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A novel method to reduce muscle fatigue during functional electrical stimulation for people with spinal cord injury


D. G. Sayenko, R. Nguyen, M.R. Popovic, K. Masani

The Journal of Spinal Cord Medicine, vol. 37, no. 5, pp. 616--649

Background/Objectives: Functional electrical stimulation (FES) is limited by the rapid onset of muscle fatigue caused by localized nerve excitation repeatedly activating only a subset of motor units. We have developed a novel method called spatially distributed sequential stimulation (SDSS) to reduce muscle fatigue during FES compared to a conventional single electrode stimulation (SES), and tested that with an individual with spinal cord injury when applied for plantar flexors. The purpose of the present study is to explore the fatigue-reducing ability of SDSS for major lower limb muscle groups in the able-bodied population, as well as individuals with spinal cord injury (SCI). Methods/Overview: SDSS was delivered through four active electrodes applied to the muscle of interest, sending a stimulation pulse to each electrode one after another with 90 phase shift between successive electrodes. For comparison, SES was delivered through one active electrode. For both modes of stimulation, the resultant frequency to the muscle as a whole was 40 Hz. Three minutes fatiguing stimulation was applied to participants (1) knee extensors, (2) knee flexors, (3) plantarflexors, and (4) dorsiflexors in 15 able-bodied individuals as well as 12 individuals with spinal cord injury. Three fatigue measures were used for comparison: fatigue index (final torque normalized to maximum torque), fatigue time (time for torque to drop by 3 dB), and torque-time integral (over the entire trial). Results: There were significant differences in all measures between SDSS and SES for knee extensors, knee flexors, and knee flexors in both experimental groups, indicating that muscle fatigue was less when using SDSS for these muscle groups. Conclusions: The present work verifies and extends reported findings on the effectiveness of using SDSS in the leg muscles to reduce muscle fatigue. Application of this technique can be easily incorporated in the clinical setup and facilitate the usefulness of FES.


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