Model-based image-matching (MBIM) technique has enabled detailed video analysis of injury situations that previously had been limited to simple visual inspection. We have analyzed anterior cruciate ligament (ACL) injury situations from ten analogue and one HD video sequences using the MBIM technique. The knee kinematical patterns were remarkably consistent, with immediate valgus and internal rotation (IR) motion occurring within 40 ms after initial contact (IC), and then external rotation was observed. Peak vertical ground reaction force (GRF) occurred at 40 ms after IC. Based on these results, it is likely that the ACL injury occurred approximately 40 ms after IC. In the one HD video available, 9 mm of abrupt anterior tibial translation at the time of injury was also detected. On the other hand, the hip kinematics were constant with an abducted, flexed and IR position during 40 ms after IC. Based on these results and other previous studies, we propose a new hypothesis for ACL injury mechanisms that valgus loading and lateral compression generate IR motion and anterior translation of the tibia, due to the joint geometry, result in ACL rupture. Moreover, it seems that the hip is relatively `locked` at IC, cannot absorb energy from GRF and thus the knee is exposed to a larger force, which leads to ACL injury. These results suggest that prevention programs should focus on acquiring a good cutting and landing technique promoting knee flexion and avoiding knee valgus and foot internal rotation, and with greater hip flexion to absorb energy from GRF. Moreover, the fact that the ACL injury occurs within 40 ms after IC suggests that "feed-forward" strategies before landing may be critical, as reflex-based "feed-back" strategies are too slow to prevent ACL injuries.
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