Functional recovery and variability in response to spinal cord transcutaneous stimulation combined with activity-based training: Insights from a multi-case study

OBJECTIVE: Spinal cord transcutaneous stimulation (scTS) combined with activity-based training (ABT) has shown promise for improving upper limb function after cervical spinal cord injury (SCI). However, factors underlying variability in functional recovery remain unclear. This study examined the functional recovery and inter-individual variability in response to scTS + ABT in individuals with chronic cervical SCI. DESIGN AND SETTING: Preliminary multi-case study with pre- and post-intervention evaluations conducted in a rehabilitation laboratory. PARTICIPANTS: Five male adults with chronic cervical SCI (C3-C7; AIS A-D). INTERVENTIONS: All participants received scTS delivered over cervical/thoracic segments combined with upper limb ABT. Intervention dosage ranged from 16 to 54 sessions (60 min per session, 2-3 sessions per week). OUTCOME MEASURES: The Graded Redefined Assessment of Strength, Sensibility, and Prehension (GRASSP), handgrip and elbow flexion strength, Neuromuscular Recovery Scale (NRS), and Modified Functional Reach Test (MFRT). Spinally evoked potentials (SEP) were recorded in one participant to evaluate spinal excitability changes. RESULTS: All participants demonstrated improvements in at least one GRASSP subdomain, with the most consistent gains observed in strength. NRS and MFRT outcomes also improved, indicating enhanced trunk stability and upper limb control. Moderate-to-strong positive correlations (ρ = 0.67-1.0) were found between intervention dosage and functional gains, although confidence intervals were broad. SEP analysis revealed increased slope and reduced activation thresholds of recruitment curves, suggesting enhanced spinal excitability. CONCLUSION: scTS + ABT led to measurable functional improvements across participants; however, substantial variability was observed, likely influenced by injury characteristics and training dosage. Given the small sample size, participant heterogeneity, and lack of long-term follow-up, these findings should be interpreted as preliminary. Further controlled studies are warranted to evaluate the efficacy of individualized neuromodulation strategies for optimizing upper limb recovery after cervical SCI.