The popliteus is a muscle of the posterior lower leg, located at the back of the knee joint. It has a specific function unique to this muscle alone. During movements in which the foot is in contact with the floor, known as closed kinetic chain movements, the popliteus functions to unlock the knee joint as it begins to move from a fully extended or locked position to a flexed position. An example of this type of movement is the beginning of the lowering phase of a squat, as one begins to bend the knees from the upright standing position.
To understand how this muscle unlocks the knee joint is to understand its anatomy. The popliteus originates on the medial tibial condyle, which is a bony protrusion found on the top inside edge of the tibia bone in the lower leg, just below and behind the inside of the knee. From there it crosses the back of the knee joint diagonally and inserts into the lateral femoral condyle, which is a similar protrusion found on the bottom outside edge of the femur bone in the thigh, just behind and to the outside of the knee. The popliteus inserts into the back of the knee via the popliteal tendon, which attaches to the lateral meniscus of the knee and pulls it backward as the knee flexes so that the meniscus isn’t crushed between the tibia and femur bones as the knee bends.
In addition to moving the meniscus out of the way of the converging tibia and femur bones, the popliteus muscle allows knee flexion to occur by pulling on the tibia bone so that it rotates slightly inward, or medially, and the femur bone so that it rotates slightly outward, or laterally. This action is necessary because in order for the knee to fully lock, the femur rotates slightly inward as the knee extends completely. Before the knee joint can flex, then, as in lowering oneself into a squat from a standing position, the popliteus must rotate the femur back laterally. Simultaneously, it twists the tibia medially, thereby unlocking the knee joint and allowing the leg to bend.
Individuals who sit for long periods, as in working behind a desk, may experience tenderness or adhesions in the popliteus, as the muscle rarely is stretched out when the knee is flexed all day. Massage or other form of myofascial release can be helpful, as can performing stretches for the backs of the legs such as hamstring and calf muscle stretches. Exercises that strengthen the popliteus include squats, leg presses, and deadlifts, as all require a transition from an extended to a flexed knee position against resistance.