Spatially Distributed Sequential Stimulation Reduces Fatigue in Paralyzed Triceps Surae Muscles: A Case Study

Abstract

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. The purpose of this study was to investigate reducing fatigue by sequentially changing, pulse by pulse, the area of stimulation using multiple surface electrodes that cover the same area as one electrode during conventional stimulation. Paralyzed triceps surae muscles of an individual with complete spinal cord injury were stimulated, via the tibial nerve, through four active electrodes using spatially distributed sequential stimulation (SDSS) that was delivered by sending a stimulation pulse to each electrode one after another with 90° phase shift between successive electrodes. For comparison, single electrode stimulation was delivered through one active electrode. For both modes of stimulation, the resultant frequency to the muscle as a whole was 40 Hz. Isometric ankle torque was measured during fatiguing stimulations lasting 2 min. Each mode of stimulation was delivered a total of six times over 12 separate days. 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). The measures were all higher during SDSS (P < 0.001), by 234, 280, and 171%, respectively. The results are an encouraging first step toward addressing muscle fatigue, which is one of the greatest problems for FES.

Maintenance of Muscle Contraction Improves during Dynamic Exercise by Multi-channel Functional Electrical Stimulation with Time Shifting Stimulation

“…The decrease of the maintenance of muscle contraction leads to a difficulty of continuing rehabilitation for enough time and the restoration of motor function may delay. Various methods have been studied to avoid reducing the strength of muscle contraction [4], [5]. It is suggested that when stimulation is applied to a motor point, less current is required to elicit the muscle contraction.…”

Minimizing muscle fatigue through optimization of electrical stimulation parameters

“…[33] This value has been used in other studies. [18,38] Nevertheless, other researchers have also used a threshold of 50%. [34,39,40] Because In the present results, at lower levels of injected PA×PD, a low signal-to-noise ratio combined with low security in torque production resulted in low bilateral symmetry (see Table 1) and higher sensitivity to measurement error (e.g., when low torque levels [< 10 Nm] are generated in Figure 3).…”

“…Our results confirm that even able-bodied subjects do not show similar muscle response to FES, mainly because of their different muscle training and the high variability in spatial motor unit recruitment between participants reported previously. [18] Therefore, increasing the number of subjects would not result in obtaining a universal optimal (PA,PD) modulation strategy, expressed as red and blue lines in Figure 4, for all subjects. The primary goal of this study was to present a protocol that can be applied on an individual basis, and thus any larger sample size would not affect the final conclusions of this study.…”

“…Presently, we applied our protocol to FT and TTI, as two previously used fatigues indices in the literature. [18,20,[33][34][35][36][37] A drop of MT from its maximum (100%) to 70.7% (3 dB) was chosen since, in the control engineering literature, when the system output drops by 3 dB, its performance is considered altered. [33] This value has been used in other studies.…”

“…It is known that fatigue in general increases with pulse frequency. [1,18] Therefore, in practice, pulse frequency is usually fixed at a sufficiently low value to minimize fatigue, yet high enough to generate fused contractions. However, the effect of PA and PD on fatigue is inconclusive.…”