Hyperthermia and central fatigue during prolonged exercise in humans.
Nybo, Lars, and Bodil Nielsen.
Department of Human Physiology, Institute of Exercise and Sport Sciences,
University of Copenhagen, DK-2200 Copenhagen Ø, Denmark
APStracts 8:0260A, 2001.
The present study investigated effects of hyperthermia on the contributions of central and
peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised
at 60% maximal oxygen consumption («vo2max») on a cycle ergometer in hot (40°C;
hyperthermia) and thermoneutral environments (18°C; control). In hyperthermia, the core
temperature increased throughout the exercise period reaching a peak value of 40.0 ±
0.1°C (mean ± SE) at exhaustion after 50 ± 3 min of exercise. In control, core
temperature stabilized at ~38.0 ± 0.1°C and exercise was maintained for 1 h without
exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of
sustained maximal voluntary contraction (MVC) either with the exercised legs (knee
extension) or with a "nonexercised" muscle group (handgrip). The degree of
voluntary activation during sustained maximal knee extensions was assessed by
superimposing electrical stimulation (EL) to n. femoralis. Voluntary knee extensor force
was similar during the first 5 s of contraction in hyperthermia and control. Thereafter,
force declined in both trials, but the reduction in maximal voluntary force was more
pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly
lower in hyperthermia compared with control. Calculation of the voluntary activation
percentage (MVC/MVC + EL) revealed that the degree of central activation was
significantly lower in hyperthermia (54 ± 7%) compared with control (82 ± 6%). In
contrast, total force of the knee extensors (MVC + force from EL) was not different in the
two trials. Force development during handgrip contraction followed the same pattern of
response as was observed for the knee extensors. In conclusion, these data demonstrate
that the ability to generate force during a prolonged MVC is attenuated with
hyperthermia, and the impaired performance is associated with a reduction in the
voluntary activation percentage.
Received 5 February 2001; accepted in final form 23 April 2001
APS Manuscript Number A0108-1.
Article publication pending J Appl Physiol
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 18 June 2001