The time course of exercise-induced expiratory and inspiratory muscle fatigue

Inspiratory muscle fatigue develops during exercise prior to intolerance. The expiratory muscles are less resistant to fatigue compared to the inspiratory muscles, but the time-course of inspiratory and expiratory muscle fatigue during exercise has not been compared. Ten healthy adults (25 ± 5 years; 2 females) cycled on three separate occasions at 25% of the difference between estimated critical power and peak ramp incremental power (severe-intensity domain) for: 1) 100% of time to the limit of tolerance (T LIM ; 10.2 ± 2.6 min); 2) 75% T LIM (7.7 ± 1.9 min); and 3) 50% T LIM (5.1 ± 1.3 min). Expiratory and inspiratory muscle fatigue were quantified as the pre- to post-exercise reduction in the gastric (Pga tw ) and diaphragm (Pdi tw ) twitch pressure response to magnetic stimulation of the thoracic and cervical nerves, respectively. Pga tw and Pdi tw were reduced from baseline values after 50% T LIM (11.9 ± 8.2% and 9.5 ± 9.2%, both P < 0.05). The magnitude of expiratory and inspiratory muscle fatigue increased progressively at 75% T LIM (20.0 ± 12.6% and 15.2 ± 10.1%, both P < 0.05) and 100% T LIM (30.3 ± 15.6% and 22.4 ± 12.5%, both P < 0.05), but there was no difference between muscle groups ( P > 0.05). Expiratory and inspiratory muscle fatigue develops relatively early during severe intensity exercise and increases progressively in magnitude by exercise intolerance. The onset and progression of respiratory muscle fatigue during exercise is not different between the expiratory and inspiratory muscles.