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

Abstract

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 3 min recovery (IPS); (ii) the same protocol but with cuff occlusion during the 3 min recovery (IPS-OCC); (iii) contractions of the left leg to failure, followed 1 min 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 α exponent (DFA α). Neuromuscular fatigue was consistently associated with a loss of torque complexity in all conditions [e.g. IPS bout 1, 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 bout 2, first minute 0.14 ± 0.03, P < 0.001). Prior contralateral contractions, with or without blood flow occlusion, had no effect on torque complexity.

Ischemic Preconditioning Blunts Loss of Knee Extensor Torque Complexity with Fatigue

“…Consequently, the complexity of neuromuscular output during fatiguing contractions was better maintained with prior IPC, which presumably preserved the adaptability of the neuromuscular system (5) and allowed the task to be sustained for a longer duration (10).…”

“…slowing of the decrease in MVC torque and increases in arEMG and mV O2) therefore suggests a slowing in the fatigue-induced increase in common synaptic input (37). We have postulated that any increase in common synaptic input should be reflected in a decrease in muscle torque complexity (10).…”

“…Any loss of complexity in muscle torque output is indicative of neuromuscular system dysfunction, and reflects a decrement in motor control which could have important implications for task performance (5,9). A loss of muscle torque complexity has been repeatedly observed with neuromuscular fatigue, occurring during both maximal and submaximal intermittent isometric contractions (6,9,10,11). Although the mechanism(s) underpinning this loss of complexity is unclear, we recently showed that caffeine ingestion delays task failure and slows the fatigueinduced loss of muscle torque complexity, concomitant to attenuation of the loss of maximum…”

Prolonged depression of knee extensor torque complexity following eccentric exercise

“…our results demonstrate that eccentric exercise results in a prolonged narrowing of system responsiveness and loss of adaptability in motor control, which could increase the risk of failing a motor task, such as dropping objects, failing to correct a fall, or, in the present experiments, failing to produce the required joint torque (Pethick et al, 2018b).…”

Differential contributions of fatigue‐induced strength loss and slowing of angular velocity to power loss following repeated maximal shortening contractions

“…Fatigue depends on many peripheral and central factors (Maffiuletti et al, ). In accordance with previous studies (Burnley, Vanhatalo, & Jones, ; Pethick, Winter, & Burnley, ; Theurel & Lepers, ), we considered that doublet torque is an index of peripheral fatigue and VA is an index of central fatigue in the current study. For the dorsiflexors, complete VA is often observed during MVC (Baudry et al, ; Chen & Power, ; Cheng & Rice, ), and this study also found “near maximal activation” throughout (Figure c) as expected.…”