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: 1) 5% below critical power (<CP; heavy intensity); 2) similar to 25% of the difference (Delta) between CP and peak ramp-incremental power (Delta 25; severe intensity "longer"); and 3) similar to 50% Delta (Delta 50; 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, Delta 25, and Delta 50 conditions, respectively. Oxygen uptake (Vo(2)) at end-exercise was lower during <CP (87 +/- 6% Vo(2peak)) relative to Delta 25 (97 +/- 4% Vo(2peak)) and Delta 50 (99 +/- 4% Vo(2peak)) (P < 0.001). The pre- to postexercise decrease in Pdi(tw) was greater after Delta 25 (-22 +/- 12%) versus <CP (-13 +/- 8%; P = 0.0499) and Delta 50 (-14 +/- 12%; P = 0.045). Conversely, the decrease in Pga(tw) from pre- to postexercise was not different between trials (<CP: -23 +/- 15%; Delta 25: -29 +/- 15%; Delta 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. NEW & NOTEWORTHY 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.