Differential Effects of Exercise Intensity and Tolerable Duration on Exercise-Induced Diaphragm and Expiratory Muscle Fatigue.

Abstract

We investigated the effect of exercise intensity and tolerable duration on the development of exercise-induced diaphragm and expiratory muscle fatigue. Ten healthy adults (25 ± 5 y; 2 females) cycled to intolerance on three separate occasions: 1) 5% below critical power (<CP; heavy intensity); 2) ~25% of the difference (Δ) between CP and peak ramp-incremental power (Δ25; severe intensity 'longer'); and 3) ~50% Δ (Δ50; severe intensity 'shorter'). Diaphragm and expiratory muscle fatigue were quantified as a pre- to 5 min post-exercise reduction in magnetically evoked transdiaphragmatic (Pditw) and gastric (Pgatw) twitch pressures, respectively. Exercise time was 34.5 ± 6.2 min, 10.2 ± 2.6 min, and 4.9 ± 0.7 min for <CP, Δ25, and Δ50 conditions, respectively. Oxygen uptake (V̇O2) at end-exercise was lower during <CP (87 ± 6% V̇O2peak) relative to Δ25 (97 ± 4% V̇O2peak) and Δ50 (99 ± 4% V̇O2peak) (P < 0.001). The pre- to post-exercise decrease in Pditw was greater after Δ25 (-22 ± 12%) vs. <CP (-13 ± 8%; P = 0.0499) and Δ50 (-14 ± 12%; P = 0.045). Conversely, the decrease in Pgatw from pre- to post-exercise was not different between trials (<CP: -23 ± 15%; ∆25: -29 ± 15%; ∆50: -25 ± 16%) (P > 0.05). In conclusion, the magnitude of exercise-induced diaphragm fatigue was greater after longer duration severe exercise than after shorter duration severe and heavy exercise. By contrast, the magnitude of exercise-induced expiratory muscle fatigue was unaffected by exercise intensity and tolerable duration.