The increase in time to task failure following endurance training is associated with adjustments in motor unit firing properties

peak. Before and after training, participants performed a non-fatiguing isometric contraction at 50% maximal voluntary contraction (MVC), followed by a sustained contraction at 30% MVC until failure, while high-density surface EMG signals were recorded from the vasti muscles. Signals were decomposed and MUs tracked across sessions. MU firing rates displayed a biphasic response to fatigue: an initial decline (first phase) followed by a later increase (second phase). Post-intervention, only the END group increased time-to-task failure and delayed the onset of the second phase (p=0.021), which correlated with time to failure (r=0.70). The END group also showed less attenuation in firing rate at failure (50% vs 30% MVC difference: END=0.56 Hz; HIIT=2.8 Hz; p=0.011), which was also associated with total endurance time (r=0.72). These findings suggest that END-induced fatigue resistance is associated with specific MU firing adaptations that enhance the central nervous system's ability to optimise MU recruitment and firing dynamics during fatigue development.