Biceps femoris

What it is

The lateral of the three hamstrings, forming the outer border of the back of the thigh. Like the biceps of the arm it has two heads, and here they are genuinely different animals.

The long head arises from the ischial tuberosity — the sitting bone — so it crosses both the hip and the knee. The short head arises from the shaft of the femur, so it crosses the knee only. They join and insert together on the head of the fibula, the small bony point on the outside of the knee.

The two heads even have different nerve supplies, which is unusual and is the strongest argument that they are functionally distinct.

What it does

Bends the knee, and — via the long head only — extends the hip.

With the knee bent, it also rotates the lower leg outward.

The long head is a two-joint muscle and behaves accordingly: it can be lengthened at the hip and shortened at the knee simultaneously, which happens on every stride you run. That combination, at speed, is where it is most vulnerable.

What loads it

Hinging at the hip with a relatively straight knee loads the long head. Bending the knee against resistance loads both heads. Running loads it heavily, particularly at high speed during the swing phase, when the muscle is lengthening while under tension.

The hip hinge and the single-leg Romanian deadlift load the long head through hip extension; the glute bridge does the same lying down. The collection has no knee-flexion movement yet, so the short head is not yet represented.

What is commonly got wrong about it

"Pulled hamstring" as one injury. Hamstring strains are among the most common injuries in running and field sport, but they are not one thing — the site, the head involved and the mechanism differ, and so do the recovery timelines. Diagnosis and rehabilitation belong with a physio, and this site does not do rehabilitation.

That tight hamstrings need stretching. A feeling of tightness at the back of the thigh has several possible sources, including sensitivity of the nervous system rather than short muscle, and including problems referred from the lower back. The reflex to stretch is not always right and occasionally makes things worse.

That the hamstrings only bend the knee. Two of the three, including this one's long head, are strong hip extensors, and hip extension is arguably their more important job.

Confusing it with the biceps of the arm. Biceps means two-headed; several muscles are.