Advantages of a smaller bodymass in humans when distance-running in warm, humid conditions

Using: a 65-kg athlete running a 2 h 10 min marathon as an example, we esti mated that imbalances between approximately 1400 W of heat production and d issipation would occur in ambient temperatures of 17 degrees C at 90% relat ive humidity (rh) to 37 degrees C at 50% rh. Because heat production during running depends on body mass and heat loss depends on surface area, interc epts between predicted heat production and maximal heat loss with increasin g speeds depend on an athlete's body mass. At 35 degrees C and 60% rh, a 45 -kg athlete could maintain thermal balance by running a 2 h 13 min marathon at 19.1 km . h(-1) but a 75-kg athlete would only be able run a 3 h 28 min marathon at 12.2 km . h(-1). In both cases. the production of 970-1020 W o f heat would necessitate the evaporation of at least 1.5-1.6 l of sweat per hour. A lower metabolic heat production in lighter runners at any given sp eed may be one reason why smallness of stature is an asset in distance runn ing.