Effects of ipsilateral and contralateral fatigue and muscle occlusion on the complexity of knee extensor torque output in humans
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 (MVC) torque. These trials involved: 1) two bouts of contractions to failure using the right leg separated by 3 min recovery (ISP); 2) the same protocol but with cuff occlusion during the 3-min recovery (ISP-OCC); 3) contractions of the left leg to failure, followed 1 min later by contractions of the right leg to failure (CONT); and 4) 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 MVCs was used to determine the degree of central and peripheral fatigue, whilst complexity was determined using Approximate Entropy (ApEn) and Detrended Fluctuation Analysis alpha-exponent (DFA alpha). Neuromuscular fatigue was consistently associated with a loss of torque complexity in all conditions (e.g., ISP bout 1 ApEn from [mean +/- SD]: 0.46 +/- 0.14 to 0.12 +/- 0.06 [P < 0.001]). In ISP-OCC, occlusion abolished the recovery from fatigue and torque complexity remained at the values observed at task failure in the preceding bout (IPS-OCC bout 2, first minute: 0.14 +/- 0.03, P < 0.001). Prior contralateral contractions had no effect on torque complexity. These results demonstrate that peripheral fatigue plays a dominant role in the loss of torque complexity.