From compensation to decompensation: The threshold effect of recurrence frequency on shock absorption and movement strategies in individuals with chronic ankle instability during single-leg drop landing.

OBJECTIVES: To compare lower limb biomechanics during single-leg drop landings between individuals with low (2-4/year) and high (≥5/year) chronic ankle instability (CAI) recurrence frequencies. DESIGN: Cross-sectional study. SETTING: Biomechanics laboratory. PARTICIPANTS: Sixty males with unilateral CAI, stratified into High-CAI (n = 30) and Low-CAI (n = 30) groups. MAIN OUTCOME MEASURES: Peak vertical ground reaction force (vGRF), time to peak vGRF, vertical loading rate (LR), leg stiffness, and lower extremity kinematics. RESULTS: High-CAI participants demonstrated greater peak vGRF, shorter time to peak vGRF, higher LR, and greater hip flexion, hip abduction, and ankle inversion at initial contact than the Low-CAI group (all p ≤ 0.048; ηp2 = 0.066-0.275). Group × side interactions occurred for knee flexion, ankle inversion, and LR (p = 0.005-0.033; ηp2 = 0.076-0.128). Simple-effects analyses showed the unstable side of the Low-CAI group exhibited less knee flexion than its stable side, whereas the unstable side of the High-CAI group exhibited greater ankle inversion and LR than both the unstable side of the Low-CAI group and its stable side (all p ≤ 0.040; d = 0.350-1.221). CONCLUSIONS: High-frequency CAI recurrence (≥5 times/year) induces a maladaptive landing strategy with altered proximal compensations and impaired shock absorption. Frequency-based stratification is essential for optimizing management.