Design and testing of an instrumentation system to reduce stimulus pulse amplitude requirements during FES
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Functional Electrical Stimulation (FES) has been used for decades to restore muscle function following neural trauma. Another promising use has been to maintain or increase muscle strength following injury. Unfortunately in the latter case there is considerable stimulation pain for the sensorially intact subject during effective levels of stimulation using surface electrodes. Recent research  has suggested using a constant long (up to 10 ms) low amplitude or ramped conditioning pulse just prior to the high amplitude stimulus pulse (100 - 200 microsec) to reduce the excitably of sensory nerve fibers. However, commercial muscle stimulators cannot be easily modified to provide such complex pulse patterns and flexible pulse train control. We have designed and implemented a novel, very flexible LabVIEW based monophasic constant current muscle stimulator that provides pulse trains with long duration prepulses and high voltage stimulus pulses with selectable shapes, amplitudes, durations and frequencies. As well, the stimulator system includes an isolated EMG amplifier to record the evoked M-waves, which are used to estimate the fraction of muscle motor units being stimulated. We are presently testing this system, by stimulating the median nerve of human subjects and measuring the evoked M-waves for a range of stimulus pulse amplitudes, preceded by either ramped or rectangular low amplitude prepulses. The results indicate that, rather than inhibiting the activation of motor axons, as has been suggested , the prepulses at very low amplitudes excite these axons to subthreshold levels. The amplitudes of the stimulus pulses can then be significantly reduced while still achieving high levels of muscle activation.
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