Differential effects of exercise intensity and tolerable duration on exercise-induced diaphragm and expiratory muscle fatigue

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 yr; 2 females) cycled to intolerance on three separate occasions: <i>1</i>) 5% below critical power (<CP; heavy intensity); <i>2</i>) ∼25% of the difference (Δ) between CP and peak ramp-incremental power (Δ25; severe intensity "longer"); and <i>3</i>) ∼50% Δ (Δ50; severe intensity "shorter"). Diaphragm and expiratory muscle fatigue were quantified as a pre- to 5 min postexercise reduction in magnetically evoked transdiaphragmatic (Pdi_tw) and gastric (Pga_tw) 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% V̇o_2peak) relative to Δ25 (97 ± 4% V̇o_2peak) and Δ50 (99 ± 4% V̇o_2peak) (<i>P</i> < 0.001). The pre- to postexercise decrease in Pdi_tw was greater after Δ25 (-22 ± 12%) versus <CP (-13 ± 8%; <i>P</i> = 0.0499) and Δ50 (-14 ± 12%; <i>P</i> = 0.045). Conversely, the decrease in Pga_tw from pre- to postexercise was not different between trials (<CP: -23 ± 15%; Δ25: -29 ± 15%; Δ50: -25 ± 16%) (<i>P</i> > 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.<b>NEW & NOTEWORTHY</b> Exercise-induced respiratory muscle fatigue contributes to limiting exercise tolerance. Accordingly, better understanding the exercise conditions under which respiratory muscle fatigue occurs is warranted. Although heavy-intensity as well as short- and long-duration severe-intensity exercise performed to intolerance elicit diaphragm and expiratory muscle fatigue, we find, for the first time, that the relationship between exercise intensity, exercise duration, and the magnitude of exercise-induced fatigue is different for the diaphragm compared with the expiratory muscles.