Prolonged depression of knee extensor torque complexity following eccentric exercise

Neuromuscular fatigue reduces the temporal structure, or complexity, of muscle torque output. Exercise-induced muscle damage reduces muscle torque output for considerably longer than

high-intensity fatiguing contractions. We hypothesised 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. These contractions were also repeated 48 hours and one week after eccentric exercise. Torque and surface EMG signals were sampled throughout each test. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis ? exponent (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 ? required 24 hours to recover to baseline levels, but only 10 minutes following isometric exercise. MVC torque remained reduced (from 233.6 (74.2) to 187.5 (64.7) N.m) 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). The prolonged depression in maximal muscle torque output is therefore accompanied by a prolonged reduction in torque complexity.