The Soleus H-Reflex as a Biomarker of Post-Injury Spasticity in Spinal Cord Injury: A Conceptual Pilot Study

Background Spinal cord injury (SCI) disrupts neuronal pathways, alters spinal reflexes, and often leads to spasticity—a condition of increased muscle tone and involuntary contractions that interfere with daily function. Spasticity typically emerges progressively after injury; however, its mechanisms remain poorly understood, particularly regarding why it develops in some individuals but not in others. Increased spinal excitability has been implicated, but prospective evidence remains scarce. Objective This pilot, proof-of-concept study explored whether early electrophysiological markers of spinal excitability, specifically the soleus H-reflex and post-activation depression (PAD), could be linked to the subsequent development of spasticity during intensive functional rehabilitation (IFR). Methods Seventeen individuals with traumatic SCI were assessed. The Hmax/Mmax ratio and PAD were measured prior to or shortly after IFR admission. At IFR admission and discharge, spasticity (Modified Ashworth Scale and clonus) and motor function (Lower Extremity Motor Score, LEMS) were evaluated. Each lower limb was analyzed separately to account for asymmetry. Correlations between early electrophysiological measures and clinical outcomes were examined. Results Spasticity measures tended to increase from IFR admission to discharge, but only LEMS improved significantly, reflecting motor recovery. Several participants did not present spasticity at admission or discharge. A higher Hmax/Mmax ratio and reduced PAD at early IFR were associated with greater clonus at both admission and discharge, whereas no significant associations were found with Ashworth scores. Across injury levels and AIS grades, AIS A participants displayed the lowest Hmax/Mmax ratios and minimal spasticity, while AIS B or D participants with H/M ratios exceeding 0.2 in early IFR developed clonus. Conclusion These exploratory findings suggest that early signs of spinal hyperexcitability reflect spasticity development, but it does not show that they precedes spasticity development, which would be required to support its prognostic value. Assessment at earlier timepoints, such as in acute care, would be necessary to validate both the Hmax/Mmax ratio and the PAD as predictive measures of spasticity in SCI. While not clinically generalizable due to the small and heterogeneous sample, this proof-of-concept study highlights the feasibility and potential value of H-reflex as early markers of spasticity. Larger, adequately powered studies stratified by injury level and severity are needed to confirm utility and clinical applicability.