Performance Fatigability at Task Failure and Beyond: Distinct Patterns of Recovery Following Constant Load Versus...

The total work completed during an exercise session is often assumed to be the primary factor influencing the kinetics of post-exercise recovery. However, the duration of work and rest periods during high intensity interval training (HIIT) have been shown to impact the magnitude of physiological stress and could also impact post-exercise recovery. The aim of this study was to characterize the recovery time course of neuromuscular function following work-to-rest matched HIIT protocols with different work interval durations and conducted to task failure. Participants (n=12, n=6 females) completed a ramp incremental exercise test to determine peak power output (PPO). In a randomized order, participants completed 3 cycling protocols at 90% PPO: (i) 3 min work, 3 min passive rest HIIT (HIIT_3min), (ii) 1 min work, 1 min passive rest HIIT (HIIT_1min), and (iii) constant load (CL) cycling. Femoral nerve electrical stimuli during maximal voluntary contractions (MVC) of isometric knee extension were performed at baseline, task failure (TF), and TF+1min, TF+4min, and TF+8min to delineate the time course of neuromuscular function recovery. MVC force declined to the same level following the three conditions at TF and demonstrated a partial recovery within TF+8min (time effect: P<0.001). The evoked muscle twitch force declined more following the CL compared to HIIT_1min at TF (P = 0.013) and showed a faster recovery within TF+8min (P<0.024). Voluntary activation decreased at TF in the HIIT_1min but not in HIIT_3min or CL (interaction effect: P<0.023) and fully recovered within TF+8min. Central and peripheral components of neuromuscular function demonstrate distinct time courses of recovery between CL exercise and work:rest matched HIIT protocols with different work interval durations.