The biceps femoris is a muscle in the posterior compartment of the thigh. It is one of the three hamstring muscles, playing a significant role in leg movements. This muscle consists of two heads, which contribute to its actions at both the hip and knee joints.
Origin Points
The biceps femoris muscle originates from two separate locations, distinguishing its long and short heads. The long head begins at the ischial tuberosity, a prominent bony protrusion on the posterior aspect of the ischium, which is part of the hip bone. This area is commonly known as the “sit bone.” The long head shares a common tendon of origin with the semitendinosus muscle.
The short head originates from the femur, or thigh bone. It arises from the lateral lip of the linea aspera and the lateral supracondylar line. The linea aspera is a roughened ridge on the posterior surface of the femoral shaft. The lateral supracondylar line extends downwards from the linea aspera towards the lateral condyle of the femur. Unlike the long head, the short head does not cross the hip joint.
Path and Insertion
The two heads of the biceps femoris converge as they extend downwards along the posterior thigh. These muscle fibers form a common tendon that descends towards the knee. This tendon inserts primarily onto the head of the fibula, the smaller of the two lower leg bones.
A smaller slip of the tendon also inserts onto the lateral condyle of the tibia. The lateral condyle is the outer portion of the expanded upper part of the tibia, or shinbone, where it articulates with the femur. The common tendon can be felt on the outer side of the hollow behind the knee, known as the popliteal fossa.
Primary Function
The biceps femoris performs several actions, primarily involving movement at the knee and hip joints. Its most prominent role is knee flexion, which involves bending the knee. Both the long and short heads contribute to this action.
The long head also plays a part in hip extension, moving the leg backward at the hip joint. This muscle contributes to the lateral rotation of the lower leg, particularly when the knee is slightly bent. The biceps femoris, along with other hamstring muscles, assists in stabilizing the pelvis.