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Article Title

THE INFLUENCE OF TRAINING STATUS ON FIRING RATES AT RECRUITMENT DURING TWO CONSECUTIVE ISOMETRIC TRAPEZOID MUSCLE ACTIONS AT 50% OF MAXIMAL VOLUNTARY CONTRACTION

Abstract

A.D. Lindskog, J.M. Hannon, K.E. McNay, M.A. Trevino, and T.J. Herda; Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS

PURPOSE: This study examined motor unit (MU) firing rates (FR) at recruitment threshold of the vastus lateralis (VL) in resistance trained (RT) and aerobically trained (AT) individuals during two consecutive contractions. METHODS: Five RT (5 males; age = 25 ± 4 yrs) and 5 AT (2 males and 3 females; age = 19 ± 1 yrs) volunteered for this study. The RT could back squat ≥ twice their body weight (207.5 ± 27.5 kg) and the AT ran an average distance of 61 ± 15 miles h∙wk-1. Electromyography (EMG) sensor (Delsys, Boston, MA) was placed over the VL. Each participant completed 3 isometric maximal voluntary contractions (MVC). The isometric trapezoid muscle actions at 50% MVC were calculated from the highest MVC. For the isometric trapezoid muscle actions, the force was increased at a rate of 10% MVC/s to the deserved force level for 12 s followed by a decrease of 10% MVC/s back to baseline. The muscle action was performed twice with 8 s rest between contractions. Decomposition of the the surface EMG signals were used to extract the firing events of single MUs. The FR (pulses per second [pps]) and force at recruitment, expressed as a percentage of MVC (%MVC), were calculated for each MU. A two-way mixed factorial ANOVA (training [AT vs. RT] x repetition (1st vs. 2nd]) was used to examine differences in FR and recruitment thresholds. RESULTS: For FR, there were no differences (P = 0.180) between the 1st and 2nd repetition for RT (mean ± SD, 1st = 6.5 ± 1.5 pps, 2nd = 6.6 ± 1.7 pps) and AT (1st = 7.2 ± 1.8 pps, 2nd = 7.4 ± 1.9 pps), however, the AT had overall higher FR than the RT (P < 0.001). For recruitment thresholds, the force at which the MUs were recruited decreased from the 1st to 2nd repetition for the AT (P < 0.001, 1st = 30.0 ± 13.0 %MVC, 2nd = 27.1 ± 11.7 %MVC) and RT (P < 0.001, 1st = 29.8 ± 10.2 %MVC, 2nd = 26.3 ± 10.3 %MVC). Finally, there were no significant recruitment threshold differences for the 1st (P = 0.539) and 2nd (P = 0.540) repetition between AT and RT. CONCLUSION: The AT had greater FR than the RT, however, there was no change in FR at recruitment from the 1st to 2nd muscle action. In contrast, there were no differences in recruitment thresholds between AT and RT, but the recruitment thresholds were lower for the 2nd muscle action in comparison to the 1st muscle action. Therefore, FR at recruitment may be influenced by training status, however, the downward shit in recruitment thresholds in a repetitive task is unlikely to be influenced by training.

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