Chapter 17: The Child: Responses and Adaptations to Physical Activity and Training

Introduction

This chapter describes the responses and adaptations of children to exercise and training. The pattern of these responses, for the most part, is similar to that of adults. The magnitude of these responses, however, may differ due to the effects of growth and maturation on body structures, and differences in body size and proportions. Physiologically, children are not small adults. This chapter highlights the quantitative and qualitative differences between children and adults in their responses to acute exercise and to aerobic training, including cardiovascular, pulmonary and metabolic changes. It then discusses the responses to anaerobic activity and resistance training from both the metabolic and neuromuscular perspective, highlighting new issues related to children’s neuromuscular activation and motor unit recruitment. The child-adult differences in the physiological responses during exercise in the heat, and their implications are also discussed. Special attention is paid to the effects of athletic training on growth, and the positive effects of exercise on bone accretion in children and adolescents.

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Accounting For Differences In Body Size

Children’s response to exercise differs from that of adults largely, although not only, because of this size difference. This chapter highlights the child-adult differences in the response to exercise, which are beyond that due to differences in body size. But in order to do that, one must somehow account for size differences. That is, in order to adequately compare the physiological response to exercise between children and adults, or longitudinally during growth and development, one must “normalize” for variation in body size.

There are different approaches to account for differences in body size but unfortunately, there is no one ideal, optimal or “correct” method. The method used will depend on the question asked.

The most commonly used approach to scaling in pediatric exercise physiology is the ratio standards. Its common use is attributed mainly to its simplicity. That is, the physiological variable of interest (e.g.,V̇O₂peak), is divided by a measure of body size (e.g., mass). A variety of anthropometric measures of body dimensions have been used as the denominator in ratio standards. For example, when comparing aerobic capacity between individuals of different sizes, body mass is often used in the denominator of (see case above). However, when comparing aerobic capacity between lean and obese individuals, it is preferable to use fat-free mass as the denominator, because the extra body does not contribute to oxygen uptake. Jumping ability is determined mainly by leg power. Therefore, when comparing jumping ability between individuals of different body size, body mass is often used as the denominator for leg power. However, when comparing strength between individuals, it is preferable to express strength relative to muscle cross-sectional area (see Section on strength later in this chapter). Body surface area is often used when adjusting metabolic rate for body size. The rationale for this is related to thermoregulation. That is, since animals of widely differing body size are characterized by the same body temperature, and since body heat loss is related to body surface area, it makes sense to relate resting metabolic rate, which controls body heat production, to body surface area (see section on thermoregulation later in this chapter). Likewise, the cardiovascular measures (cardiac output and stroke volume) have also traditionally been expressed relative to body surface area. This approach is based on empirical data of the cardiovascular response to exercise in children, although its biological rationale is not clear (see section on cardiovascular response later in this chapter).

 Ratio standards, while simple to use and to conceptualize, are arguably, not always appropriate. The underlying assumption in using ratio standards is that the physiological variable of interest is linearly related to the denominator. Often, this is not the case (see (Rowland, 2005)(Nevill, 1997; Welsman and Armstrong, 2008) for details). Allometric scaling is recommended when the physiological variable of interest is related to body size in a non-linear fashion. For example, Welsman and Armstrong (2000) demonstrated that in their sample of children, V̇O₂peak was related to body mass to the of 0.66 (mass^0.66), rather than to mass to the of 1 (mass¹), which is the commonly used normalizing factor. Allometric scaling involves using the equation Y=aM^b, where a is the physiological variable of interest, M is the body dimension (e.g., mass), and b is the scaling exponent. Y is the normalized physiological variable of interest. The main limitation of allometric scaling, besides its mathematical and conceptual complexity relative to ratio standards, is the fact that scaling exponents for a given physiological variable calculated in different data sets vary widely. For example, empirically derived scaling exponents for VO₂peak in children vary from 0.37 to 1.07, depending on the sample size, body composition, training status, etc. (Rowland, 2005). Moreover, it is not clear what exponent actually reflects the “true” biological relationship between VO₂peak and body mass. Therefore, while the use of allometric scaling in pediatric exercise physiology is increasing, the use of ratio standards is still prevalent. The optimal way to account for, or “normalize” for body size is yet to be determined. Therefore, for the most part, ratio standards are used in order to compare between children and adults. The body dimension used in the denominator varies in accordance with the physiological variable and the question of interest.

Aerobic Exercise and Training

Aerobic exercise Aerobic exercise and endurance training have long been associated with multiple health-related physiological outcomes in adults. It is assumed that in children, as in adults, aerobic exercise is associated with short- and long-term health outcomes. Therefore, the physiological responses to prolonged exercise and to aerobic training in children are of particular interest. Generally, the pattern of responses and adaptations to aerobic exercise and training in children are similar to those of adults. The magnitude of these responses is, however, different due to the effect of growth and maturation of body structures, and differences in body size. This section presents the main physiological responses associated with aerobic activities, highlighting the differences in response between children and adults.

Cardiovascular Responses

The role of the cardiovascular system during exercise is to deliver blood to the working muscles, providing O₂ and removing CO₂ from the muscles, transporting nutrients and hormones, retaining osmotic and acid-base balance and regulating the body’s core temperature (see Chapters 9 and 11). Traditionally, the cardiovascular response to exercise has been studied via the Fick equation:

Where V̇O₂ is the rate of oxygen uptake, Q̇ is the rate of cardiac output, which is determined by the stroke volume (SV) and heart rate (HR), and (a-v̅)O₂ is the arterio-venous oxygen difference. The central (or cardiac) response is reflected by changes in Q̇, SV, and HR, while the peripheral (or vascular) response at the level of the tissue is reflected by changes in (a –v̅)O₂ difference. The central and peripheral factors determining V̇O₂ have been previously described in detail in the commentary by Bassett and Howley (Bassett and Howley, 2000).

In children, as in adults, all components of the Fick equation increase during exercise, reflecting the fact that as exercise intensity increases, the body makes central and peripheral adjustments in order to supply its energy needs. However, the magnitude and the rate of change in the cardiovascular response differ between children and adults.

Cardiac Response

In an elegant study by Turley and Wilmore (Turley and Wilmore, 1997), children’s cardiac response to different exercise intensities was compared to that of adults. This study demonstrated that at an exercise intensity requiring a given

On the other hand, it is also important to understand the comparative responses of children during a given absolute (e.g., running at 8 km∙hr^-1) versus relative (e.g., 60% of V̇O₂max) exercise intensity. As can be seen in Figure 17.1a, as exercise intensity increases, heart rate increases in both children and adults. Importantly, at any given absolute exercise intensity, children’s heart rate is higher compared with adults. For example, at 10 km∙hr^-1, a young man’s heart may beat at 180 beats∙min^-1, whereas a 9 year-old boy’s heart may beat at close to 200 beats∙min^-1. Likewise, children’s maximal heart rate is also higher compared with adults. For example, it is not uncommon for children to reach a heart rate >200 beats∙min^-1 during very intense exercise. In adults, on the other hand, an observation of such a high heart rate is quite rare. Figure 17.1b demonstrates that if children and adults are running at the same relative intensity, the difference in heart rate is very small. For example, at an exercise intensity of 80% V̇O₂max ( vigorous intensity), children’s heart rate is just slightly greater than adults’ (e.g., 180 vs. 174 beats∙min^-1). The implication of this age-related difference is that if heart rate is to be used as an indicator of exercise intensity, as is often recommended for adults, one must keep in mind that for children, the targeted heart rate, as well as the heart rate at a given relative exercise intensity would be higher.

With the increase in exercise intensity, there is also an increase in stroke volume. However, unlike the linear increase in heart rate, stroke volume increases from rest to moderate intensity exercise (about 50% of maximum workload) and plateaus thereafter. This appears to be the case in both children and adults, although, of course, in view of children’s smaller body size, and, thus, their smaller left ventricular mass (LVM), stroke volume is much lower in children. Indeed, any increases in body size with growth are accompanied by increases in the size of the heart, particularly, LVM. These increases are associated with the enhanced stroke volume observed with growth and (Daniels et al., 1995; de Simone et al., 1998; Dekkers et al., 2002).

Cardiac output, which is the product of heart rate and stroke volume, increases linearly with exercise intensity, in both children and adults (Figure 17.1c). At any given exercise intensity, a child’s cardiac output will be much lower than an adult’s. For example, in the case of Anna and Rachel, while running at 8 km∙hr^-1, Anna’s cardiac output was 11.5 L∙min^-1, while Rachel’s cardiac output was 18 L∙min¹. Even when running at the same relative intensity, children’s cardiac output is lower than that of adults. For example, in a study comparing 7 year-old boys with young men, while running at about 55% of V̇O₂max, the boys’ cardiac output was about 8.5 L∙min^-1, while that of the men’s was about 19.3 L∙min^-1 (Figure 17.1c) (Turley and Wilmore, 1997). However, as is the case for stroke volume, cardiac output at rest and during exercise is also size-dependent. Therefore, child-adult comparisons require size adjustments. Traditionally, the cardiac response is adjusted to body surface area, resulting in stroke index (ml∙beat^-1∙m^–2) or cardiac index (ml∙min^-1∙m^–2). Indeed, when cardiac index is considered, there are no appreciable differences between children and adults exercising at moderate to vigorous intensity (Drinkwater et al., 1977; Rowland et al., 2008). The reasoning for adjusting the cardiac response to body surface area likely relates to the thermoregulatory perspective. That is, cardiac output and blood supply to the periphery (to the skin) are instrumental to maintain body temperature, especially at rest (see section on thermoregulation). On the other hand, it may be argued that cardiac output and blood supply is also necessary for the proper functioning of all organs, specifically, working muscles during exercise. Hence, some researchers believe that, during exercise, the cardiac response should be adjusted to body mass, or fat-free mass (reflecting muscle mass). When cardiac output is related to the fat-free mass, children’s cardiac output is higher than that of adults’, at any given relative exercise intensity (Figure 17.1d). For example, in the study by Turley and Wilmore (1997), while running at 55%

In summary, similar to adults, children’s cardiac output increases with increasing intensity of exercise, in proportion with the increase in stroke volume and heart rate. These responses reflect the increase in V̇O₂ (Fick equation).At a given exercise intensity, as well as during maximal exercise intensity, children display higher heart rate lower absolute stroke volume and lower absolute cardiac output. The lower cardiac output does not necessarily denote cardiac insufficiency. Rather, when adjusted for body size, no appreciable differences in cardiac output are observed between children and adults (Vinet et al., 2002). Therefore, from the perspective of the physiological cardiac response, despite their smaller size and different activity habits, children are not disadvantaged in the performance of aerobic activities.

Peripheral Vascular Response

In accordance with the  Fick equation, the peripheral response to exercise is reflected in the arterio-venous oxygen difference. In other words, in the amount of oxygen extracted from the blood by the working muscles ((a –v̅)O₂ difference). This section discusses age-related differences in (a –v̅)O₂ difference, which are dependent on blood flow to working muscles (vascular response) and on muscle enzymatic activity. Age-related differences in the vascular response, namely blood pressure and total peripheral resistance, are also discussed in this section. Muscle enzymatic activity is discussed later in the chapter.

During exercise there is an increase in blood flow to exercising muscles, cardiac muscle and skin, while there is a decrease in blood flow to internal organs and non-exercising muscles. Similar to adults, the blood flow redistribution in children facilitates greater oxygen transport to active muscle. The percent cardiac output that is directed to the exercising muscles appears to be higher in children than adults (Koch, 1980; Leclair et al., 2012). Specifically, Koch (Koch, 1980) showed that post-exercise blood flow to the vastus lateralis was greater in children than young adults. When the same children were tested 1 and 4 years later the difference had decreased, indicating maturation towards the adult response to blood flow distribution (Koch, 1980). These findings have been supported by Pichler et al. (Pichler et al., 2004), who demonstrated that forearm blood flow at rest decreased with increasing age (from 7 to 17 years old).

Furthermore, at an exercise intensity requiring a given V̇O₂, children also demonstrate slightly greater (a-v̅)O₂ difference than adults. This may be attributed to the fact that a greater proportion of their cardiac output goes to the exercising muscles, although disparity in muscle enzymatic activity may also affect (a-v̅)O₂ difference. During submaximal exercise, children’s higher (a-v̅)O₂ difference partially compensates for their lower cardiac output (relative to adults).

At rest, both diastolic and systolic blood pressure is lower in children than in adults (Paradis et al., 2004). During dynamic aerobic exercise there is an increase in the systolic blood pressure but no change in diastolic blood pressure. This response pattern is similar in children and adults (Rowland and Whatley Blum, 2000). The magnitude of increase in systolic blood pressure depends on the intensity of exercise with no apparent differences between children and adults. A possible exception to the latter may occur during high intensity static exercise (e.g., isometric or 1RM lifting), during which the increase in systolic blood pressure is less in children compared with adults (Turley et al., 2002).

Total peripheral resistance (TPR) is directly proportional to mean arterial blood pressure (MAP) and inversely proportional to cardiac output.

TPR = MAP∙Q̇^-1                                                                           Equation 17-2

During exercise, total peripheral resistance decreases, due to vasodilation in working muscles. While this response is similar in children and adults, the absolute TPR is higher in children at any absolute or relative exercise intensity, reflecting their smaller blood vessels. However, it should be remembered that TPR during exercise should be related to the blood supply in the working muscles. Thus, at a given relative exercise intensity, when TPR is scaled to muscle mass, TPR is lower in children compared with adults (Rowland and Whatley Blum, 2000; Turley and Wilmore, 1997). The implication of this lower TPR, is that at a given relative work rate, muscle blood flow is higher in children.

In summary, during aerobic exercise, children demonstrate a lower blood pressure response and lower size-adjusted total peripheral resistance. Their (a-v̅)O₂ difference is somewhat higher during submaximal exercise. Therefore, in terms of the peripheral vascular response children are certainly not disadvantaged in the performance of aerobic activities, and in fact, may even have a slight advantage.

Pulmonary Responses

Pulmonary ventilation (V̇E), or the amount of air flowing in and out of the lungs every minute, is lower in children compared with adults at rest and during a given exercise intensity. However, when V̇_E is adjusted to body size (for example, per body mass), girls exhibit a higher ventilatory response compared to women during submaximal and maximal exercise (Rowland and Green, 1988). This could also be assumed for boys since pre-pubertal girls are of similar size and pulmonary response as boys. This higher size-adjusted V̇_E is due to age-related differences in the airways. That is, gas exchange takes place in the alveoli. Therefore, all airways leading to the alveoli do not serve in gas exchange and are therefore termed anatomical dead space. In children, the length (or volume) of these airways, where no gas-exchange occurs, is greater relative to the volume of the alveoli, when compared with adults.

Due to their greater relative anatomical dead space, children’s fast breathing is considered relatively shallow. In terms of the measured lung volumes and capacities, this means that the percentage of vital capacity used in each breath (tidal volume) is lower in children, making their breathing pattern inefficient. This breathing inefficiency is reflected in children’s lower tidal volume and higher breathing rate at any given submaximal load (Mercier et al., 1991; Rowland and Cunningham, 1997). Importantly, this means that during exercise, the amount of air needed to breathe in and out in order to supply the body with a given volume of oxygen (the ventilatory equivalent, V̇_E∙V̇O₂^-1) has been found to be greater in children than in adults (Macek and Vavra, 1985). Other studies (Rowland and Green, 1988) also found higher V̇_E/V̇O₂, and thus, inferior breathing efficiency during submaximal running (at a given speed), in girls compared to women. In other words, there is some evidence that the cost of breathing in children is higher than in adults. During exercise, the cost of breathing adds to the cost of exercise. That is, at any given exercise intensity, this higher cost of breathing contributes to the higher oxygen cost (orV̇O₂) observed in children compared with adults. This is especially apparent during locomotion-type of exercise (Bar-Or and Rowland, 2004).

The ventilation determines the gas exchange in the lungs. Despite their lower breathing efficiency, children’s ventilation is higher than adults’, at any given exercise intensity (Rowland and Cunningham, 1997). This higher ventilation may be due to a lower CO₂ threshold drive for respiration. Importantly, children’s higher ventilation is beneficial for clearing of CO₂ during exercise.

In summary, compared with adults, children demonstrate an exaggerated ventilatory response to a given metabolic rate, likely due to differences in airway dimensions or proportions, resulting in inefficient breathing (Gratas-Delamarche et al., 1993). Despite their inefficient ventilation, children’s ventilation is greater than adults’, allowing them to benefit from greater gas exchange during exercise.

Aerobic Capacity

Aerobic capacity is a multifactorial quality. The indicators of aerobic capacity include: maximal aerobic (V̇O₂max), endurance performance, time to exhaustion, and lactate or ventilatory threshold. Amongst those, maximal aerobic is the most often used indicator of aerobic capacity in both children and adults. There are many factors that determine the

Absolute V̇O₂max (in L of oxygen per minute) increases during growth and (Figure 17.3a). For example, a “typical”V̇O₂max for a healthy young man may be 3.5 L∙min^-1. For a 10 year-old healthy boy, a “typical” V̇O₂max may be 1.4 L∙min^-1. This age-related difference reflects the differences in body size and specifically, in the exercising muscle mass.

While there are no available “normal values” forV̇O₂max in children, due to differences in measurement protocols used in different studies, numerous studies have demonstrated an increase in V̇O₂max with an increase in age. In boys, this increase appears to be linear. However, in girls, V̇O₂max appears to increase linearly until the age of 12-14 years, at which time it plateaus until young adulthood (Figure 17.3a). This pattern is evident in the many cross-sectional studies, as well as in the few longitudinal studies conducted on this topic in Europe and North America (Malina et al., 2004). At every age, boys V̇O₂max is higher than girls’, with the difference increasing as boys and girls get older. The apparent sex-related difference in V̇O₂max among young children is a reflection of boys’ larger heart size (specifically, LVM), resulting in higher stroke volume and cardiac output. As children mature, specifically during the pubertal period, the disparity in hemoglobin content and muscle mass between boys and girls increases. This is reflected in the increasing sex-related difference in V̇O₂max with increased age.

Since V̇O₂ is largely dependent on the size of the active muscle, V̇O₂max is usually adjusted for body mass. Size-adjusted V̇O₂max, expressed in mL of oxygen·kg^-1·min^-1 does not change with increasing age in boys, while in girls, V̇O₂max decreases, particularly after the age of 12-14 years (Figure 17.3b). In other words, in males, maximal aerobic capacity, relative to body mass, does not differ between boys and men. Typical V̇ O₂max values for healthy, normally active boys and men are 45-50 mL∙kg^-1 ∙ min^-1. In females, girls exhibit higher V̇ O₂max compared with women. Part of the explanation for this age-related difference is the fact that women have a higher percentage of body (and a lower percentage of muscle mass), compared with girls. This difference in body composition is not taken into account when adjusting

Metabolic Response –

Fuel utilization during exercise

During exercise, energy is supplied to the working muscles via aerobic and anaerobic metabolic pathways. As exercise intensity increases, there is a shift from aerobic to anaerobic metabolism (see Chapter 6). This shift in metabolism is often reflected by an accumulation of lactate in the blood above a certain exercise intensity, termed the lactate threshold (see Chapters 10 and 16). At any given relative exercise intensity (e.g., 50% of maximal aerobic power), as well as at maximal exercise intensity, the blood lactate concentration in children is lower than in adults. For example, in adults, the lactate threshold is said to occur around a blood lactate concentration of 4mM. While this value may vary among individuals, the equivalent threshold concentration in children is suggested to be around 2.5mM (Williams and Armstrong, 1991). More importantly, when expressed as a percentage of V̇O₂max, the lactate threshold in children occurs at a higher percentage of maximal aerobic (%V̇O₂max) compared with adults. For example, Anderson and Mahon (Anderson and Mahon, 2007) reported that among 9-10 year old boys, the lactate threshold occurred at 64% ofV̇O₂max whereas in young adult men with similar aerobic fitness (similar V̇O₂max), the lactate threshold occurred at 55% ofV̇O₂max.

The determination of the lactate threshold requires invasive measurements. Therefore, the ventilatory threshold is often used to estimate the exercise intensity at which the rate of production of lactate exceeds the rate of removal (see Chapter 10). As is the case with the lactate threshold, the ventilatory threshold is also reported to occur at a higher percentage of V̇O₂max in children compared with young adults. For example, Klentrou et al. (Klentrou et al., 2006) reported that 10-11 years old boys had higher ventilatory threshold than the adults (65% versus 58% of V̇O₂max) of similar V̇O₂max. Similarly, in the study by Anderson and Mahon (Anderson and Mahon, 2007), boys’ ventilatory threshold occurred at 68% of V̇O₂max, compared with men’s 59% of V̇O₂max.

Recall that one of the differences between aerobically trained and untrained individuals is the higher lactate threshold in the former (see Chapter 10). Along these lines, it may be argued that children’s higher lactate threshold may indicate an ability to rely on aerobic metabolism at higher intensities of submaximal exercise.

Metabolic control at the muscle level is often studied non-invasively using V̇O₂ kinetics at the onset of exercise (see Chapter 10). Indeed, as illustrated in Figure 17-4, children have been shown to have faster V̇O₂ kinetics at the onset of intense exercise compared with adolescents and young adults (Armstrong and Barker, 2009; Fawkner and Armstrong, 2004). These findings reflect a greater reliance on oxidative energy pathways, resulting in lower accumulation of metabolic by-products, such as lactate. Therefore, the fasterV̇O₂ kinetics observed in children are consistent with the observation that their lactate and ventilatory thresholds occur at a higher percentage of V̇O₂max.

Magnetic resonance technology has also been used to investigate muscle metabolism non-invasively. Using this technology, exercise scientists (Ratel et al., 2008) studied the  kinetics  of phosphocreatine resynthesis after exercise and found that the rate constant of phosphocreatine recovery and the maximum rate of aerobic ATP production were about 2-fold higher in young boys than in men, indicating that muscle oxidative capacity was higher in children than in adults (see Chapter 10). Collectively, the findings described above of higher lactate and ventilatory thresholds, faster V̇O₂ kinetics at the onset of exercise and faster post-exercise phosphocreatine kinetics all point to a greater reliance on aerobic metabolic pathways in children at any given relative exercise intensity. This reliance on aerobic, rather than anaerobic metabolism is instrumental for enhanced exercise endurance. Therefore, from a metabolic perspective, children appear to be more aerobically inclined compared with adults.A case in point is children’s fuel utilization during exercise. Recall that, among adults, endurance-trained athletes are typically able to rely more on fat, rather than on carbohydrate utilization during submaximal exercise. This adaptation to training is considered beneficial for endurance performance. Interestingly, during submaximal exercise of the same relative intensity (e.g., 60-70% V̇O₂max), children, like endurance athletes, rely more on utilization and less on carbohydrates utilization, compared with adults (Martinez and Haymes, 1992; Timmons et al., 2003). In other words, among exercising children, lipid oxidation contributes relatively more to the total energy expenditure, compared with adults. For example, Timmons et al. (Timmons et al., 2003) demonstrated that during 60 min of cycling at a moderate-vigorous intensity (70% V̇O₂max), oxidation contributed about 40% of the energy requirement for the boys, whereas for the men, only about 20% of the energy came from oxidation (Figure 17.5a). Riddell et al. (Riddell et al., 2008) demonstrated that the oxidation rate during a wide range of exercise intensities is higher in boys compared with men, and that the maximal oxidation rate decreases during puberty. Although the exact mechanisms for this age- or maturation-related change in substrate utilization are not well understood, the higher utilization during exercise has been linked to factors such as higher aerobic enzymatic (SDH) activity in muscle, higher mitochondrial volume or mitochondria/myofibrillar volume, greater intramuscular triglyceride deposits, or possibly, greater recruitment of type I fibres (vs. type II fibres) in children (see highlight box).

The greater reliance on oxidation in children was also apparent when a sports drink (containing carbohydrate) was consumed during exercise. Interestingly, the boys utilized more of the exogenous (i.e., ingested) carbohydrates for their energy needs, compared with the men (Figure 17.5b). That is, while consuming a sports drink, reliance on oxidation was still higher in the boys (30%) compared with the men (17%). However, the sports drink provided 22% of the energy for the boys during exercise and only 15% of the energy consumed by the men (Timmons et al., 2003). While the reason for the boys’ greater relative utilization of the carbohydrates from the sports drink is unknown, it may be beneficial in preserving the limited endogenous (i.e., stored within the body) carbohydrate stores in the muscle during prolonged exercise, ultimately enhancing endurance capability.

Table 17-1  Child-adult exercise performance and metabolic differences, and possible explanatory roles of age-related neuromuscular activation differences.

Adaptations to Aerobic Training

The question is often asked, how effective is aerobic training in increasing children’s V̇O₂max and aerobic performance? Any change in V̇O₂max is effected through central (cardiac) and/or peripheral (muscle) adaptations. Presumably, an enhanced V̇O₂max, results in improved aerobic performance. This section discusses the physiological adaptations (central and peripheral) to aerobic training in children and the observed changes in V̇O₂max and aerobic performance.

As previously discussed, V̇O₂ is determined by cardiac output (stroke volume and heart rate) and by the arterio-venous oxygen difference. Young athletes have been shown to have higher maximal cardiac output, as well as higher blood volume, compared with untrained peers (Eriksson and Koch, 1973; Nottin et al., 2002b; Rowland et al., 1998). Others have shown increased stroke volume and a decreased heart rate during prolonged, submaximal exercise in endurance trained children (Obert et al., 1998). These are typical responses that would be seen in adults on an aerobic training program. However, it is possible that these cross-sectional studies reflect selection bias (genetic advantage), rather than true adaptations to training. That is, it is possible that the young athletes have genetically greater cardiovascular responses to exercise and that this is one of the reasons they were drawn to, and succeeded in endurance sports. There are very few intervention studies that have examined the cardiac or hemodynamic adaptations to endurance training in children. Some studies have shown that LVM increases as a result of endurance training in children, as it does in adults (Nottin et al., 2002a; Obert et al., 2003), enhancing stroke volume at rest and presumably also during exercise. Exercise training does not seem to have a significant impact on maximum heart rate, on arterio-venous oxygen difference or on total peripheral resistance in children, although there is very limited information on these responses in children.

Changes in left ventricular dimensions and stroke volume will result in increased maximal cardiac output and potentially, greater aerobic capacity. The magnitude of change in maximal aerobic capacity following aerobic training is influenced by genetics, the training program itself, and the interaction between these two factors. Among adults, the training effect on V̇O₂max ranges between 0-50% (Bouchard, 1995), although a crude average improvement is about 20% improvement in V̇O₂max. In children, numerous studies have investigated the effect of aerobic training on V̇O₂max, but only few demonstrated more than 10% improvement. In fact, the crude average improvement in children is about 8% of V̇O₂max, with many studies demonstrating no effect at all (Pfeiffer et al., 2008). The reason for the lower trainability in children is not clear. The most likely explanation is that, in most training studies the training program has been designed based on adult guidelines. For example, participants train 3 times per week, 45-60 minutes each session, at an intensity eliciting 75-80% of maximal heart rate.  While these guidelines may be effective for adults they may be insufficient to stress the oxygen delivery system in children. That is, in view of the observed higher (lactate or ventilatory threshold) in children, as discussed previously, children may need to train at a higher relative intensity in order to elicit adaptations in the cardiovascular system. Indeed, those studies in which a higher training intensity was used (for example, HR>85% of maximal HR) demonstrated significant increases in

In spite of the relatively low training effect on V̇O₂max in children, training does result in improved endurance performance. For example, following a training program, children will be able to complete a distance run in a shorter duration, even if V̇O₂max does not increase. Possible reasons for this improved performance include better movement economy, improved “tactics” and higher anaerobic ability. Better movement economy is a bioenergetic term associated with the energy cost of locomotory activity. This energy cost is reflected in the O₂ cost at a given submaximal speed, which is shown to be higher in children (Frost et al., 2002) than in adults. It is possible that following training, children’s running style becomes less mechanically “wasteful”, or more economical, resulting in the ability to perform at a greater speed for a given energy expenditure. Another possible explanation for children’s improved performance may be a “learning effect”. That is, children, through practice, learn how to pace themselves during prolonged exercise, thus being able to utilize better tactics to improve performance. Finally, every aerobic performance includes an anaerobic component. Long-distance races are characteristically ended by a “sprint to the finish”. Rotstein et al (Rotstein et al., 1986) demonstrated that in children, an aerobic training intervention will result in aerobic, as well as in anaerobic adaptations. Therefore, it is possible that the improved performance (shorter time) in prolonged exercise tasks (e.g., 5 km run), reflects, at least in part, improved anaerobic capacity.

Anaerobic Capacity and Recovery from Intense Exercise

As opposed to aerobic fitness, which is widely accepted to be reflected by maximal aerobic uptake (V̇O₂max), there is no accepted physiological index of anaerobic fitness. Rather, anaerobic fitness is assessed via anaerobic-type performance. The most widely accepted test of anaerobic performance is the Wingate Anaerobic Test (WAnT) (see chapter 10). Also, unlike aerobic fitness, which is known to be related to health, anaerobic fitness, as reflected by peak or mean mechanical during short duration, high intensity exercise, is not specifically related to any health measure. Perhaps this is the reason for the much lower number of studies that have investigated anaerobic fitness or performance compared to aerobic fitness in children and adolescents.

Anaerobic performance increases with growth and maturation. This increase has been demonstrated in numerous cross-sectional studies (e.g.,(Inbar and Bar-Or, 1986; Van Praagh and Dore, 2002)), as well as in few longitudinal studies (e.g. (Armstrong et al., 2001; Falk and Bar-Or, 1993; Martin et al., 2004)). For example, Van Praagh and Dore (Van Praagh and Dore, 2002), demonstrated such an increase in peak short-term in over 500 boys and girls between 7 and 21 years of age. Importantly, even after correcting for body size, the increase in anaerobic performance with age is readily apparent (Figure 17.6). Regardless of the manner in which body size is taken into account (e.g., body mass, height, muscle cross-sectional area), increases with growth and development. Thus, as opposed to maximal aerobic (but similar to strength, see section on strength), scaling for body size does not eliminate the age-related increase in maximal anaerobic power. Therefore, factors other than body size must explain the age-related increase in anaerobic power.

Factors Affecting the Increase in Anaerobic Power with Growth

There are numerous factors, other than body size, which may explain changes in anaerobic power. Similar to the factors affecting strength (see section on strength), possible increases in type-II fibres distribution, increases in musculotendinous stiffness, and/or possible decreases in co-contraction may result in enhanced anaerobic power. However, the most accepted and widely cited factor explaining the lower anaerobic performance in children compared with adults is the former’s “metabolic immaturity”. Several studies have demonstrated lower activity in enzymes which are central to anaerobic metabolism, including phosphofructokinase (PFK, (Eriksson, 1980; Eriksson et al., 1973)) and lactate dehydrogenase (LDH, (Kaczor et al., 2005). Indeed, rate of lactate production in the muscles during exercise has been shown to be lower in children compared with adults. Furthermore, the rate of muscle glycogen utilization during intense exercise has been shown to be much lower in children compared with adults (Eriksson et al,. 1971). Therefore, it is believed that enhanced enzymatic activity, especially during adolescence, is at least partly responsible for the increased anaerobic performance seen during this period. It is still unclear however, what is the reason behind the changes in enzymatic activity. We have recently proposed that these changes may be a reflection of increased utilization of type-II motor units, which is hypothesized to occur with growth and (see highlight box).

It is interesting to note that while maximal aerobic does not change with growth, specifically in boys, maximal does, even when corrected for the increases in body size. This difference in pattern of change is illustrated in Figure 17.6. In a mixed-longitudinal study, Falk and Bar-Or (Falk and Bar-Or, 1993) describe the increase that occurred in peak anaerobic power, as determined in the WanT, among 36 pre- to late-pubertal boys. At the same time, there was no change in the maximal aerobic in these boys. For example, while boys’ maximal mechanical on a cycle ergometer at the end of a maximal progressive aerobic test remains about 4 watts∙kg^-1 as they grow, their peak anaerobic power, as determined by the Wingate Test, can increase from 9 watts∙kg^-1 at 10 years, to over 14 watts∙kg^-1 at 18 years. The authors suggested that metabolic specialization occurs during puberty, possibly as a result of increased utilization of type-II motor units (see highlighted box).

Recovery from Intense Exercise

 Recovery from intense exercise is notably faster in children compared with adults. That is, following intense exercise, boys are able to repeat their output within a shorter period of time compared with men (Hebestreit et al., 1996; Ratel et al., 2002; Ratel et al., 2004; Zafeiridis et al., 2005). From a methodological perspective, it is difficult to determine whether children’s physiological recovery, that is, the return to homeostasis, is also faster than adults’. For example, as discussed earlier in this chapter, maximal heart rate, an indicator of cardiovascular stress, is higher in children than in adults. On the other hand, blood lactate concentration, an indicator of metabolic stress, is lower in children than in adults. Further complicating the comparison is that resting levels (e.g., heart rate) may also differ between children and adults. Therefore, it may be argued that children and adults do not recover from the same level, nor do their physiological responses need to return to the same level. One approach has been to examine net changes in the physiological response to exercise (e.g., heart rate, ventilation, RER). Using this approach, Hebestreit et al. (Hebestreit et al., 1996) demonstrated that following the WAnT, children’s physiological response recovers faster. Another approach is to match post-exercise values. For example, by using a variable duration WAnT, Dotan et al. (Dotan et al., 2003) matched post-exercise blood lactate levels in men and boys. The authors demonstrated that although the rate of disappearance of blood lactate was similar in the two groups, blood lactate concentration peaked earlier in the boys, thus allowing them to begin their recovery process earlier.

Part of the explanation for children’s faster recovery is that their maximal output is lower than adults’ (see above). Therefore, for children, there is less disturbance to homeostasis during exercise so they have ‘less to recover from’ (Falk and Dotan, 2006). Children have a lower proportion of muscle mass. Additionally, their lower output may be a result of a lower utilization of the higher-threshold, more powerful motor units (see highlight box). However, there are other child-adult differences, which may result in faster recovery of performance (Figure 17-7). Some of these differences are simply related to differences in body size, some are metabolic differences and some are qualitative differences related to maturation. For example, children’s muscle fibres are smaller in diameter. That means that the diffusion distance of metabolites from the muscle fibre to the blood is shorter. This shorter distance, along with presumed greater capillary density around the muscle fibre, allows metabolites to clear from the muscle faster in children. Likewise, children’s shorter circulation distance means that metabolites (e.g., lactate) can be dispersed (e.g., get to inactive muscle, where they can be metabolized) within a shorter time period. From a metabolic perspective, children’s faster oxygen kinetics (Armstrong and Barker, 2009; Willcocks et al., 2010), along with their lower concentration of metabolites in the blood (lactate, H⁺) also directly results in faster recovery. Thus, children’s faster recovery, in terms of performance, as well as the physiological response is due to quantitative and qualitative reasons.

Muscle Strength and Resistance Training

Ironically, with the general decrease in physical activity, there is an increase in organized sports participation among children and adolescents. With this increase, there is an increase in the physical and training demands of each sport. More importantly, this increase in demands occurs at a younger and younger age. In other words, young athletes today are training more intensely and longer hours than in the past. Their performance level is also higher than in the past. An integral component of the higher demands of sport performance today is the inclusion of resistance training. In fact, many young athletes today, whether they are swimming, playing hockey or various ball-game sports, are involved in resistance training. On the other end of the continuum, resistance training has also become a part of the management of chronic childhood conditions, such as obesity, cerebral palsy and even cancer. Some of the important questions to consider when contemplating resistance training for youth include,

• Is resistance training effective in terms of increasing strength during childhood and adolescence?
• Is resistance training safe during childhood and adolescence?
• Does resistance training affect growth during childhood and adolescence?

The answers to these questions are discussed below. However, before embarking on resistance training and its related issues, one must consider the changes that take place in muscle structure and function during growth and maturation. This background will provide a framework for consideration of the impact of resistance training.

Muscle Structure and Function During Growth and Maturation:

Myogenesis occurs predominantly pre-natally. Muscle fibres develop from embryonic mesodermic cells and the number of muscle fibres in each muscle is believed to be achieved by birth, or soon thereafter. During childhood and adolescence, there is no increase in the number of muscle fibres so post-natal increase in muscle mass is a result of muscle fibre hypertrophy, rather than hyperplasia (Lexell et al., 1992). This means that sarcomeres are added in parallel and in series (Figure 17.8). When sarcomeres are added in parallel, this is achieved first by increased size of myofibrils, then by increased number of myofibrils when these myofibrils split.

Muscle mass increases linearly during childhood with very small differences between boys and girls. During puberty, the increase in muscle mass is greater in boys compared with girls (Tanner et al., 1981). In fact, the increase in muscle mass during puberty can be about twice as large in boys compared with girls (Johnston and Malina, 1966). At the same time, there is a parallel increase in maximal strength (Figure 17-8). There is a lot of literature on the increase in maximal isometric strength during growth and maturation (see (Blimkie, 1989; Farpour-Lambert and Blimkie, 2008; Sale D.G, 1996)). However, most daily activities, as well as sporting skills, require dynamic, rather than isometric strength. Performance in many activities (e.g., running, jumping, throwing) requires a quick generation of force in a very short time window. Usually, maximal strength is not attained during this short time window. That is, these activities require a high rate of force development, or explosive strength. Surprisingly, we know very little about the changes in dynamic, and especially in explosive strength with growth and development, although we assume that these changes are parallel to the changes in isometric strength.

Factors Explaining Increased Muscle Strength

There are numerous factors that are involved in the increase in strength during growth, above and beyond the increase in body size. Indeed, as seen in Figure 17-8, while muscle diameter and muscle mass can increase about two-fold from age 10 to 15 years, strength increases almost three-fold. When comparing children with adults, their absolute maximal strength is obviously much lower due to their size difference. However, when maximal strength is normalized for body size (for example, using ratio standards with body mass or fat-free mass in the denominator), children are still not as strong as adults (see (Blimkie, 1989; Farpour-Lambert and Blimkie, 2008; Inbar and Bar-Or, 1986; Van Praagh and Dore, 2002) for review). Therefore, factors other than body size must also be considered to explain the strength difference between children and adults. These factors include muscle composition (i.e., fibre type distribution) and neuromuscular activation pattern. Additionally, tendon stiffness  may also affect explosive strength. These factors are discussed below.

Type II fibres fibres are known to be larger and able to produce greater force per fibre than type I fibres (see Chapter 5). Therefore, a lower proportion of type II fibres in children may potentially explain the strength difference observed between children and adults. However, the fibre-type distribution is widely considered to be determined by one year of age (Bell et al., 1980; Brooke and Engel, 1969; Dubowitz, 1965; Vogler and Bove, 1985). While few studies suggest that there can be a small increase in the proportion of type II fibres during childhood and adolescence, particularly in males (Jansson, 1996; Lexell et al., 1992), this small increase is insufficient to explain the large age-related differences in static or dynamic strength. Nor are they sufficient to explain the large differences observed in explosive strength between children and adults.

A lower level of neuromuscular activation could also potentially explain children’s lower maximal strength. Several studies have suggested that there is lower motor unit activation in children compared with adults (Grosset et al., 2008; O’Brien et al., 2009; O’Brien et al., 2010). For example, using the interpolated twitch technique, O’Brien and colleagues (O’Brien et al., 2010), recently demonstrated that during maximal voluntary knee extension, adults will activate 85% of their total motor neuron pool, while children activate only 70% of the available motor units. In accordance with the Henneman’s size principle (Henneman et al., 1965)(see Chapter 3), it was recently proposed that the deficit in motor unit activation in children is made up of proportionately larger amount of type-II motor units (Dotan et al., 2012). This suggests that children utilize less of their higher-threshold, type-II motor units compared with adults (see highlight box). This difference could potentially explain not only children’s lower maximal strength, but also their lower explosive strength.

A lower tendon stiffness (higher compliance) is associated with a lower ability to produce force quickly (Wilson et al., 1994). This means that the lower the tendon stiffness, the lower the explosive strength. Therefore, a lower musculo-tendinous stiffness in children may play a partial role in the lower explosive strength typical of children. Indeed, several studies have demonstrated lower musculo-tendinous stiffness in children compared with adults (Grosset et al., 2007; Lambertz et al., 2003). Lambertz and colleagues (Lambertz et al., 2003) found not only a difference between children and adults, but a progressive increase in musculo-tendinous stiffness in children from 7 to 10 years of age. Thus, children’s lower explosive strength, and particularly, the slower of force may be partly explained by the lower musculo-tendinous stiffness, although it cannot account for all the observed age-related differences.

Effects of Resistance Training in Children

One of the myths in resistance training is that it is ineffective in children. In other words, engagement in resistance training among children was not considered to result in enhanced strength. Several early studies supported this view by failing to demonstrate any increase in strength in children who participated in resistance training (Docherty et al., 1987; Vrijens, 1978). However, a closer look at these studies would reveal that either the volume or the intensity of the training program was too low to result in increased strength. Since then, there have been numerous studies which have disputed this myth, demonstrating considerable strength gains in children following resistance training (for reviews see (Behm et al., 2008; Blimkie, 1992; Blimkie, 1993; Falk and Eliakim, 2003; Falk and Tenenbaum, 1996; Malina, 2006)).

One of the most impressive and comprehensive studies on the effects of resistance training in children was reported by Ramsay and colleagues (Ramsay et al., 1990), in which pre-pubertal boys engaged in resistance training over a period of 20 weeks, which is considered a particularly lengthy training duration for a pediatric research study. Significant strength gains were observed in the training group (with minimal strength gains in the non-training group), whether strength was measured using 1RM tests, isometric or isokinetic testing (Figure 17-9). That is, in each of eight different exercises, boys who trained improved by 15-60%, depending on the type of exercise, significantly more than the boys who did not train.

Today, the effectiveness of resistance training in children is accepted among coaches, as well as among health professionals. Indeed, numerous professional organizations, such as the American College of Sports Medicine, the International Federation of Sports Medicine, the National Strength and Conditioning Association, the British Association of Sport and Exercise Sciences, and many more have published position statements in favour of resistance training for children. The Canadian Society for Exercise Physiology came out with a position statement (Behm et al., 2008) advocating the benefits of several types of resistance training for children.

Accepting that resistance training is indeed effective in enhancing strength in children, the obvious next question is, how effective is it compared to its effectiveness in adults. In order to answer this question, one would have to design a study in which children and adults, preferably at the same baseline level, participated in an identical training program. Technically and logistically, this would be very difficult to do. Indeed, Lillegard and colleagues (Lillegard et al., 1997) examined the effect of resistance training in early and late-pubertal children but did not observe any clear effect of maturity status. Notably, the number of participants in each group was relatively small, making it difficult to observe real differences.

In an effort to examine the relative effects of resistance training on children compared with adults, Sale (Sale, 1989) merged data from two separate studies in which the effect of resistance training on evoked twitch torque (resulting from electrical stimulation) of the elbow flexors and knee extensors was investigated. Although scientifically combining data from two studies is not recommended, in this case the data were collected by the same research group, using the same equipment and testing techniques, and the training programs were similar, so the approach is justified. In both studies, the training group’s strength improved significantly more than the control group, which in view of the above discussion is not surprising. In terms of absolute torque gain at the end of the training period, it is evident that the children improved 50% less than the adults. However, if one compares the relative gain as a percentage of pre-training torque, it is evident that the children improved much more than the adults (about twice as much). Therefore, the answer to whether resistance training is as effective in children as it is in adults is not straight forward. In absolute terms, children improve less. However, relative to their starting level, children may improve more than adults as a result of resistance training.

Mechanisms Explaining the Effects of Resistance Training

Generally, there are two types of mechanisms, which may explain the strength gains observed following resistance training: morphological and neurological adaptations. Both of these mechanisms play a role in children and in adults, but their relative contribution in each age group differs. Morphological adaptations, specifically muscle hypertrophy, are often evident in adults, specifically in men. In children, on the other hand, most studies report no increases in muscle size following resistance training (see (Behm et al., 2008; Falk and Eliakim, 2003; Falk and Tenenbaum, 1996; Sale D.G, 1996; Sale, 1989) for review). Notably, the studies that examined muscle hypertrophy in children have generally used anthropometric techniques (Blimkie, 1989; Granacher et al., 2011; McGovern, 1984; Ozmun et al., 1994; Ramsay et al., 1990; Sailors and Berg, 1987; Siegel et al., 1989). Two studies in which more sensitive methods of measurements were utilized (magnetic resonance imaging and ultrasound) have suggested that muscle hypertrophy, although minimal, may indeed occur among children following resistance training (Mersch and Stoboy, 1989; von Fukunaga et al., 1992). However, even if muscle hypertrophy does occur as a result of resistance training in children, the extent of that hypertrophy is minimal and is likely insufficient to explain the apparent strength gains.

In view of the limited evidence of muscle hypertrophy and its small potential contribution, the main mechanism believed to explain children’s strength  gain following resistance training, involves neural adaptation. These adaptations are difficult to define, measure and characterize. Therefore, neurological adaptations in children are mainly based on indirect evidence. There are only three studies that attempted to directly demonstrate neurological changes in children following resistance training. Using the interpolated twitch technique (Chapter 13), Ramsay et al. (Ramsay et al., 1990) demonstrated an increase in motor unit activation of the elbow flexors and knee extensors, following 20 weeks of training. Using the same technique, Waugh et al. (Waugh et al., 2014) could not demonstrate such an increase following 10 weeks of resistance training in young boys, although the number of boys in the study was relatively small. Ozmun et al. (Ozmun et al., 1994) used integrated electromyographic amplitude (IEMG (Chapter 3)) to demonstrate an increase in neuromuscular activation in agonist muscles following eight weeks of training. However, in all cases, the extent of the increased neuromuscular activation of the agonist muscles was much smaller than the increases in strength.

An additional neuromuscular adaptation which may explain some of the strength gains in children may involve improved inter-muscle coordination, which likely plays a more influential role in complex, multi-joint actions. Mitchell and colleagues (Mitchell et al., 2011) observed that young male gymnasts, who presumably engage in extensive resistance training, particularly explosive training, demonstrate much faster rate of muscle activation compared with endurance-trained or untrained boys. Gymnasts were also characterized by a faster rate of force (explosive strength) (Figure 17-10). These findings are supported by a recent study which demonstrated that resistance training (free weights or plyometric training), added to regular soccer training, resulted in enhanced rate of force development, along with increases in the rate of muscle activation (McKinlay et al. 2018).

Safety and Resistance Training

Resistance training has been touted to be unsafe for children and adolescents. Admittedly, all types of physical activity are associated with some risk of injury, including resistance training. The question is whether resistance training is associated with greater risk than other activities. Hamill and colleagues (Hamill, 1994) specifically studied the incidence of injuries among adolescent boys engaged in resistance training, weight lifting and other, more pervasive sports such as rugby and soccer. The authors reported that the incidence of injuries in resistance training and weight lifting was several orders of magnitude lower than the more common sports (0.017-0.035 vs. 0.14 to 8 injuries per 1000 participation-hours). Notably, the authors also reported a higher injury incidence in physical education classes compared with resistance training. More recently, Malina (Malina, 2006) reported similarly low incidence among pre-pubertal boys engaged in resistance training. Thus, it appears that, when properly performed and supervised, resistance training is associated with a lower risk of injury compared with many other prevalent activities. On the other hand, it should be noted that the most serious type of injury, which has been reported to result from resistance training (or weight lifting) is an injury to the epiphyseal (growth) plate. Obviously, this type of injury can appear only in children or adolescents, not adults, and, if left untreated, can have potentially serious consequences. Therefore, in addition to the safety rules, which are recommended for adults, there are several safety precautions, which are particularly important for children. These include adherence to the proper technique, the use of properly-sized equipment, the engagement of properly qualified exercise specialists and a higher instructor-trainee ratio. For a detailed list of recommendations and safety precautions, see the Canadian Society of Exercise Physiologists (CSEP) position statement on resistance training in children and adolescents (Behm et al., 2008).

Thermoregulation During Exercise in the Heat

Thermoregulation is the process by which an organism keeps its body temperature within certain boundaries, even when the surrounding temperature is very different (Chapters 9 & 16). Thermoregulation is affected by environmental factors, such as temperature, humidity and wind, as well as by physiological processes such as metabolic rate (exercise), circulation and sweating. In efficient thermoregulation, heat dissipation (via sweating or skin blood flow) must equal heat production (by the muscles). These processes occur in children, as in adults. This section highlights the differences between children and adults.

The amount of heat dissipated is dependent on the available surface area. On the other hand, the amount of heat produced is dependent on the exercising muscle mass. The smaller the body, the larger the body surface area-to-mass (BSA/M) ratio. Thus, children’s BSA/M ratio can be 30-40% greater than adults’. This geometric difference affects the means of heat dissipation in different environmental conditions (see below).

Physiologically, heat can be dissipated via increasing blood flow to the skin (promoting “dry” heat exchange, also known as conduction and convection), or via sweat evaporation (Chapter 9). By far, the most prominent and consistently observed difference between children and adults in regards to temperature regulation is the former’s much lower sweat rate. In any given environmental condition, whether at rest or during exercise, children consistently demonstrate lower sweating rate. For example, during moderate exercise (50% V̇O₂max) in dry heat (42C, 20% humidity), sweating rate was lower in pre-pubertal boys compared with men (Figure 17.11a). Sweating rate increases from pre- to late-puberty yet is still lower in late-pubertal boys compared with young men (Falk et al., 1992a; Falk et al., 1992c). Since the number of sweat glands is determined by the age of 2-3 years, the sweating rate per gland is also lower in children (Figure 17.11b). Children’s sweating rate remains lower after taking into account differences in body surface area. Thus, absolute and relative sweating rate is considerably lower in children compared with adults. This age-related difference has been demonstrated consistently in various environmental conditions (see (Falk and Dotan, 2008a; Falk and Dotan, 2008b; Falk and Dotan, 2017) for review). The explanation for the difference in sweating rate may be related to age-related differences in gland size, sensitivity or capacity. But it may also reflect the fact that children may not need to sweat as much.

As mentioned above, heat dissipation is achieved via increasing skin blood flow and via sweating. Children’s blood flow to the skin is greater during exercise in the heat (Drinkwater et al., 1977), and it increases with maturity (Falk et al., 1992b; Falk et al., 1992c). Therefore, it is possible that due to their ability to increase their blood flow to the skin, they do not need to rely on sweat evaporation for effective thermoregulation. In other words, the “strategy” used for thermoregulation is different in children and adults: Children rely more on dry heat loss (via increased skin blood flow), and less on sweat evaporation in order to dissipate the heat produced by the exercising muscles (Falk and Dotan, 2008a; Rowland, 2008).

Does this different thermoregulatory strategy affect the effectiveness of thermoregulation during exercise? In extreme environmental conditions, hot or cold, children may be more vulnerable, but under most conditions, they are at no disadvantage relative to adults. In fact, there are no epidemiological data demonstrating higher rate of heat injury in children, even during heat waves (see (Falk and Dotan, 2008a; Falk and Dotan, 2017; Rowland, 2008) for review).

In view of children’s lower reliance on evaporative heat loss, it may be argued that they may be able to conserve water, relative to adults, during exercise in the heat. However, the issue of proper hydration should not be overlooked. For example, Decher et al. (Decher et al., 2008) demonstrated that among children and adolescents participating in summer sports camps, 50% arrived at their first practice of the day in a hypohydrated state. During exercise, children (as do adults) do not usually consume sufficient fluids and become inadvertently hypohydrated, a phenomenon called “voluntary dehydration”. Wilk et al. (Wilk et al., 2001) demonstrated that about 50% of well-trained acclimated adolescent athletes finished a triathlon at 2-3% hypohydration. More importantly, 7% of the athletes were at a more severe (>3%) state of hypohydration at the end of the competition. The same group of authors also demonstrated that even 1% hypohydration is sufficient to negatively affect performance in 10-12 yr old runners (Wilk et al., 2002).

While the rate of dehydration may be similar in children and adults, the implication of this fluid loss is harsher in children. As demonstrated in Figure 17.12, at a similar state of hypohydration (e.g., 1% body mass), children’s body temperature increases about 50% more than adults’ (e.g., 0.6 vs. 0.4C, respectively). Thus, while children may “conserve” fluids by relying on a lower sweat rate, their need for maintaining proper hydration level is higher. It is unclear whether children’s habitual fluid replacement habits are sufficient during competition, specifically in a hot environment. Therefore, it is up to coaches, teachers and parents to ensure that children rehydrate sufficiently.

Summary

In summary, this chapter describes the responses and adaptations of children to exercise and training. These responses, for the most part, are similar in pattern to the responses of adults. The magnitude of these responses is, however, different due to the effect of growth and maturation of body structures, and differences in body size. Therefore, physiologically, children are not just small adults. Most of the anatomical and physiological differences observed between children and adults are summarized in Table 17.2.

The chapter first discusses the responses to acute aerobic exercise and to aerobic training. Similar to adults, children’s cardiac output increases during exercise, reflecting the increases in stroke volume and heart rate. These responses allow for the increase in V̇O₂ in the exercising muscles. At a given exercise intensity, as well as during maximal exercise intensity, children display higher heart rate, lower absolute stroke volume and lower absolute cardiac output. The lower cardiac output does not necessarily denote cardiac insufficiency. Rather, when adjusted for body size, no appreciable differences in cardiac output are observed between children and adults. In addition, children demonstrate lower blood pressure and size-adjusted total peripheral resistance during aerobic exercise. Their (a-v̅)O₂ difference is somewhat higher during submaximal exercise. Therefore, despite their smaller size and different activity habits, children are not disadvantaged in the performance of aerobic activities. In fact, due to their slightly favorable peripheral vascular response, as well as their greater reliance on utilization, they may even have a slight advantage in this type of activities.

Children demonstrate an exaggerated ventilatory response to a given metabolic rate, likely due to differences in airway dimensions or proportions, resulting in inefficient breathing. However, despite their inefficient ventilation, children’s ventilation is greater than adults’, allowing them to benefit from greater gas exchange during exercise.

Boys’ maximal aerobic capacity, relative to body mass, is not different than that of men. Girls’ mass-adjusted maximal aerobic capacity is actually higher than women’s. In addition, in spite of the relatively low training effect on V̇O₂max in children, training does result in improved endurance performance. Possible reasons for this improved performance include higher movement economy, improved “tactics” and higher anaerobic ability. In addition, recovery from intense exercise is notably faster in children compared with adults.

Anaerobic performance and strength increase with growth and maturation. In fact, as opposed to maximal aerobic power, scaling for body size does not eliminate the age-related increase in either, maximal or strength. Therefore, factors other than body size must explain the age-related increase in and strength. These factors include possible differences in neuromuscular activation, greater co-contraction of the antagonist muscles in children, as well as lower musculo-tendinous stiffness. While children’s performance in high-intensity exercise is lower than adults’, their recovery is much faster. The faster recovery may be partly due to their lower amount of work output, but is also due to qualitative age-related differences, mainly due to growth and maturation.

Resistance training has been consistently demonstrated to enhance strength in children. However, while adults’ strength enhancement is a result of muscle hypertrophy and neuromuscular adaptations, this is not the case in children. Rather, children’s strength appears to increase solely or mostly as a result of neuromuscular adaptations. The nature of these adaptations has not been fully elucidated. In absolute terms, children’s strength improves less than adults’, but relative to their starting level, children may improve more than adults.

Children are able to tolerate and properly thermoregulate during exercise in the heat. While their sweat rate is much lower than adults’, their blood flow to the skin is higher. Children appear to use a different ‘strategy’ to thermoregulate: they rely more on dissipating heat via dry heat exchange (increasing skin blood flow and losing heat by conduction and convection), and less on sweat evaporation. Therefore, they are able to conserve body water. However, they appear to be more sensitive to a given level of hypohydration, emphasizing the need to encourage fluid consumption during physical activity in a hot environment. This is particularly important for athletes competing in outdoor tournaments.

In conclusion, as demonstrated in Table 17.2, children’s performance and responses to exercise and training differ from those of adults. These age-related differences are quantitative, as well as qualitative, demonstrating that in many ways, children are not small adults and should not be regarded as such.

Table 17-2  Anatomical and Physiological comparison between children and adults.

Exercises or Topics for Discussion

What are the cardiovascular factors that may change as a result of aerobic training and may explain the typical improvement in V̇O₂max seen after training? Are the effects of aerobic training on each of these factors similar in children and in adults? Explain.

You have volunteered to help coach your school’s soccer team (grades 4-6 boys and girls). The teacher who has been coaching soccer for the last few years is happy to have you aboard, as she knows you have recently become a kinesiologist. She therefore asks your advise on whether to devote any practice time to develop aerobic ability in the children (they are not particularly well-trained). She asks whether you expect any benefits of aerobic training in these children. What is your advise?

Considering osteoporosis is characteristic of old(er) age, explain why we should be concerned with bone health during childhood. What can we do in order to reduce the risk of osteoporotic fractures in old(er) age?

In your opinion, does athletic training affect (enhance/delay) growth? Justify.

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About the Authors

Panagiota (Nota) Klentrou,Dept of Kinesiology, Faculty of Applied Health Sciences

Brock University

Dr. Panagiota (Nota) Klentrou is a professor in the Department of Kinesiology, and the Associate Dean Research and Graduate Studies of the Faculty of Applied Health Sciences at Brock University, Ontario, Canada. She received a BSc (1981) in Physical Education and Sport Science from the National University of Athens, Greece, and an MSc (1987) and PhD (1991) in Exercise Physiology from the University of Montréal, Québec, Canada. Her research program uses applied and basic science approaches to study human performance and the implications of sport training primarily in children and youth. With funding from NSERC, SSHRC, CIHR and other sports agencies, her current research projects examine how sexual maturation, exercise, inflammation, adiposity and nutrition affect musculoskeletal growth and development. In particular, this research is trying to identify the cellular mechanisms that explain how exercise training and dietary choices during childhood and adolescence affects skeletal and lifelong bone health. Dr. Klentrou is a leader in her field with >100 publications and >2,000 citations covering a range of topics in pediatric exercise physiology, exercise interventions, musculoskeletal growth and maturation, nutritional supplements and inflammation. A retired international rhythmic gymnast herself, she has been involved with many local, provincial, national and International organizations including Osteoporosis Canada, the International Gymnastics Federation and the Athens 2004 Summer Olympic Games. Dr. Klentrou served as Chair of the Board of Directors of the Canadian Society for Exercise Physiology 2013-2015 after serving 10 years (2003-2013) as Treasurer.

Bareket Falk, Dept of Kinesiology, Faculty of Applied Health Sciences, Brock University

Dr. Bareket Falk is a pediatric exercise physiologist, with a keen interest in children’s responses to exercise and the physiological effects that physical training may have on healthy children, as well as on children with chronic diseases. She obtained a Bachelor degree in Physical Education from McGill University in Montreal (1982), M.Sc. (1986) from Tel Aviv University, and Ph.D. (1991) from McMaster University. She headed the Physiology Department and the Sports Medicine and Research Centre at the Wingate Institute in Israel and is currently a professor in the Department of Kinesiology at Brock University.

Bareket has been teaching topics in growth and development, exercise physiology and specifically, pediatric exercise physiology for about 30 years at the undergraduate and graduate level.

Her recent areas of research include the effect of exercise and physical training on bone health and on neuro-muscular function during growth and maturation. She has published over 180 scientific papers and book chapters encompassing various areas in pediatric exercise physiology, including the effects of physical activity and training on growing bone and on muscle function, as well as in areas such as thermoregulation and metabolism.