Recovery Of Neuromuscular Efficiency After Severe- And Extreme-intensity Knee Extension Exercise

The recovery of voluntary force generation after severe-intensity exercise is slower compared to extreme-intensity exercise. Whether the efficiency of converting motor unit excitation into mechanical force plays a role in these intensity-dependent recovery timelines remains unknown. PURPOSE: To test the hypothesis that recovery of neuromuscular efficiency (NME) is faster after extreme-intensity exercise compared to severe-intensity exercise. METHODS: Eighteen participants (10 M/8F; 21 ± 1 yrs.) performed two bouts of severe-intensity knee-extension exercise (S1: 7.5 ± 1.5% 1RM and S2: 11.3 ± 3.6% 1RM) until task failure. A subset (n = 8) performed a single bout of extreme-intensity knee-extension exercise (EXT; 70% 1RM) until task failure. Baseline (BL) and post-exercise maximal voluntary contractions (MVCs) were performed at 0, 30, 60, 90, 120, and 150 s after task failure. Motor unit excitation was quantified using the root mean square (RMS) of electromyography signals. NME was calculated as the ratio of force output to RMS. Comparisons to BL were made during recovery using one-way repeated-measures ANOVAs and Tukey’s tests. RESULTS: MVC was lower than BL (401 ± 148 N) at all timepoints after S1 and S2 (204 ± 70 N and 227 ± 68 N at 150 s; p < 0.0001). Following EXT, MVC was lower than BL (375 ± 123 N) at 0 s (298 ± 88 N; p < 0.05) but not at any timepoint after 30s of recovery (all, p > 0.12). RMS was not different than BL (0.26 ± 0.11 mV) 0 s after task failure (p > 0.36) but was lower than BL 30s (S1: 0.17 ± 0.10 mV; S2: 0.20 ± 0.12 mV; both p < 0.01) and 60s (S1: 0.17 ± 0.09 mV; S2: 0.21 ± 0.13 mV; both, p < 0.05) after task failure. RMS was not different than BL after 90s of recovery from S2 (p = 0.16) and after 120 s of recovery after S1 (p = 0.09). Following EXT, RMS was not decreased from BL (0.31 ± 0.14 mV) at any recovery timepoint (p = 0.40). NME was less than BL (1.63 ± 0.83 N/V) at all timepoints after S1 (0.97 ± 0.36 N/V at 150 s; p < 0.0001) and S2 (1.01 ± 0.46 N/V at 150 s; p < 0.0001). Following EXT, NME was less than BL at 0 s (0.79 ± 0.29 N/V; p < 0.01) but not at any timepoint after 30s of recovery (all, p > 0.71). CONCLUSION: The recovery of NME was remarkably fast (within 30s) after extreme intensity exercise. Despite the recovery of motor unit excitation, NME did not recover within 150 s after severe-intensity exercise.