Brian Benitez1, Minyoung Kwak1, Taylor K. Dinyer-McNeely2, Pasquale J. Succi1, Lindsay McCallum1, Haley C. Bergstrom1. 1University of Kentucky, Lexington, KY. 2Oklahoma State University, Stillwater, OK.

BACKGROUND: Discrepancies between the individual and the composite (mean) responses in electromyographic (EMG) amplitude (AMP) and mean power frequency (MPF) have been reported for lower-body dynamic constant external resistance (DCER) exercise. This study compared the composite and individual patterns of responses for EMG AMP and MPF during fatiguing DCER forearm flexion performed at 30% of the 1-repetition maximum (1RM). METHODS: 8 recreationally active women (age=23.25±3.58yrs;weight=67.48±13.44kg; height=166±7.15cm) performed a DCER 1RM bilateral forearm flexion test and repetitions to failure (RTF) at 30% 1RM. EMG signals were recorded for the biceps brachii of the right arm. Separate, polynomial regression analyses (linear and quadratic) were used to define the individual and composite normalized EMG AMP and MPF versus repetitions relationship during the RTF (alpha=0.05). RESULTS: The mean ± standard deviation for the number of repetitions completed during the RTF trials at 30% 1RM was 112±87 repetitions. There was a positive, linear relationship (r2=0.976;p=<0.001) between normalized EMG AMP and repetitions for the composite. For the individual EMG AMP responses, it was revealed that 6 subjects demonstrated a positive, linear relationship (r2=0.236-0.958;p=<0.001-0.023), and 2 subjects demonstrated a positive, quadratic relationship (R2=0.911-0.929;p=<0.001-0.002). There was a negative, linear relationship (r2=0.942;p=<0.001) between normalized EMG MPF and repetitions for the composite. For the individual EMG MPF responses, it was revealed that 4 subjects demonstrated a negative linear relationship (r2=0.503-0.942;p=<0.001-0.015), 3 subjects demonstrated a negative, quadratic relationship (R2=0.236-0.948;p=<0.001-0.020), and 1 subject did not demonstrate a significant linear or quadratic relationship (p>0.05) for EMG MPF. CONCLUSIONS: Under fatiguing conditions, increases in EMG AMP may reflect indices of motor unit recruitment, rate coding, and/or synchronization, while decreases in EMG MPF may reflect decreased action potential conduction velocities along the sarcolemma. Most participants demonstrated a similar direction (88-100%), but fewer demonstrated a similar pattern (50-75%) of response for EMG AMP and MPF. These findings suggested that inferences made regarding the patterns of neuromuscular responses to fatiguing DCER muscle actions should be considered on a subject-by-subject basis.

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