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

Inspiratory muscle fatigue develops during exercise before 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 yr; 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 <i>1</i>) 100% of time to the limit of tolerance (T_LIM; 10.2 ± 2.6 min); <i>2</i>) 75% T_LIM (7.7 ± 1.9 min); and <i>3</i>) 50% T_LIM (5.1 ± 1.3 min). Expiratory and inspiratory muscle fatigue were quantified as the pre- to postexercise 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 <i>P</i> < 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 <i>P</i> < 0.05) and 100% T_LIM (30.3 ± 15.6% and 22.4 ± 12.5%; both <i>P</i> < 0.05), but there was no difference between muscle groups (<i>P</i> > 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 are not different between the expiratory and inspiratory muscles.<b>NEW & NOTEWORTHY</b> During severe exercise, expiratory and inspiratory muscle fatigue develops by ∼50% of the tolerable exercise duration. The magnitude of expiratory and inspiratory muscle fatigue increases progressively toward exercise intolerance but is not different between muscle groups; this is despite the expiratory muscles being less fatigue-resistant. Inspecting esophageal and gastric twitches via cervical stimulation, we speculate that the progressive magnitude of exercise-induced inspiratory muscle fatigue is a function of recruitment and fatigue of the accessory inspiratory muscles.