Fatigue during hyperthermia may be due in part to a failure of the central nervous system to fully activate the working muscles. We investigated the effects of passive hyperthermia on maximal plantar flexor isometric torque (MVC) and voluntary activation (VA) to determine the roles of local skin temperature (T_sk), core temperature (T_c) and peripheral muscle temperature (T_m) in fatigue. Nine healthy subjects were passively heated from 37.2 to 39.5°C (T_c) and then cooled back down to 37.9°C using a liquid conditioning garment, with the right leg (TN) kept at a thermoneutral temperature throughout the protocol while the left leg (H) was allowed to heat and cool. Passive heating resulted in significant decreases in MVC torque from (mean (SD)) 172 (39) to 160 (44) Nm and VA from 96 (2) to 91 (5)% in the heated leg, and MVC decreased similarly from 178 (37) to 165 (38) Nm and VA from 97 (2) to 94 (5)% in the thermoneutral leg. The initiation of cooling, which produced a rapid decrease in skin temperature and cardiovascular strain (heart rate reserve decreased from 58 (12) to 31 (12)%), did not immediately restore either torque or VA. However, when T_c was lowered back to normal, torque and VA were restored to baseline values. It was concluded that an increase in T_c is a factor responsible for reducing VA during brief voluntary isometric contractions and that temperature-induced changes in the contractile properties of muscle and local thermal afferent input from the skin do not contribute significantly to the decrement in torque.