Neuromuscular electrical stimulation producing low evoked force elicits the repeated bout effect on muscle damage markers of the elbow flexors

This study examined the repeated bout effect (RBE) on muscle damage markers following two bouts of neuromuscular electrical stimulation (NMES) in untrained individuals. Following familiarization, participants received 45 consecutive NMES to the biceps brachii at an intensity that produced low evoked force for the elbow flexors. Muscle damage markers (maximal voluntary isometric contraction [MVIC], elbow range of motion [ROM], muscle soreness via visual analogue scale [VAS] scores, pressure pain threshold [PPT], and muscle thickness) were measured before (PRE), after (POST), 1 day after (24 POST), and 2 days after (48 POST) NMES. Following 1 week of rest, procedures were replicated. Separate repeated measures two-way ANOVAs examined each measure. There were no interactions or bout main effects for MVIC or ROM. Time main effects indicated PRE MVIC was greater than POST (p = 0.002) and 24-POST (p=0.024), and PRE ROM was greater than POST (p = 0.036). There was no interaction for muscle thickness. Respective time and bout main effects indicated muscle thickness at PRE was less than POST (p = 0.017), and second-bout muscle thickness (p = 0.050) was less compared to the initial-bout. For PPT, there was an interaction (p = 0.019). Initial-bout PRE PPT was less than POST (p = 0.033). Initial-bout 48-POST PPT was less than second-bout 48-POST (p = 0.037). There was a significant interaction for VAS (p = 0.009). Initial-bout PRE VAS was less than POST (p = 0.033) and 24-POST (p = 0.015). Initial-bout POST and 24-POST VAS were greater than second-bout POST (p = 0.023) and 24-POST (p = 0.006), respectively. The results support RBE on muscle damage markers related to inflammation, but not MVIC and ROM.