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Abstract

It is well established that knee muscle strength and power (i.e., muscle performance) are negatively impacted after a sport-related knee injury, potentially elevating reinjury and post-traumatic osteoarthritis risk. Despite knowing this, important gaps remain in our understanding of how and why muscle performance changes after knee injury, and the clinical implications of these changes. This dissertation includes four studies that characterize and explore knee extensor and flexor muscle performance after a variety of knee injuries in active individuals, including the trajectory of muscle performance after injury, possible mechanisms underlying persistent deficits, and how muscle performance impacts leg-related function. Study one (prospective cohort study) characterizes changes in knee muscle strength following a variety of knee injuries and identifies a 10% deficit (compared to uninjured controls) at two years, regardless of injury type. Study two (cross-sectional study) explores quadriceps muscle morphology after an anterior cruciate ligament (ACL) tear and reveals no differences in muscle fiber size and distribution or extent of muscle fibrosis compared to participants without an ACL tear. Study three (cross-sectional study) assesses the relationship between knee muscle (extensor and flexor) strength and power, and self-reported and performance-based clinical measures of leg-related function, and demonstrates a link between strength and power, and hop performance, but not self-reported function. Study four (prospective cohort study) examines the relationship between increases in knee muscle strength and leg-related function improvements and uncovers that there is no clinically meaningful association between strength increases over the first year after injury and improvements in leg-related function. Collectively, these studies provide evidence that knee extensor and flexor strength and power deficits can persist for several years after a variety of knee injuries, refines the list of candidate mechanisms underlying these persistent deficits, and suggests that improving leg-related function after knee injury may require more than addressing knee muscle performance.