Effects of Work-to-Rest Ratios on Peak Torque and Neuromuscular Responses during Submaximal, Isometric Muscle Actions.

Abstract

PURPOSE: The purpose of the present study was to examine the effects of work-to-rest ratios on the maximal voluntary isometric contraction (MVIC) torque and neuromuscular responses to intermittent, submaximal, isometric muscle actions. METHODS: Twenty resistance-trained men and women (mean age ± SD = 23 ± 2 years; resistance training = 15 ± 5 days per month) visited the lab on two occasions (separated by ≥ 48 hours) and performed two MVICs before (pretest) and after (posttest) randomly performing two protocols (4-s on, 4-s off vs. 4-s on, alternating 6- and 2-s off) that consisted of 50 intermittent, submaximal (65% of MVIC), isometric muscle actions of the dominant forearm flexors on a calibrated isokinetic dynamometer. In addition, bipolar surface electromyographic (EMG) and mechanomyographic (MMG) signals were simultaneously recorded from the biceps brachii muscle. Separate 2 X 2 (Time [pretest, posttest]) X (Protocol) repeated measures ANOVAs were used to analyze the MVIC torque, EMG amplitude (AMP), EMG mean power frequency (MPF), MMG AMP, and MMG MPF data. RESULTS: There were no significant (p > 0.05) interactions for any of the variables. There were, however, significant main effects for time (marginal means collapsed across protocol). Specifically, as a result of the 50 repetitions MVIC torque, EMG MPF, and MMG MPF decreased 22, 20, and 16%, respectively, while MMG AMP increased 21% from pretest to posttest. CONCLUSIONS: Work-to-rest ratios had no effects on MVIC torque or any of the neuromuscular parameters. As a result of the 50 submaximal muscle actions, however, there were decreases in MVIC torque, possibly due to a decrease in global motor unit firing rate (MMG MPF). The decrease in MMG MPF may also have resulted in decreased muscle stiffness and, therefore, allowed greater oscillations of the activated motor units which increased MMG AMP. The non-significant increase in EMG AMP suggested that muscle activation was unaffected by the submaximal muscle actions, while the decreases in action potential conduction velocity (EMG MPF) may reflect a fatigue-induced buildup of metabolic byproducts.