The popliteus muscle is a small, triangular muscle located in the back of the knee. It originates at the posterior aspect of the femur (thigh bone) and inserts into the lateral aspect of the tibia (shin bone). The popliteus muscle is one of several muscles that make up the posterior compartment of the leg.

The main function of the popliteus muscle is to medially rotate (inwardly rotate) the tibia on the femur, which helps to unlock the knee joint and allows the leg to move from a straightened position to a bent position. This movement is essential for activities such as walking, running, and climbing stairs. The popliteus muscle also helps to stabilize the knee joint during weight-bearing activities. It is also used in activities that require lateral stability of the knee, such as standing on one leg or performing lateral squats.