Prolonged depression of knee extensor torque complexity following eccentric exercise

Neuromuscular fatigue reduces the complexity of muscle torque output. Exercise-induced muscle damage reduces muscle torque output for considerably longer than high-intensity fatiguing contractions. We therefore hypothesized that muscle damaging eccentric exercise would lead to a persistent decrease in torque complexity, whereas fatiguing exercise would not. Ten healthy participants performed five isometric contractions (6 s contraction, 4 s rest) at 50% maximal voluntary contraction (MVC) before, immediately after, 10, 30 and 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. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA). Global, central and peripheral perturbations were quantified using MVCs with femoral nerve stimulation. Complexity decreased following both eccentric (ApEn, mean (SD), from 0.39 (0.10) to 0.20 (0.12), P < 0.001) and isometric exercise (from 0.41 (0.13) to 0.09 (0.04); P < 0.001). After eccentric exercise ApEn and DFA α required 24 hours to recover to baseline levels, compared to only 10 minutes following isometric exercise. MVC torque remained reduced (from 233.6 (74.2) N.m to 187.5 (64.7) N.m) and submaximal EMG amplitude increased (from 51.2 (6.9)% to 68.4 (11.3)%) 48 hours after eccentric exercise, with such changes only evident up to 60 minutes following isometric exercise (MVC torque, from 246.1 (77.2) to 217.9 (71.8) N.m; submaximal EMG from 52.9 (6.4)% to 66.2 (9.0)%). The prolonged depression in maximal muscle torque output is therefore accompanied by a reduction in torque complexity, suggesting that eccentric exercise diminishes motor control as well as muscle force-generating capacity.