Association between injury severity and recovery of objective functional measures following adolescent lateral ankle sprain: a secondary cohort analysis

Background/objectives Lateral ankle sprain (LAS) is common in adolescents, but the extent to which structural injury severity is associated with recovery of objective functional measures remains unclear. This study explored the association between Grade I–III injury severity and short-term recovery of objective functional measures following adolescent LAS using a post hoc secondary analysis of a randomized controlled trial (RCT). Methods This exploratory secondary cohort analysis included 65 adolescents aged 12–17 years with acute LAS who had participated in a RCT comparing two rehabilitation strategies. The severity-based analysis was not prespecified in the trial protocol. Participants were analyzed according to Grade I–III injury severity, irrespective of original treatment allocation. Injury severity was classified using clinical and imaging findings. Functional performance was assessed at 1–2, 5–7, and 12–15 weeks using Biodex isokinetic dynamometry for inversion (IN) and eversion (EV) peak torque normalized to body weight (PEAK TQ/BW) and ankle range of motion (ROM), and the Y-Balance Test composite score (YBT-CS). Outcomes were expressed as signed side-to-side differences (uninjured−injured), with values closer to zero indicating greater interlimb symmetry. Changes were analyzed using mixed repeated-measures ANOVA and covariate-adjusted LMMs including Severity, Time, Severity × Time, age, and treatment allocation, with participant as a random effect. Injured- and uninjured-limb values were examined separately. Results Significant changes over time were observed for most outcomes. No significant Severity × Time interactions were identified for signed side-to-side differences in either repeated-measures ANOVA or covariate-adjusted LMMs. In the ANOVA, Severity main effects were observed for ROM at 120°/s ( p = 0.024) and YBT-CS ( p = 0.014); after adjustment, the Severity effect remained significant for YBT-CS ( p = 0.045) but not ROM at 120°/s ( p = 0.090). In injured-limb analyses, all outcomes improved over time (all p < 0.001), without significant Severity or Severity × Time effects. Conclusions No statistically significant severity-related differences in short-term recovery trajectories were identified for the measured outcomes, including in covariate-adjusted analyses. The Severity effect for YBT-CS, but not ROM at 120°/s, remained significant after adjustment. Given the exploratory design and limited power for interaction effects, these findings should be considered hypothesis-generating. Trial registration NCT07287020; registered 3 December 2025 (retrospectively registered).