Association Between Interlimb Asymmetry in Normalized Vastus Medialis sEMG Amplitude and Functional Mobility in Patients with Post-Traumatic Knee Contracture: A Cross-Sectional Study

Background/Objectives: Patients with post-traumatic knee contracture often present with limited range of motion (ROM) and impaired functional mobility, but functional performance may not be fully explained by ROM and pain alone. This study aimed to examine whether normalized vastus medialis (VM) surface electromyographic (sEMG) amplitude and its exploratory interlimb ratio were associated with objective functional mobility and patient-reported knee function. Methods: This single-center cross-sectional study included 80 patients with post-traumatic knee contracture. Assessments included passive total range of motion (ROM), activity-related pain assessed using the visual analog scale (VAS), Timed Up and Go (TUG) performance, the Simplified Chinese version of the International Knee Documentation Committee Subjective Knee Form (SC-IKDC-SKF), and bilateral VM sEMG amplitude during active knee extension. VM sEMG amplitude was normalized to side-specific maximal voluntary isometric contraction (MVIC) and expressed as %MVIC; the exploratory interlimb ratio was calculated as the affected-side normalized VM sEMG amplitude divided by the unaffected-side value × 100. Spearman correlation, multivariable linear regression, and exploratory hierarchical regression analyses were performed. Results: Affected-side normalized VM sEMG amplitude was significantly lower than unaffected-side amplitude (23.8 ± 3.7% vs. 34.0 ± 4.7%, p < 0.001), with a mean exploratory interlimb ratio of 70.2 ± 8.8%. In the adjusted regression model, total ROM, activity-related VAS pain score, time to first rehabilitation intervention, and the exploratory interlimb ratio were associated with TUG time; for the interlimb ratio, standardized β = −0.247 and p = 0.004. In contrast, SC-IKDC-SKF score was independently associated only with total ROM and activity-related VAS pain score. In the exploratory hierarchical analysis, addition of the interlimb ratio increased the explained variance in TUG time by 4.8% (ΔR2 = 0.048, p = 0.004) but did not increase the explained variance in the SC-IKDC-SKF score (ΔR2 < 0.001, p = 0.891). Conclusions: Patients with post-traumatic knee contracture demonstrated lower affected-side normalized VM sEMG amplitude and interlimb asymmetry. A lower exploratory interlimb ratio was associated with longer TUG time and increased the explained variance by 4.8% in the exploratory hierarchical analysis, but was not associated with the SC-IKDC-SKF score.