The hamstring muscle group is composed of three distinct muscles: the Biceps Femoris, the Semitendinosus, and the Semimembranosus. These structures work together to facilitate movement at both the hip and knee joints. The specific relationship between the Biceps Femoris and the Semitendinosus is defined by both shared proximal anatomy and distinct distal attachments. Understanding their differences clarifies how these muscles contribute to complex lower limb mechanics.
General Location Within the Hamstring Group
The three hamstring muscles are located directly beneath the gluteal fold. The Biceps Femoris is situated on the lateral, or outer, side of the posterior thigh.
The Semitendinosus is positioned on the medial, or inner, side of the posterior thigh. Distally, these muscles define the superior borders of the popliteal fossa behind the knee joint. The Biceps Femoris forms the superior-lateral border, while the Semitendinosus and Semimembranosus form the superior-medial border.
Distinct Structural Attachments
The relationship between the Biceps Femoris and the Semitendinosus begins with a shared proximal anatomy at the pelvis. The long head of the Biceps Femoris and the Semitendinosus both originate from a common tendon on the ischial tuberosity, or “sit bone.” This conjoint tendon secures both muscles high on the posterior hip, allowing them to act on the hip joint.
The Biceps Femoris, which includes a long head and a short head, inserts primarily onto the head of the fibula, the smaller, lateral bone of the lower leg. The short head of the Biceps Femoris is unique among the hamstrings because it originates lower on the femur and only crosses the knee joint.
The Semitendinosus inserts onto the medial surface of the tibia, the large shin bone. Its tendon joins with the tendons of the Sartorius and Gracilis muscles to form the pes anserinus structure. The contrast between the lateral insertion of the Biceps Femoris and the medial insertion of the Semitendinosus is a primary anatomical distinction.
Functional Roles in Movement
The shared origin at the pelvis means both muscles perform the actions of knee flexion and hip extension. These actions are integral to walking, running, and standing up from a seated position.
The differing insertion points, however, give each muscle a distinct role in rotation of the lower leg. The Biceps Femoris, inserting on the lateral side, is responsible for external, or lateral, rotation of the tibia when the knee is bent.
The Semitendinosus, inserting on the medial side of the tibia, provides the opposing rotational force. It is the primary muscle responsible for internal, or medial, rotation of the tibia when the knee is flexed. This contrast between lateral rotation (Biceps Femoris) and medial rotation (Semitendinosus) is a primary functional distinction in their otherwise synergistic relationship.
Significance for Injury and Rehabilitation
The functional and structural relationship between these two muscles has direct implications for common muscle strains. Hamstring strains, which are common in sports, typically occur during the eccentric phase of muscle contraction, where the muscle is actively lengthening under tension, such as during high-speed running.
The long head of the Biceps Femoris is the most frequently injured hamstring muscle, often at the junction where the muscle transitions into the tendon. This vulnerability is thought to be due to the greater stretch and force experienced by the Biceps Femoris during sprinting motions. The Semitendinosus is the second most commonly injured muscle in the group.
Rehabilitation protocols must address the specific functional deficits created by an injury to one of these muscles. Recovery often involves eccentric strengthening exercises, such as the Nordic hamstring exercise, to increase muscle strength and lengthen the muscle fibers. Since the Biceps Femoris and Semitendinosus control opposing rotational forces, rehabilitation must target both shared strength and the unique rotational function of each muscle to ensure proper balance and minimize the risk of re-injury.