Work-heat tolerance of distance runners.

Physical training in a cool environment by subjects not previously trained improves their work-heat tolerance, but can not replace heat acclimatization to the standard heat stress conditions employed by a variety of investigators. This is attributed to the inability of these subjects to sustain prolonged work at high metabolic rates. Thus, they are not maintaining high core body temperatures long enough to bring about an adaptive change to heat. On the other hand, the intense and prolonged (years) training of long distance runners in a temperature environment at high metabolic rates has acclimatized them for at least 4 hours of mild work (MR 160 kcal/m2-hr) in both hot dry (50/27 C db/wb) and hot wet (36.7/33.1 C db/wb) environments, but not for work at high energy expenditures (MR 540 kcal/m2-hr) in a less severe thermal stress (35/21 C db/wb). These highly trained athletes can improve their work-heat tolerance at high metabolic rates in a warm climate by training at competitive speeds in a cool environment dressed in sweat clothing or by training at near competitive speeds in the heat. In either of these situations the athlete is cautioned to consume water at frequent intervals to offset the dehydration associated with excessive sweating under these conditions.