Biomechanics of the knee joint: flexion and extension

Flexion and extension of the knee refer to the movements of the leg at the knee joint. Flexion of the knee occurs when a person bends the leg, bringing the heel closer to the back of the thigh. It reduces the angle between the femur and tibia. Extension of the knee happens when a person straightens the leg, moving the heel away from the thigh. This increases the angle between the femur and tibia. The range of knee flexion varies according to the position of the hip and whether the movement is active or passive. This video shows active flexion with the hip extended, and the range of this movement is up to 120 degrees. If the hip is flexed, the range can reach up to 140 degrees. Knee extension is a bit more complex than flexion. There is no absolute extension because, in the reference position, the leg is fully extended (with the axis of the leg in line with the axis of the thigh). Active extension is rarely achieved, and if it occurs, it is only slight. The leg seldom goes beyond the reference position. The term relative extension can be used to describe the movement that brings the knee into full extension from any position of knee flexion.