Training and performing in the heat
Critically high levels in body temperature is the main factor that limits endurance performance in hot environments (1). Both dehydration and hypothermia effect the athlete in their mental and physical performance. The effects become more apparent, as the duration increases making it harder to achieve optimum performance (2). Preparations for these conditions are essential if there is to be any chance of a performing at their physical best.
What happens physiologically:
During exercise, blood flow is shared between the muscles to provide oxygen, and the skin for heat loss. When cardiac output is insufficient to meet the demands of both of these, blood flow to the muscles takes priority over blood flow to the skin, leading to reduced heat loss and a rise in body temperature (2). Fatigue experienced during exercise in the heat has been shown to be related to high internal body temperature (1).
What about dehydration:
Dehydration during prolonged exercise can occur when there is fluid deficit induced prior to the beginning of exercise, or when fluid losses are not replaced during exercise. The consequences of dehydration is increased heart rate and core temperature (2). The combined effects of progressive dehydration and a rising body temperature pose a considerable threat to the runner (2)
The ability to sustain a high rate of work output in the heat requires replacement of water to prevent dehydration. However the rates of fluid replenishment are limited by the rates of gastric emptying and intestinal absorption. An isotonic or moderately hypotonic solution containing glucose and sodium will be most effective.
Heat acclimation:
It is good to note that trained subjects are already partially adapted regardless of their usual training climate, however full adaptation is only seen after a period of time spent training in the heat.
Major adaptions can be seen within 6-8 days of exposure including a reduced heart rate and body core temperature, and redirection of a larger proportion of blood flow to the skin.
When performing in the heat:
For athletes who habitually live and train in temperate climates and have athletic competitions scheduled for hot climates, there is a need for the athlete to prepare for the conditions by having a suitable acclimatisation strategy as well as consulting with a sports dietitian to work out their individualised re-hydration strategy.
Regular simulated exposure to hot humid conditions need to be included in the training plan. Sauna, bikram yoga, treadmill running or indoor cycling in a hot humid room are all viable options. It is important to note that if possible arriving at the event location even a week prior will instigate the major adaptions. Preparations are essential if there is to be any chance of success (2).
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References:
1. Gonzalez-Alonso. J, Teller. C, Anderson. S, Jensen.F, Hyldig. T, Nielsen. B (1999). Influence of body temperature on the development of fatigue during prolonged exercise in the heat. Journal of Applied Physiolog
2. Terrados. N, Maughan. R (1995) Exercise in the heat: Strategies to minimize the adverse effects on performance. Journal of Sports Sciences
3. Tatterson. A, Hahn. A, Martini. D, Febbraio. M (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. Journal of Science and Medicine in Sport, 3(2), 186–193.
4. Susan I. Barr (1999) Effects of Dehydration on Exercise Performance Canadian Journal of Applied Physiology
