This study analyzed changes in action potential of supraspinal neuron and motor unit depending on maximum tolerance isometric contraction(MTIC) by electrical stimulation and examined influence of functional electrical stimulation (FES) on spinal neuron adaptation. It selected 40 university students in their twenties and divided into experimental groups of 25% MTIC(I), 50% MTIC I (II), 75% MTIC(III) and 100% MTIC(IV) depending on MTIC by electrical stimulation, and performed isometric contraction of plantar flexor muscle to each experimental group with given contraction for 20 times. It measured V/Mmax and MDF pre and post exercise, compared volume of contraction. 1. V/Mmax ratio showed no significant difference in comparison among experimental groups. 2. There was significant difference in median frequency of gastrocnemius and soleus in action potential motor unit according to comparison among experimental groups(p<.001). When contraction by electrical stimulation was maximum, change was greatest. This results suggest that muscle contraction by electrical stimulation was influence to action potential of spinal motor neuron system which appear optimal level though aspect and difference degree were not in accordance. Consequently, optimal stimulation level of MTIC(50%) by FES would be lead to central nerve adaptation. muscle contraction by electrical stimulation was influence highly to MDF which should be consider to fatigue of motor unit for muscle contraction by electrical stimulation.