Interferential Electrical Stimulation Applied to the Soleus Muscle in Humans: Preliminary Study on the Relationship Among Stimulation Parameters, Force Output, and Pain Sensation

This study presents a method of providing deep muscle stimulation with an interferential current via surface electrodes, and analyzes the response of the skeletal muscles and the pain sensation experienced by subjects. Three healthy adult males were subjected to isometric contraction to generate the interferential current in the soleus muscle group, and the degree of muscle contraction by the plantarflexion torque and the pain sensation of the subjects were evaluated. Two square waves with different pulse widths were generated at the same time, and acted as the stimulation current with a relatively small amplitude of ±16mA. The experiments confirmed that a larger output torque could be obtained by generating interferential waves, compared to the condition where two identical waves (non-interferential current) were used. In addition, it was confirmed that the output torque and the pain sensation of the subjects decreased with the increase in the average frequency of the interferential current. When an interferential current was generated under the condition of interferential frequency fLow = 30 Hz and average frequency fHigh=\pmb50 ~ 100 Hz with the proposed electrode arrangement, subjects produced the largest output without pain. The results can contribute to assisting ankle pushoff movement in walking during the rehabilitation of patients with central motor paralysis.