Inspiratory muscle fatigue following moderate-intensity exercise in the heat

Abstract

Heavy 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 PointsThe combined effects of a heat load and exercise on inspiratory muscle strength were investigated in untrained female subjects.Previous studies have shown that a very high exercise intensity (> 80% VO2max) is required to elicit reductions in inspiratory muscle strength.Prolonged submaximal exercise (40-min/60% VO2max) in a hot environment significantly reduced inspiratory muscle strength in untrained females whereas the same intensity in a thermo-neutral environment had no effect on inspiratory muscle function.These reductions in inspiratory muscle strength may be related to competition for blood flow among the locomotor, inspiratory, and cutaneous circulations.

Keywords: Control of breathing; endurance; respiratory function; thermal load.

Figure 1.

Maximal sustained inspiratory mouth pressure (PI_max) obtained pre and post endurance exercise during the thermo-neutral (closed circles) and hot (open circles) environmental conditions. Values are means ± SE. # significantly different from baseline (p = 0.001); * significantly different from the thermo-neutral condition (p = 0.003).

Figure 2.

Rectal temperature (T_RE) and heart rate (HR) responses during the endurance rides for both thermo-neutral (closed circles) and hot (open circles) environmental conditions. Values are means + SE. * significantly different from the thermo-neutral condition (p < 0.01).

Figure 3.

Tidal volume (V_T) and breathing frequency (F_b) responses during the endurance rides for both thermo-neutral (closed circles) and hot (open circles) environmental conditions. Values are means ± SE. * significantly different from the thermo-neutral condition (p < 0.05).

Figure 4.

Ratings of perceived exertion (RPE) and dyspnea (RPD) during the endurance rides for both thermo-neutral (closed circles) and hot (open circles) environmental conditions. Values are means ± SE. * significantly different from the thermo-neutral condition (p < 0.05).