Summary
Origins
Insertions
- Long head: Merges into the common hamstring tendon on the head of the fibula with slips to the lateral condyle of the tibia
- Short head: Joins the long head in the shared tendon on the head of the fibula and fans into the fascia at the lateral condyle of the tibia
Actions
- Flexes the knee
- Extends the hip (long head)
- Laterally rotates the leg when the knee is flexed
The biceps femoris spans the outer hamstring. It steadies the knee against inward pull from the inner hamstrings. It balances semitendinosus. It also balances semimembranosus.
Long head
The long head begins on the pelvis. It anchors at the ischial tuberosity. It crosses the hip. It fuses with the short head to form a dense lateral cord. Together with gluteus maximus it drives the thigh. The pair pushes the hip into extension. That teamwork helps you rise from a hinge, stride forward, or sprint.
Short head
The short head starts on the back of the femur below the gluteal tuberosity. It does not cross the hip. Instead it reinforces the shared tendon near the knee. It aids the long head in bending the knee. It also turns the lower leg outward. The fibers join at the fibula. They sweep across the outer tibia.
Comparing the heads
The long head arises from the pelvis. The short head springs from the femur. Both insert into the same lateral tendon on the fibula and tibia.