Body composition: growth and development of tissues

To understand the physical capabilities of children and the potential impact that sport activity can have on young athletes, we must first consider the physical state of their bodies.

Height and weight

Specialists in the field of growth and development have spent considerable time analyzing the changes in height and weight that accompany growth. These two variables are most useful when we examine the rates of change. Change in height is assessed in terms of centimeters per year and change in weight in terms of kilograms per year. Figure below shows that height increases rapidly during the first two years of life. In fact, the child reaches about 50% of adult height by age 2. After this, height increases at a progressively slower rate throughout childhood; thus, there is a decline in the rate of its change. Just before puberty, the rate of change in height increases markedly, followed by an exponential decrease in rate until full height is attained at a mean or average age of about 16 years in girls and 18 years in boys. Some boys do not reach their full height until their early 20s. The peak rate of growth in height occurs at approximately 12 years in girls and 14 years in boys. The peak rate of growth in body weight occurs at approximately 12.5 years in girls and 14.5 years in boys – slightly later than for height.

Bone

Bones, joints, cartilage and ligaments form the body’s structural support. Bones provide points of attachment for muscles, protect delicate tissues, and act as reservoirs for calcium and phosphorus; and some are involved in blood cell formation. Early in fetal development, the bones begin to develop from cartilage. Some flat bones, such as those of the skull, develop from fibrous membranes, but the vast majority of bones instead develop from hyaline cartilage. During fetal development, as well as during the initial 14 to 22 years of life, membranes and cartilage are transformed into bone through the process of ossification, or bone formation. The average ages at which the different bones in our bodies complete ossification differ widely, but bones typically begin to fuse in the preteens, and all are fused by the early 20s. On average, girls achieve full bone maturity several years before boys.

The structure of mature long bones is complex. Bone is a living tissue that requires essential nutrients, so it receives a rich blood supply. Bone consists of cells distributed throughout a matrix or lattice-type arrangement, and it is dense and hard because of deposits of lime salts, mainly calcium phosphate and calcium carbonate. For this reason, calcium is an essential nutrient, particularly during periods of bone growth and in the later years of life when bone tends to become brittle because of bone mineral loss associated with aging. Bones also store calcium. When our blood calcium level is high, excess calcium can be deposited in our bones for storage; and when calcium levels are too low, bone is resorbed, or broken down, to release calcium into the blood. When injury occurs or when extra stress is placed on a bone, more calcium is deposited. Thus, throughout life, our bones are constantly changing.

Weight-bearing exercise is essential for proper bone growth. Although exercise has little or no influence on bone lengthening, it does increase bone width and bone density by depositing more mineral in the bone matrix, which increases the bone’s strength. There is preliminary evidence indicating that the prepubertal years may be the most opportune time to lay down bone in response to an exercise stimulus.

Muscle

From birth through adolescence, the body’s muscle mass steadily increases, along with the youngster’s weight. In males, the skeletal muscle mass increases from 25% of total body weight at birth to about 40% to 45% or more in young men(20-30 years). Much of this gain occurs when the muscle development rate peaks at puberty. This peak corresponds to a sudden, almost 10-fold increase in testosterone production. Girls don’t experience such rapid acceleration of muscle growth at puberty; but their muscle mass does continue to increase, although more slowly than boys’ ; to about 30% to 35% of their total body weight as young adults. This rate difference is largely attributed to hormonal differences at puberty. These percentage values for both men and women decrease with aging due to loss of muscle mass and gains in fat mass.

Increases in muscle mass with age appear to result primarily from hypertrophy(increase in size) of existing fibers, with little or no hyperplasia(increase in fiber number). Fiber hypertrophy results from increases in the myofilaments and myofibrils. Increases in muscle length as young bones elongate result from increases in the number of sarcomeres(which are added at the junction of the muscle and the tendon) and from increases in the length of existing sarcomeres. Muscle mass peaks in females at age 16 to 20 years and in males at 18 to 25 years, unless it is increased further through exercise, diet, or both.

Fat

Fat cells form and fat deposition starts in these cells early in fetal development, and this process continues indefinitely thereafter. Each fat cell can increase in size at any age from birth to death. Early studies investigating fat cell and fat mass development suggested that the number of fat cells becomes fixed early in life. This led many scientists to believe that maintaining a low total body fat content during the early period of development would minimize the total number of fat cells that develop, greatly reducing the likelihood of obesity as an adult. But subsequent studies provided evidence showing that the number of fat cells can continue to increase throughout life. In light of this evidence, it is important to maintain good dietary and exercise habits throughout life.

The amount of fat that accumulates with growth and agind depends on:

• Diet;
• Exercise habits;
• Heredity.

Heredity is unchangeable, but both diet and exercise can be altered to either increase or decrease fat stores.

At birth, 10% to 12% of total body weight is fat. At physical maturity, the fat content reaches approximately 15% of total body weight for males and approximately 15% of total body weight for males and approximately 25% for females. This sex difference, like that seen in muscle growth, is primarily attributable to hormonal differences. When girls reach puberty, their estrogen levels increase, which promotes the deposition of body fat. The trend of body fat increasing with age is shown in the figure below, which illustrates the relationship between subcutaneous fat(measured at the triceps and subscapular sites) and age for boys and girls from ages 2 to 18 years. The amount of subcutaneous fat is representative of total body fat. It is important to realize that both fat mass and fat-free mass increase during period 8-20 years of age, so an increase in absolute fat does not necessarily mean an increase in relative fat.

Nervous system

As children grow, they develop better balance, agility, and coordination as their nervous systems develop. Myelination of the nerve fibers must be completed before fast reactions and skilled movement can occur because conduction of an impulse along a nerve fiber is considerably slower if myelination is absent or incomplete. Myelination of the cerebral cortex occurs most rapidly during childhood but continues well beyond puberty. Although practicing an activity or skill can improve performance to a certain extent, the full development of that activity or skill depends on full maturation(and myelination) of the nervous system. The development of strength is also likely influenced by myelination.