Because the human body is not 100% efficient in converting stored energy into useful work, heat is generated during pedaling activity and must be dissipated if the rider is to sustain pedaling effort. As indicated earlier, much of the work of pedaling a bike goes into overcoming air resistance. This implies that in an open vehicle there is adequate air flowing over the cyclist to provide the heat exchange required to remove waste heat. The diagram opposite assumes that the body surface temperature is 35 C, the air temperature 15 C, the relative humidity 80%, and the effective body surface area of the cyclist is 1.8 m². The heat flux plotted includes both the convective and the evaporative components of heat transfer to the air.

From: Abbott & Wilson, "Human-Powered Vehicles,"
Human Kinetics (1995)