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Mandy E. Parra, MS – Graduate Student, University of Kansas

Jonathan D. Miller, MSE – Doctoral student, University of Kansas

Mackenzie L. Hatcher – Student, University of Kansas

Stephanie A. Sontag, CSCS, NSCA-CPT – Student, University of Kansas

Matthew J. Hermes, MS – Doctoral Student, University of Kansas

Trent J. Herda – Associate Professor, University of Kansas


PURPOSE: To examine the association between the slopes of the motor unit action potential amplitudes (MUAPAMPS)-recruitment threshold (RT) relationships and isokinetic peak torque.

METHODS: Twelve healthy, recreationally active males (age=20.7±2.6 yrs; height=183.8±5.7 cm; weight= 88.5±11.4 kg) volunteered for this investigation. A 5 pin electromyographic (EMG) sensor was placed over the vastus lateralis (VL). Each participant performed two isometric maximal voluntary contractions (MVCs) of the leg extensors. The highest peak torque (PT) of the contractions was used to normalize trapezoid muscle actions at 70% MVC. The surface EMG signals were decomposed to extract MUAPAMP and corresponding RTs. Only MUs decomposed with accuracies >90% from the reconstruct-and-test and possessed stable AP waveforms via a spike trigger average procedure were included for analysis.  Linear regressions were performed to determine the slopes of the MUAPAMP-RT relationships separately for each subject. Each subject performed three isokinetic muscle actions at 1.05 and 3.14 rad·s-1 (PT1.05 and PT3.14). Isokinetic PT was calculated as the highest mean value over a 25 ms epoch during the load range with the average of the three trials used for statistical purposes. Pearson product moment correlations were used to determine possible significance relationships between isokinetic PT (PT1.05 and PT3.14) and the slopes of the MUAPAMP-RT relationships. Alpha was set at 0.05.

RESULTS: The average RTs of observed MUs from the 70% MVC were 24.7%-57.5% MVC and all relationships were significant (r=0.845±0.163) with positive slopes (0.0051±0.0030 mV/%MVC) for each subject. The isokinetic PT values were 209.6±30.7 Nm and 153.5±36.8 Nm for PT1.05 and PT3.14, respectively. There were positive correlations between the slopes of the MUAPAMP-RT relationships and PT1.05 (r=0.579; p=0.048) and PT3.14 (r=0.716; p=0.009).  

CONCLUSIONS: The slopes of the MUAPAMP-RT relationships explained a significant portion of the variance in isokinetic PT at 1.05 and 3.14 rad·s-1. The greater slopes indicated recruitment of larger MUs with increments in RTs. Thus, recruitment of relatively larger higher-threshold MUs during a 70% MVC were associated with PT recorded during the isokinetic muscle actions. Previously, the slopes of MUAPAMP-RT relationships were positively correlated with muscle cross-sectional area. It is plausible that individuals with greater slopes for the MUAPAMP-RT relationships possessed greater muscle fiber diameters of higher-threshold MUs that leads to greater torque development during isokinetic muscle actions.

PRACTICAL APPLICATIONS: The sizes of higher-threshold MUs, or the diameter of muscle fibers that comprise the MUs, are of importance for maximal muscular strength and power. Increasing the sizes of higher-threshold MUs via high-intensity resistance training may partially explain the subsequent increases in muscular strength and power.


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