The knee joint is primarily unlocked through rotation of the tibia (lower leg) externally on the femur (thigh bone). This external rotation is facilitated by the popliteus muscle.
Here's a more detailed explanation:
The "Screw-Home" Mechanism
The knee's fully extended position is often referred to as the "locked" position. This occurs due to a phenomenon called the "screw-home" mechanism. This mechanism provides stability in full extension and involves:
- Shape of the Femoral Condyles: The medial femoral condyle (inner side of the thigh bone) is larger than the lateral condyle (outer side).
- Lateral Pull of the Quadriceps: The quadriceps muscle pulls slightly laterally on the patella and tibia.
These factors cause the tibia to externally rotate during the last few degrees of knee extension, effectively "locking" the knee joint. This locked position provides passive stability, reducing the need for constant muscle activation to maintain a standing posture.
Unlocking the Knee: The Role of the Popliteus Muscle
To initiate knee flexion (bending), the knee joint must be "unlocked." This is the primary function of the popliteus muscle, which:
- Attaches: Originates on the lateral femoral condyle and inserts on the posterior aspect of the tibia.
- Action: Contracts to internally rotate the tibia on the femur (or externally rotate the femur on the tibia if the foot is planted). This action reverses the "screw-home" mechanism, allowing the knee to flex.
Other Considerations
While the popliteus muscle is the primary "unlocking" mechanism, other factors contribute to smooth knee motion:
- Menisci: These C-shaped pieces of cartilage act as shock absorbers and help guide knee movement.
- Ligaments: The ligaments (ACL, PCL, MCL, LCL) provide stability and prevent excessive movement.
- Other Muscles: Hamstrings and other muscles surrounding the knee assist in controlling flexion and rotation.
Loose Bodies and "Joint Mice"
While not directly related to unlocking the knee joint in the biomechanical sense, the question's reference to "loose bodies" and "joint mice" is relevant to conditions that can affect knee movement. A loose body is a fragment of cartilage or bone that floats freely within the joint space. These fragments can cause:
- Clicking or Catching: As the loose body moves around the joint.
- Pain: If the loose body becomes trapped between joint surfaces.
- Locking: In some cases, a loose body can become lodged in a position that prevents full extension or flexion, mimicking a "locked" knee.
