Brian Benitez1, Pasquale J. Succi1, Taylor K. Dinyer-McNeely2, Caleb C. Voskuil3, Minyoung Kwak1, Haley C. Bergstrom1. 1University of Kentucky, Lexington, KY. 2Oklahoma State University, Stillwater, OK. 3Texas Christian University, Fort Worth, TX.

BACKGROUND: During fatiguing exercise, increases in electromyographic (EMG) amplitude (AMP) reflect fatigue-induced recruitment of additional motor units and/or increases in firing rate (muscle excitation), while decreases in EMG mean power frequency (MPF) reflect a reduction in the muscle fiber action potential conduction velocity. To further elucidate the patterns of response in EMG AMP and MPF during fatiguing exercise, the present study examined the time course of changes in the EMG AMP and MPF responses during 2 cycle ergometry trials to exhaustion (TTE). METHODS: 11 subjects (age 24.9±3.6yrs) completed a graded exercise test to determine peak power (PP) and peak EMG AMP and MPF, followed by TTE at 85% and 100%PP. EMG signals were collected from the vastus lateralis of the right limb, normalized to their respective peak EMG signal, and examined using 10 time points that reflected 10% of the time to exhaustion (TLim) (0-10%⋯90-100%). Separate, 2 (TTE:85%vs100%PP) x 10 (time:%TLim) repeated measures (RM) ANOVAs with follow-up 1-way RM ANOVAs and Student Newman-Keuls (SNK) tests were used to examine the time course of changes in EMG AMP and MPF. Differences in EMG signals between intensities were examined with post-hoc Tukey tests. RESULTS: The TLim was 8.5±2.0 min at 85%PP and 4.0±0.9 min at 100%PP. There were significant 2-way interactions for EMG AMP (p=0.002) and EMG MPF (p=0.010). Follow-up analyses for EMG AMP at 85%PP indicated significant increases relative to the initial value (10%TLim) from 70% to 100%TLim, while at 100%PP, EMG AMP increased relative to the initial value from 40% to 100%TLim. EMG AMP was also significantly greater at 100%PP relative to 85%PP, from 50% to 100%TLim. For EMG MPF, there were significant follow-up 1-way RM ANOVAs, but the SNK did not reveal significant differences across time within each intensity. Across intensities, the EMG MPF at 85%PP was significantly greater than 100%PP at 40%TLimand again from 60% to 100%TLim. CONCLUSIONS: There were unique neuromuscular fatigue responses at 85%PP compared to 100%PP reflecting an earlier onset of fatigue-induced increases in muscle excitation and a greater normalized EMG AMP for the trial at 100%PP compared with 85%PP. Despite no changes in EMG MPF across time, EMG MPF was lower for the last 50%TLim at 100%PP compared to 85%PP, which may reflect a greater influence of metabolic byproduct accumulation for the trial at 100%PP.

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