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Cerebral changes during exercise in the heat.

Bodil Nielsen¹, Lars Nybo

¹Institute of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark. bnielsen@aki.ku.dk

Sports Medicine (Auckland, N.Z.)

|December 13, 2002

Summary

High body temperature during exercise causes central fatigue by reducing brain activity, not muscle force. This impacts performance by altering neural drive and brain function, not peripheral muscle capacity.

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Area of Science:

Exercise Physiology
Neuroscience
Environmental Physiology

Background:

High core body temperature during exercise (hyperthermia) is a significant factor in exercise-induced fatigue.
Trained individuals often reach exhaustion around 40°C core temperature.
Central fatigue, originating from the central nervous system (CNS), is implicated in hyperthermia-induced fatigue.

Purpose of the Study:

To review cerebral changes during combined exercise and heat stress.
To explore the relationship between these changes and fatigue development.
To understand the role of central fatigue in exercise in hot environments.

Main Methods:

Review of existing literature on exercise, heat stress, and fatigue.
Analysis of studies measuring core temperature, central activation, electroencephalogram (EEG), electromyogram (EMG), and cerebral blood flow.
Correlation of physiological measures with perceived exertion and fatigue.

Main Results:

Hyperthermia reduces central activation during maximal contractions, indicating reduced neural drive from the CNS.
Dynamic exercise in heat shows EEG slowing, correlated with perceived exertion, suggesting altered brain activity.
Cerebral blood flow decreases during hyperthermic exercise, potentially impairing brain heat removal but not glucose/oxygen uptake.

Conclusions:

Hyperthermia-induced fatigue is primarily linked to central mechanisms, specifically reduced CNS neural drive and altered brain activity (EEG changes).
Peripheral muscle function remains largely intact, but sustained force production is limited by central factors.
Further research is needed to clarify the interplay between circulatory changes, brain activity, and hyperthermia-induced central fatigue.

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