The semitendinosus is a long, slender muscle located in the back of the thigh, forming one of the three muscles collectively known as the hamstrings. This muscle group is situated along the posterior compartment of the leg, playing a significant part in the mechanics of walking, running, and overall lower body posture. The semitendinosus contributes to the dynamic movements of both the hip and the knee joints.
Anatomical Location and Structure
The semitendinosus muscle is found along the posterior and medial side of the thigh, positioned between the other two hamstring muscles. It originates from the ischial tuberosity, the bony prominence commonly referred to as the “sitting bone” located at the base of the pelvis. From this high attachment point, the muscle fibers run vertically down the back of the leg.
A distinguishing feature is its exceptionally long, cord-like tendon, which begins approximately halfway down the thigh. This structure gives the muscle its name, as “semi” refers to half and “tendinosus” refers to its tendon. The muscle belly is relatively superficial and lies on top of the semimembranosus, the deepest and most medial hamstring muscle.
The long tendon curves around the inside of the knee joint and inserts onto the medial surface of the superior tibia, the larger bone of the lower leg. This insertion point is shared with the tendons of the gracilis and sartorius muscles, forming a common structure known as the pes anserinus, or “goose’s foot.” Its location on the inside of the thigh distinguishes it from the biceps femoris, which is situated on the lateral side of the posterior thigh.
Primary Role in Movement
The semitendinosus is a biarticular muscle, meaning it crosses and acts upon two major joints: the hip and the knee. Its primary functions are knee flexion (bending the knee) and hip extension (drawing the leg backward). These actions are fundamental to almost every lower-body movement, from standing up to sprinting.
Hip extension is used during the propulsion phase of walking or running, when the leg moves behind the body to push off the ground. The muscle contracts to help extend the thigh at the hip joint, working in conjunction with the gluteal muscles.
Knee flexion is employed during the recovery phase of gait, as the foot is lifted off the ground and brought forward for the next step. When the knee is bent, the semitendinosus assists with internal rotation of the tibia, helping to turn the lower leg inward.
This rotational function contributes to the dynamic stability and control of the knee joint during complex movements. The muscle also plays a role in stabilizing the pelvis and preventing the trunk from flexing too far forward at the hip.
Common Injuries and Recovery
The semitendinosus, like the other hamstrings, is highly susceptible to strain injuries, often referred to as a “pulled hamstring.” These injuries most frequently occur during explosive activities, such as high-speed running or sudden acceleration, which place an intense eccentric load on the muscle. The injury involves a tear in the muscle fibers that can range in severity.
A Grade I strain is a mild overstretch causing minor pain and discomfort but little loss of strength. A Grade II strain involves a partial tear of the muscle fibers, resulting in noticeable pain, swelling, and some loss of function.
The most severe, a Grade III tear, is a complete rupture of the muscle or its tendon. This often causes intense pain and an inability to bear weight, sometimes requiring surgical intervention.
Initial management for mild to moderate strains involves the RICE principle: Rest, Ice, Compression, and Elevation. Resting the muscle immediately helps prevent further damage and allows healing to begin. Applying ice helps limit swelling and reduce pain in the first 48 hours. For severe pain or an inability to walk, seeking professional medical advice is necessary to accurately diagnose the injury and determine the appropriate rehabilitation plan.