Passive hyperthermia reduces voluntary activation and isometric force production

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Abstract
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 [V̇O2max=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 39.4°C rectal temperature (T re) and then cooled back to 37.4oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T c of 39.4oC reduced VA by 11 (11)% and MVC by 13 (18)% (P T c back to normal did VA and MVC return to baseline (P 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.