PERFORMANCE PHYSIOLOGY

delay sexual maturation but evidence for this hypothesis has yet to be conclusively provided. In the majority of girls menarche commences when the velocity of growth is declining and tends to occur post PHV and when maximum growth and height decelerations are experienced.

Timing and tempo of maturation One of the most important factors related to maturation is the timing and tempo as the time of entering puberty will vary considerably between members of the same sex. For example, girls enter and end puberty approximately 2 years ahead of boys, whilst the average age of breast development commences in girls at 10.5 years it can start as early as 8 or as late as 13 years. It is also entirely possible for one child to move through all five Tanner stages within 2 years whilst another individual might take six years. Therefore, it is extremely important to treat each child’s growth and maturation on an individual basis and not merely to compare to average normative scores.

The majority of evidence examining the effect of training on growth appears to support the contention that training does not affect growth. However, not all researchers support this proposition (7). These authors argue that certain adolescent athletes may be at risk of restricted growth and delayed maturation when training is very intense and nutrition is inadequate. Although the body can compensate for this deficit by diverting nutrients from the normal growth processes eg. height, final adult height is

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deceleration

boys girls

© British Gymnastics Association

Figure 2: Three gymnasts showing the wide variation of maturation for the same chronological age

not likely to be reduced. Daly and col- leagues also argue that as most studies are merely reporting the average changes, there is a failure by researchers to pay appropriate attention to the interindividual differences in skeletal growth patterns, training volumes and nutritional strategies.

KEY POINT

The definitive study to investigate the occurrence of inadequate growth in young athletes has yet to be per- formed but it is clearly warranted.

acceleration

peak height velocity

deceleration

termination of growth

initiation of adolescent spurt

3 5 7 9 11 13 15 15 17 19 age, years

Figure 1: Velocity curve for boys and girls stature 8

CARDIOVASCULAR, RESPIRATORY AND MUSCULO- SKELETAL GROWTH The monitoring of heart rate is a function that many coaches and children are involved in during training. Therefore understanding the changes in the heart’s size is important. As children have smaller hearts than adults, they have a higher rest- ing heart rate to compensate for a smaller stroke volume (amount of blood pumped per beat). Girls appear to have a resting heart rate on average 3-5 beats per minute higher than boys. After maturity there is a decrease in resting heart rate with age of approximately 0.8 beats per minute per year (8). Children also have a higher maxi- mal heart rate than adults, approximately 195-220 beats per minute, but there does not appear to be a sex difference in childrens’ maximal heart rate. Typical resting stroke volumes at birth, 5 and 15 years are 5 millilitres (mL), 25 mL and 85 mL respectively. With an increase in age, increases in heart size are therefore well

correlated to the stroke volume increases.

Before puberty there is little difference in lung function scores between the sexes both at rest and in exercise. Young children tend to hyperventilate (rapid breathing) more than adults during exercise and ven- tilate more air in order to consume one litre of oxygen. As a result of increasing body size, the ability of the body to maximally consume oxygen during exercise increases. By approximately 11 years of age boys have a maximal aerobic capacity of 2.0 litres per minute and girls is 1.85 litres per minute. By age 17 years maximal aerobic power is 3-3.5 litres per minute for boys and 2-2.5 litres per minute for girls (Figure 3). Although there are differences between the sexes after puberty, the differences are smaller between age groups. This indicates that the body size and composition are important determi- nants of aerobic power. Early maturing boys and girls, who are bigger in size, especially if the increase in body size is due to increases in muscle rather than fat, will have a higher aerobic power output.

During puberty there is a substantial increase in the muscle mass of boys’ com- pared to girls. At age 5, boys’ muscle mass is estimated to be as high as 42% of total body mass, increasing to 54% by age 17. For girls, muscle mass increases from approximately 30% before puberty to 40- 45% at age 17 years. The other important change in body composition is the increase in body fat between boys and girls. At 6 years of age, boys have ~11% of body fat which increases to 15% at 17 years of age. Girls have been found to have a body fat of

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height gain,cm/yr