Background Spinal cord injuries (SCI) can affect muscle control, often resulting in significant functional impairments. Damage to sensorimotor pathways in the spinal cord can lead to muscle weakness, which may gradually recover during the first year post-injury. Functional electrical stimulation therapy (FEST) aims to enhance muscle strength, particularly when natural recovery diminishes. However, the recovery profiles of individual muscles treated with FEST during the chronic phase are still not well understood. The primary objective of this study was to characterize the timing and magnitude of gains in individual muscle strength during FEST. The secondary objective was to identify factors predictive of the response.

Methods In this cohort study, we examined the strength recovery profiles of 136 muscles treated with FEST, from 17 participants with cervical SCI at the chronic phase of the injury. Electrophysiology was conducted at baseline to assess corticospinal tract (CST) integrity and the excitability of the lower motor neuron pools using surface electromyography (sEMG).

Results Our findings reveal that 69 muscles did not respond to the treatment, while 67 muscles showed a median strength increase of one muscle motor score (MMS). Among responder muscles, achieving a 1 MMS increase required approximately 59 days (19.2 FEST sessions). A prediction model highlighted key predictors of responsiveness to FEST, including baseline MMS, characteristics of lesion location/severity, and neurophysiological indicators of CST integrity.

Conclusion Our findings underscore the need for refined guidelines in rehabilitation medicine, particularly for FEST in cervical SCI. The data demonstrates that achieving a 1-point increase in muscle strength requires ≈59 days and 19 FEST sessions. Further evidence also suggest that CST integrity and the responsiveness of lower motor neuron pools may influence FEST outcomes. These insights will enable more personalized and effective rehabilitation strategies, optimizing outcomes and resource allocation for individuals living with cervical SCI.

The authors have declared no competing interest.

NCT 05462925

This work was supported by the Wings for Life Spinal Cord Research Foundation (Project #210).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The results reported in this article include participants who received FEST from 3 sources, all using the same instrumentation and training protocols. (1) A clinical program at our institution. In this case, the therapy was delivered clinically and the study team conducted additional sEMG and strength testing assessments. (2) A FEST clinical trial taking place at our institution at the time of this study (Clinicaltrials.gov: NCT 03439319). The team for the present study conducted additional sEMG and strength testing assessments. (3) Later, to increase the sample size and as a result of available funding, additional participants were recruited whose therapy was delivered by the same clinical team but funded through this study (Clinicaltrials.gov: NCT 05462925). Regardless of the therapy delivery scenario, all of the data reported in this article were collected under protocol 19-5395 approved by the Research Ethics Board of the University Health Network. All participants provided written informed consent.

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