[Purpose] Post-activation potentiation is the increase in muscle twitch force in response to electrical stimulation after a conditioning contraction (CC) of maximum or near-maximum intensity. It is controversial whether muscular force during low-level voluntary contractions increases after maximum contraction. This study investigated the changes in electromyographic activity after CC. [Subjects and Methods] Fifteen healthy adults participated in this study. In the first experiment, a supramaximal electrical pulse was applied to the tibial nerve, and the isometric twitch force of plantar flexion was measured. After maximum voluntary contraction (MVC), i.e., CC, the twitch force elicited by electrical stimulation was monitored. In the second experiment, the surface electromyogram were recorded at an intensity of 20% MVC before and after CC. [Results] Results indicated the twitch force significantly increased by 30% within 30 s after CC. In the second experiment, the electromyographic activity significantly decreased by 16% for the gastrocnemius muscle but did not change for the soleus. Correlation coefficients between changes in the twitch force and electromyographic activity at 20%MVC were -0.54 for the gastrocnemius muscle and 0.04 for the soleus muscle. [Conclusion] Post-activation potentiation might work for daily physical activities compensating for reduced force output of the fatigued muscle.