Which muscle is the deepest one in the popliteal space?

• A. Popliteus
• B. Plantaris
• C. Soleus
• D. Gastrocnemius

Answer: (A)

In the popliteal space, the deepest muscle is the popliteus. This small, thin muscle sits tucked under the larger muscles that cross the knee—deep to the gastrocnemius and plantaris—and forms part of the floor of the popliteal fossa. It originates from the lateral femoral condyle and inserts on the posterior surface of the tibia above the soleal line, so it lies right against the knee joint structures.

Its job is to stabilize the knee and help with movement: it weakly flexes the knee and, importantly, unlocks the knee from its fully extended (locked) position by rotating the tibia medially on the femur (or, depending on perspective, by rotating the femur laterally on the tibia). This action explains why the popliteus is the deepest muscle in the popliteal space, beneath the more superficial gastrocnemius, plantaris, and parts of the soleus.