Biceps Femoris
| Pronuncation | BYE-seps FEM-or-iss |
| Derivation | The two- (bi) headed (ceps) muscle of the femur. |
| Origin | The long head arises from the ischial tuberosity. The short head arises from a line on the back of the femur lying completely under the long head. |
| Insertion | Head of the fibula. |
| Action | Pulls the thigh backwards and bends the leg at the knee. |
The belly of biceps femoris lies next to semitendinosus on the back of the thigh before inserting to the lateral side of the leg on the fibula. The short head lies underneath the long head but continues lower, making its mass visible under the tendon.
This tendon is prominent on the bent leg and forms the lateral wall on the outside of the hollow of the knee. It points to the lateral border of biceps femoris, not its central axis. When flexed strongly, the belly of the long head can be seen to fall next to it.