Changes in torque complexity with fatigue are related to motor unit behaviour

Physiological complexity is believed to reflect a system’s adaptability to environmental challenges having been proposed as an indirect indicator of the functional capacity of the neuromuscular system. This study aimed to investigate the association between torque complexity’s changes with neuromuscular fatigue and motor unit parameters. Twenty-one healthy and young adults visited the laboratory on one occasion. Knee extension maximum voluntary isometric contractions and isometric contractions at 30% of maximum were collected at baseline and immediately after a fatiguing knee extension protocol, which consisted of a series of concentric and eccentric knee extensions at 90°/s until exhaustion. Torque signals were sampled continuously, and torque complexity was assessed through an entropy measure. Motor unit-related parameters were extracted from the submaximal trials and further analysed. Our findings demonstrate that torque complexity’s alteration pre-to-post neuromuscular fatigue is highly correlated with vastus lateralis and medialis average firing rate (r = − 0.618 and r = − 0.659, respectively) and peak motor unit action potential amplitude (r_s = − 0.801 and r_s = − 0.703, respectively) pre-fatigue. Moreover, alterations in torque complexity were observed, indicating a loss of adaptability within the neuromuscular system with neuromuscular fatigue. Overall, our findings supported our hypothesis by demonstrating alterations in torque complexity with neuromuscular fatigue, rendering the system less adaptable. Moreover, our results added to the current knowledge by highlighting the association between torque complexity’s changes with neuromuscular fatigue and motor unit parameters.