The muscle responsible for unlocking the knee during motion is?

• A. Popliteus
• B. Digital Flexor
• C. Gastrocnemius
• D. Soleus

Answer: (A)

Unlocking the knee from a fully extended, “locked” position is accomplished by the popliteus. In standing, the knee locks partly because the tibia externally rotates on the femur to stabilize the joint. The popliteus sits at the back of the knee and, when it contracts, twists the tibia internally (or, equivalently, rotates the femur externally) to release that lock. This action starts the knee flexion cycle, allowing movement to proceed. The other muscles listed aren’t responsible for this unlocking mechanism: the gastrocnemius and soleus mainly cross the ankle to plantarflex, while the digital flexor is involved with finger movement, not the knee.