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Hyperthermia and central fatigue during prolonged exercise in humans
1Department of Human Physiology, Institute of Exercise and Sport Sciences, University of Copenhagen, DK-2200 Copenhagen Ø, Denmark. lnnielsen@aki.ku.dk
Journal of Applied Physiology (Bethesda, Md. : 1985)
|August 18, 2001
Summary
Hyperthermia impairs exercise performance by reducing central drive, leading to decreased voluntary activation. This study shows heat significantly lowers maximal voluntary contraction force due to reduced central activation, not peripheral factors.
Area of Science:
- Exercise Physiology
- Environmental Physiology
Background:
- Understanding the physiological mechanisms behind exercise-induced fatigue is crucial for optimizing athletic performance and preventing heat-related illness.
- Hyperthermia, or elevated core body temperature, is known to affect physiological functions, but its specific impact on central versus peripheral fatigue mechanisms requires further elucidation.
Purpose of the Study:
- To investigate the differential effects of hyperthermia on central and peripheral contributions to neuromuscular fatigue during prolonged exercise.
- To determine whether impaired force production under heat stress is primarily due to reduced central neural drive or peripheral muscle factors.
Main Methods:
- Fourteen healthy men performed cycling exercise at 60% maximal oxygen consumption in both a hot (40°C) and a thermoneutral (18°C) environment until exhaustion.
- Following cycling, participants performed sustained maximal voluntary contractions (MVC) of knee extensors and handgrip muscles.
- Central activation was assessed using superimposed electrical stimulation of the femoral nerve during knee extensions, calculating the voluntary activation percentage (MVC / (MVC + electrical stimulation)).
Main Results:
- In the hot environment, core temperature rose to 40.0°C at exhaustion (50 min), whereas in the thermoneutral condition, it stabilized around 38.0°C with 60 min of exercise.
- Maximal voluntary force of knee extensors declined more rapidly in hyperthermia, being significantly lower from 30 to 120 seconds compared to control.
- The voluntary activation percentage was significantly reduced in hyperthermia (54%) compared to the control condition (82%), indicating impaired central drive. Peripheral factors, assessed by total force (MVC + stimulation), were not different between conditions.
Conclusions:
- Hyperthermia significantly attenuates the ability to generate force during prolonged maximal voluntary contractions.
- The observed decline in performance under heat stress is primarily attributed to a reduction in central voluntary activation, rather than peripheral muscle fatigue.
- These findings highlight the critical role of central neural regulation in exercise performance during heat exposure.