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The Heat Trap - Why Your Kid's Workout Is Making Him Sick And How To Fix It

The Heat Trap - Why Your Kid's Workout Is Making Him Sick And How To Fix It

In his search for excellence in his sport, your child or student may be pushing himself too hard for his own good. Chills and heat strokes are signs that his body may have problems coping with the heat. Find out how you can help him. 20000704_boys.gif (22243 bytes)

Dr Michael Chia
Assistant Professor & Exercise Physiologist
School of Physical Education, NIE, NTU

What's Thermoregulation?
During exercise, the heat produced by the exercising muscles can be as high as 20 times as that produced at rest (Falk, 1998). Exercising in a hot environment therefore places an added stress on the thermoregulatory processes, while exercising in the cold relieves some of the thermal stress.

The trouble with young athletes in school is that they are not to be treated as adults in miniature. At the age of 10 or 12, they are still growing. For instance, they have smaller sweat glands, they tend to lose heat faster in warm and cold environments, and they have less blood flow to the muscles during exercise.

Thermoregulation is how well the body maintains a stable core temperature in various environments during rest and also during exercise. Much depends on the size of the person, the body surface-to-mass ratio, how sensitive the person to hot or cold stimuli, as well as his state of fitness and hydration.

While exercising at between 21 degrees Celsius and 25 degrees Celsius do not pose a great health risk, the situation changes in hotter environments.

Why Young Athletes Are Vulnerable To Heat Exhaustion
a) Large surface-to-body mass ratio
The main physical difference between young people and adults is that young people have a larger surface-to-body mass ratio. That just means that for every unit of body mass, a young athlete has a lot more skin surface that is exposed to the environment compared with adult. During exercise, a young person who is smaller in size absorbs heat from a hot environment and loses heat in a cold environment more rapidly than an adult who is larger in size.

This means that dry heat exchange (conduction, convection and radiation) is relatively greater in young people than in adults.

For young swimmers, their problem is that their higher surface area-to-body mass ratio makes them vulnerable to rapid heat loss, because the heat conductance of water is 25 times that of air (Armstrong and Welsman, 1997). This can of course happen even in very hot ambient air temperatures.

b) More heat is produced when they move about
Another thermoregulatory disadvantage is the fact that more heat per unit of body mass is created when the child is moving about.

In other words, young people are less efficient when it comes to locomotion where they tend to expend more energy than adults do in accomplishing the same exercise task. For instance, Bar-Or (1983) demonstrated that at speed of 8 km/h, a seven-year-old child expends 15-20% more energy per unit of body mass than a 17 year-old adolescent does. This additional heat energy poses an additional strain when then child is exercising in a hot environment.

This may be advantageous when he is exercising in the cold in the short term. But if the exercise is of a long duration, exercising in the cold will lead to less available energy a child may have in reserve (Smolander et al, 1992).

c) Young people have a lower cardiac output
With lower cardiac output, less blood is flowing out to the muscles and skin that need it.

This may limit loss through convection and radiation from the skin surface (Armstrong and Welsman, 1997) to keep the body cool.

The competition for blood flow to the skin to maintain adequate cooling may also result in reduced blood flow to the exercising muscles and the central nervous system, probably explaining in part the lower heat tolerance reported in children (Drinkwater et al, 1977). However in a cold environment, the smaller blood volume reported in young people does not appear to affect the effectiveness of thermoregulation (Falk, 1998).

d) Young children sweat less
During exercise in a hot environment, evaporative cooling of sweat off the skin is the main mechanism of thermoregulation in adults (Bar-Or, 1989; Falk, 1998). Young people do not sweat as much or as quickly as adults. For instance Bar-Or (1989), reported that children's sweat rate is only between 60 and 70% of that of adults.

Young people also have smaller sweat gland size compared to adults (Sato and Sato, 1983). It appears that sweat gland size is directly related to sweating rate and also sensitivity to sweating activation.

In other words, a young athlete starts sweating later and sweats much less when trying to keep his body cool. He would be disadvantaged when it comes to sports performance in the heat.

Adapting To Heat Stress
Adaptation to heat stress is achieved mainly through the process of natural acclimatisation or acclimation (i.e. repeated exposures to heat).

Bear in mind that the rate of acclimation to exercise in the heat is slower in young people than in adults. For instance, research shows that 10-year-old boys can reach an acclimation level that is similar to young adult men following a two-week, three-times-per-week acclimation protocol that involved exercising in the heat (430C and 21% relative humidity), even though the rate of acclimation is slower in the boys. Interestingly, it appears that young people can also acclimatise to the heat by merely being passively exposed (e.g. sitting or standing) to the hot environment without the need for exercise (Inbar et al, 1985).

Implications For PE Teachers and Parents
The physical and physiological differences that exist between young people and adults in thermoregulation suggest an increased vulnerability to heat illness such as heat strain, heat exhaustion and in extreme cases, heat stroke.20000704_boy1.gif (6835 bytes)

Be careful that young people who are engaged in exercise or physical activity in hot environments are not pushing themselves beyond what their body can endure. Young people are also at risk of cold exposure (hypothermia) if they spend a long time in water activities or if they are exposed to wind chill. Table 2 provides a summary of prudent practices that the PE teacher, sports trainer or activity organiser may find useful when working with young people in our hot and sometimes 'unforgiving' climate.

Most importantly, take precautionary procedures such as making sure that there is enough water available before, during and after exercise.

In Singapore, where the climate can be described as "hot and hotter", the Ministry of Education "strongly advises" that physical education classes be held in the mornings between 7.30 am and 10.30 am and in the afternoons, from 4.30 p.m. onwards (Principals' Handbook, MOE). The Co-Curricular Activities Centre (CCAC) also furnishes guidelines to schools on how outdoor activities including annual cross-country races, swimming events and sports days should be organised to avoid any inappropriate exposure of young people to the elements.

Keeping to those guidelines as well as those outlined in Table 2 in practice will help safeguard the well being of young people, and ensure they have a better chance to excel in their sport.

About the author
Dr Michael Chia is an assistant professor and paediatric exercise scientist at the School of Physical Education and a former Association of Commonwealth Universities' scholar.


  1. Falk B (1998). Effects of thermal stress during rest and exercise in the paediatric population. Sports Medicine, Vol 25 (4): 221-240.
  2. Docherty, D , Eckerson, J & Hayward, J (1986). Physique and thermoregulation in prepubertal males during exercise in a warm and humid environment. American Journal of Physical Anthropology, 70, 19-23.
  3. Armstrong, N., & Welshman, J. (1997). Young People and Physical Activity. Oxford University Press.
  4. Smolander, J, Bar-Or, O & Korhnen, O (1992). Thermoregulation during rest and exercise in the cold in pre- and early pubescent boys and in young men. Journal of Applied Physiology, 72: 1589-1594.
  5. Drinkwater, B, Kupprat, I & Denton, J (1977). Response of pre-pubertal girls and college women to work in the heat. Journal of Applied Physiology, 43: 1046-1053.
  6. Falk, B, Bar-Or, O & MacDougall, J (1992). The thermoregulatory response of pre-, mid- and late pubertal boys to exercise in dry heat. Medicine and Sports in Sports and Exercise, 24: 688-694.
  7. Bar-Or, O (1989). Temperature regulation during exercise in children and adolescents. In CV Gisolfi and DR lamb (eds). Perspectives in Exercise Science and Sports Medicine, Vol 2, Youth, Exercise and Sport. Indianapolis, IN, Benmark Press, pp. 335-367.
  8. Sato, K & Sato, F (1983). Individual variations in structure and function of human eccrine sweat glands. American Journal of Physiology, 245: R203-208.
  9. Inbar, O, Bar-Or, O, & Dotan, R (1985). Conditioning versus exercise in heat as methods for acclimatising 8-10 year-old boys to dry heat. Journal of Applied Physiology, 1981, 50: 406-411.
  10. Principals' Handbook, Ministry of Education.

Date reviewed: 07 April 2000

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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