Spinal cord injury (SCI) can be sudden— a bad blow to the back and now you are faced with permanent debilitating symptoms. The nervous system is delicate and complicated, and injuries to the cells within it have proven to be a difficult task to remedy. Although a method to complete recovery has not been achieved, there are forms of therapy, such as functional electrical stimulation (FES), that aid in activating motor axons that have lost function. A major downside to this approach is that FES causes rapid muscle fatigue, making it difficult for patients to endure on top of their already existing injury. In the Journal of Neural Engineering, Buckmire et al., 2018, approached this issue and proposed the use of multiple electrodes over a singular one in order to activate the full-array of motor axons. They suggest that in conventional FES, because only a few motor axons are being activated, a smaller volume of muscle is being innervated, causing increased fatigue and reduced endurance (Buckmire et al., 2018). The results of this study support this hypothesis, indicating that we may be taking a step forward in SCI recovery; however, more recent studies open a caveat into this breakthrough.
A facet of FES is FES-cycling, which involves a specially equipped ergometer that allows the patient to pedal when the electrical simulations are applied. Rather than simply having the patients contract a muscle, the component of cycling allows for a greater application of mobility. A case study from 2021 utilizes this form of FES while also observing different deliveries of stimulation. Ceroni et al., 2021, follow the journey of one SCI subject, and under consent, they test both single and multiple electrode stimulation during FES-cycling. The two types of single electrode stimulation they apply are constant frequency trains (CFTs) and doublet frequency trains (DFTs). These differ by rhythm of the pulses, CFTs containing shorter intervals. The multiple electrode stimulation they use is called Spatially Distributed Sequential Stimulation (SDSS), which essentially utilizes the several electrodes to activate a variety of different motor neurons within the same group of muscles. This was the first study to observe the effects of CFTs, DFTs, and SDSS on muscle fatigue within FES-cycling. Contradictory to Buckmire et al., 2018 (and several other previous studies: Popovic et al., 2009, Nguyen et al., 2011), they found in this case that SDSS had the lowest performance rate based on the cycling cadence that was recorded. Ceroni at el., 2021, acknowledge that these are novel findings and suggest that the variable in question is the form of muscle movement: contractions as opposed to cycling.
While evidence has been built in the favor of multiple electrode use in FES, Ceroni et al., 2021, introduce a new obstacle. When movement evolves from a simple contraction to a more complex combination of relaxations and contractions, perhaps the use of a single electrode is more effective. This seems counterintuitive: multiple electrodes for simple movements and singular electrodes for complex movements, but this underlines the complexity of the nervous system we have yet to fully understand. Ceroni et al., 2021, do raise into question the limitation of their findings coming from a case study, which again shows how unique each SCI can be. Treatment for SCI, although making great strides, faces many roadblocks ahead. It seems unlikely to develop a therapy that can be applied to every SCI patient, as shown by the contradiction of Buckmire et al., 2018, and Ceroni at al., 2021. These differences, however, will help to underline what caveats must be overcome.
References:
Buckmire et al., “Mitigation of excessive fatigue associated with functional electrical stimulation,” J. Neural Eng., 2018, 15 066004.
Ceroni et al., "Comparing Fatigue Reducing Stimulation Strategies During Cycling Induced by Functional Electrical Stimulation: a Case Study with one Spinal Cord Injured Subject," 2021 43rd Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC), 2021, pp. 6394-6397, doi: 10.1109/EMBC46164.2021.9630197
Nguyen, K. Masani, S. Micera, M. Morari, and M. R. Popovic, “Spatially distributed sequential stimulation reduces fatigue in para- lyzed triceps surae muscles: a case study,” Artificial Organs, vol. 35, pp. 1174–1180, Dec. 2011.
Popovic and N. M. Malesevic, “Muscle fatigue of quadriceps in paraplegics: comparison between single vs. multi-pad electrode surface stimulation,” Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual International Conference, vol. 2009, pp. 6785–6788, 2009.
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