Torque regulation is affected by joint angle during isometric contraction in young male adults

Research demonstrates that the magnitude of torque variability is influenced by joint angle. However, complexity-based measures seem to provide distinct information than the traditional magnitude-based measures of variability, being more sensitive to detect subtle changes in physiological outputs’ dynamics. Therefore, this study aimed to investigate the effect of joint angle on torque complexity and intermuscular coordination. Twenty-five male adults performed a sustained isometric knee extensor task at 110% of their end-test torque for 30 s at five different joint angles. Torque complexity and variability were assessed using Sample Entropy (SampEn) and Coefficient of Variation (CV), respectively, while intermuscular coordination was evaluated through Co-Contraction Index (CCi). Results revealed that SampEn at OA−30º was significantly lower than at OA (p = 0.014) and OA + 15º (p = 0.022), indicating reduced torque complexity at shortened muscle lengths. CV and CCi exhibited similar behaviors by presenting a trend for a U-shape relationship with joint angle. Our findings suggest a decrease in the individuals’ capacity to regulate the torque production with the shortening of the knee extensors which can possibly be explained by the mechanical properties of the muscle and central and peripheral neuromuscular factors or, most importantly, by the complex interaction between all the mechanisms involved in torque regulation.