The Relationship between Ankle Muscle Regulation and Standing Postural Control

－Focusing on linear and nonlinear analysis－

Abstract

Purpose：Motor control in various movements relies on the nervous systemʼs regulation of muscle output, critical for performing controlled movements. While conventional measures like error and coefficient of variation（ CV） assess quantitative aspects of muscle force regulation, this study incorporated sample entropy （SampEn） as a nonlinear analysis tool to evaluate qualitative aspects. It aimed to clarify the relationship between muscle force regulation in a foot visual pursuit task and standing postural control. Methods： Twenty healthy adults performed an ankle plantar flexion task at 20%, 40%, and 60% of maximal voluntary contraction （MVC） for 5 seconds, following a visual target line. Quantitative measures included absolute error mean and CV between target and response outputs, while SampEn assessed qualitative aspects. Standing postural control was evaluated using a center-of-gravity sway meter during 30 seconds of one-leg standing, calculating total trajectory length （LNG） and rectangular area （REC）. Statistical analysis involved one-way ANOVA and Pearson’s correlation. Results：No significant differences were found in absolute error means or CVs across %MVC levels. However, SampEn was significantly higher at 20% MVC than at 40% and 60% MVC （p＜0.001）. A positive correlation was observed between 20% MVC SampEn and LNG （r=0.54, p＜0.01）. Conclusion：Higher SampEn at low loads may reflect complex neuromuscular coordination, highlighting its independence from absolute error and CV. The correlation with standing postural control suggests that low-load regulation training could enhance postural control. Further research in dynamic tasks is recommended.