**Core Concept**
The popliteus muscle is the only muscle responsible for unlocking the knee joint during flexion by rotating the tibia laterally and initiating the extension of the knee. It acts as a key component in the initial phase of knee extension, particularly when the joint is flexed and the patella is in a reduced position.

**Why the Correct Answer is Right**
The popliteus muscle, located in the posterior compartment of the knee, originates from the lateral aspect of the femur and inserts into the posterior surface of the tibia. It is unique in that it unlocks the knee joint by rotating the tibia laterally, allowing the joint to move from a flexed position into extension. This action is essential for the initial unlocking of the knee during gait, especially when the joint is flexed and the patella is not yet in full alignment. Without this action, the knee would remain locked in flexion.

**Why Each Wrong Option is Incorrect**
Option B: The quadriceps femoris is responsible for knee extension but does not initiate unlocking; it acts during full extension and is not involved in the initial release of the knee joint.
Option C: The semitendinosus and semimembranosus are part of the hamstring group and primarily function in knee flexion and hip extension, not in unlocking.
Option D: Like semitendinosus, semimembranosus contributes to knee flexion and stabilizes the posterior joint, but has no role in unlocking.

**Clinical Pearl / High-Yield Fact**
The popliteus is the only muscle that unlocks the knee joint — a key point in orthopedic exams. Remember: "Popliteus unlocks, hamstrings flex, quadriceps extend."

✓ Correct Answer: A. Popliteus