The possibility of using M-waves related to double pulses for evaluating muscle fatigue in FES control

One of the major problems in the clinical application of FES is muscle fatigue. M-waves have been found to be useful for detecting muscle fatigue in voluntary movements and FES control. We discuss a new method of using M-waves to detect and evaluate muscle fatigue in FES control. The method used double pulses with various inter-pulse intervals (IPIs), which were made by additional pulses inserted into a stimulus pulse train. In this paper, the new method was examined using four neurologically intact subjects by applying electrical stimulation through surface electrodes. M-waves from the vastus lateralis muscle and the isometric muscle force at the ankle joint were measured when the muscle was stimulated at supra maximum amplitude or the maximum amplitude that subjects could bear stimulation over 200s. We found that the peak-to-peak amplitude of M-wave, both V(aft) and V(bef), which were evoked by the stimulus pulses just after or before the double pulse, respectively, showed good correlation with isometric muscle force. The peak- to-peak amplitude of the M-wave evoked by the second pulse, V(2nd), decreased sharply in the earlier stage as the IPI decreased. It was shown that V(bef) could predict electrically induced muscle force. A new index to evaluate muscle fatigue using V(2nd), FI(2nd), was found to be useful. The results showed that the method of using double pulses with various IPIs has the ability to detect and evaluate muscle fatigue in more detail, and in the early stage of muscle fatigue.