The biceps femoris
is a muscle of the posterior (the back) thigh
. As its name implies, it has two parts, one of which (the long head) forms part of the hamstrings
Origin and insertion
It has two heads of origin;
The fibers of the long head form a fusiform belly, which passes obliquely downward and lateralward across the sciatic nerve to end in an aponeurosis which covers the posterior surface of the muscle, and receives the fibers of the short head; this aponeurosis becomes gradually contracted into a tendon, which is inserted into the lateral side of the head of the fibula, and by a small slip into the lateral condyle of the tibia.
At its insertion the tendon divides into two portions, which embrace the fibular collateral ligament of the knee-joint.
From the posterior border of the tendon a thin expansion is given off to the fascia of the leg. The tendon of insertion of this muscle forms the lateral hamstring; the common peroneal nerve descends along its medial border.
Both heads of the Bicep Femoris perform knee flexion.
Since the long head originates in the pelvis it is also involved in hip extension.
The long head of the bicep femoris is a weaker knee flexor when the hip is extended (because of active insufficiency
For the same reason the long head is a weaker hip extender when the knee is flexed.
When the knee is semi flexed, the Biceps femoris in consequence of its oblique direction rotates the leg slightly outward
The short head may be absent; additional heads may arise from the ischial tuberosity
, the linea aspera, the medial supracondylar ridge
of the femur, or from various other parts.
A slip may pass to the gastrocnemius.