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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<0.05), 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.

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Acknowledgements

The authors wish to express their gratitude to the subjects who participated in the experiments. The project was supported by a Discovery Grant (S.S.C. and G.G.S.) from the Natural Sciences and Engineering Research Council (NSERC).

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Authors and Affiliations

  1. Human Performance Laboratory, Faculty of Kinesiology, University of New Brunswick, Fredericton, NB, E3B 5A3, Canada

    Shawnda Morrison & Gordon G. Sleivert

  2. Director of Sport Science and Medicine, Canadian Sport Centre Victoria, PacificSport, 100-4636 Elk Lake Drive, Victoria, BC, V8Z 5M1, Canada

    Gordon G. Sleivert

  3. Environmental Ergonomics Laboratory, School of Health and Human Performance, Dalhousie University, 6230 South St., Halifax, NS, B3H 3J5, Canada

    Stephen S. Cheung

Authors
  1. Shawnda Morrison
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  2. Gordon G. Sleivert
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  3. Stephen S. Cheung
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Correspondence to Gordon G. Sleivert.

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Morrison, S., Sleivert, G.G. & Cheung, S.S. Passive hyperthermia reduces voluntary activation and isometric force production. Eur J Appl Physiol 91, 729–736 (2004). https://doi.org/10.1007/s00421-004-1063-z

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