The semitendinosus is one of the three hamstring muscles located at the back of the thigh. It is a long, slender muscle situated between the other two hamstring muscles: the biceps femoris on the lateral side and the semimembranosus on the medial side.
Structure
- Origin: The semitendinosus originates from the ischial tuberosity, which is the lower part of the pelvis, specifically from a shared point of origin known as the conjoined tendon, which it shares with the long head of the biceps femoris.
- Course: From its origin, the semitendinosus muscle extends downward along the length of the thigh. Its fibers run vertically and are somewhat twisted. The muscle transitions into a long, round tendon approximately at the middle of the thigh, hence the name ‘semitendinosus’ (semi meaning ‘half’ and tendinosus indicating a tendon-like structure).
- Insertion: This tendon then continues to travel medially and inferiorly to insert into the medial aspect of the superior part of the tibia, specifically at the pes anserinus, which is an area where the tendons of three muscles (sartorius, gracilis, and semitendinosus) converge.
Location
The semitendinosus is located at the back of the thigh. It lies medial to the biceps femoris and lateral to the semimembranosus. It is one of the prominent structures of the posterior thigh compartment and is easily palpable when the knee is flexed and the thigh is extended.
Function
The semitendinosus muscle, as part of the hamstring group, has several key functions related to movement of the hip and knee:
- Knee Flexion: It aids in bending the knee. When the knee flexes, the semitendinosus muscle contracts, pulling the lower leg towards the back of the thigh.
- Hip Extension: It assists in extending the hip, which involves moving the thigh backward from the pelvis. This movement is crucial for walking, running, and jumping.
- Medial Rotation of the Knee: When the knee is flexed, the semitendinosus can medially rotate the lower leg, turning it inward. This action is essential for activities that require turning or twisting movements of the leg.
- Thigh Extension: By acting on the hip joint, the semitendinosus also helps in extending the thigh, contributing to the stabilization of the pelvis, especially during walking or standing from a seated position.
Clinical significance
Hamstring Injuries: The semitendinosus, along with the other hamstring muscles, is commonly involved in hamstring strains or tears, especially among athletes. Such injuries typically occur during activities that involve sudden starts, stops, or changes in direction. Treatment often requires rest, ice, compression, elevation (RICE), physical therapy, and, in severe cases, surgery.
Anterior Cruciate Ligament (ACL) Reconstruction: The semitendinosus tendon is frequently used as a graft source in ACL reconstruction surgery. The tendon’s strength and suitable length make it an excellent choice for replacing the damaged ACL in the knee. Post-surgery, patients often undergo rehabilitation to restore strength and flexibility.
Tendon Harvesting and Grafting: Beyond ACL reconstruction, the semitendinosus tendon is also utilized in other surgical repairs, such as in reconstructive surgeries of the knee, shoulder, and other joints. Its use necessitates subsequent rehabilitation to compensate for the loss of the original tendon.
Muscle Tightness and Imbalances: Tightness in the semitendinosus and other hamstring muscles can lead to postural issues and lower back pain. Clinically, addressing such tightness through stretching, strengthening, and therapeutic exercises is crucial to restore proper function and alleviate discomfort.
Trigger Points and Myofascial Pain: The semitendinosus can develop trigger points, leading to myofascial pain syndrome in the thigh and knee. Treatment may include manual therapy, dry needling, or myofascial release techniques.
Biomechanical Implications: Imbalances or weaknesses in the semitendinosus and the hamstring group can affect gait and biomechanics, leading to various musculoskeletal problems. Clinically, this underscores the importance of balanced muscle strength and flexibility in injury prevention and rehabilitation.
