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3. 8. 2012.

Physiological responses to acute exercise compared to sex differences

The function of almost all physiological systems improves until full maturity is reached or shortly before. After that, physiological function plateaus for a period of time before starting to decline with advancing age. Focus will be on following areas:
  • Strength
  • Cardiovascular and respiratory function
  • Metabolic function, including aerobic capacity, running economy, and anaerobic capacity


Strength improves as muscle mass increases with age. Peak strength usually is attained by age 20 in women and between ages 20 and 30 in men. The hormonal changes that accompany puberty lead to marked increases in strength in pubescent males because of the increased muscle mass noted before. The extent of development and the performance capacity of muscle also depend on the relative maturation of the nervous system. High levels of strength, power, and skill are impossible if the child has not reached neural maturity. Myelination of many motor nerves is incomplete until sexual maturity, so the neural control of muscle function is limited before that time.
Figure below illustrates changes in leg strength in a group of boys from the Medford Boys’ Growth Study. The boys were followed longitudinally from age 7 to 18. The rate of strength gain(slope of line) increased noticeably around age 12, the typical age for onset of puberty. Similar longitudinal data for girls over this same age span are not available. Cross-sectional data, however, indicate that girls experience a more gradual and linear increase in strength and do not exhibit a marked change in their rate of strength gain with puberty, also illustrated on the picture below.

Cardiovascular and respiratory function

Cardiovascular function undergoes considerable change as children grow and age. Let’s consider some of these changes during submaximal and maximal exercise.

Rest and submaximal exercise

Blood pressure at rest and during submaximal levels of exercise is lower in children than in adults but progressively increases to adult values during the late teen years. Blood pressure is also directly related to body size. Larger people generally have higher blood pressures, so size is at least partially responsible for children’s lower blood pressure values. In addition, blood flow to active muscles during exercise in children can be greater for a given volume of muscle than in adults because children have less peripheral resistance.
Recall that cardiac output is the product of heart rate and stroke volume. A child’s smaller heart size and total blood volume result in a lower stroke volume, both at rest and during exercise, than in adult. In an attempt to compensate for this, the child’s heart rate response to a given rate of submaximal work(such as on a cycle ergometer), where the absolute oxygen requirement is the same, is higher than an adult’s. As the child ages, heart size and blood volume increase along with body size. Consequently, stroke volume also increases, as body size increases, for the same absolute rate of work.
However, a child’s higher submaximal heart rate cannot completely compensate for the lower stroke volume. Because of this, the child’s cardiac output is also somewhat lower than the adult’s for a given absolute rate of work or a given oxygen consumption. To maintain adequate oxygen uptake during these submaximal  levels of work, the child’s arterial-mixed venous oxygen difference, or (a-ṽ)O2 difference, increases to further compensate for the lower stroke volume. The increase in (a-ṽ)O2 difference is most likely attributable to increased blood flow to the active muscles – a greater percentage of the cardiac output goes to the active muscles. These submaximal relationships are illustrated in the figure below, in which the responses of a 12-year-old boy are compared to those of a fully mature man.

Maximal exercise

Maximum heart rate(HRmax) is higher in children than in adults but decreases linearly as children age. Children under age 10 frequently have maximum heart rates exceeding 210 beats/min, whereas the average 20-year-old has maximum approximately 195 beats/min. With further aging(25-30 years and older), however, results of cross-sectional studies suggest that maximum heart rate decreases by slightly less than 1 beat/min per year. Longitudinal studies, on the other hand, suggest that maximum heart rate decreases only 0.5 beats/min per year. Longitudinal studies, in which the same people are followed over time, generally provide more accurate estimates of the true changes.
During maximal exercise, as also seen with submaximal exercise, the child’s smaller heart and blood volume limit the maximal stroke volume that he or she can achieve. Again, the high HRmax cannot fully compensate for this, leaving the child with a lower maximal cardiac output than the adult. This limits the child’s performance at high absolute rates of work(e.g., pedaling at 100W on a cycle ergometer or trying to achieve the same absolute VO2max) because the child’s capacity for oxygen delivary is less than an adult’s. However, for high relative rates of work in which the child is responsible for moving only his or her body mass(e.g., running on a treadmill at the same speed with no grade), this lower maximal cardiac output is not as serious as limitation. In running, for example, a 25kg(55lb) child requires(in proportion to body size) considerably less oxygen than a 90kg(198lb) man would require, yet the rate of oxygen consumption when scaled for body size is about the same for both.

Lung function

Lung function changes markedly with growth. All lung volumes increase until growth is complete. Peak flow rates follow the same pattern. The changes in these volumes and flow rates are matched by the changes in the highest ventilation that can be achieved during exhaustive exercise, which is referred to as maximal expiratory ventilation(VEmax), or maximal minute ventilation.  VEmax increases with age until physical maturity and then decreases with aging. For example, cross-sectional data show that VEmax averages about 40L/min for 4-to 6-year-old and increases to 110 to 140L/min at full maturity. Girls follow the same general pattern, but their absolute values are considerably lower post-puberty, primarily because of their smaller body size. These changes are associated with the growth of the pulmonary system, which parallels the general growth patterns for children. As body size increases with growth and development, so do lung size and function.

Metabolic function

Metabolic function also changes as the child and adolescent grow larger, as we would expect from the changes that we have just reviewed in muscle mass and strength and cardiorespiratory function.

Aerobic capacity

The purpose of the basic cardiovascular and respiratory adaptations that can occur in response to varying levels of exercise(rates of work) is to accommodate the exercising muscles’ need for oxygen. Thus, increases in cardiovascular and respiratory function that accompany growth suggest that aerobic capacity(VO2max) similarly increases. In 1938, Robinson demonstrated this phenomenon in a cross-sectional sample of boys and men ranging in age from 6 to 91 years. He found that VO2max peaks between ages 17 and 21 and then decreases linearly with age. Other studies subsequently confirmed these observations. Studies of girls and women have shown essentially the same trend, although in females the decrease begins at a much younger age, generally age 12 to 15, probably attributable to an earlier assumption of a more sedentary lifestyle. The changes in VO2max with age, expressed in liters per minute, are illustrated in the figure a below.

Expressing VO2maxrelative to body weight( ml x kg-1 x min-1) provides a considerably different picture, as shown in the figure b. Values change little in boys from age 6 to young adulthood. For girls, however, little change occurs from age 6 to 13; but after age 13, aerobic capacities show a gradual decrease. Although these observations are of general interest, they might not accurately reflect the development of the cardiorespiratory system as children grow and their physical activity levels change. Several concerns have been raised about the validity of using body weight to account for changes in the size of the cardiorespiratory and metabolic systems, as when one is dividing absolute values by body weight, for example, VO2 per kilogram.
Arguments against using body weight to scale VO2max for differences in size include the following: First, although VO2max values expressed relative to body weight remain relatively stable or decline with age, endurance performance steadily improves. The average 14-year-old boy can run the mile(1.6km) almost twice as fast as the average 5-year-old boy, yet the two boys’ VO2max values expressed relative to body weight are similar. Second, although the increases in VO2max that accompany endurance training in children are relatively small compared with those in adults, the performance increases in these children are relatively large. Therefore, body weight is likely not the most appropriate way to scale VO2max values for differences in body size in children and adolescents. The relationships between VO2max, body dimensions, and system functions during growth are extraordinarily complex.

Running economy

How do growth-related changes in aerobic capacity affect a child’s performance? For any activity that requires a fixed rate of work, such as cycling on an ergometer, the child’s lower VO2max limits endurance performance. But as noted earlier, for activities in which body weight is the major resistance to movement, such as distance running, children should not be at a disadvantage, because their VO2max values expressed relative to body weight are already at or near adult values.
Yet children cannot maintain a running pace as fast as adults can because of basic differences in economy of effort. At a given speed on a treadmill, a child will have a substantially higher submaximal oxygen consumption when expressed relative to body weight than an adult. Even if the child’s lactate thresholdoccurred at the same relative oxygen consumption as the adult’s(at the same percentage of their respective VO2max values), the child would be running at a much slower pace. Also, as children age, their legs lengthen, their muscles become stronger, and their running skills improve. Running economy increases, and this improves their distance-running pace, even if the children are not training and if their VO2max values don’t increase. Rowland argues that increased stride frequency as children and adolescents grow is the most important factor in explaining these changes in running economy. It is also possible that scaling oxygen consumption to body weight is inappropriate during growth and development.

Anaerobic capacity

Children have a limited ability to perform anaerobic-type activities. This is demonstrated in several ways. Children cannot achieve adolescent or adult concentrations of lactate in either muscle or blood for maximal and supramaximal rates of exercise. This suggests that children have a lower glycolitic capacity. The lower lactate levels might reflect a lower concentration of phosphofructokinase, the key rate-limiting enzyme of anaerobic glycolysis. Lactate dehydrogenase activity also seems to be lower in children. However, lactate threshold, when expressed as a percentage of VO2max, does not appear to be a limiting factor in children because children’s lactate thresholds are similar to, if not somewhat higher than, those of similarly trained adults. Also, children’s resting levels of adenosine triphosphate(ATP) and phosphocreatine(PCr) are similar to those of adults, so activities of less than 10 to 15s should not be compromised. Thus, only activities that tax the anaerobic glycolytic system – those from 15s to 2min in duration – will be lower.
Children cannot achieve high respiratory exchange ratios during maximal or exhaustive exercise. Maximal respiratory exchange ratios in children are seldom above 1.10 and are sometimes below 1.00, but adult ratios are usually more than 1.10 and often greater than 1.15. This indicates that less carbon dioxide is produced in children for the same oxygen consumption, which in turn indicates less buffering of lactate.
Anaerobic mean and peak power output, as determined by the Wingate aerobic power test(a 30s, all-out maximal effort on a cycle ergometer), is also lower in children than in adults. Figure below illustrates the results of a similar cycle ergometer anaerobic power test that is potentially a better discriminator of peak power output capacity. In this figure, peak power is statistically adjusted for body mass to account for differences in body size when we compare values for preteenagers, teenagers, and adults. This figure demonstrates the very low peak power outputs for preteenagers(9-10 years of age) compared with both teenagers(14-15 years of age) and adults(mean age of 21 years). Teenagers were much closer to the values for adults than the preteenagers. Again, these values were adjusted for body size, so they should accurately reflect anaerobic power.

Bar-Or summarized the development of both the aerobic and anaerobic characteristics of boys and girls from ages 9 through 16, using 18 years of age as the criterion for 100% of the adult value. The changes with age are shown in the figure below. Aerobic power is represented by the child’s performance on the Margaria step-running test(a field test). Maximal energy expenditure per kilogram represents the maximal energy-generating capacities of the aerobic and anaerobic systems, scaled to body weight to account for body size differences with growth. Notice that aerobic fitness remains constant for the boys but declines for the girls from 12 to 16 years. Nine-to 12-year-old girls have a higher aerobic capacity than the 18-year-old reference adult value; thus, their values are 110% of the adult value. For both boys and girls, anaerobic capacity increases from 9 through 15 years of age.

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