Design and Testing of an Instrumentation System to Determine the Efficacy of Pain Reduction During Muscle Stimulation

Abstract

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 sensory intact subject during effective levels of stimulation using surface electrodes. Recent research [1,2] has suggested using a constant long (up to 10 msec) low amplitude or ramped conditioning pulse just prior to the high amplitude stimulus pulse (100 – 200 μsec) will 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 pre-pulses and high voltage stimulus pulses with selectable shapes, amplitudes, durations and frequencies. The stimulator hardware uses a standard voltage to current converter circuit with an efficient high voltage DC/DC converter which can present up to 100 ma pulses to surface electrodes. 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. The system is presently being tested in a study to determine the efficacy of the pain reduction stimulation strategy described above.