Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T-sk) and body core temperature (T-c) on these factors. Twenty-two males [(V) over dot O-2max = 64.2 (8.9) ml kg(-1) min(-1), body fat = 8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4degreesC rectal temperature (T-re) and then cooled back to 37.4degreesC using, a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5degreesC intervals during both heating and cooling. Passive heating to a T-c of 39.4degreesC reduced VA by I I (11) % and MVC by 13 (18) % (P < 0. 0 5), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T-c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T-c reduces VA during isometric MVC, and neither T-sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.