Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA159

Poster Communications

Prolonged depression of knee extensor torque complexity following eccentric contractions of the knee extensors in man

J. Pethick1, S. Winter1, M. Burnley1

1. School of Sport and Exercise Science, University of Kent, Chatham Maritime, Kent, United Kingdom.

The temporal structure, or complexity, of torque output is thought to reflect the ability to adapt motor output rapidly and accurately in response to alterations in task demand. Fatiguing isometric contractions have been demonstrated to reduce torque complexity (Pethick et al., 2015), which could have important implications for system functionality. Exercise-induced muscle damage reduces muscle torque output for considerably longer than high-intensity fatiguing contractions. If the fatigue-induced loss of torque complexity is related to the functional capacity of the muscle, then muscle damage should result in a prolonged loss of torque complexity. We therefore hypothesised that muscle damaging eccentric exercise would lead to a persistent decrease in torque complexity, but that fatiguing exercise would not. Ten healthy participants performed a set of five isometric contractions of the knee extensors at 50% of their maximal voluntary contraction (MVC) with a duty cycle of 0.6 (6 s contraction; 4 s rest) before, immediately after, 10, 30, 60 minutes and 24 hours after eccentric (muscle damaging) and isometric (fatiguing) exercise. Further measures were taken 48 hours and one week after eccentric exercise. Torque and surface EMG signals were sampled continuously during the contractions at 50% MVC. Complexity and fractal scaling of torque were quantified by calculating approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent for the contractions at 50% MVC. Global, central and peripheral fatigue were quantified using MVCs with femoral nerve stimulation. Values are means ± SEM, compared by ANOVA and t-test. Complexity decreased as a result of both eccentric and isometric contractions (decreased ApEn, increased DFA α, all P < 0.001). However, following the eccentric contractions ApEn and DFA α required 24 hours to recover to baseline levels, compared to only 10 minutes following isometric contractions. MVC torque was reduced and submaximal EMG amplitude increased up to 48 hours following eccentric exercise (both P < 0.001), indicating prolonged mechanical disruption and adjustments in motor unit activation, respectively. In contrast, decreased MVC torque and increased submaximal EMG were only evident up to 60 minutes following isometric exercise (both P < 0.001). These findings indicate that eccentric exercise perturbs the complexity, and thus the adaptability, of motor control. Eccentric exercise resulted in a more prolonged loss of complexity, decreases in torque output and adjustments in motor unit activity than isometric exercise. The prolonged reduction in complexity following eccentric exercise was likely due to an effect of muscle damage. However, whether this was due to mechanical disruption itself or to mechanical disruption impairing/influencing neural drive is yet to be fully elucidated.

Where applicable, experiments conform with Society ethical requirements