Motor Unit Action Potential Amplitude Vs Recruitment Threshold Relationships In Endurance Runners And Sedentary Females

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PURPOSE: To examine potential differences in the motor unit action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationship between female cross- country athletes and healthy controls. METHODS: Eight chronically-endurance trained (ET) females (age=20±0.93 yrs; height=166.18±5.93 cm; body mass=54.44±5.47 kg) nine healthy sedentary (SED) females (age=19.7±2.12 yrs; height=167.2±8.9 cm; body mass=66.61±11.9 kg) volunteered for the study. The ET group consisted of NCAA Div. I collegiate cross-country runners that ran 10-20 hrs/week. The SED subjects participated in minimal regimented physical activity in the previous 6 months. Subjects performed three isometric maximal voluntary contractions (MVCs) followed by a 70% isometric trapezoidal muscle action performed at 70% MVC. Surface electromyography was recorded from the vastus lateralis via 5-pin surface sensor array. The EMG signals were decomposed and APAMP (mV) and RT (expressed as %MVC) were calculated for each MU. For each subject, MUAPAMPS were linearly regressed against RTs with the slope and y-intercept calculated and used for statistical analysis. Two independent samples t-tests were used to examine potential between-group differences in the slopes and y-intercepts from the MUAPAMP vs. RT relationships. Statistical significance was set at p ≤0.05. RESULTS: The average RT ranges for observed MUs were 23.4 – 55.7% and 16.1 – 44.4% MVC for the ET and RT, respectively. There were no significant differences between groups for the slopes (ET=0.0035±0.0016 mV/%MVC, SED=0.028±0.0.0021 mV/%MVC, p=0.47) or y-intercepts (ET=-0.032±0.068 mV/%MVC, SED=- 0.018±0.0.067 mV/%MVC, p=0.67). CONCLUSIONS: Previously, increases in the slopes from the MUAPAMPS vs. RT relationships have been correlated with increases in muscle cross-sectional area following resistance training, which was suggested to indicate MU hypertrophy. The similar MUAPAMP vs RT relationships observed between the cross-country runners and sedentary controls may suggest that chronic endurance training did not result in hypertrophy of higher-threshold MUs of the vastus lateralis.