The Effect of Muscle Contraction Type on Neuromuscular Complexity in Trained Individuals

Abstract

Physiological systems exhibit high levels of complexity characterized by non-linearity and persistent fractal correlations (low levels corresponding to states such as disease, injury, and fatigue) and has become recognized as a defining feature of healthy physiological functioning. Neuromuscular complexity is affected by fatigue and intensity of contractions, although no study has investigated the effect of contraction type on complexity. PURPOSE: The purpose of this study was to investigate the effect of contraction type on neuromuscular complexity. METHODS: Twelve collegiate-aged resistance-trained females (21 ± 1 years, 63.3 ± 7.4 kg) were recruited to visit the laboratory on two occasions, the first for familiarization purposes. In session two, participants performed three maximal knee extensor contractions on an isokinetic dynamometer for each contraction type [concentric (CON), eccentric (ECC), and isometric (ISO)] in random order. Relative knee angle was standardized to 120° during ISO contractions. Angular speed was standardized to 30°∙s-1 and range of motion to 90° (90° - 180° at full extension) during CON and ECC contractions. Each contraction lasted three seconds with three seconds rest between contractions. Electromyographic (EMG) signals were recorded from the vastus lateralis using a bipolar electrode configuration. Sample entropy (SE), a unitless measure of statistical irregularity was used as an index of physiological complexity. A one-way repeated measures ANOVA was performed to investigate differences in EMG SE among contraction types. Alpha level was set to 0.05. RESULTS: Contraction type was observed to have a significant effect on EMG SE (F(2,22) = 7.212, p = 0.004). Specifically, CON contractions (1.671 ± 0.193) displayed significantly greater EMG SE than ECC (1.497 ± 0.321, p = 0.017) and ISO (1.569 ± 0.223, p = 0.028) contractions. CONCLUSION: These findings indicated that neuromuscular complexity is contraction-type dependent, being significantly higher during CON than ECC and ISO contractions. In addition, there exists no generally accepted framework to explain the underlying factors regulating complexity and thus, further investigation may provide valuable insight into these potential mechanisms.