The biceps femoris is a major muscle of the posterior thigh and a significant component of the hamstring group. This muscle is fundamental for controlling lower body movement, particularly during walking, running, and jumping. Understanding its structure and function provides insight into its role in lower limb mechanics and why it is frequently involved in athletic injuries. It is unique among the hamstrings because of its two distinct heads and its position on the outer side of the thigh.
Where the Biceps Femoris is Located
The biceps femoris is positioned on the lateral, or outer, aspect of the back of the thigh, extending from the pelvis down to the lower leg bones. It is distinct from the other hamstring muscles—the semitendinosus and semimembranosus—which lie toward the middle of the thigh. The muscle is named for its two points of origin, or “heads.”
The long head originates from the ischial tuberosity, the bony prominence on the lower pelvis often called the “sit bone.” Because the long head starts at the pelvis, it crosses both the hip and knee joints, making it a two-joint muscle. The short head, in contrast, originates lower down on the femur, specifically from the lateral lip of the linea aspera and the lateral supracondylar line.
The two heads merge as they travel downward and laterally toward the knee joint. The combined tendon inserts primarily onto the head of the fibula, the smaller bone of the lower leg. A smaller portion of the tendon also attaches to the lateral condyle of the tibia, the larger lower leg bone. This final tendon forms the prominent cord felt on the outer back side of the knee, often referred to as the lateral hamstring.
The Primary Functions of the Muscle
The dual-headed structure of the biceps femoris allows it to perform actions that cross both the hip and knee joints. The main functions are hip extension and knee flexion. These actions are essential for coordinated movement, particularly during the propulsion and swing phases of walking and running.
The long head is responsible for extending the hip joint, moving the thigh backward, and works with the gluteus maximus to generate powerful push-off movements. Both the long and short heads contribute to knee flexion, which is the action of bending the lower leg backward. The short head can only perform knee flexion because it originates solely on the femur and does not cross the hip joint.
A secondary action occurs when the knee is already bent. In this position, the muscle can cause the lower leg to rotate slightly outward, or laterally. This lateral rotation helps stabilize the knee and contributes to the control of the lower limb. The long head also stabilizes the pelvis during forward leaning movements.
Common Biceps Femoris Injuries
The biceps femoris is the most frequently injured muscle within the hamstring group. Injuries often occur during activities that require explosive speed or sudden deceleration. These injuries are categorized as hamstring strains, involving the tearing or overstretching of muscle fibers. The long head is especially susceptible to strain, particularly near the musculotendinous junction.
A strain usually presents as a sudden, sharp pain felt in the back of the thigh during intense movement like sprinting. The severity of a hamstring strain is classified into three grades. A Grade 1 strain is a mild pull with minimal fiber damage, causing tightness and slight discomfort but little strength loss.
A Grade 2 injury is a partial tear of the muscle fibers, resulting in immediate pain, bruising, and a moderate loss of strength, making it difficult to bear weight. The most severe injury, a Grade 3 tear, is a complete rupture of the muscle or tendon, marked by significant pain, swelling, and an inability to walk without assistance. In some cases, the tendon can completely pull away from the pelvis bone, known as an avulsion injury.
Strengthening and Stretching the Muscle
Targeted exercises are an effective way to strengthen the biceps femoris, improving performance and helping prevent injury. Strengthening the muscle is most effective when exercises focus on eccentric contraction, where the muscle lengthens while under tension. The Nordic hamstring curl is an example that emphasizes this eccentric strength, which is relevant for injury prevention in sports.
Exercises that involve hip extension and knee flexion, such as Romanian deadlifts and lying leg curls, are effective for building strength. When performing these movements, maintain a controlled pace and gradually increase the resistance or intensity. Stretching is equally important for maintaining the muscle’s flexibility and range of motion.
A simple stretching technique is the seated straight-leg stretch, where one sits with the legs extended and gently leans forward until tension is felt in the back of the thigh. Research suggests that to achieve the greatest lengthening of the long head, stretching should be performed with the hip bent and the knee fully straightened. Proper stretching and strengthening, combined with a thorough warm-up before activity, contribute to lower body health.