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Robert E. Hight, Travis W. Beck, Debra A. Bemben, FACSM, and Christopher D. Black. Department of Health and Exercise Science, University of Oklahoma, Norman, OK; e-mail: robbyhight@gmail.com

Unaccustomed eccentric exercise results in exercise-induced muscle damage. Following an acute bout of damaging exercise, an adaptation occurs which significantly attenuates the magnitude of muscle damage induced by future bouts of eccentric exercise. This phenomenon is termed the “repeated-bout effect (RBE).” There is evidence neural adaptations may underlie the RBE, however, the specific changes in motor-unit recruitment strategies remain unclear. PURPOSE: The primary aim of this study was to examine changes in motor-unit activation (mean firing rate, recruitment threshold, and their inter-relationship) of the biceps brachii 3 weeks following a bout of eccentric exercise. METHODS: Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50, 80, and 100% maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were collected and decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and the percent of MVC at which it began firing (recruitment threshold; RT) was examined using linear regression. Participants then performed an exercise protocol consisting of eccentric dumbbell curls with a weight corresponding to 120% of concentric 1-RM until MVC had decreased by ~40%. Indirect markers of muscle damage [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. RESULTS: A RBE was observed with a second bout of eccentric exercise resulting in smaller reductions in MVC (-32±14% vs -25±10%; p=0.034) and ROM (-11% vs 6%; p = 0.01), and reduced soreness (31.0±19mm vs 19±12mm; p=0.015) compared to the initial bout of eccentric exercise. When compared to the initial assessment, a decrease in the slope (-0.60±0.13 vs -0.70±0.18; p=0.029) and increase in the y-intercept (46.5±8.3 vs 53.3±8.8; p=0.020) of the MFR vs. RT relationship during contractions at was observed during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% or 100% of MVC. CONCLUSION: These findings indicate a long lasting shift in the relationship between MFR and RT of motor-units after the induction of muscle damage occurs concomitant with the RBE. The observed shift is consistent with preferential activation of slow-twitch motor-units which have been shown to be less susceptible to damage than fast-twitch fibers. These findings support the hypothesis that neural adaptations, at least partially, underlie the RBE.

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