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 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 and 10, 30 and 60 min and 24 h after eccentric (muscle-damaging) and isometric (fatiguing) exercise. These contractions were also repeated 48 h and 1 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 α exponent (DFA α). Global, central and peripheral perturbations were quantified using MVCs with femoral nerve stimulation. Complexity decreased after 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 h to recover to baseline levels, but after isometric exercise they required only 10 min. The MVC torque remained reduced [from 233.6 (74.2) to 187.5 (64.7) N m] 48 h after eccentric exercise, with such changes evident only up to 60 min after 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.