We can rotate our leg at the knee because, although primarily a hinge joint, the knee possesses inherent structural features that allow for a limited degree of rotational movement.
The knee's rotation isn't as free as a ball-and-socket joint like the hip. Instead, it's a subtle movement permitted by a combination of factors:
- Ligament "Play": The knee joint is stabilized by numerous ligaments. While they primarily dictate forward, backward, and side-to-side motion, they also allow a small amount of give that contributes to rotation. Specifically, the medial collateral ligament (MCL) and lateral collateral ligament (LCL) contribute to this stability, but allow a small degree of rotation when the knee is flexed. Cruciate ligaments (ACL and PCL) also play a role.
- Joint Capsule Flexibility: The capsule surrounding the knee joint has some elasticity, enabling slight movement and, therefore, minimal rotation. This is like a loose bag that allows for some wiggle room.
- Shape of the Femoral Condyles & Tibial Plateau: The shapes of the bones involved in the knee joint (femur and tibia) allows for slight rotation during flexion.
- Muscles around the Knee: Specific muscles around the knee joint such as the hamstring and quadriceps allow for external and internal rotation.
In essence, the knee isn't designed for extensive rotation. The slight rotation we experience is a consequence of the slight "play" within the ligaments, the flexibility of the joint capsule, and the complex interaction between the bones and muscles around the knee. This movement is more pronounced when the knee is bent (flexed) compared to when it is fully extended (locked).
