Optimal stimulus parameters in functional electrical stimulation, which can minimize muscle fatigue and can achieve effective muscle contraction were determined to restore the motor function in atrophied muscles caused by central nervous system injury. Fatigue index of muscle contraction was measured from 2 min of tetanic contraction in T6 spinal cord injured Sprague-Dawley rats.

In control rats, the soleus muscle was classified into slow muscle and the medial gastrocnemius muscle was fast muscle according to their mechanical property of contraction. Optimal parameters of electrical stimulation for slow muscle were 20Hz, 0.2ms square pulse. For fast muscle, 40Hz, 0.3ms square pulse was optimal to reduce muscle fatigue. The atrophied soleus and medial gastrocnemius muscles by spinal cord injury showed significantly decreased muscle weight and tension, however, optimal parameters of electrical stimulation were the same as those of control rats. All of these findings suggest that optimal parameters in electrical stimulation which restore the motor function of atrophied muscle should be 20Hz, 0.2ms in slow muscle and 40Hz, 0.3ms in fast muscle.