HOW TO TRAIN AND RACE IN THE HEAT - part 2

Body Temperature Limit
One of the most vital components of a successful training in the heat is an ability to sustain the temperature of the body at a level allowing its efficient functioning. Professor T.Noakes in his book "Lore of Running"(1) wrote: "To live, we humans must keep our body temperatures within a narrow range (35 to 42° C) despite wide variations in environmental temperatures and differences in levels of physical activity. However, during exercise, the conversion of chemical energy stored in adenosine triphosphate (ATP) into the mechanical energy that permits exercise is extremely inefficient; as much as 70 per cent of the total chemical energy used during muscular contraction is released as heat rather than as athletic endeavour. It is clear that in order to control the heat rise associated with exercise and so doing prevent overheating and heatstroke, the body must be able to call upon a number of very effective heat-diffusing mechanisms. "

It is well known that exercise increases muscle work, which increases blood flow and heat of the body by bringing heated blood to the organs and to the skin. After that blood flow distributes heat over the body and to the skin where "circulating air currents then carry this heat away by convection. (Convection is simply the transfer of the heat energy into the surrounding air.)"(1). The body's heat is lost when the temperature of surrounding air is lower, and by this difference it's attracting the heat. On the opposite, when the temperature of the air is higher, then the body is attracting the heat from surrounding air. As we know, at rest, the body skin temperature is about 33°C. This fact clearly indicates that heat can't be lost if the surrounding air temperature is greater than 33°C, and this fact should be a warning signal for a runner to make adjustments for his training.

During an exercise the athlete's body accumulates heat until the body temperature gets to the critical level when heatstroke can occur. This "terminal" temperature is about 41°C and the body's sensors send a signal to the brain, which shuts down physical activity in order to prevent heatstroke. This preventive mechanism in some cases, as it is mentioned by Dr.Noakes (1), is over run by a strong will of the runner, which leads to heatstroke.

One of the efficient mechanisms of reducing the heat of the body is sweating, but "it is important to appreciate that sweating itself does not cause heat loss: it is evaporation of the sweat into the atmosphere that causes heat to be lost"(1). Otherwise we are just losing water, but not the heat, and this is another problem.

Another important parameter of the body heat development is the air humidity. With increasing air humidity the body's ability to lose heat by sweating is decreasing. It happens because humid air can't absorb additional water (1). This factor is making the heat problem even more complex and should be on a priority list in training planning.

Acclimatization to heat takes about 7 to 14 days by Armstrong and Maresh (2). Scientists' opinion is that training should be done in the heat for gradually increasing periods of between 30 to 100 minutes for an initial period of 10 to 14 days (3). Dr.Noakes (1) thinks that full heat acclimatization is always superior in those who have always lived in hot environments. Nielsen (4) considers that daily experience to rising body temperature and prolonged exercise induces adaptation. The adaptation to heat is lost in approximately about 28 days.

The important changes that occur with heat acclimatization are that the heart rate, body temperature and sweat salt (sodium chloride) content during exercise decrease (1,2,4). This data could be used to identify an adaptation of the body to the heat, and your ability to run faster and longer, as well. Another indication of improved ability to train in the heat is the rate of perceived exertion (RPE) or your perception of difficulty of training, which together with other data could be good indicators of the training progress in the heat.

Dr.Romanov

References:

1. T.Noakes, 2002, Lore of Running, Forth Edition, Oxford University Press, p.222, 223, 227.)
2. Armstrong, L.E., Maresh, C.M.(1991). The indication and decay of heat acclimatization in trained athletes. Sports Medicine12, 302-312.
3. Shapiro, Y., Moran, D., Epstein, Y.(1998). Acclimatization strategies - preparing for exercise in the heat. International Journal of Sport Medicine 19 (Suppl 2), S161-S163.
4. Nielsen, B. (1998). Heat acclimation -mechanisms of adaptation to exercise in the heat. International Journal of Sport Medicine 19 (Suppl 2), S154-S156.