Although children (C) are often thought of as little adults (A), many functions in their bodies are not completely developed, and they cannot perform certain types of exercise as well as they will be able to do when they are A. For the purpose of this article, C are described as being from 6 years old until the onset of puberty, which is the period when C undergo the process of sexual maturation. It is important to understand that the onset and duration of puberty is quite variable. According to Kail and Cavanaugh (1), puberty begins at ages 10 to 11 years in girls and is completed at ages 15 to 17 years; the corresponding ages for boys are 11 to 12 and 16 to 17 years. The period from puberty to becoming a legal adult is generally called adolescence.
In general, the basic physiological responses to exercise and to training are similar at all ages. Nevertheless, there are quantitative differences between C and A. Some of these differences are associated with body size, whereas others are associated with biological maturation. As an example, when V˙O2peak is expressed in absolute terms (L·min−1), the values in C are lower but increase as they grow. When V˙O2peak is expressed relative to body weight (mL·kg−1·min−1), C and A have similar values, especially when there are similar levels of physical activity. These comparisons are summarized in the Table.
Comparison of children (C) and adults (A) at rest and during exercise
||C < A
||C ~ A
|V˙O2 at rest
||C > A
|V˙O2 (mL·kg−1·min−1) walk or run
||C > A
|ATP (amount stored and rate of utilization)
||C ~ A
|CP (amount stored and rate of utilization)
||C ~ A
|Glycogen (amount stored and rate of utilization)
||C < A
|Maximal lactate concentration
||C < A
|Rate of rise in V˙O2 at onset of exercise
||C > A
|Rate of recovery from exercise
||C > A
Before puberty, there is little difference in absolute or relative V˙O2peak between boys and girls of similar body size and activity level. After puberty, there are greater differences between the sexes (a) in absolute terms because males tend to be larger and (b) in relative terms because females tend to accumulate more body fat. Interestingly, if V˙O2peak is expressed relative to fat-free weight (mL·kg FFW−1·min−1) to eliminate the difference in body fat, then there is little difference between the sexes.
Fitness professionals should be aware of the differences between children and adults in how they respond to different types of exercise.
COMPARISONS DURING REST AND EXERCISE
It is an interesting phenomenon in nature that smaller animals use more energy at rest and during exercise relative to their body weight. This is not different when comparing C and A. The reason why C need more energy at rest is not clear, but they may be less efficient during exercise (2).
Looking at the skeletal muscle of C and A, the concentrations of adenosine triphosphate (ATP) and creatine phosphate (CP) and their rates of utilization are similar. The same is true for the oxidative enzymes. On the other hand, the amount of glycogen stored in the muscles of C is less. More importantly, the rate of glycogen use is much less in C. This is reflected in lower maximal values of lactate in their muscles and blood. As a result, C have a limited capacity to work at high intensities for long times. During and after puberty, there is a rise in maximal lactate; this maturation is associated with higher levels of testosterone and a parallel increase in the activity of the glycolytic enzymes.
C tend to respond more rapidly at the beginning of intense exercise, their perception of the effort involved in exercise is lower, and they tend to recuperate faster after exercise. As an example, the late Dr. Oded Bar-Or, who developed the 30-second Wingate Anaerobic Test (WAT), showed that C could totally recuperate from the WAT after 2 to 3 minutes, whereas young A needed 10 minutes (2). Part of this faster recovery may be associated with the lower levels of lactate in C. These comparisons are summarized in the Table.
Bar-Or suggested that prepubertal C are metabolic nonspecialists (3). In other words, those C who have a good aerobic capacity also tend to have a good anaerobic capacity. As a result, they can perform well in many sports. It seems that only after puberty do they demonstrate the metabolic and morphologic traits associated with specific types of exercise. This has obvious implications for identifying athletic potential before puberty.
REGULATION OF BODY TEMPERATURE
A major difference between C and A is in the way each regulates body temperature. Metabolism produces heat (heat gain) and is always positive, whereas evaporation is the principal mechanism for heat loss during exercise and is always negative. The other forms of heat exchange (gain or loss) with the environment (radiation, conduction, and convection) can be positive or negative, depending on whether the environmental temperature is higher or lower than that of the body.
Body mass produces and stores heat, whereas the body surface exchanges heat with the environment. The higher the ratio of body surface to body mass, the greater will be the exchange of heat to the body in a hot environment and from the body in a cold environment. Given that C have a surface-to-mass ratio that is 36% larger than that of A per unit of body mass, this means that they will lose more heat to a cold environment and gain more heat from a hot environment.
Why is this important? Consider these facts. C use more energy and as a result produce more heat per kilogram of body weight during exercise. C have a higher blood flow to the skin where heat is exchanged than A. As mentioned above, evaporation is the major avenue to lose heat during exercise in A. C start to sweat at higher body temperatures. When they do begin to sweat, they sweat less and lose less heat through evaporation. As a result, their body temperature will be higher at the same level of exercise. Because C do not sweat as much, they exchange more heat through radiation, conduction, and convection, i.e., it is a “drier” heat exchange (4). Interestingly, the rate of sweating per sweat gland increases with maturation, with the greatest rise in sweating during the onset of puberty.
One other factor that is important relative to regulating body temperature is that C require more time to acclimatize to heat (3). Therefore, they will be at higher risk for heat problems during exercise in the first days of high environmental temperatures.
To summarize, C have fewer problems in a neutral environment but do not do as well as A in hot or cold temperatures. For more detailed information on these factors, please see the article (5), chapter (3), and book (2) by Bar-Or.
Because many responses to exercise are different before and after puberty, it is difficult to predict the athletic potential of children before they become adults.
WHAT IS THE IMPORTANCE OF THESE DIFFERENCES?
C can perform very intense activities that last a few seconds because their ATP–CP system is adequate.
C cannot perform high-intensity activities that last ≥30 to120 seconds as well as A because they cannot produce high levels of lactate normally associated with anaerobic glycolysis; this changes during puberty.
C can perform moderate-intensity activities of moderate duration because their aerobic system is adequate.
C cannot perform endurance activities of long duration as well as A because their thermoregulatory system cannot use the sweating mechanism to the same extent; this changes during puberty.
Although C are not aware of their physiological differences, their pattern of play reflects these differences. C tend to do activities of low to moderate intensity with brief periods of high-intensity activities. A study done by Dr. R.C. Bailey (6) and his colleagues from UCLA found that during daily observation periods of 12 hours, C did 22 minutes of high-intensity activity with an average duration of 3 seconds. There were no periods of intense activity that lasted 10 minutes and 95% of the intense activity periods lasted less than 15 seconds. They concluded that C engaged in very short bursts of intense activity, interspersed with varying intervals of low to moderate intensity. A study by Ruiz and colleagues (7) also showed that preschool C ages 2 to 5 years tend to be active in spurt-like bouts of activity followed by brief rest periods.
It is mainly during competitive situations (especially with pressure from A) that C will do (a) high-intensity exercise for more than 30 seconds or (b) prolonged endurance exercise.
There are some big differences in how children and adults respond during exercise in a hot environment, especially when it is also humid.
The observations of how C play suggests that high-intensity interval exercise (HIIE) might be a useful form of exercise for C. Tarp et al. (8) looked at accelerometry data on about 30,000 individuals 4 to 18 years old. Although they did not differentiate between C and adolescents, they found that HIIE was a major factor for reducing cardiometabolic risk factors. In a study on C, Cockcroft and coworkers (9) found that C enjoyed and preferred HIIE more than moderate-intensity exercise. Bond et al. (10) reviewed studies on HIIE done by C and adolescents. Although most studies involved adolescents, several were done with C. They concluded that HIIE is a “feasible and potent method” to improve many cardiometabolic risk factors in both groups.
Previously, C seemed to naturally play when they were allowed to be C. Today, they are told to sit and be quiet and do not have the opportunities to play. As a result, they spend a lot of time seated while watching television or playing computer games. C should be encouraged to actively play and enjoy themselves. If this is done, there is a better chance that they will be more active, healthier, and will develop habits that will continue into their adult years. Unfortunately, encouragement alone may not be enough to ensure active C. Therefore, developmentally appropriate interventions in early life are needed to provide the opportunities to be more active and to develop fundamental movement skills. This suggests that youth fitness professionals should be involved in physical activity programs for C.
C should not be encouraged to specialize in sport until after puberty. As mentioned earlier, C with high or low aerobic abilities tend to have corresponding high or low anaerobic abilities. As mentioned above, it is more important to emphasize the development of basic motor skills, perhaps under the direction of trained youth development coaches/specialists (11). If done well, C may be better prepared after puberty when they do begin to specialize. As well, they will have a good basis for lifelong physical activity.
Personal trainers working with C should be aware of environmental temperatures and how they may affect the ability of C to exercise. Because (a) evaporation is the major mechanism for losing heat during exercise and (b) evaporation is reduced with higher humidity, personal trainers should also be aware of the relative humidity. The amount and intensity of exercise should be decreased when it is hot and humid.
From this brief review, it should be obvious that C are not little A, as their responses to some forms of exercise and to a hot environment are different from those seen in A.
I saw a Dennis the Menace cartoon in which he and his friend Joey were walking by his mother who was exercising. Dennis said, “When you are too old to play, you have to exercise.” In agreement with Faigenbaum et al. (11), we should provide as many opportunities as possible for C to play and perhaps there will be fewer problems associated with physical inactivity as they are C and when they mature into A.
BRIDGING THE GAP
The ability of children to perform brief, high-intensity exercise and moderate-intensity exercise is similar to that of adults. Children do not perform high-intensity exercise lasting more than 30 seconds or prolonged endurance exercise as well as adults. These problems diminish as children grow and mature.
1. Kail RV, Cavanaugh JC. Human Development: A Lifespan View
. 5th ed. Boston (MA): Wadsworth Cengage Learning; 2010. p. 96.
2. Helgestreit HU, Bar-Or O. Differences between children and adults for exercise testing and exercise prescription. In: Skinner JS, editor. Exercise Testing and Exercise Prescription for Special Cases. Theoretical Basis and Clinical Application
. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005. p. 68–84.
3. Bar-Or O. Pediatric Sports Medicine for the Practitioner: From Physiological Principles to Clinical Applications
. New York (NY): Springer-Verlag; 1983. p. 376.
4. Falk B, Dotan R. Temperature regulation
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7. Ruiz RM, Sommer EC, Tracy D, et al. Novel patterns of physical activity in a large sample of preschool-aged children. BMC Public Health
8. Tarp J, Child A, White T, et al. Physical activity intensity, bout-duration, and cardiometabolic risk markers in children and adolescents. Int J Obes (Lond)
. 2018 Sep;42(9):1639–50.
9. Cockcroft EJ, Williams CA, Jackman SR, et al. A single bout of high-intensity interval exercise and work-matched moderate-intensity exercise has minimal effect on glucose tolerance and insulin sensitivity in 7- to 10-year-old boys. J Sports Sci
10. Bond B, Weston KL, Williams CA, et al. Perspectives on high-intensity interval exercise for health promotion in children and adolescents. Open Access J Sports Med
11. Faigenbaum AD, MacDonald JP, Carvalho C, et al. The pediatric inactivity triad: a triple jeopardy for modern day youth. ACSMs Health Fit J