The purpose of the present study was to determine electrophysiological differences in muscular fatigue between the soleus (Sol) and tibialis anterior (TA) muscles in normal human subjects (n=5) . The subjects were asked to make four 20-sec maximum voluntary contractions (MVCs), each separated by 3-min intervals of rest. A 3-sec MVC (V_task) or a 3-sec supramaximum electrical stimulation (E_task) was imposed at 1-min intervals during the resting period. The plantar flexion and dorsiflexion forces were significantly decreased throughout both the V_ and E_tasks. In particular, the decrease in the dorsiflexion force during the V_task was found to be very steep. However, wide inter-subject variations were found in the time course of the decrease in the MVC force in all tasks. Changes in mean power frequency (MPF) of the electromyographic (EMG) recordings in the Sol were found to be small in both tasks. In contrast, the MPF was significantly decreased during the 20-sec MVC in the TA. The root mean square (RMS) of the EMG gradually declined during both the V_ and E_tasks. The ratio of the root mean square of EMG (RMS) and the exerted force (RMS/F) was also determined during both tasks and for both muscles. The RMS/F was markedly increased during the V_task in the TA. An increase was also found in the Sol, but the magnitude of the increase was small. A small but consistent decrease in the M-wave was found in the V_task. The Sol H-reflex was decreased until the second 20 s MVC, then reached a plateau, and further decreased at the end of the fourth 20s MVC. It was suggested that the electrophysiological differences in the Sol and TA during muscular fatigue induced by the repetitive 20-sec MVC reflected differences in the physiological properties of these muscles. The RMS/F was suggested to be a useful parameter for determining the local muscular fatigue in intact human lower leg muscles.