Association between knee angles at initial contact and post-landing knee ranges of motion in athletes with and without anterior cruciate ligament reconstruction.

Altered knee kinematics is associated with Anterior Cruciate Ligament (ACL) injury risk. Knee angle at initial contact (AIC) and range of motion (RoM) after landing are often used interchangeably as ACL risk indicators, yet their relationship remains unclear. Moreover, no consensus exists about the time window to be considered for the RoM. This study explored the degree of association between knee AIC and two RoMs (unstandardized: from IC to maximum knee flexion angle, ARoMFULL; standardized: within 100 milliseconds after IC, defining a Risk Time Window for ACL injury, ARoMRTW). Eleven ACL-reconstructed soccer players and 20 healthy controls performed Single Leg Hop (SLH) and Single Leg Cross Drop Landing (SLCDL) tasks on a force plate. Knee kinematics was recorded in the sagittal, frontal, and transverse planes. Correlation analysis showed moderate-to-very-high positive correlations between ARoMFULL and ARoMRTW across all tasks, limbs, and planes (r: 0.56–0.99), supporting their similarity in capturing early post-landing motion. Conversely, significant correlations between AIC and both RoMs were only found in frontal plane (r: 0.48–0.69), suggesting that AIC alone may not reliably reflect knee dynamics after impact. Since ACL injuries typically occur within 100 ms post-landing, ARoMRTW may offer a practical metric for injury screening and rehabilitation monitoring.