Sunggun Jeon, Alex Olmos, Stephanie Sontag, Lyric Richardson and Michael Trevino

Oklahoma State University, Stillwater, OK

The repeated bout effect (RBE) is the adaptation where a bout of exercise protects against muscle damage in subsequent exercise bouts. This effect can transfer to the non-exercised contralateral muscle. However, it’s unknown if neuromuscular electrical stimulation (NMES) can induce a protective effect on contralateral muscles. PURPOSE: To examine if NMES can elicit the contralateral RBE (conRBE) on maximal strength of the elbow flexors and motor unit (MU) behavior of the biceps brachii (BB). METHODS: Ten untrained adults (age: 25.2 ± 6.2 yrs; weight: 90.3 ± 23.8 kg; height: 170.1 ± 10.8 cm) participated in this study. Following a familiarization, participants completed two experimental visits. For the first visit (1st bout), participants completed 3 maximal voluntary isometric contractions (MVIC) and a submaximal isometric trapezoidal muscle action at 70% of MVIC before (PRE) and after (POST) 45 transcutaneous electrical stimuli to the BB (5 sec on/10 sec off at maximal tolerable intensity). Visit 2 (2nd bout) was performed a week later with the same experimental procedures as visit one, but on the contralateral arm. Surface electromyography (EMG) was recorded from the BB for the isometric contractions and the signals were decomposed for the 70% MVICs to analyze MU recruitment thresholds (RT) and mean firing rates (MFR). Y-intercepts (y-ints) and slopes were calculated for the MFR vs. RT relationships. EMG amplitude (EMGRMS) during the plateau of the trapezoid was normalized (N-EMGRMS) to MVC. 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 perfomed on N-MVIC, N-EMGRMS, y-ints, and slopes. Alpha = 0.05. RESULTS: For N-MVIC, the two-way ANOVA showed a significant time × bout interaction (p = 0.005). The paired samples t-tests showed N-MVIC for the 2nd bout (93.8% ± 7.1%) was reduced less than the 1st bout (85.5% ± 7.1%) at POST (p = 0.003). However, there were no significant interactions (p > 0.05) or main effects (p > 0.05) for N-EMGRMS, and the y-int or slope coefficients for the MFR vs. RT relationships. CONCLUSION: NMES induced a protective effect on maximal strength of the contralateral elbow flexors, but MU behavior during a high-intensity contraction did not support a conRBE.

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