Fatigue reduces the complexity of knee extensor torque fluctuations during maximal and submaximal intermittent isometric contractions in man

Key points Healthy physiological systems generate time series possessing complex structures, as seen for example in heart rate variability, respiratory rate and gait. A loss of complexity in physiological time series has been associated with system dysfunction, and this loss is a characteristic feature of torque output from the ageing neuromuscular system. We sought to determine the effect of neuromuscular fatigue on the complexity of knee extensor torque output in healthy young humans performing repeated maximal and submaximal contractions. Fatigue resulted in a substantial loss of knee extensor torque complexity, with the noise in the torque signal becoming increasingly Brownian in character. Complexity has been associated with system adaptability, and the fatigue‐induced loss of complexity, the physiological origin of which is obscure, may contribute to the inability to sustain physical exercise.Abstract Neuromuscular fatigue increases the amplitude of fluctuations in torque output during isometric contractions, but the effect of fatigue on the temporal structure, or complexity, of these fluctuations is not known. We hypothesised that fatigue would result in a loss of temporal complexity and a change in fractal scaling of the torque signal during isometric knee extensor exercise. Eleven healthy participants performed a maximal test (5 min of intermittent maximal voluntary contractions, MVCs), and a submaximal test (contractions at a target of 40% MVC performed until task failure), each with a 60% duty factor (6 s contraction, 4 s rest). Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling of torque were quantified by calculating approximate entropy (ApEn), sample entropy (SampEn) and the detrended fluctuation analysis (DFA) scaling exponent α. Fresh submaximal contractions were more complex than maximal contractions (mean ± SEM, submaximal vs . maximal: ApEn 0.65 ± 0.09 vs . 0.15 ± 0.02; SampEn 0.62 ± 0.09 vs . 0.14 ± 0.02; DFA α 1.35 ± 0.04 vs . 1.55 ± 0.03; all P  < 0.005). Fatigue reduced the complexity of submaximal contractions (ApEn to 0.24 ± 0.05; SampEn to 0.22 ± 0.04; DFA α to 1.55 ± 0.03; all P  < 0.005) and maximal contractions (ApEn to 0.10 ± 0.02; SampEn to 0.10 ± 0.02; DFA α to 1.63 ± 0.02; all P  < 0.01). This loss of complexity and shift towards Brownian‐like noise suggests that as well as reducing the capacity to produce torque, fatigue reduces the neuromuscular system's adaptability to external perturbations.