Loss of knee extensor torque complexity during fatiguing isometric muscle contractions occurs exclusively above the critical torque

Abstract

The complexity of knee extensor torque time series decreases during fatiguing isometric muscle contractions. We hypothesized that because of peripheral fatigue, this loss of torque complexity would occur exclusively during contractions above the critical torque (CT). Nine healthy participants performed isometric knee extension exercise (6 s of contraction, 4 s of rest) on six occasions for 30 min or to task failure, whichever occurred sooner. Four trials were performed above CT (trials S1-S4, S1 being the lowest intensity), and two were performed below CT (at 50% and 90% of CT). Global, central, and peripheral fatigue were quantified using maximal voluntary contractions (MVCs) with femoral nerve stimulation. The complexity of torque output was determined using approximate entropy (ApEn) and the detrended fluctuation analysis-α scaling exponent (DFA-α). The MVC torque was reduced in trials below CT [by 19 ± 4% (means ± SE) in 90%CT], but complexity did not decrease [ApEn for 90%CT: from 0.82 ± 0.03 to 0.75 ± 0.06, 95% paired-samples confidence intervals (CIs), 95% CI = -0.23, 0.10; DFA-α from 1.36 ± 0.01 to 1.32 ± 0.03, 95% CI -0.12, 0.04]. Above CT, substantial reductions in MVC torque occurred (of 49 ± 8% in S1), and torque complexity was reduced (ApEn for S1: from 0.67 ± 0.06 to 0.14 ± 0.01, 95% CI = -0.72, -0.33; DFA-α from 1.38 ± 0.03 to 1.58 ± 0.01, 95% CI 0.12, 0.29). Thus, in these experiments, the fatigue-induced loss of torque complexity occurred exclusively during contractions performed above the CT.

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

“…As observed previously (Pethick et al., ), the values of each of ApEn, the DFA α exponent and the potentiated doublet torque reached consistently low values at task failure across experimental conditions, despite each commencing with different levels of fatigue and complexity. It has been suggested that consistent levels of peripheral fatigue at the termination of exercise might reflect the achievement of a ‘sensory tolerance limit’ (for review, see Hureau, Romer & Amann, 2016).…”

“…The sustained loss of torque complexity only after ipsilateral exercise and femoral occlusion adds weight to our previous suggestion that peripheral fatigue is a major contributor to the loss of neuromuscular complexity (Pethick et al., ). This loss of torque complexity was associated with both peripheral and central fatigue at the onset of contractions in IPS‐OCC2.…”

“…The fatigue-induced loss of torque complexity has been observed only during contractions performed above the critical torque (Pethick, Winter, & Burnley, 2016), a threshold above which metabolitemediated peripheral fatigue (assessed using the potentiated doublet torque) appears to be the dominant mechanism of force/torque loss (Burnley, 2009;Burnley, Vanhatalo, & Jones, 2012). This suggests that metabolite-mediated peripheral fatigue could be a major contributor to the loss of torque complexity during high-intensity contractions (Pethick et al, 2016).…”

Prolonged depression of knee‐extensor torque complexity following eccentric exercise

“…Common synaptic input to muscles and motor unit synchronization have been proposed to be major determinants of force variability (Dideriksen, Negro, Enoka, & Farina, ; Farina & Negro, ) and have been demonstrated to increase with fatigue (Castronovo, Negro, Conforto, & Farina, ). We have, therefore, previously speculated a link between motor unit synchronization and torque complexity (Pethick et al., , ). However, direct measurement of individual motor units via high‐density surface EMG electrodes will be necessary to confirm a link between motor unit synchronization and torque complexity, rather than the analysis of motor unit action potential trains recorded using bipolar EMG, as was done in the present study.…”

“…Several computer simulation studies have suggested that increased motor unit synchronization substantially increases EMG amplitude (Yao, Fuglevand, & Enoka, ; Zhou & Rymer, ). Moreover, motor unit synchronization has previously been speculated to be a potential cause of the fatigue‐induced loss of torque (Pethick et al., , ) and EMG (Beretta‐Piccoli et al., ; Mesin, Cescon, Gazzoni, Merletti, & Rainoldi, ) complexity.…”