Detection of Weak Muscle Fatigue Using Bioimpedance Spectroscopy Based on Current Concentration

Low-load muscle exercises are a safer alternative to high-load exercises and effectively promote health across various age groups. However, low-load exercises induce minimal physiological changes, making it difficult to detect muscle fatigue. In this study, we aimed to increase the electrical conductivity of target muscles owing to exercise hyperemia during exercises and detect the weak muscle fatigue signals as following approaches. First, the spacing between current electrodes on the human thigh was determined through electromagnetic simulations to clarify the high current concentration in muscle. Next, fatigue detection sensitivities during low-load knee extension exercise with and without current concentration were evaluated. The evaluation parameter was bioelectrical impedance (BI), which was recorded before exercise and after each set of exercise. In the narrow-space electrodes with high-current concentration, extracellular water resistance (R_ECW) derived from the BI decreased significantly with exercise. In contrast, in the wide-space electrodes with uniform current, R_ECW showed no significant change. These findings suggest that localized BI electrode enhances fatigue detection sensitivity and provides a reliable approach to assessing the effects of low-load exercise.