Changes In Motor Unit Recruitment And De-recruitment Strategies Are Not Associated With The Repeated-bout Effect

Abstract

The “repeated bout effect” (RBE) is an adaption that attenuates muscle damage following eccentric exercise. Several neural adaptations have been proposed to underlie the RBE. PURPOSE: This study used decomposition of surface EMG signals (dEMG) to examine the relationship between recruitment (RT) and de-recruitment thresholds (DRT) and changes in firing rates or motor units during recruitment and de-recruitment prior to and following eccentric exercise resulting in the RBE. METHODS: Nine participants performed 5 sub-maximal isometric trapezoid contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between RT and DT was examined using linear regression. The slope of the change in mean firing rate (MFR) during the ramp-up and ramp-down phase of the trapezoid contraction was also examined. RESULTS: No changes were found in the slope of the RT vs DT relationship for 50% MVC (1.13±0.17 vs 1.29±0.43; p=0.42) and 80% MVC (1.09±0.18 vs 1.25±0.41; p=0.25). There were also no changes in the y-intercept of the RT vs DT relationship at 50% (-13.79±6.77 vs -15.37±17.07; p=0.80) and at 80% (-9.29±10.71 vs -23.07±20.28; p=0.06) of MVC. The mean slope of the increase in firing rate during recruitment did not change between bouts 10.2±1.8 vs 10.5±2.2 pps/s (p=0.77) and 8.4±0.7 vs 9.0±1.6 pps/s (p=0.28) for 50% and 80% of MVC, respectively. The slope of the decrease in firing rate during de-recruitment did not differ at 50% of MVC -9.7±1.5 vs -10.2±1.8 pps/s (p=0.48), but became steeper during contractions at 80% of MVC -7.3±0.9 vs -8.7±1.7 pps/s (p=0.04). However, no relationship was observed between the change in DT slope and the magnitude of the RBE. CONCLUSION: A bout of eccentric exercise conferred protection from a subsequent identical bout. Few changes in motor-unit recruitment and de-recruitment behavior were observed suggesting changes in these parameters are not responsible for the RBE.