Effect Of Repeated Neuromuscular Electrical Stimulation On Muscle Damage And Motor Unit Control Strategies

PURPOSE: To examine the repeated bout effect (RBE) on muscle damage markers and motor unit (MU) firing properties in biceps brachii (BB) following two bouts of neuromuscular electrical stimulation (NMES). METHODS: Nine untrained adults (age: 26.89 ± 6.89 yrs, height: 173.74 ± 6.54 cm, weight: 95.54 ± 37.43 kg) participated. Following a familiarization, participants received 45 electrical stimuli (5-sec on/10-sec off at the maximal tolerable intensity) to BB for the first visit (1st bout). Before (Pre) and after (Post) NMES, participants completed visual analogue scales (VAS) for muscle soreness, 3 maximal voluntary isometric contractions (MVICs), and a submaximal isometric trapezoidal contraction at 70% of MVIC. Visit 2 (2nd bout) was performed a week later with the same experimental procedures as Visit 1. Surface electromyographic (EMG) signals were recorded from the BB during MVIC and trapezoidal contraction and decomposed to analyze motor unit (MU) recruitment threshold (RT) and mean firing rate (MFR) relationship. Linear regression analyses were used to calculate slope coefficients and y-intercepts (y-ints). Post-MVIC was normalized (N-MVIC) to Pre-MVIC for the respective visit. Separate two-way repeated measures analysis of variance (ANOVAs) (time [Pre vs. Post] × bout [1st bout vs. 2nd bout]) were used to examine N-MVIC, VAS, slopes, and y-ints. Alpha was set at 0.05. RESULTS: The two-way ANOVAs showed significant time × bout interactions for N-MVIC (p = 0.037) and VAS (p = 0.031). The paired sample t-tests showed N-MVIC for 2nd bout (85.98 ± 7.72%) was reduced less than 1st bout (79.85 ± 4.02%) at Post (p = 0.037), and VAS for 2nd bout (4.44 ± 4.13 mm) was lower than 1st bout (11.33 ± 9.27 mm) at Post (p = 0.031). No significant intercations (p > 0.05) or main effecs for bout (p > 0.05) were found for the slopes or y-ints of MFR vs. RT relationships. However, there were main effects for time for slopes (p = 0.029) and y-ints (p = 0.005). The follow-up pairwise comparisons indicated the slopes were more negative at Pre (-0.51 ± 0.020) than at Post (-0.37 ± 0.14, p = 0.029) and the y-ints were lower at Post (31.28 ± 6.86) than at Pre (41.47 ± 11.32, p = 0.005). CONCLUSION: The results support RBE on muscle damage markers, but suggest that neural adaptation may not be involved in the potential mechanisms underlying RBE following NMES.