Final answer:
The popliteus muscle is responsible for flexing the knee and causing the slight medial rotation necessary to unlock the knee joint from its fully extended position.
Step-by-step explanation:
The muscle responsible for flexing the knee and causing a slight medial rotation to "unlock" the knee joint is the popliteus. This small but vital muscle of the posterior compartment of the leg aids in initiating the bending of the knee by laterally rotating the femur on the tibia, which in turn "unlocks" the fully extended knee. The popliteus muscle works in opposition to the small medial rotation that occurs when the knee is fully extended and "locked".
The action of the popliteus is essential for smooth and coordinated knee movements, particularly when transitioning from a standing position to walking or running. For example, when you sit cross-legged, the popliteus muscle is responsible for the lateral rotation of the femur that allows your knee to 'unlock' and initiate the flexion of the knee joint.