15.19: Muscles that Move the Leg
The movement of the legs is facilitated by numerous muscles located within the anterior, medial, and posterior compartments of the thigh.
Anterior Compartment
The quadriceps femoris, the most visible muscle of the anterior compartment, is integral for leg extension and thigh flexion. It is formed by merging four distinct muscles — the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris. The quadriceps tendon, a shared tendon of the four quadriceps muscles, is affixed to the patella. This tendon extends beyond the patella, transforming into the patellar ligament that attaches securely to the tibial tuberosity. Additionally, the anterior compartment houses the sartorius muscle, which is involved in knee flexion and lateral hip rotation. It enables sitting cross-legged and is fittingly dubbed the "tailor’s muscle," reflecting its function.
Medial Compartment
The medial compartment of the thigh, also known as the adductor compartment, houses several key muscles, including the gracilis, adductor longus, adductor brevis, adductor magnus, and obturator externus. The gracilis, the most superficial and medial of the thigh muscles, is known for its long, strap-like appearance. It primarily aids in adducting the thigh, flexing the leg, and medially rotating the tibia during knee flexion. The adductor longus, often the most anterior of the adductor muscles, originates from the pubis and extends to the middle third of the linea aspera on the femur. The adductor brevis lies deep and works alongside the adductor longus to adduct and medially rotate the thigh. It has a shorter and more oblique course from the pubis to the femur.
Posterior Compartment
The posterior compartment houses muscles that enable flexion at the knee and extension at the hip, including the three hamstring muscles — the biceps femoris, semitendinosus, and semimembranosus. Hamstring tears or "pulled hamstrings" are common in sports due to their significant involvement in leg movement. The popliteal fossa, a diamond-like area on the backside of the knee, is defined by boundaries set by various tendons. It is flanked on either side by the biceps femoris muscle tendons and medially bordered by the tendons of the semitendinosus and semimembranosus muscles.
Finally, the popliteus is a small, triangular muscle located at the back of the knee. It is known for its crucial role in initiating knee flexion and providing stability to the joint. It originates from the lateral femoral condyle, a bony protrusion on the thigh bone, and then inserts onto the posterior surface of the tibia, just below the knee joint. It contributes to leg movement by causing the medial rotation of the tibia, a key mechanism for unlocking the knee during flexion.
