Effects of ipsilateral and contralateral fatigue and muscle blood flow occlusion on the complexity of knee-extensor torque output in humans

Neuromuscular fatigue reduces the temporal structure, or complexity, of torque output during muscular contractions. To determine whether the fatigue-induced loss of torque complexity could be accounted for by central or peripheral factors, nine healthy participants performed four experimental trials involving intermittent isometric contractions of the knee extensors at 50% of the maximal voluntary contraction torque. These trials involved: (i) two bouts of contractions to failure using the right leg separated by 3min recovery (IPS); (ii) the same protocol but with cuff occlusion during the 3min recovery (IPS-OCC); (iii) contractions of the left leg to failure, followed 1min later by contractions of the right leg to failure (CONT); and (iv) the same protocol but with cuff occlusion applied to the left leg throughout both the recovery period and right leg contractions (CONT-OCC). Supramaximal electrical stimulation during maximal voluntary contractions was used to determine the degree of central and peripheral fatigue, whilst complexity was determined using approximate entropy (ApEn) and detrended fluctuation analysis exponent (DFA). Neuromuscular fatigue was consistently associated with a loss of torque complexity in all conditions [e.g. IPS bout1, ApEn from (mean +/- SD) 0.46 +/- 0.14 to 0.12 +/- 0.06 (P<0.001)]. In IPS-OCC, occlusion abolished the recovery from fatigue, and torque complexity remained at the values observed at task failure in the preceding bout (IPS-OCC bout2, first minute 0.14 +/- 0.03, P<0.001). Prior contralateral contractions, with or without blood flow occlusion, had no effect on torque complexity.