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

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-alpha scaling exponent (DFA-alpha). 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-alpha 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-alpha 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.