Motor unit adaptations contribute to the repeated bout effect following damaging resistance exercise

An initial bout of eccentric exercise (EE) is known to protect against exercise-induced muscle damage (EIMD) following the performance of a subsequent bout of similar volume and intensity, a phenomenon known as the repeated bout effect (RBE). We examined whether aspects of motor unit (MU) behavior and reticulospinal tract (RST) drive are neural components of this protective effect. Twenty-three participants (6 females; age 26 ± 5 yr) performed two bouts of EE (10 repetitions × 10 sets) with the dorsiflexors separated by 3 wk. Maximal voluntary isometric torque (MVIC), muscle soreness (DOMS), MU behavior (quantified from MUs identified via high-density electromyography decomposition), and RST drive (visual-auditory vs. visual-startle reaction time) were recorded at baseline, 24, 48, and 72 h postexercise. Symptoms of EIMD were elevated following bout 1; MVIC was reduced, and perceived soreness was increased. Despite comparable work performed (∼1,300 J; P = 0.721), MVIC (P < 0.001) and soreness (P < 0.001) recovered quicker following bout 2. The attenuated symptoms of EIMD were coupled with reduced variability in MU discharge rate (P = 0.001) and torque (P < 0.001). MU adjustments were not accompanied by any change in RST drive (-8 ms; P = 0.634). Lower MU discharge variability and an attenuated increase in firing rate in bout 2 support a neural contribution to the RBE. The present study cannot infer whether such adaptations actively protect against muscle damage or merely reflect the reduced mechanical and nociceptive disturbance. Nonetheless, we confirm that MU adjustments are involved in the RBE phenomenon.NEW & NOTEWORTHY Unfamiliar eccentric exercise causes muscle damage, but repeating the same exercise later substantially reduces symptoms-an adaptive phenomenon known as the repeated bout effect (RBE). We investigated the neural contributions to this response. We observed lower motor unit discharge variability and smaller increases in firing rate after the repeated bout. These findings provide evidence that motor unit-level neural adjustments are a critical component of the RBE, offering a new perspective on this protective adaptation.