A pilot study of the stability of the ankle joint moment and M-wave evoked by intermittent stimulation

We examined the stability of the ankle joint moment (AM) and M-wave amplitude evoked by electrical stimulation of the gastrocnemius and soleus muscle. The use of functional electrical stimulation (FES) is associated with some problems, including the fact that the muscle activity evoked by electrical stimulation decreases with muscle fatigue. Also, when electrical stimulation is delivered via surface electrodes, the stimulation reaching the muscle can vary according to the impedance between the electrode and the skin. In this paper, we performed experiments to (1) examine the relation between AM and stimulation intensity and between M-wave amplitude and stimulation intensity; (2) determine the change in this relation over a long duration; and (3) identify a strategy to allow the muscle to recover from fatigue. In the first experiment, M-wave amplitude and AM changed similarly with respect to the stimulation voltage. In the second experiment, the pattern of the M-wave and AM both showed downward. The pattern, however, was not similar at the soleus. In the third experiment, M-wave and AM remained almost constant compared with fatigued response in the second experiment. We showed that it is possible to generate a stable AM using electrical stimulation of the muscle. To ensure the stability of AM, it is necessary to consider the characteristics of the muscle (e.g., the proportion of fast and slow twitch fibers), the stimulation area, and the state of the muscles (e.g., fatigue).