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exercise has been shown to elicit reductions in inspiratory muscle
strength in healthy subjects. Our purpose was to determine the combined
effects of moderate-intensity endurance exercise and a thermal load
on inspiratory muscle strength in active subjects. Eight active, non
heat-acclimatized female subjects (23.5 ± 1.4 yr; VO2max
= 39.8 ± 2.4 ml.kg-1.min-1) randomly performed
two 40 min endurance exercise bouts (60% VO2max) in either
a thermo-neutral (22°C/21% RH) or hot (37°C/33% RH) environment on
separate days. Maximal sustained inspiratory mouth pressure (PImax)
was obtained pre and post exercise as an index of inspiratory muscle
strength. Additional variables obtained every 10 min during the endurance
exercise bouts included: rectal temperature (TRE), heart
rate (HR), minute ventilation (VE), oxygen uptake (VO2),
tidal volume (VT), breathing frequency (Fb),
and ratings of perceived exertion and dyspnea (RPE/RPD). Data were
analyzed with repeated measures ANOVA. PImax was significantly
reduced (p < 0.05) after exercise in the hot environment when compared
to baseline and when compared to post exercise values in the thermo-neutral
environment. PImax was unchanged from baseline following
exercise in the thermo-neutral environment. HR and TRE
were significantly higher (p < 0.05) in the hot compared to the
thermo-neutral environment. VE and VO2 were
not significantly different between conditions. VT was
unchanged between conditions whereas Fb was higher (p <
0.05) in the hot condition compared to thermo-neutral. RPE was not
significantly different between conditions. RPD was significantly
higher (p < 0.05) in the hot compared to the thermo-neutral environment.
We conclude that moderate-intensity endurance exercise (60% VO2max)
in a hot environment elicits significant reductions in inspiratory
muscle strength in unfit females. This finding is novel in that previous
studies conducted in a thermo-neutral environment have shown that
an exercise intensity of >80% VO2max is required to
elicit reductions in inspiratory muscle strength. In addition, dyspnea
perception during exercise is greater in a hot environment, compared
to thermo-neutral, at a similar level of VE and VO2.
KEY
WORDS: Control of breathing, endurance, respiratory function,
thermal load.
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