The Anatomy of Biceps Femoris: The Only Hamstring with Two Heads

The biceps femoris is the only hamstring muscle with two separate heads that don’t even share the same nerve supply — a genuine anatomical oddity that explains both its unique rotational function and why it’s the single most frequently strained muscle in sprinters and kickers.

Biceps Femoris Anatomy & Location

The biceps femoris is one of three muscles that make up the hamstring group, alongside:

1. Semitendinosus — a long, slender muscle with an unusually long tendon of insertion, running along the inner side of the thigh
2. Semimembranosus — the broadest of the three hamstrings at its origin, lying partly beneath the semitendinosus+

The biceps femoris runs along the outer (lateral) side of the back of the thigh, making it the easiest of the three hamstrings to distinguish by feel — bend your knee against resistance, and you can trace its tendon down toward the outer side of the knee. According to StatPearls’ anatomical reference on the hamstring muscle group, the biceps femoris forms the upper-lateral border of the popliteal fossa, the diamond-shaped space behind the knee. To see exactly how it fits within the posterior thigh musculature, our interactive muscle anatomy tool lets you explore it alongside the other two hamstrings. If terms like origin, insertion, and innervation aren’t yet fully familiar, our muscle anatomy basics guide covers what they mean and why they matter for training.

Origin, Insertion, Innervation & Blood Supply

Origin (long head)	Ischial tuberosity, sharing a common tendon with the semitendinosus and the sacrotuberous ligament
Origin (short head)	Lateral lip of the linea aspera and the lateral supracondylar line of the femur
Insertion	Head of the fibula, with a smaller attachment onto the lateral condyle of the tibia
Innervation	Long head: tibial division of the sciatic nerve. Short head: common fibular (peroneal) division of the sciatic nerve
Blood supply	Perforating branches of the profunda femoris (deep femoral) artery

Biceps Femoris Structure & Muscle Architecture

The dual innervation of the biceps femoris isn’t just a trivia point — it reflects a genuine developmental difference between the two heads. The short head actually develops within a different embryonic compartment than the long head, which is why anatomists sometimes describe the biceps femoris as a “composite muscle”: two separate developmental origins that fuse into one functional unit sharing a common distal tendon. This is part of why the short head can occasionally be affected independently of the long head in strain injuries.

The long head’s origin overlaps directly with the semitendinosus at the ischial tuberosity, with both tendons sharing a connection to the sacrotuberous ligament before running down the thigh together for a distance and eventually separating.

Functions of the Biceps Femoris

Like the other hamstrings, the biceps femoris flexes the knee, and its long head also extends the hip. But it has one function the other two hamstrings don’t share: because of its lateral position and fiber orientation, it’s the primary external rotator of the leg at the knee when the knee is bent — the semitendinosus and semimembranosus instead perform internal rotation in that same position. This rotational tug-of-war between the lateral and medial hamstrings helps keep the knee stable during rotation in activities like cutting and changing direction.

Biceps Femoris Synergists & Antagonists

Synergists (muscles that work alongside the biceps femoris to help produce the same movement):

For knee flexion: semitendinosus, semimembranosus, and the gastrocnemius (secondary role). For hip extension: gluteus maximus, the primary mover in hip-dominant lifts.

Antagonists (muscles that oppose the biceps femoris’s action, working in the opposite direction):

The quadriceps femoris directly opposes knee flexion by performing knee extension. At the knee’s rotational axis specifically, the semitendinosus and semimembranosus act as functional antagonists to the biceps femoris, producing internal rather than external rotation.

Common Biceps Femoris Exercises

Exercise	Role of the Biceps Femoris
Romanian deadlift	Prime mover, hip-dominant bias via the long head’s hip-extension role
Nordic hamstring curl	Prime mover — eccentric loading at long muscle length, where biceps femoris strains typically occur
Lying leg curl	Prime mover — research comparing hamstring exercises has found prone-lying leg curls tend to bias the biceps femoris more than the medial hamstrings
Glute-ham raise	Major contributor, combining hip and knee-dominant demand
Single-leg Romanian deadlift	Strong contributor, with added demand on rotational stability since the biceps femoris also controls external rotation unilaterally

For a complete breakdown of hamstring training options, see our full guide: Best Hamstring Exercises.

Stretching & Mobility

For hamstring stretching progressions that address the biceps femoris alongside the rest of the group, see our guide: Top 10 Hamstring Stretching Exercises. For general tightness, foam rolling can also help — see our guide: Foam Roller Hamstring Roll.

Bodybuilder’s Take

“Why is the biceps femoris always the hamstring that gets strained in sprinters?” Its dual-joint role, combined with its unique fiber architecture and the rapid stretch of its short head during high-speed running, makes it disproportionately vulnerable to strain compared with the semitendinosus and semimembranosus. It’s consistently identified as the most frequently injured hamstring in sprinting and kicking sports.

“Does it matter that the two heads have different nerves?” For general training purposes, not much — both heads are recruited together during almost every hamstring exercise. It matters more clinically, since an injury or nerve issue can occasionally affect one head without the other, which is useful diagnostic information even if it doesn’t change how you train.

“My hamstrings cramp specifically during leg curls — is that a biceps femoris issue?” Not necessarily specific to this muscle — cramping during isolated knee flexion is common and usually related to training in a shortened position, hydration, or unfamiliar volume rather than a problem with any one hamstring in particular. We cover this in detail in the article on why hamstrings cramp during leg curls.

Common Biceps Femoris Conditions

This section is educational and should not replace professional medical advice.

Biceps femoris strain — the most frequently injured of the three hamstring muscles, most often affecting the long head near the myotendinous junction during high-speed running or kicking.

Post-surgical relevance in ACL reconstruction — the semitendinosus is one of the most commonly harvested tendons for ACL graft surgery. Interestingly, research using functional MRI has found that the biceps femoris shows measurably increased activation after this type of graft harvest, appearing to partially compensate for the reduced size and function of the semitendinosus — a good example of how interconnected the three hamstring muscles are, even though they can be trained somewhat independently.

Persistent posterior thigh pain, a sudden sharp pain during a sprint, or bruising along the outer back of the thigh should be evaluated by a healthcare professional rather than trained through.

Common Biceps Femoris Myths

Myth: All hamstring exercises train the biceps femoris the same way as the rest of the hamstrings. Research shows real differences between exercises — lying leg curls tend to bias the biceps femoris more, while Nordic curls tend to bias the semitendinosus more, meaning a complete hamstring program benefits from including more than one movement pattern.

Myth: The “hamstrings” are basically one muscle that works as a unit. The biceps femoris has a distinct nerve supply for each of its two heads, a different rotational function from the other two hamstrings, and a different injury profile — treating all three hamstring muscles as interchangeable overlooks real anatomical and functional differences.