Knee osteoarthritis is an age-related disease which will affect nearly 50% of individuals in their lifetime. Because there are currently no treatments to substantially slow the progression of this disease, it is important to identify mechanisms to reduce the risk of osteoarthritis initiation. Osteoarthritis is a disease which is at least partially mediated by mechanical factors which may result from age-related changes in gait. The extent to which habitual physical activity can modify the impact of age on gait, knee mechanics, and thus cartilage loading is unknown. The aim of this dissertation was to examine the effects of age and habitual physical activity level on biomechanical risk factors for knee osteoarthritis including knee mechanics during gait, knee extensor muscle function, neuromuscular control, coordination, and the physiological and biomechanical response to a bout of exercise. Three groups of 20 healthy individuals each were recruited:​ young adults, highly active older adults, and less active older adults. Overground gait mechanics and knee extensor muscle torque and power were collected before and after a 30 minute treadmill walk designed to allow for observation of changes in gait and muscle function in response to muscle fatigue. At baseline, both older adult cohorts displayed decreased concentric knee extensor power compared to young adults. Older adults, especially in the less active group, had more femoral anterior translation relative to the tibia during the stance phase of gait, a measure that has previously been linked to osteoarthritis risk, incidence, and progression. Movement coordination was more affected by age than physical activity level as older adults from both physical activity cohorts displayed differences in coordination and its variability, particularly in movement coordination about the hip and ankle during periods of single-support. When comparing males and females across different age and physical activity cohorts, sex was identified as a determinant of hip and knee mechanics, and baseline knee extensor muscle function. The results of this dissertation provide evidence that, even in relatively young, high-functioning older adults, age and low physical activity levels are associated with a shift towards markers of increased knee osteoarthritis risk.