Neuromuscular fatigue and loss of torque complexity during severe‐intensity exercise: The role of muscle size

Abstract This study investigated the effect of severe‐intensity exercise on neuromuscular fatigue (NMF) and torque complexity in two muscle groups of different sizes. The NMF of 11 healthy males was assessed through maximal voluntary contractions at the beginning, every minute and at task failure during intermittent isometric exercise performed at an intensity 15% above the critical torque for knee extensors (KE) and handgrip (HG) muscles. Torque complexity was assessed using submaximal contractions from the first and last minutes of the exercise through approximate entropy (ApEn) and detrended fluctuation analysis (DFA‐α). The NMF reduced torque complexity (ApEn from 1.55 ± 0.09 to 1.25 ± 0.20, P < 0.001; DFA‐α from 1.40 ± 0.11 to 1.52 ± 0.06, P = 0.001) of the KE muscles. However, in HG muscles, the torque complexity was reduced only for DFA‐α analysis (1.01 ± 0.16 to 1.24 ± 0.16, P < 0.001). The loss of complexity was accompanied by greater central (KE −14.9 ± 12.4% × HG −7.66 ± 5.23%, P < 0.001) and peripheral fatigue (KE −63.8 ± 20.9% × HG −29.0 ± 14.5%, P < 0.001) in KE muscles than HG muscles, despite a similar decline in maximal torque production and exercise tolerance. These findings suggest that the magnitude of NMF may influence the loss of torque complexity between different muscle groups, resulting in impaired motor control ability. Furthermore, the greater impairment observed may be influenced by differences in muscle size, which could affect various physiological responses.