This article outlines the influence of muscle activity on knee-joint loading, describes the deficits in muscle function observed in people with knee osteoarthritis, and summarizes available evidence pertaining to the role of muscle in the development and progression of knee osteoarthritis. The article focuses on whether muscle deficits can be modified in knee osteoarthritis and whether improvements in muscle function lead to improved symptoms and joint structure. This article concludes with a discussion of exercise prescription for muscle rehabilitation in knee osteoarthritis.