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EXERCISE-HEAT TOLERANCE OF CHILDREN AND ADOLESCENTS
Pediatric Exercise Science, 1995, 7, 239-252.
By Lawrence E. Armstrong and Carl M. Maresh

This excellent review of literature regarding children and exercise-heat tolerance provides some insights that coaches of youth sports should be familiar with. "The greatest risk of health illness for children is heat exhaustion and not heat stroke."



Review

"Exercise-heat tolerance (EHT) is defined as the ability to tolerate exercise in a hot environment." "Despite two reports in the 1960’s of heat illness in infants, exercise-heat tolerance has not been explored extensively in children and adolescents. This is surprising because children are habitually more active and spend more of their active hours outdoors than adults do."

The following statements represent the consensus of opinion regarding studies of children and adolescents, which Armstrong and Maresh evaluated and qualified their meaning:

1. "Children have a greater mass- relative submaximal oxygen demand than adults, during walking or running, but a lower anaerobic capacity.
2. Children have a smaller Mass to Surface Area (M/SA) ratio.
3. Children have a smaller sweating rate and total cardiac output, but a higher heart rate and core temperature (however, Armstrong and Maresh point out that core temperature is not supported by scientific evidence).
4. Children are not capable of complete heat acclimatization and thus acclimatize at a slower rate.
5. Children report a lower rating of perceived exertion during exercise in the heat.
6. Adolescents exhibit EHT similar to that of adults, but specific organ systems have not matured to full function or size."

Some investigators have reported that children exhibit adequate thermoregulation and EHT that is similar to that of adults, while others have reported decreased EHT and inadequate thermoregulation in children. It is also significant that children thermoregulate as effectively as adults in cool conditions of 20 to 25 degrees C, and in warm conditions when the ambient temperature exceeds the skin temperature by five to seven degrees celsisus and humidity is below 50% relative humidity. However, in a very hot environment, when the ambient temperature exceeds the skin temperature by more than 10 degrees celsisus, the EHT of children is reduced.

1. Children have a greater mass-relative submaximal oxygen demand than adults, during walking or running, but a lower anaerobic capacity.

Drinkwater et al. in one of the few studies that focused on the cardiovascular responses of both children and adults who had similar VO2 max. The girls, age 12, exhibited overt indications of cardiovascular difficulty when they stopped exercising. Two other investigators have reported similar cardiovascular insufficiency in boys.

The primary risk of heat illness for children appears to be heat exhaustion, not hyperthermia or heatstroke. Because there are virtually no reports of heatstroke among children at outdoor contests, it appears that children, or coaches and parents, selectively halt exercise when cardiovascular insufficiency results in great discomfort, dizziness, or extreme fatigue.

2. Children have a smaller Mass to Surface Area ratio.

"M/SA is a theoretical means of identifying who is susceptible to heat injury, because mass is proportional to heat production and surface area is proportional to heat dissipation. If both boys and girls have smaller mass than adults, theoretically they should dissipate exercise-generated heat more efficiently via radiative and convective heat loss, because the ratio of heat-generating mass is lower per unit of skin surface area."

However, there is an overlap in the M/SA values of children and adults. This indicates that the classic M/SA explanation above must be qualified by describing factors such as the age, fitness level and body composition of children.

3. Children have a smaller sweating rate and total cardiac output, but a higher heart rate and core temperature.

The sweating rate of prepubertal boys is significantly lower than in men. Prepubertal girls, however, exhibit sweating rates similar to those of women. When groups that differ markedly in maximal aerobic power (VO2 max) are given a task with the same relative exercise intensity (%VO2 max), the effect of age can be confounded because sweating rate is proportional to the absolute metabolic rate. This research design is common, however, because core temperature and heart rate are proportional to %VO2max.

There have been no scientific studies that have observed large exercise-induced differences between the rectal temperature of children and adults. This is important because rectal temperature is one of the standards by which exercise-heat tolerance and heat acclimatization historically have been evaluated.

4. Children are not capable of complete heat acclimatization and thus acclimatize at a slower rate.

This is an area for more research. The studies that have shown that children thermoregulate like adults in cool environments but have limited theromoregulatory capacities in hot environments had flaws. In fact, at least six studies did not observe adult responses in identical tests. The current literature does not support the concept that exercise in a hot environment is dangerous for them.

5. Children report a lower rating of perceived exertion during exercise in the heat.

Perceived exertion was not focused on in any study cited. However, it was noted that to improve cardiovascular dynamics during exercise-heat exposure, children could either enhance their physical fitness or acclimatize to heat.

6. Adolescents exhibit EHT similar to that of adults, but specific organ systems have not matured to full function or size.

Anatomical and physiological factors that change during puberty and young adulthood include muscle mass, body fat percent, M/SA, whole-body sweating rate (males), development of sweat glands, horomonal responses, economy of upright exercise, cardiac dimensions of O2 delivery chain, pulmonary dimensions, VO2 max, and anaerobic metabolic capacity. This suggests that all of the physiological factors critical to homeostasis, thermoregulation, EHT undergo change during puberty, the general result being that EHT is enhanced.

Other investigators have concluded that the complete transition to adult thermoregulatory status and trainability probably occurs later than puberty—in young adulthood.

The authors conclude with a series of research questions and a well-developed list of references.