C.E. Corrigan, S.M. Hurd, J.E. Burton, 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) peak firing rates (PFR) and duration of MU activity of the vastus lateralis (VL) in resistance trained (RT) and aerobically trained (AT) individuals during repetitive 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) prior and 1 following the repetitive tasks. The isometric trapezoid muscle actions at 50% MVC were calculated from the highest MVC (peak force [N]). 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 20 consecutive times or until the desired force level was not matched (8 s rest between contractions). Decomposition of the the surface EMG signals were used to extract the peak firing events of single MUs. The PFR (pulses per second [pps]) and duration of MU activity (s) were calculated for each MU. A two-way mixed factorial ANOVA (training [AT vs. RT] x time (pre vs. post)] was used to examine differences in MVCs. A two-way mixed factorial ANOVA (training [AT vs. RT] x repetition (1st vs. Final]) was used to examine differences in PFR and duration of MU activity. RESULTS: The five AT were able to successfully complete 20 repetitive muscle actions, however, none of the RT were able to complete the 20 muscle actions (mean ± SD, 14.0 ± 4.8). There was no change pre- (480.8 ± 149.5 N) to post-MVC (426.2 ± 96.2 N) for the AT (P = 0.158), however, MVC decreased (pre = 1035.4 ± 197.1 N, post = 667.0 ± 102.2 N) for the RT (P = 0.002). For PFR, there was a decrease from the 1st to last repetition for the AT (P = 0.001, 1st = 21.0 ± 4.3 pps, 20th = 19.4 ± 5.2 pps) and RT (P = 0.008, 1st = 19.0 ± 4.2 pps, last = 18.1 ± 4.8 pps), however, duration of MU activity decreased from the 1st to last repetition for only the RT (P < 0.001, 1st = 14.8 ± 2.1 s, last = 13.2 ± 2.0 s) and not for the AT (P = 0.124, 1st = 16.1 ± 1.6 pps, 20th = 16.0 ± 1.5 s). Finally, the 1st and last repetition PFR and duration of MU activity were greater for the AT than the RT (P < 0.001). CONCLUSION: The AT had greater PFR and longer duration of MU activity than the RT. PFR decreased in a similar manner during the repetitive tasks for the AT and RT, however, MVC and duration of MU activity decreased for the RT but not for the AT. Thus, duration of MU activity may be a more sensitive indicator of fatigue than PFR.

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