SAGITTAL PLANE BIOMECHANICS DURING SPORT MOVEMENTS DOES NOT EXPLAIN HIGHER INCIDENCE OF ACL INJURY IN FEMALES Scott G. McLean 1,2 , Anne Su 1 and Antonie J. van den Bogert 1,2 1 Department of Biomedical Engineering, The Cleveland Clinic Foundation, Cleveland OH 2 The Orthopaedic Research Center, The Cleveland Clinic Foundation, Cleveland OH email: mcleans@bme.ri.ccf.org web: www.lerner.ccf.org/bme INTRODUCTION Anterior cruciate ligament (ACL) injury is a common and traumatic sports injury, particularly for females. Female neuromuscular control patterns can contribute to their increased risk of ACL injury. For example, women tend to land in a more extended (hip and knee) position than men (e.g. Malinzak et al., 2001). This places the patellar tendon in a more anterior orientation (Herzog and Read, 1993), which is theorized to increase the amount of quadriceps-induced ACL force, and thus contribute to ACL injury (DeMorat et al., 2004). During athletic movements, however, the quadriceps force and ground reaction force are not independent of flexion angle, or of each other, and this is not incorporated within this theory. The purpose of this study is, therefore, to collect data from human subjects of both genders, and quantify the contributions of sagittal plane knee biomechanics to the force in the ACL during an athletic movement that has been associated with ACL injury. METHODS Ten male and ten female NCAA basketball players had lower limb 3D kinematics and kinetics quantified during the stance phase of ten sidestep cutting maneuvers. A dynamic model of the pelvis and lower extremity was generated for each subject as described previously (McLean et al. 2003). Joint angles were obtained from Mocap Solver (Motion Analysis Corporation, Santa Rosa, CA) and intersegmental 3-D forces and moments at the knee were solved using standard inverse dynamics. Intersegmental loads were defined as components of external load on the joint, expressed in a joint coordinate system. Anterior tibial shear force (F ant ), external flexion (M flex ), valgus (M val ), and internal rotation moments (M int ) were used for further analysis. ACL force (F ACLsag ) due to the sagittal plane joint loading mechanism was estimated as: ) ( sin ) ( flex ant ACLsag θ α θ ×         + = d M F F , where d is the moment arm of the patellar tendon as a function of knee flexion angle θ, and α represents the angle between the patellar tendon and the long axis of the tibia in degrees, also a function of knee flexion (Herzog and Read, 1993). This equation does not incorporate the force contributions of the hamstring muscles, and thus represents a worst-case scenario in terms of ACL injury potential during sidestepping. Peak loads and initial contact knee flexion data were obtained from each trial, averaged for each subject, and compared between genders using a Student t-test (p<0.01 after Bonferroni correction). RESULTS AND DISCUSSION Women landed with less initial knee flexion (Table 1), similar to previous observations (Malinzak et al., 2001). No significant