LAUSR

It is vital to measure muscular fatigue for human–machine interaction (HMI) and avoiding muscle impairment. This article presents a novel approach to comprehensively evaluate muscle fatigue through the fusion of mini-grid surface electromyography (sEMG), mechanomyography (MMG), and near-infrared spectroscopy (NIRS). A hybrid mini-grid sEMG, MMG, and NIRS sensor system was developed, forming an instrument to measure motor unit action potential trains (MUAPts), low-frequency myofiber vibration, and oxygen metabolism. Muscle fatigue-induced experiment was performed via a sustained isometric contraction at 50% of maximal voluntary contraction (MVC) force with ten subjects. MUAPts were decomposed from the four-channel array of sEMG signals, and the firing rate (FR) of specific motor units (MUs) was calculated to measure fatigue process. Furthermore, root mean square (rms) and median frequency (MDF) of sEMG/MMG, blood volume (BV), and muscle oxygenation ( $\triangle $ HbO2) extracted from NIRS, were proposed as fatigue metrics. The variation and correlation analysis among fatigue metrics provided a detailed and reliable physiological assessment on muscle fatigue, gaining a synthetic understanding of muscle fatigue mechanisms. The outcomes of this study have great potential applications in sports, clinical diagnosis, and HMI.