Eric M. Mosier1, Michael A. Trevino1, Johnathon D. Miller1, and Trent J. Herda1. 1University of Kansas, Lawrence, KS; e-mail: emosier4@ku.edu

PURPOSE: This study examined the acute effects of vibration (VIB) on motor unit (MU) behavior of the vastus lateralis (VL) during 40% maximal voluntary contraction (MVC). METHODS: Five males (age= 22.0±2.4 yrs) and three women (age=20.7±3.1 yrs) volunteered for this study. An electromyographic (EMG) sensor was placed over the VL. Participants preformed 3 isometric MVCs followed by isometric trapezoid muscle actions at 40% MVC (CON). Procedures were preformed twice separated by 3-5 days. VIB was then applied at 55 Hz. to the patella tendon for 10 min. prior to testing and continued throughout the remainder of testing, including a 40% MVC (VIB) at the same absolute force as the 40% CON. Decomposition techniques were applied to the EMG signals to extract action potentials and firing events of single MU. Only MUs decomposed with accuracies >92% were included for analysis. Linear regressions were performed on the MU recruitment threshold (REC) vs. peak firing rate (PFR) relationships during the first, middle, and final sec of the steady force segment for each subject. Slopes and y-intercepts were calculated for each subject and time point. Two separate 2-way repeated measures ANOVAs (treatment [CON vs. VIB] x time [first vs. middle vs. final sec]) were used to examine possible differences among y-intercepts and slopes. When appropriate, follow-up analyses for the ANOVA models were preformed using paired samples t-test with Bonferroni corrections. The level of significance was set at P ≤ 0.05. RESULTS: The analyses for the y-intercepts indicated that there was no 2-way interaction (P = 0.681) and no main effect for time (P = 0.803), however, there was a main effect for treatment (P = 0.047). The y-intercepts were greater for the VIB (29.49 ± 1.74 pps) than CON (26.49 ± 1.83 pps). The analyses for the slopes indicated no 2-way interaction (P = 0.684) or main effects for time (P = 0.729) and treatment (P = 0.055). CONCLUSION: The firing rates of the motoneuron pool were greater during VIB than the CON at the same absolute force. Thus, supporting the hypothesis that muscle spindles provide inhibitory effects on MU firing rates. When Ia afferent feedback from muscle spindles is removed the firing rates of the motoneuron pool increase. In addition, there was no time-dependent alterations in the REC vs. PFR relationships as function of VIB.

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