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Article Title

MOTOR UNIT CONTROL STRATEGIES FOLLOWING CONTRALATERAL REPEATED BOUTS ON ARM AND HAND MUSCLES

Abstract

Sunggun Jeon1, William Miller2, Jun Seob Song2, & Xin Ye3

1Oklahoma State University, Stillwater, OK; 2University of Mississippi, University, MS; 3University of Hartford, West Hartford, CT

PURPOSE: To compare the potential contralateral repeated bout effect (RBE) on motor unit control strategies in both biceps brachii (BB) and first dorsal interosseous (FDI) muscles. METHODS: Thirteen adults (Age: 25.5 ± 4.7 yrs; Weight: 73.3 ± 12.2 kg; Height: 175.4 ± 7.6 cm) participated in this study. Participants were randomly assigned into either arm (n = 8) or hand group (n = 5). After the first visit as the familiarization, participants performed 6 sets of 10 repetitions eccentric exercise at 50% maximal voluntary isometric contraction (MVIC) during Visit 2. Before (Pre) and after (Post) the exercise, all participants completed MVIC and submaximal isometric trapezoid contraction task at 30% of MVIC. Specifically, the trapezoid contraction task required the participants to gradually increase the force from 0 to 30% MVIC in 3 seconds, held it for 10 seconds, and then gradually decreased the force to 0% in 3 seconds. One week after Visit 2, the exact same exercise and measurements were conducted for the contralateral muscles. Surface EMG signals from the BB and FDI muscles were collected and decomposed into constituent motor unit action potential trains. The relationship between the motor unit recruitment threshold (RT) and mean firing rate (MFR) was examined using linear regression analyses. Separate three-way (Time [pre vs. post] × Bout [1st bout vs. 2nd bout] x Group [Arm vs. Hand]) mixed factorial ANOVA were conducted to examine the muscle strength and the slope and y-intercept of the RT vs. MFR relationship. RESULTS: The threeway ANOVA showed only significant time main effect for the relative strength (F = 27.46, p < 0.001), the linear slope coefficients (F = 34.66, p < 0.001), and the y-intercepts (F = 9.72, p = 0.010). The follow-up pairwise comparisons indicated relative strength was significantly lower in Post than in Pre (100.0 ± 0.00 % vs. 68.8 ± 6.00 % p < 0.001), the slope coefficient was significantly more negative in Pre than in Post (-0.60 ± 0.07 vs. -0.98 ± 0.08, p < 0.001) and the y-intercepts was significantly lower in Pre than in Post (23.31 ± 2.02 vs.29.71 ± 2.01, p = 0.010). CONCLUSION: Our results for muscle strength and motor unit firing properties did not support the evidence for the contralateral RBE in the BB and FDI muscles.

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