ACL Injury Mechanisms and the Kinetic Chain Linkage: The effect of proximal joint stiffness on ACL injury risk.
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Recent literature has suggested that core and gluteal neuromuscular deficits are involved in the mechanism of non-contact ACL injury. Several research groups have identified dynamic valgus of the lower extremity to be an injurious posture that is predictive of future non-contact ACL injury risk. Aberrant kinematics of segments proximally in the kinetic chain, namely the trunk and hip, have also been observed to drive dynamic valgus during dynamic activities. Comprehensive investigation of the neuromuscular deficits postulated as the mechanism of injurious mechanics are lacking in the literature. Given that certain motions can be created by infinite muscle activation combinations, and that muscle activation contributes both force and controlling stiffness, this work aims to characterise any such deficits by examining the ability to modulate proximal joint stiffness to dynamically control distal segments of the kinetic chain. Three-dimensional lumbar spine stiffness and hip stiffness were quantified in participants deemed as ‘high valgus’ and ‘low valgus’ based on their frontal plane knee displacement during each task. The risk of non-contact ACL damage is highest among active females, justifying their choice to study. Eighteen female participants completed drop vertical jump (DVJ), stop jump (SJ), single leg drop (SLD) and single leg crossover drop (SLCD) tasks in order to measure medial knee displacement and associated proximal joint stiffness values. It was hypothesized that those with high valgus would not generate sufficient joint rotational stiffness at the lumbar spine, hip, or both, and thus aberrant kinematics and the injurious dynamic valgus motion would result. Those who were able to develop sufficient stiffness at the lumbar spine and hip had greater control over the kinetic chain and in doing so reduced dynamic valgus and likely their risk of future ACL injury. However, variance within subjects was found, specifically the same person would show a valgus landing on one trial, but not on another. This necessitated a change in analysis to one considering the landings as case studies, and groupings of landings by whether valgus occurred or not, rather than by subjects. This was an unexpected, and therefore exciting part of the thesis journey. The results here provide insight into the motor control component of avoiding dynamic valgus and is the first work to confirm, and specifically characterize, a neuromuscular deficit at the core or hip. That deficit appears to be an inability to generate sufficient joint rotational stiffness in order to control the linkage. Given this insight, appropriate interventions and training programs may be designed to reduce one’s risk of ACL injury.