Measurement of Stimulus Frequency-Force Characteristics for FES Control Method Including Differences between Fast and Slow Muscle Properties

摘要

Functional Electrical Stimulation (FES) is one of effective methods for restoring motor functions of paralyzed extremities. A portable multi-channel FES system for clinical application was developed by our research group in Japan. In the FES system, muscle contraction is controlled by amplitude modulation of stimulus pulse trains with constant pulse width (0.2ms) and stimulus frequency (20Hz). This method, however, does not consider the muscle activation in the normal body such as recruitment order of the motor unit and variation of firing rate. The purposes of this study are to form an FES control method having capability of dealing with different muscle properties and to improve control ability of the FES system. As the FES control method considering muscle property and having improved control ability, we use an amplitude modulation method with pulse frequency modulation. In this paper, the difference between fast and slow muscle responses at various stimulus frequencies was investigated with rabbits. The response of muscle was force at the ankle joint under the isometric condition, when electrical stimulation was applied to the common peroneal nerve for the fast muscle (m. tibialis anterior) and the tibial nerve for the slow muscle (m. triceps sirae). First, force of electrically stimulated muscles was examined by the fusion index in order to evaluatefast and slow muscle propertoes. Then the properties of force versus frequency were studied. It was made clear from results that the fast muscle developed ,uch larger force than the slow muscle did when stimulus frequency was increased. And more, the fast muscle increased force steeply as the frequency was increased, while the slow muscle increased force gently. These results will be considered as a useful information, when a new FES control method will be made up.