The effects of muscle temperature on the development of muscular power are discussed. Temperature influences power (both metabolic and mechanical) by means of its effects on the rate of ATP hydrolysis and/or resynthesis. One would therefore expect reduced power outputs at cold muscle temperatures in humans. However, this is not the case during submaximal aerobic exercise. In fact, no changes in metabolic power output at any given submaximal work load were found at cold muscle temperatures, despite the reduced rate of ATP resynthesis and/or splitting. To explain this, it has been postulated that the fraction of active muscle mass at any given time instant could increase in the cold, thus compensating for the reduced ATP splitting rate. This means that the aerobic ATP resynthesis in the cold may be carried out at a slower rate by a greater activated muscle mass. This compensation cannot be operational when maximal power is attained, for in this case the instantaneously activated muscle mass is constant and limited. In fact the maximal aerobic power and the maximal instantaneous anaerobic power decrease with decreasing muscle temperature, as indicated by an average Q10 of 1.4 in the physiological muscle temperature range. The reduction in maximal aerobic power in the cold may be the consequence particularly of a decrease in O2 supply associated with reduced maximal cardiac output and muscle blood flow. On the other hand, the Q10 of the maximal anaerobic power should strictly depend on the reduced rate of ATP hydrolysis. The Q10 of the latter, however, according to Arrhenius law, should be 2 to 3 instead of 1.4.