Inspiratory Muscle Fatigue Is Not Different In Response To Long Vs. Short-duration High-intensity Exercise

Abstract

Maximal isokinetic power (PISO) at the limit of short duration (<6 min) high-intensity constant-power exercise (TlimSHORT) is not different from task power. Conversely, PISO at the limit of long duration (>7 min) high-intensity constant-power exercise (TlimLONG) exceeds task power. This suggests that while TlimSHORT is predominantly limited by locomotor neuromuscular fatigue, other physiological mechanisms contribute to exercise limitation in TlimLONG. One possibility for this difference in the mechanism of exercise intolerance is that the severity of exercise-induced inspiratory muscle fatigue is greater in TlimLONG vs. TlimSHORT due to a larger cumulative work of breathing in the longer-duration task. PURPOSE: To determine whether the magnitude of exercise-induced inspiratory muscle fatigue is greater in TlimLONG vs. TlimSHORT. METHODS: Ten healthy adults (3 females; 25 ± 3 yr) completed a maximal ramp-sprint test (RIT) to determine critical power (CP), V̇O2peak and peak ramp power (RITpeak). Maximal constant-power exercise was then performed at 1) 50% (TlimSHORT), and 2) 25 % (TlimLONG) of the difference between CP and RITpeak. PISO (6 s effort at 80 r/min) was measured at intolerance. Inspiratory muscle fatigue was assessed as the pre- to post-test reduction in magnetically evoked transdiaphragmatic (Pditw) twitch pressure. RESULTS: TlimLONG was longer than TlimSHORT (10.3 ± 2.6 vs. 5.2 ± 1.1 min; P < 0.001), but V̇O2peak was not different between tests (3.7 ± 0.8 vs. 3.7 ± 0.8 L/min; P > 0.05). PISO at intolerance was not different from task power in TlimSHORT (294 ± 101 vs. 241 ± 58 W; P = 0.11). Conversely, PISO at intolerance was greater than task power in TlimLONG (341 ± 106 vs. 215 ± 53 W; P = 0.008). Cumulative diaphragm pressure-time product was higher in TlimLONG vs. TlimSHORT (5945 ± 1956 vs. 2729 ± 1004 cmH2O/s; P < 0.001). Both TlimLONG and TlimSHORT induced a reduction in Pditw (-15 ± 13% vs. -19 ± 13%, respectively; P < 0.05). However, the magnitude of exercise-induced inspiratory muscle fatigue was not different between tests (P > 0.05). CONCLUSIONS: Despite PISO at intolerance being greater than task power in TlimLONG but not TlimSHORT, inspiratory muscle fatigue was not different between tests and therefore may not contribute to differences in exercise limitation in TlimLONG vs. TlimSHORT.