The overall goal of this dissertation is to use novel motion analysis systems to investigate the underlying mechanisms that cause an anterior cruciate ligament (ACL) injury and then to explore movement modification methods that might prevent ACL injuries from occurring. This injury causes immediate functional impairment and also increases the long term risk of developing osteoarthritis (OA), a degenerative joint disease. Thus, understanding the causes of this injury and investigating methods to prevent it from occurring are important goals and could lead to improved health and quality of life. Additionally, novel motion analysis systems can provide new information about ACL injuries and therefore should be used to help analyze these injuries from a different perspective. This thesis provides the results from multiple experimental studies that used two novel motion analysis systems to investigate the underlying causes of ACL injury and potential injury prevention methods. These results add to the understanding of the ACL injury mechanism and also suggest potential preventative methods that could decrease the overall incidence of ACL injury.

Using a markerless motion capture system, the first investigation determined that increasing the coefficient of friction of the shoe-surface condition will change a subject’s movement strategies during a sidestep cutting task in specific ways that may increase the risk of ACL injury. Additionally, increased running speed combined with increased floor friction further alters a subject’s movement in biomechanical measures associated with risk for ACL injury, and these changes are different between females and males. This investigation provides a biomechanical basis for the increased incidence of ACL injuries on high friction surfaces, and suggests that the biomechanical causes change based on the speed of the maneuver. In terms of gender, this investigation suggests that females are more at risk for ACL injury when cutting on high friction surfaces at different speeds.

In terms of novel motion analysis systems, there is a need for simple, cost effective methods to identify athletes at a higher risk for ACL injury during jumping tasks. Wearable systems offer many advantages over traditional motion capture systems:​ they are simpler to use, do not require complex post-processing, and make it feasible to test subjects in a natural environment. As such, the second study assessed the capacity of a wearable inertial-based system to evaluate ACL injury risk during jumping tasks. This system accurately detected crucial temporal events and measured total jump height with a precision comparable to dedicated optical devices. Additionally, the proposed system measured the knee flexion and the trunk lean, and demonstrated good concurrent validity and discriminative performance in terms of the known risk factors for ACL injury. This study also reported the angular velocity of the thigh and shank segments during bilateral and unilateral drop jumps for the first time, and showed that angular velocity was consistent between subjects. Furthermore, this study illustrated there is an association between the coronal segment angular velocity and knee abduction moment, and that the coronal segment angular velocity can differentiate between subjects at higher risk for ACL injury.

Recent studies have shown that the incidence of ACL injury can be decreased through the use of intervention programs, but the quality of the feedback provided to the participants in these programs can vary depending on the skill of the observer. Therefore, the objective for the final study was to determine if an independent inertialbased system can be used to modify jump landing mechanics in order to decrease the risk for ACL injury by providing real-time feedback based on known kinematic and kinetic injury risk factors. This study found that the subjects reduced their risk for ACL injury after training with the system because there were significant increases in the maximum knee flexion angle and the maximum trunk lean. The subjects also reduced their risk for injury by decreasing their thigh coronal angular velocity, which was correlated with a decrease in their knee abduction moment. This study suggests that an inertial-based system could be used for interventional training aimed at reducing the risk for ACL injury.