The knee joint is comprised of four bones: the femur, patella, tibia, and fibula (Figure 1). They combine to create one of the least structurally stable joints in the body. Accordingly, the knee can only move safely in a limited range and pattern of motion, unlike the shoulder or the hip.
While the musculature and connective tissues that keep the knee stable are complex, the movements that the knee produces are quite simple: flexion and extension (Figure 2).
In life beyond the superficial movements associated with some gym equipment, such as a leg extension machine, knee flexion and extension rarely occur in isolation. It is more common to see these movements coordinated with other movements of the lower limbs, such as in the squat, where the knees flex and extend along with the hips and ankles.