Nerve supply of the short head of biceps femoris
The short head of biceps femoris, a muscle at the back of the thigh, takes the common fibular branch of the sciatic nerve; the other hamstrings take the tibial branch.
Remember
Biceps femoris has a split supply: its short head takes the common fibular branch of the sciatic nerve, while its long head, like the other hamstrings, takes the tibial branch.
The sciatic nerve (the thick nerve running down the back of the thigh) is really two nerves bundled in one sheath, the tibial nerve and the common fibular nerve. The sciatic nerve supplies all the hamstrings, the muscles at the back of the thigh that extend the hip and flex the knee, but it does so through its two parts.
Biceps femoris is one of the three hamstrings, and it has two heads. The long head arises from the ischial tuberosity (the bony prominence of the pelvis that you sit on) and takes the tibial nerve, as do semitendinosus and semimembranosus, the other hamstrings. The short head arises from the linea aspera (the roughened ridge on the back of the femur) and takes the common fibular nerve. That makes the short head the odd one out among the hamstrings.
The same common fibular nerve goes on to wrap around the neck of the fibula. Damage to it can cause foot drop, where the foot cannot be lifted at the ankle, and numbness over the dorsum (the top) of the foot. A deep injury to the common fibular share of the sciatic nerve in the thigh leaves the short head without its supply, while the hamstrings that take the tibial nerve still work. Knee flexion is therefore weakened rather than lost. An injury lower down, at the neck of the fibula below the knee, falls beyond the branch to the short head.
Key facts
- Short head
- arises from the linea aspera and takes the common fibular branch of the sciatic nerve.
- Long head
- arises from the ischial tuberosity and takes the tibial branch.
- Other hamstrings
- semitendinosus and semimembranosus take the tibial branch.
- Common fibular damage
- can cause foot drop and numbness over the dorsum of the foot.
- Thigh injury to the common fibular share
- knee flexion is weakened, not lost, because the tibial-supplied hamstrings still work.
- Injury at the neck of the fibula
- falls beyond the thigh branch to the short head, so the short head is spared.
Exam traps
- The tibial branch does not supply the short head of biceps femoris, because the short head comes from the other half of the sciatic nerve, the common fibular branch, while the tibial branch supplies the long head and the other hamstrings.
- Giving the short head of biceps femoris the tibial branch swaps the nerve supplies of the two heads: the tibial branch supplies the long head, whereas the short head depends on the common fibular branch.
- Semitendinosus does not take the common fibular branch, because it is supplied by the tibial branch, so it is spared by an injury to the common fibular share of the sciatic nerve.
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