediatric obesity epidemic. In boys and girls, BMI declines during early childhood, reaching its nadir at about ages 5-6, and then increases through adolescence. A gender difference emerges during puberty, with males gaining greater fat-free mass than females. Both the period of �iposity rebound” (the increase in BMI in midchildhood following the decline in early childhood) and puberty are times of risk for excess fat gain that correlates with future adiposity (Rolland-Cachera et al., 1984). Physical activity and BMI are inversely correlated in children and adolescents, although the correlations are modest (Lohman et al., 2006), reflecting the difficulty of measuring physical activity, as well as variation in body composition and physical activity at a given weight (Rowlands et al., 2000). Indeed, when studied separately, fat mass index (FMI, or fat mass divided by height squared) and fat-free mass index (FFMI, or fat-free mass divided by height squared) are both inversely related to physical activity. With FMI controlled, however, FFMI is positively related to physical activity, indicating that, for a given level of body fat, individuals with more fat-free mass are more active (Lohman et al., 2006). BMI cut-points for defining overweight and obesity have historically been based on age- and gender-specific population distributions of BMI. Recent work has shown good correspondence between BMI standards and percent fat standards that are referenced to health criteria (Laurson et al., 2011). These new standards should prove useful for identifying children and adolescents at risk for higher levels of cardiometabolic risk factors.

Percent body fat

Direct measures of body fat as a percent of weight provide a better index of adiposity and health risk than BMI (Zeng et al., 2012), which is confounded by variation in lean tissue mass relative to height. Recently, percent fat growth curves were established for representative samples of U.S. boys and girls using National Health and Nutrition Examination Survey (NHANES) data (Laurson et al., 2011 Ogden and Flegal, 2011). Median percent fat for boys aged 5-18 ranged from 14 to 19 percent and for girls across the same ages 15 to 28 percent. In both boys and girls, percent fat increases slowly during early childhood, with girls having a consistently greater relative fatness than boys after ages 5-6. In girls, percent fat increases gradually throughout adolescence in the same manner as fat mass. In boys, percent fat increases gradually until the adolescent growth spurt and thereafter gradually declines until about age 16-17, reflecting the rapid growth in fat-free mass relative to fat mass. After age 17, percent fat in males gradually increases again into adulthood.

The increased prevalence of child and adolescent obesity as defined by BMI presumably also reflects increased adiposity, although the degree is not certain as population-based estimates of percent fat have only recently been developed (Laurson et al., 2011). Health-related percent fat standards recently were developed by determining levels of body fat associated with greater occurrence of chronic disease risk factors defined by metabolic syndrome (Going et al., 2011). In boys and girls aged 12-18, body fat above 20-24 percent and above 27-31 percent, respectively, was predictive of metabolic syndrome.

Physical activity is inversely correlated with percent body fat (Rowlands et al., 2000 Lohman et al., 2006), although the correlations are modest, and changes in overall fatness as well as subcutaneous adipose tissue with habitual physical activity are reasonably well documented in children and adolescents (Gutin and Humphries, 1998 Gutin and Owens, 1999 Dionne et al., 2000). In youth, as in adults, the effects of exercise without caloric restriction are modest and are influenced by the initial level of body fat and the duration and regimen of exercise (Going, 1999). Experimental studies have documented reductions in percent body fat with aerobic exercise, especially in children and adolescents who are overweight or obese at the initiation of an exercise program (Davis et al., 2012). Regular physical activity also affects adipose tissue metabolism (Gutin and Owens, 1999). Individuals who engage in aerobic endurance exercise training have an increased ability to mobilize and oxidize fat, which is associated with increased levels of lipolysis (Depres and Lamarche, 2000). Similar information on adipose tissue metabolism in children and youth is lacking, although one can reasonably expect similar adaptations in older adolescents.

Metabolic syndrome

The tendency for risk factors for cardiometabolic disease to cluster, now called metabolic syndrome, is well recognized in adults (Alberti and Zimmet, 1998). Similar clustering occurs in older children and especially adolescents (Cook et al., 2003), and interest in metabolic syndrome has increased, driven by the increased prevalence of pediatric obesity and the increasing incidence and earlier onset of type 2 diabetes in youth. There is as yet no accepted definition of metabolic syndrome for use in pediatric populations (Jolliffe and Janssen, 2007). Typically, adult definitions are extrapolated to children and adolescents, with appropriate adjustments of the thresholds for the defining variables. Perhaps the most common approach is to emulate the National Cholesterol Education Program (NCEP), which defines metabolic syndrome as exceeding thresholds on three of five components: waist circumference, blood pressure (systolic or diastolic), blood lipids (high-density lipoprotein [HDL] and triglycerides), and blood glucose levels (NIH, 2001).

The concept of metabolic syndrome is useful as it provides an integrated index of risk, and it recently was used to derive health-related percent-body-fat standards (Laurson et al., 2011). Based on NHANES data, the prevalence of metabolic syndrome varies with the degree of obesity, and it is estimated at 4-6 percent of children and adolescents (Cook et al., 2003 Dubose et al., 2007) among obese youth it may be as high as 30-50 percent (Weiss et al., 2004). Youth with metabolic syndrome have an increased risk of type 2 diabetes and cardiovascular disease. In adults a loss of 5-10 percent of body weight through calorie restriction and exercise has been shown to reduce the risk of cardiometabolic disease by improving risk factors (Diabetes Prevention Program Research Group, 2002 Ross and Janiszewski, 2008). In particular, weight loss results in reduced visceral adipose tissue, a strong correlate of risk (Knowler et al., 2002), as well as lower blood pressure and blood glucose levels due to improved insulin sensitivity. Even without significant weight loss, exercise can have significant effects in adults by improving glucose metabolism, improving lipid and lipoprotein profiles, and lowering blood pressure, particularly for those who are significantly overweight (Ross and Bradshaw, 2009). Similar benefits have been observed in adolescents.

A growing body of literature addresses the associations of physical activity, physical fitness, and body fatness with the risk of metabolic syndrome and its components in children and especially adolescents (Platat et al., 2006 McMurray et al., 2008 Rubin et al., 2008 Thomas and Williams, 2008 Christodoulos et al., 2012). Studies in adults have shown that higher levels of physical activity predict slower progression toward metabolic syndrome in apparently healthy men and women (Laaksonen et al., 2002 Ekelund et al., 2005), an association that is independent of changes in body fatness and cardiorespiratory fitness (Ekelund et al., 2007). Few population studies have focused on these relationships in children and adolescents, and the use of self-reported activity, which is imprecise in these populations, tends to obscure associations. In a large sample of U.S. adolescents aged 12-19 in the 1999� NHANES, for example, there was a trend for metabolic syndrome to be more common in adolescents with low activity levels than in those with moderate or high activity levels, although the differences among groups were not statistically significant (Pan and Pratt, 2008). Moreover, for each component of metabolic syndrome, prevalence was generally lower with higher physical activity levels, and adolescents with low physical activity levels had the highest rates of all metabolic syndrome components.

The association between cardiorespiratory fitness and metabolic syndrome also was examined in the 1999� NHANES (Lobelo et al., 2010). Cardiorespiratory fitness was measured as estimated peak oxygen consumption using a submaximal treadmill exercise protocol, and metabolic syndrome was represented as a 𠇌lustered score” derived from five established risk factors for cardiovascular disease, an adiposity index, insulin resistance, systolic blood pressure, triglycerides, and the ratio of total to HDL cholesterol. Mean clustered risk score decreased across increasing fifths (quintiles) of cardiorespiratory fitness in both males and females. The most significant decline in risk score was observed from the first (lowest) to the second quintile (53.6 percent and 37.5 percent in males and females, respectively), and the association remained significant in both overweight and normal-weight males and in normal-weight females. Other studies, using the approach of cross-tabulating subjects into distinct fitness and fatness categories, have examined associations of fitness and fatness with metabolic syndrome risk (Eisenmann et al., 2005, 2007a,b Dubose et al., 2007). Although different measures of fitness, fatness, and metabolic syndrome risk were used, the results taken together across a wide age range (7�) show that fitness modifies the influence of fatness on metabolic syndrome risk. In both males and females, high-fit/low-fatness subjects have less metabolic syndrome risk than low-fit/high-fatness subjects (Eisenmann, 2007).

That many adult chronic health conditions have their origins in childhood and adolescence is well supported (Kannel and Dawber, 1972 Lauer et al., 1975 Berenson et al., 1998 IOM, 2004). Both biological (e.g., adiposity, lipids) and behavioral (e.g., physical activity) risk factors tend to track from childhood and especially adolescence into adulthood. Childhood BMI is related to adult BMI and adiposity (Guo et al., 1994, 2000 Freedman et al., 2005), and as many as 80 percent of obese adolescents become obese adults (Daniels et al., 2005). Coexistence of cardiometabolic risk factors, even at young ages (Dubose et al., 2007 Ramírez-Vélez et al., 2012), has been noted, and these components of metabolic syndrome also have been shown to track to adulthood (Bao et al., 1994 Katzmarzyk et al., 2001 Huang et al., 2008). Landmark studies from the Bogalusa Heart Study (Berenson et al., 1998 Li et al., 2003) and others (Mahoney et al., 1996 Davis et al., 2001 Morrison et al., 2007, 2008) have demonstrated that cardiometabolic risk factors present in childhood are predictive of adult disease.

The benefits of exercise for prevention and treatment of cardiometabolic disease in adults are well described (Ross et al., 2000 Duncan et al., 2003 Gan et al., 2003 Irwin et al., 2003 Lee et al., 2005 Sigal et al., 2007 Ross et al., 2012). Prospective studies examining the effects of exercise on metabolic syndrome in children and adolescents remain limited, and it is important to refrain from extrapolating intervention effects observed in adults to youth, although one might reasonably assume the benefits in older adolescents to be similar to those in young adults. Indeed, based on the inverse associations of physical activity and physical fitness with metabolic syndrome (Kim and Lee, 2009) and on the available intervention studies, some experts have recommended physical activity as the main therapeutic tool for prevention and treatment of metabolic syndrome in childhood (Brambilla et al., 2010). Comparative studies in adults have shown that the effect of exercise on weight is limited and generally less than that of calorie restriction (Brambilla et al., 2010). Moreover, the relative effectiveness of diet and exercise depends on the degree of excess fatness (Brambilla et al., 2010). Comparative studies in children and youth are few, as behavioral interventions in overweight children and adolescents commonly combine exercise and dietary restriction, making it difficult to disentangle their independent effects. Nonetheless, diet and exercise have different effects on body composition: While both contribute to fat loss, only exercise increases muscle mass and thus has a direct effect on metabolic health. In children and youth, as in adults, the effect of exercise on cardiometabolic risk factors is greater in overweight/obese youth than in their normal-weight peers (Kang et al., 2002 Lazaar et al., 2007).

Exercise also may have important benefits even without significant modification of body composition (Bell et al., 2007). Experimental studies in overweight and obese youth have shown that exercise leads to reductions in visceral fat (Owens et al., 1999 Gutin et al., 2002 Lee at al., 2005 Barbeau et al., 2007 Kim and Lee, 2009) without a significant change in BMI, as well as improvement in markers of metabolic syndrome, primarily fasting insulin and insulin resistance (Treuth et al., 1998 Ferguson et al., 1999 Carrel et al., 2005 Nassis et al., 2005 Meyer et al., 2006 Shaibi et al., 2006 Bell et al., 2007). Results from experimental studies of the effects of exercise on lipids and lipoproteins (Stoedefalke et al., 2000 Kelley and Kelley, 2008 Janssen and LeBlanc, 2010) are mixed. Although some studies have shown improved lipid and lipoprotein profiles, primarily a decrease in low-density lipoprotein (LDL) cholesterol and triglyceride concentrations and an increase in HDL cholesterol (Ferguson et al., 1999), other studies have shown no improvement in these outcomes (Kelley and Kelley, 2008). In part, such conflicting results are likely due to initial differences in body composition and severity of hyperlipidemia. Well-controlled exercise training studies in obese children (Escalante et al., 2012) and children with adverse blood lipid and lipoprotein profiles have shown positive alterations in their profiles (Stoedefalke et al., 2000), whereas results in normolipid-emic children and adolescents are equivocal. Similarly, exercise has little effect on resting blood pressure in normotensive children and adolescents (Kelley and Kelley, 2008), whereas reductions in resting systolic and sometimes diastolic pressures have been reported in youth with high blood pressure (Hagberg et al., 1983, 1984 Danforth et al., 1990 Ewart et al., 1998 Farpour-Lambert et al., 2009 Janssen and LeBlanc, 2010).

In adults, physical activity is inversely associated with low-grade inflammation (Wärnberg et al., 2010 Ertek and Cicero, 2012), which is now recognized as a significant feature of metabolic syndrome and an independent predictor of cardiometabolic disease (Malina, 2002). In obese children and adolescents, as in their adult counterparts, elevation of inflammatory markers is evident, and observational studies have shown significant relationships among physical activity, physical fitness, and inflammation (Isasi et al., 2003 Platat et al., 2006 Ruiz et al., 2007 Wärnberg et al., 2007 Wärnberg and Marcos, 2008). These relationships are better studied and stronger in adolescents than in children. In one study of boys and girls aged 10-15, those who were obese and unfit had the highest levels of systemic inflammation, whereas those who were obese yet fit had levels as low as those who were lean and fit (Halle et al., 2004). In another study, low-grade inflammation was negatively associated with muscle strength in overweight adolescents after controlling for cardiorespiratory fitness, suggesting that high levels of muscle strength may counteract some of the negative consequences of higher levels of body fat (Ruiz et al., 2008). Experimental studies of the effects of exercise and markers of low-grade inflammation in children and adolescents are lacking. Improved cardiorespiratory fitness in adults (Church et al., 2002), however, has been shown to be inversely related to concentration of C-reactive protein (CRP), a marker of low-grade inflammation. In a small study of a lifestyle intervention entailing 45 minutes of physical activity 3 times per week for 3 months, a small reduction in body fat and an overall decrease in inflammatory factors (CRP, interleukin [IL]-6) were seen in obese adolescents (Balagopal et al., 2005).

Performance-Related Fitness

Speed, muscle power, agility, and balance (static and dynamic) are aspects of performance-related fitness that change during body development in predictable ways associated with the development of tissues and systems discussed above (Malina et al., 2004). Running speed and muscle power are related, and both depend on full development of the neuromuscular system. Running speed and muscle power are similar for boys and girls during childhood (Haubenstricker and Seefeldt, 1986). After puberty, largely because of differences in muscle mass and muscle strength, males continue to make significant annual gains, while females tend to plateau during the adolescent years. Sociocultural factors and increasing inactivity among girls relative to boys, along with changes in body proportion and a lowering of the center of gravity, may also contribute to gender differences (Malina et al., 2004).

Balance—the ability to maintain equilibrium—generally improves from ages 3 to 18 (Williams, 1983). Research suggests that females outperform males on tests of static and dynamic balance during childhood and that this advantage persists through puberty (Malina et al., 2004).

Motor performance is related in part to muscle strength. Increases in muscle strength as a result of resistance exercise were described above. A question of interest is whether gains in strength transfer to other performance tasks. Available results are variable, giving some indication that gains in strength are associated with improvement in some performance tasks, such as sprinting and vertical jump, although the improvements are generally small, highlighting the difficulty of distinguishing the effects of training from changes expected with normal growth. Changes in body size, physique, and body composition associated with growth and maturation are important factors that affect strength and motor performance. The relationships vary among performance measures and with age, and these factors often are inadequately controlled in studies of components of performance-related fitness and performance tasks.


Contents

Development before birth, or prenatal development (from Latin natalis 'relating to birth') is the process in which a zygote, and later an embryo and then a fetus develops during gestation. Prenatal development starts with fertilization and the formation of the zygote, the first stage in embryonic development which continues in fetal development until birth.

Fertilization Edit

Fertilization occurs when the sperm successfully enters the ovum's membrane. The chromosomes of the sperm are passed into the egg to form a unique genome. The egg becomes a zygote and the germinal stage of embryonic development begins. [2] The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation. [3]

The zygote contains a full complement of genetic material, with all the biological characteristics of a single human being, and develops into the embryo. Briefly, embryonic development have four stages: the morula stage, the blastula stage, the gastrula stage, and the neurula stage. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of rapid mitotic cell divisions called cleavage. [4] A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's uterus. This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species.

Embryonic development Edit

Human embryonic development refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. [2] The embryonic stage covers the first eight weeks of development at the beginning of the ninth week the embryo is termed a fetus.

The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. [3] During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryonic development continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.

In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryonic development involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.

Fetal development Edit

A fetus is a stage in the human development considered to begin nine weeks after fertilization. [5] [6] In biological terms, however, prenatal development is a continuum, with many defining feature distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final location.

Maternal influences Edit

The fetus and embryo develop within the uterus, an organ that sits within the pelvis of the mother. The process the mother experiences whilst carrying the fetus or embryo is referred to as pregnancy. The placenta connects the developing fetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply to fight against internal infection and to produce hormones which support pregnancy. The placenta provides oxygen and nutrients to growing fetuses and removes waste products from the fetus's blood. The placenta attaches to the wall of the uterus, and the fetus's umbilical cord develops from the placenta. These organs connect the mother and the fetus. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development. [7] The homology of such structures in various viviparous organisms is debatable, and in invertebrates such as Arthropoda, is analogous at best.

Infancy and childhood Edit

Childhood is the age span ranging from birth to adolescence. [8] In developmental psychology, childhood is divided up into the developmental stages of toddlerhood (learning to walk), early childhood (play age), middle childhood (school age), and adolescence (puberty through post-puberty). Various childhood factors could affect a person's attitude formation. [8]

The Tanner stages can be used to approximately judge a child's age based on physical development.

    (breast development) 11y (8y–13y) (pubic hair) 11y (8.5y–13.5y)
  • Growth spurt 11.25y (10y–12.5y) (first menstrual bleeding) 12.5y (10.5y–14.5y)
  • Wisdom tooth eruption 15y (14y-17y)
  • Adult height reached 15y (14y–17y)
    (testicular enlargement) 12y (10y–14y) (pubic hair) 12y (10y–14y)
  • Growth spurt 13y (11y–18.5y) (first ejaculation) 13.5y (11.5y–15.5y)
  • Wisdom tooth eruption 17y (15y-19y)
  • Completion of growth 17y (15y–19y)

Puberty Edit

Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs.

On average, girls begin puberty around ages 10–11 and end puberty around 15–17 boys begin around ages 11–12 and end around 16–17. [9] [10] [11] [12] [13] The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13 [14] [15] [16] [17] for males, it is the first ejaculation, which occurs on average at age 13. [18] In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys. [19] This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition, [20] or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors. [21] [22] Puberty which starts earlier than usual is known as precocious puberty, and puberty which starts later than usual is known as delayed puberty.

Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body from girl to woman, from boy to man.

Adulthood Edit

Biologically, an adult is a human or other organism that has reached sexual maturity. In human context, the term adult additionally has meanings associated with social and legal concepts. In contrast to a "minor", a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible. The typical age of attaining adulthood is 18, although definition may vary by legal rights and country.

Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory a person may be biologically an adult, and have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, one may legally be an adult but possess none of the maturity and responsibility that may define an adult, the mental and physical development and maturity of an individual is proven to be greatly influenced by the circumstances in which they exist.

Development of human organs and organ systems begins in the embryo and continues throughout the human lifespan.


Do all body organs grow in proportion during the period of physical development? - Biology

Regular trips to the shoe shop and trousers that rapidly become too short are common occurrences during puberty.

In their teens, children put on an amazing growth spurt to reach their final adult height. At their fastest, boys can grow taller by as much as 9cm a year and girls at a rate of 8cm a year. It's no wonder teenagers are clumsy. Their body is shooting upwards at a speed their brain simply cannot keep up with.

This phenomenal growth starts at the outside of the body and works in. Hands and feet are the first to expand. Needing new shoes is the first sign of trouble.

Next, arms and legs grow longer, and even here the 'outside-in' rule applies. The shin bones lengthen before the thigh, and the forearm before the upper arm.

Finally the spine grows. The very last expansion is a broadening of the chest and shoulders in boys, and a widening of the hips and pelvis in girls.

Growing up and tripping over

Many teenagers shoot up so fast that their brains cannot keep up. As their height increases, their centre of gravity lifts. This happens so quickly that the brain does not get a chance to calculate the new rules for balancing. Clumsiness is often unavoidable.

Rapidly increasing height is a sign that a teenager is experiencing puberty. Growth is triggered in both boys and girls by increased levels of the sex hormone testosterone. This chemical also triggers the sexual organs to develop. In fact, the relationship between growth of the skeleton and puberty is so strong that a teenager's developmental age can be measured by looking at the maturity of the bones in their hand and wrist.

Watch a child's hand grow into an adult's.
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Timing is everything. No teenager wants to be developing too quickly, or lagging behind. In reality, many of them grow up much earlier or later than the average and this is perfectly normal.

The average boy is growing fastest between 14 and 15. Girls start earlier, growing fastest when 12 and 13. Girls also end their growth spurt earlier at 18, while boys need another two years before they finish growing aged 20.


Growth and Development in a Child | Psychology

In this article we will discuss about the growth and development in a child.

General-Nature of Growth and Development:

We often use ‘growth’ and ‘development’ interchangeably, as synonymous terms. In the strictest sense of the word, ‘growth’ is different from ‘development’. In this strict sense ‘growth’ means an increase in size. When we say that a body or any of its parts has “grown”, it means that it has become larger and heavier.

Thus increase in size height, length and weight which can be measured, contributes ‘growth’. Development, in the strict sense of the word, implies change in shape, form or structure resulting in improved working or functioning. Improved functioning implies certain qualitative changes leading to maturity. For example, ‘arms’ do not grow larger but they also develop because they improve in their functions. Increase in size and structure of arms enables the human individual to use them for more complex functions which were not possible earlier.

There are many thinkers who give a wider connotation to the term growth. One of them is Gesell. According to him growth carries a more dynamic connotation which organically ties the present with past and directs it towards the future it places an emphasis on the total economy of the individual. Thus, in a wider sense, growth and development can be used synonymously.

Causes of Development:

Development is the result of the interaction between maturation and learning. According to Hurlock, by maturation is meant the development or unfolding of traits potentially present in the individual, because of his hereditary endowment from his parents and other ancestors. It is not directly dependent on the child’s experience, but is stimulated and influenced to some degree by the different environmental factors with which he comes in contact.

Thus maturation is the inner growth process unaffected by training. Most of pre­natal development takes place because of this process. Before a child develops the ability to walk, the muscles of his legs must reach a certain degree of maturity. Once he has reached this maturity, he starts walking, as it were all of a sudden. Similarly, the mouth and larger muscles mature before the child starts speaking.

Another factor that causes growth is ‘learning’. Learning implies exercise and experience on the individual’s part, the child’s own activities. Learning may result from practice or mere repetition of an act which, in time, may bring about a change in the individual’s behaviour, or it may result from training which is nothing but a selective, directed and purposive type of activity. ‘Learning’ is responsible for ‘walking’ in a particular manner — an indication of the development.

It must be noted that maturation and learning are closely related: one influences or retards the other. Traits potentially present will not develop to their maximum without effort (learning). No amount of effort or exercise on the individual’s part or no amount of training will be adequate to bring up a trait to a desired standard if the trait is limited in its potentialities.

Thus, maturation, in the words of Hurlock, provides the raw material for learning and determines to a large extent the more general patterns and sequences of the individual’s behaviour. This is not, however to minimise the importance of learning or environmental influences on the growth or development.

Characteristics or Principles of Growth and Development:

The process of development has been studied experimentally and otherwise. The studies and researches have highlighted certain significant facts or principles underlying this process.

(i) Development follows a pattern:

Development follows a pattern peculiar of the species Development occurs in orderly manner and follows a certain sequence. For example, the human body cuts his molars before his incisors, can stand before he walks and can draw, a circle before he can draw a square. In physical development one can see the cephalocaudal sequence in the prenatal life of the human child.

This means that control of the body as well as improvements in the structure itself develops first in the head and progresses later to parts further from the bread. The cephalocaudal sequence may be illustrated by the development of motor functions. When the baby is placed in a prone position, he can lift his head by his neck before he can do to by lifting his chest. The control of muscles of the trunk precedes that of the muscles of the arms and legs.

Even the specific phases of development such as motor, social and play follows a pattern also. Group play activity follows the self-centered play activity. The child is interested in himself first before he can develop interest in other children. He babbles before he talks, he is dependent on others before he achieves dependence on self.

(ii) Development proceeds from general to specific responses:

It moves from a generalised to localised behaviour. This can be observed in the behaviour of infants and young children. This new-born infant moves his whole body at one time instead of moving only one part of it. The baby waves his arms in general and makes random movements before he is capable of such a specific response as reaching out for a specific object.

He makes random kicking with his legs before he can co-ordinate the leg muscles well enough to crawl or to walk. When given an unpleasant stimulus on any part of the body i.e. a pin­prick he reacts with the entire body before he learns to restrict the movement to the particular part of the body which is stimulated. In the emotional field, the baby first responds to all strange objects with a general fear. Gradually, his fear becomes specific. He reaches out for the object as a whole before he can hold its specific parts.

(iii) Development is a continuous process:

Development does not occur in spurts. Although, it is suggested that there are definite developmental stages such as ‘gang age’ or ‘adolescence’, yet it is a fact that growth continues from the moments of conception until the individuals reaches maturity. It takes place at a slow regular pace rather than by ‘leaps and bounds’. Development of both physical and mental traits continues gradually until these traits reach their maximum growth.

For example, speech does not come over-night. It has gradually developed from the cries and other sounds made by the baby at birth. The first teeth seem to appear suddenly, but they start developing as early as the fifth fetal month: they cut through the gums about five months after birth. There may be a break in the continuity of growth due to illness, starvation or malnutrition or other environmental factors or some abnormal conditions in the child life.

(iv) Although development is continuous process, yet the tempo of growth is not even:

There are periods of accelerated growth and periods of accelerated growth. During infancy and the early preschool years, growth moves swiftly. Later on it slackens Growth from three to six is rapid but not so rapid as form birth to three years. In early adolescence it is again rapid as compared to the period covering eight to twelve years.

(v) Different aspects of growth develop at different rates:

Neither all parts of the body grow at the same rate, nor do all aspects of mental growth proceed equally. They reach maturity at different times. For example, the brain attains its mature size around the age of six to eight years. It gains much in organisation after that. The feet, hands and nose reach their maximum devolvement early in adolescence.

This can explain the awkwardness, clumsiness and self-consciousness characteristic of this period. Similarly, creative imagination develops rapidly during childhood it seems to reach its peak during youth. Reasoning develops at a relatively slower rate. Rote memory and memory for concrete objects and facts develop more quickly than memory for abstract and theoretical materials.

General intelligence reaches its peak, in most cases, about the age of 16 years. Children probably learn more new things in the first five years of life than in all the rest of their lives. Adolescence is marked by the most rapid development of the genital systems and of certain definite social interests and emotional capacities which is not so in other stage of development.

(vi) Most traits are correlated in development:

Generally it is seen that the child whose intellectual development is above average is o in health size, sociability and special aptitudes. Mental defectives tend to be smaller in stature than the normal child. Idiots and imbeciles are often the smallest of the feeble­minded group. There is a correlation between high intelligence and sexual maturity.

(vii) Growth is complex. All of its aspects are closely inter-related:

“It is impossible to understand the physical child without understanding him at the same time as a child who thinks and has feeling.” His mental development is intimately related to his physical growth and its needs. Again, there is a close relationship between his total adjustment to school and his emotions, his physical health and his intellectual adequacy. An emotional disturbance may contribute to difficulties in eating or sleeping. A physical defect may be responsible for the development certain attitudes and social adjustments.

(viii) Growth is a product of the interaction both heredity and environment:

Neither heredity alone, nor the mere environment is the potent factor in the development of an individual. But it is not possible to indicate exactly in what proportion heredity and environment contribute to the development of an individual.

The two work hand in hand from the very conceptions. The environment bears upon the new organism from the beginning. Among the environmental factors, one can mention nutrition, climate, the conditions in the home, the type of social organisation in which individual move and live, the roles they have to play and other.

(ix) Each child grows in his own unique way: There are wide individual difference:

How much and how little individuals vary one from another has not yet been discovered as definitely as the fact that they do differ. It is definitely indicated in various studies that the differences in physical structure are less than the differences in intellectual capacity. Similarly, it has been found out, that personality differences are far more marked than either physical or intellectual differences. Differences in special aptitudes seem to be the most marked of all.

Individual differences are caused by differences in hereditary endowment and environmental influences. Among the environmental influences, the most important factors are food, climate health conditions, opportunities for learning, motivation to learn, social relationships, codes of behaviour set up by the social group to which the individual belongs, and the strength of social approval or disapproval.

Individual differences in rate of development remain constant. For example, a child may be slow in learning in early childhood. It is wrong to presume that he will catch up with the average. Evidence shows that the rate of growth is consistent and those who grow rapidly at first will continue to do so and those who develop slowly in early years will continue to do so, in later years. This observation is not applicable when the growth has been regarded by some condition which may be remedied, if the treatment is given in time.

(x) Growth is both quantitative and qualitative:

These two aspects are inseparable. The child not only grows in ‘size’ he grows up or matures in structure and function. Breckenridge and Vincent have given a nice example to illustrate this principle. The baby’s digestive tract not only grows in size, but also changes in structure, permitting digestion of more complex foods and increasing its efficiency in converting foods into simpler forms which the body can use.

The younger the child, the simpler the emotions. With growth, there is an increase of experiences and these produce more and more complex emotional reactions to more and more complicated situations.

(xi) Development is predictable:

We have seen that the rate of development for each child is fairly constant. The consequence is that it is possible for us to predict at an early age the range within which the mature development of the child is likely to fall. But it may be noted that all types of development, particularly mental development, cannot be predicted with the same degree of accuracy. It is more easily predictable for children whose mental development falls within the normal range rather than for those whose mental development shows marked deviation from the average.

(xii) Principle of spiral versus linear arrangement:

The child doesn’t proceed straightly on the path of development with a constant or steady pace. Actually he makes advancement, during a particular period but takes rest in the next following period to consolidate his development. In advancing further, therefore, he turns back and then makes forward again like a spiral.

Developmental psychologists have also observed that each developmental phase has certain traits characteristic if it, that it has certain undesirable forms of behaviour which are usually found at that age and which are outgrown as the individual passes into the next stage, and that every individual normally passes through each stage of development.

Jersild, while writing about the principles of development, remarks that one feature of the growing ability is its spontaneous use and wholeheartedness. As a child’s capacities for doing, thinking and feeling mature he has an impulse to put them to use, and he often does it wholeheartedly. This is described by Jersild as ‘Indigenous motivation’. Another feature of human development is its struggle. The process of growth involves conflicting impulses and demands.

The child struggles against these in his striving toward maturity. The process of development is also characterised by anticipation, in that it is also geared to the needs of the future, by the capacity for self-repair by the developmental revision of habits, by the persistence of archaic behaviour trends and by its quality of ‘becoming’ its dynamic rather static nature, made so by the changes that occur in the individual at every step.

Development is affected by many factors. Some of these factors play a more important role than others. These factors are intelligence, sex, glands of internal secretion, nutrition, fresh air and sunlight, injuries, accidents and diseases, position in the family, psychological conditions in the family matrix, social roles and cultural demands. These factors affect different phases of development at different stages in varying degrees.

Educational Implications of Principles of Growth and Development:

1. Education is not only a process and a product of growing, it means growing. It aims at the fullest possible realisation of all the potentialities of children. This implies that teachers and parents must know what children are capable of and what potentialities they possess. Equipped with this knowledge they should provide suitable opportunities and favourable environmental facilities which are conducive to the maximum growth of children. Apart from these opportunities, it is necessary that their attitudes are helpful, encouraging and sympathetic.

2. School programmes, procedures and practices should be adjusted to the growth and maturational levels of children, bearing in mind the individual variations in rates of growth. Since various aspects of growth are interrelated, parents and teachers should pay attention to all aspects.

Good physical growth, for example, through the provision of play, games and sports, is conducive to effective intellectual development, malnutrition has been found to be an important factor that retards development: hence, teachers and parents should cooperate in cultivating among pupils habits of balanced eating.

3. The principles of development have highlighted the importance of “individual differences” from one child to the other and from one stage to another. This fact justifies the provision of diversified courses for the development of specific talents, abilities and interests and a rich and varied programme of co-curricular activities. Similarly, the curricular activities should be based on the needs and interests of various stages of growth i.e., childhood, boyhood or later childhood, pre-adolescence and adolescence.

4. Each stage of growth has its possibilities and limitations. This implies that teachers and parents should not demand of pupils or children what is beyond their stage of growth. If they do so, they will only cause frustrations, heighten tension and nervousness in children. For example, it is wrong to expect a primary school child to appreciate abstract concepts and theories.

5. The inter-relatedness of growth’ demands presentation of knowledge in an interrelated manner and its integration with action. Since each child grows in his own unique way, it is but opposite that parents and teachers should treat each child as a unique individual and provide for this special needs and interests.


Aging changes in organs, tissues, and cells

All vital organs begin to lose some function as you age during adulthood. Aging changes occur in all of the body's cells, tissues, and organs, and these changes affect the functioning of all body systems.

Living tissue is made up of cells. There are many different types of cells, but all have the same basic structure. Tissues are layers of similar cells that perform a specific function. The different kinds of tissues group together to form organs.

There are four basic types of tissue:

Connective tissue supports other tissues and binds them together. This includes bone, blood, and lymph tissues, as well as the tissues that give support and structure to the skin and internal organs.

Epithelial tissue provides a covering for superficial and deeper body layers. The skin and the linings of the passages inside the body, such as the gastrointestinal system, are made of epithelial tissue.

Muscle tissue includes three types of tissue:

  • Striated muscles, such as those that move the skeleton (also called voluntary muscle)
  • Smooth muscles (also called involuntary muscle), such as the muscles contained in the stomach and other internal organs
  • Cardiac muscle, which makes up most of the heart wall (also an involuntary muscle)

Nerve tissue is made up of nerve cells (neurons) and is used to carry messages to and from various parts of the body. The brain, spinal cord, and peripheral nerves are made of nerve tissue.

Cells are the basic building blocks of tissues. All cells experience changes with aging. They become larger and are less able to divide and multiply. Among other changes, there is an increase in pigments and fatty substances inside the cell (lipids). Many cells lose their ability to function, or they begin to function abnormally.

As aging continues, waste products build up in tissue. A fatty brown pigment called lipofuscin collects in many tissues, as do other fatty substances.

Connective tissue changes, becoming more stiff. This makes the organs, blood vessels, and airways more rigid. Cell membranes change, so many tissues have more trouble getting oxygen and nutrients, and removing carbon dioxide and other wastes.

Many tissues lose mass. This process is called atrophy. Some tissues become lumpy (nodular) or more rigid.

Because of cell and tissue changes, your organs also change as you age. Aging organs slowly lose function. Most people do not notice this loss immediately, because you rarely need to use your organs to their fullest ability.

Organs have a reserve ability to function beyond the usual needs. For example, the heart of a 20-year-old is capable of pumping about 10 times the amount of blood that is actually needed to keep the body alive. After age 30, an average of 1% of this reserve is lost each year.

The biggest changes in organ reserve occur in the heart, lungs, and kidneys. The amount of reserve lost varies between people and between different organs in a single person.

These changes appear slowly and over a long period. When an organ is worked harder than usual, it may not be able to increase function. Sudden heart failure or other problems can develop when the body is worked harder than usual. Things that produce an extra workload (body stressors) include the following:

  • Illness
  • Medicines
  • Significant life changes
  • Sudden increased physical demands on the body, such as a change in activity or exposure to a higher altitude

Loss of reserve also makes it harder to restore balance (equilibrium) in the body. Drugs are removed from the body by the kidneys and liver at a slower rate. Lower doses of medicines may be needed, and side effects become more common. Recovery from illnesses is seldom 100%, leading to more and more disability.

Side effects of medicine can mimic the symptoms of many diseases, so it is easy to mistake a drug reaction for an illness. Some medicines have entirely different side effects in the elderly than in younger people.

No one knows how and why people change as they get older. Some theories claim that aging is caused by injuries from ultraviolet light over time, wear and tear on the body, or byproducts of metabolism. Other theories view aging as a predetermined process controlled by genes.

No single process can explain all the changes of aging. Aging is a complex process that varies as to how it affects different people and even different organs. Most gerontologists (people who study aging) feel that aging is due to the interaction of many lifelong influences. These influences include heredity, environment, culture, diet, exercise and leisure, past illnesses, and many other factors.

Unlike the changes of adolescence, which are predictable to within a few years, each person ages at a unique rate. Some systems begin aging as early as age 30. Other aging processes are not common until much later in life.

Although some changes always occur with aging, they occur at different rates and to different extents. There is no way to predict exactly how you will age.

TERMS TO DESCRIBE TYPES OF CELL CHANGES

  • Cells shrink. If enough cells decrease in size, the entire organ atrophies. This is often a normal aging change and can occur in any tissue. It is most common in skeletal muscle, the heart, the brain, and the sex organs (such as the breasts and ovaries). Bones become thinner and more likely to break with minor trauma.
  • The cause of atrophy is unknown, but may include reduced use, decreased workload, decreased blood supply or nutrition to the cells, and reduced stimulation by nerves or hormones.
  • Cells enlarge. This is caused by an increase of proteins in the cell membrane and cell structures, not an increase in the cell's fluid.
  • When some cells atrophy, others may hypertrophy to make up for the loss of cell mass.
  • The number of cells increases. There is an increased rate of cell division.
  • Hyperplasia usually occurs to compensate for a loss of cells. It allows some organs and tissues to regenerate, including the skin, lining of the intestines, liver, and bone marrow. The liver is especially good at regeneration. It can replace up to 70% of its structure within 2 weeks after an injury.
  • Tissues that have limited ability to regenerate include bone, cartilage, and smooth muscle (such as the muscles around the intestines). Tissues that rarely or never regenerate include the nerves, skeletal muscle, heart muscle, and the lens of the eye. When injured, these tissues are replaced with scar tissue.
  • The size, shape, or organization of mature cells becomes abnormal. This is also called atypical hyperplasia.
  • Dysplasia is fairly common in the cells of the cervix and the lining of the respiratory tract.
  • The formation of tumors, either cancerous (malignant) or noncancerous (benign).
  • Neoplastic cells often reproduce quickly. They may have unusual shapes and abnormal function.

As you grow older, you will have changes throughout your body, including changes in:


Understanding Puberty

Your daughter is asking about getting her first bra, and your son comes home from soccer practice smelling like he's been digging on a road crew all day. What's going on?

Welcome to puberty, the time when kids sprout up, fill out, and maybe even mouth off.

Puberty was awkward enough when you were the one going through it. So how can you help your child through all the changes?

Stages of Puberty

Sure, most of us know the telltale signs of puberty &mdash hair growth in new places, menstruation, body odor, lower voice in boys, breast growth in girls, etc. But we may not fully comprehend the science behind all of these changes. Here's a quick look at how it works.

Usually after a girl's 8th birthday or after a boy turns 9 or 10, puberty begins when an area of the brain called the hypothalamus starts to release gonadotropin-releasing hormone (GnRH). When GnRH travels to the pituitary gland (a small gland under the brain that produces hormones that control other glands throughout the body), it releases two more puberty hormones &mdash luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

What happens next depends on gender:

  • Boys: Hormones travel through the bloodstream to the testes (testicles) and give the signal to begin production of sperm and the hormone testosterone.
  • Girls: Hormones go to the ovaries (the two oval-shaped organs that lie to the right and left of the uterus) and trigger the maturation and release of eggs and the production of the hormone estrogen, which matures a female's body and prepares her for pregnancy.

At about the same time, the adrenal glands of both boys and girls begin to produce a group of hormones called adrenal androgens. These hormones stimulate the growth of pubic and underarm hair in both sexes.

For a Boy

The physical changes of puberty for a boy usually start with enlargement of the testicles and sprouting of pubic hair, followed by a growth spurt between ages 10 and 16 &mdash on average 1 to 2 years later than when girls start. His arms, legs, hands, and feet also grow faster than the rest of his body. His body shape will begin to change as his shoulders broaden and he gains weight and muscle.

A boy may become concerned if he notices tenderness or swelling under his nipples. This temporary development of breast tissue is called gynecomastia and it happens to about 50% of boys during puberty. But it usually disappears within 6 months or so.

And that first crack in the voice is a sign that his voice is changing and will become deeper.

Dark, coarse, curly hair will also sprout just above his penis and on his scrotum, and later under his arms and in the beard area. His penis and testes will get larger, and erections, which a boy begins experiencing as an infant, will become more frequent. Ejaculation &mdash the release of sperm-containing semen &mdash will also occur.

Many boys become concerned about their penis size. A boy may need reassurance, particularly if he tends to be a later developer and he compares himself with boys who are further along in puberty. If a boy is circumcised, he may also have questions about the skin that covers the tip of an uncircumcised penis.

For a Girl

Puberty generally starts earlier for girls, some time between 8 and 13 years of age. For most girls, the first evidence of puberty is breast development, but it can be the growth of pubic hair. As her breasts start to grow, a girl will initially have small, firm, tender lumps (called buds) under one or both nipples the breast tissue will get larger and become less firm in texture over the next year or two. Dark, coarse, curly hair will appear on her labia (the folds of skin surrounding the vagina), and later, similar hair will begin growing under her arms.

The first signs of puberty are followed 1 or 2 years later by a noticeable growth spurt. Her body will begin to build up fat, particularly in the breasts and around her hips and thighs, as she takes on the contours of a woman. Her arms, legs, hands, and feet will also get bigger.

The culminating event will be the arrival of menarche, her first period (menstruation). Depending on the age at which they begin their pubertal development, girls may get their first period between the ages of 9 and 16.

Common Puberty Concerns

The physical changes kids experience as they move toward adulthood often are accompanied by emotional consequences.

Some girls are excited about their budding breasts and new training bras others may worry that all eyes are focused on their breasts. Some boys love the sight of themselves all lathered up with shaving cream others may be uncomfortable with the attention they get for a few new shoots of hair.

Pimples are common for most teens. Acne is caused by glands in the skin that produce a natural oil called sebum. Puberty hormones make the glands produce extra sebum, which can clog the pores. Washing gently with water and mild soap can get rid of excess sebum and help reduce breakouts.

Over-the-counter and prescription medications are available for more severe cases of acne. Your family doctor can recommend a dermatologist (a doctor who specializes in treating the skin) if basic skin care and OTC medications don't keep acne under control.

Kids who once associated bath time with play need to learn to wash regularly and to apply deodorant or antiperspirant. A teen who's learning to shave will need to learn how to keep a razor clean, to throw a disposable one away before it becomes dull and ineffective, and to not share it with others.

Boys, capable of having erections since infancy, can now experience ejaculation. Usually, this first happens between the ages of 11 and 15, either spontaneously in connection with sexual fantasies, during masturbation, or as a nocturnal emission (also called a wet dream). If he doesn't know about wet dreams before he has one, a boy may think he has urinated accidentally or that something has gone wrong with his body.

As kids mature physically and emotionally, they become increasingly curious about their sexuality and their own bodies. Although infants and young kids do touch their own genitals from time to time because they like the way it feels, masturbation is more common in older kids, from the preadolescent and teen years and beyond.

As far as the myths and beliefs about masturbation: No, it won't cause kids to grow hair on their hands, become infertile, go blind, or develop new emotional problems. A small number of kids and teens with already existing emotional problems may become preoccupied with masturbation &mdash just as they may become overly occupied with other behaviors or thoughts. Constant or obsessive masturbation may be a sign of anxiety or other emotional problem.

But, other than that, masturbation is generally considered by doctors to be a common form of normal sexual self-exploration. Although some preteens and teens may choose to masturbate, others may not.

Because masturbation is often considered a private topic, many kids might feel too embarrassed to talk about it because they're concerned that their parents will be angry or disappointed with them. Some kids may prefer to talk to older siblings, friends, or their doctors rather than a parent. If you are concerned or have questions about masturbation, consult your doctor.

Talking to Kids About Puberty

Boys and girls can see these changes happening to each other &mdash in some cases, they can smell them. It's important to talk to your child about how bodies change &mdash sooner, rather than later.

Be prepared to talk to a girl about the expected events of puberty, including menstruation, when you see the first signs of breast development, or earlier if she seems ready or has questions. A boy should know about normal penile development, erections, and nocturnal emissions before age 12 &mdash sooner, if he's an early developer. And it's also important to talk to your child about what's happening to members of the opposite sex.

It's best not to have "The Talk" as one grand summit but rather as a series of talks, ideally beginning when your child is young and starting to ask questions about body parts. Each time you talk, offer more and more detail, depending upon your child's maturity level and interest in the topic.

And, if your child has a question, answer it honestly. If you feel uncomfortable, need answers to questions, or are uncertain about how to have these talks with your child, ask your doctor for advice.


The First Trimester: Fetal Development

The most dramatic changes and development happen during the first trimester. During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester, it becomes a fetus that is fully formed, weighing approximately 0.5 to 1 ounce and measuring, on average, 3 to 4 inches in length.

First Trimester Fetal Growth and Development Benchmarks

The chart below provides benchmarks for most normal pregnancies. However, each fetus develops differently.

  • All major systems and organs begin to form.
  • The embryo looks like a tadpole.
  • The neural tube (which becomes the brain and spinal cord), the digestive system, and the heart and circulatory system begin to form.
  • The beginnings of the eyes and ears are developing.
  • Tiny limb buds appear, which will develop into arms and legs.
  • The heart is beating.
  • All major body systems continue to develop and function, including the circulatory, nervous, digestive, and urinary systems.
  • The embryo is taking on a human shape, although the head is larger in proportion to the rest of the body.
  • The mouth is developing tooth buds, which will become baby teeth.
  • The eyes, nose, mouth, and ears are becoming more distinct.
  • The arms and legs can be easily seen.
  • The fingers and toes are still webbed, but can be clearly distinguished.
  • The main organs continue to develop and you can hear the baby's heartbeat using an instrument called a Doppler.
  • The bones begin to develop and the nose and jaws are rapidly developing.
  • The embryo is in constant motion but cannot be felt by the mother.
  • After 8 weeks, the embryo is now referred to as a fetus, which means offspring.
  • Although the fetus is only 1 to 1.5 inches long at this point, all major organs and systems have been formed.
  • The external genital organs are developed.
  • Fingernails and toenails appear.
  • Eyelids are formed.
  • Fetal movement increases.
  • The arms and legs are fully formed.
  • The voice box (larynx) begins to form in the trachea.

The fetus is most vulnerable during the first 12 weeks. During this period of time, all of the major organs and body systems are forming and can be damaged if the fetus is exposed to drugs, infectious agents, radiation, certain medications, tobacco and toxic substances.

Even though the organs and body systems are fully formed by the end of 12 weeks, the fetus cannot survive independently.



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