Locomotor and diaphragm muscle fatigue in endurance athletes performing time-trials of different durations

Purpose: Fatigue in leg muscles might differ between running and cycling due to inherent differences in muscle activation patterns. Moreover, postural demand placed upon the diaphragm during running could augment the development of diaphragm fatigue. Methods: We investigated quadriceps and diaphragm fatigue in 11 runners and 11 cyclists (age: 29±5years; $\dot{V}$ O2,peak: 66.9±5.5mlmin−1kg−1) by assessing quadriceps twitch force (Q tw) and transdiaphragmatic twitch pressure (P di,tw) before and after 15- and 30-min time-trials (15TT, 30TT). Inspiratory muscle fatigue was also obtained after volitional normocapnic hyperpnoea (NH) where postural demand is negligible. We hypothesized that running and cycling would induce different patterns of fatigue and that runners would develop less respiratory muscle fatigue when performing NH. Results: The reduction in Q tw was greater in cyclists (32±6%) compared to runners (13±8%, p0.05). Conclusion: Different levels of leg muscle fatigue in runners and cyclists could in part be related to the specific muscle activation patterns including concentric contractions in both modalities but eccentric contractions in runners only. Diaphragm fatigue likely resulted from the large ventilatory load which is characteristic for both exercise modalities and which was higher in 15TTs than in 30TTs (+27%, p<0.01) while postural demand appears to be of less importance