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

Abstract

High-intensity interval training (HIIT) and continuous endurance exercise (END) induce distinct neuromuscular adaptations, with END particularly enhancing fatigue resistance during sustained submaximal contractions. However, the motor unit (MU) mechanisms underlying these effects remain unclear. This study investigated MU firing adaptations associated with changes in time to task failure following END and HIIT. Sixteen healthy men were randomly assigned to END or HIIT (n ¼ 8/group) and completed six sessions over 14 days. HIIT involved 8–12 × 60-s intervals at 100% peak power output, separated by a 75-s recovery; END involved 90–120 min of continuous cycling at ~65% peak oxygen uptake (V_ O2peak). Before and after training, participants performed a nonfatiguing 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 were tracked across sessions. MU firing rates displayed a biphasic response to fatigue: an initial decline (first phase) followed by a later increase (second phase). Postintervention, only the END group increased time-to-task failure and delayed the onset of the second phase (P ¼ 0.021), which correlated with time to failure (r ¼ 0.70). The END group also showed less attenuation in firing rate at failure (50% vs. 30% MVC difference: END ¼ 0.56 Hz; HIIT ¼ 2.8 Hz; P ¼ 0.011), which was also associated with total endurance time (r ¼ 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.