The semitendinosus muscle sits on the back of your thigh and forms part of the hamstring group. It gets its name from its long tendon, which begins about halfway down the muscle. This article explains the anatomy, function, and clinical relevance of the semitendinosus muscle in clear, practical terms.
The semitendinosus is one of the three muscles that make up the hamstring group in the posterior compartment of the thigh. The other two are the semimembranosus and the biceps femoris. Together, these muscles power hip extension and knee flexion during walking, running, climbing, and rising from a seated position.
Unlike the semimembranosus, which has a broad flat tendon, the semitendinosus has a strikingly long, cord-like tendon that starts early and continues to the knee. This anatomical feature makes it a popular graft source in reconstructive knee surgery.
The semitendinosus muscle has clear, well-documented attachments and a reliable nerve supply. Knowing these details helps clinicians diagnose injuries and plan surgical procedures.
The semitendinosus originates from the ischial tuberosity, the bony prominence you feel when you sit on a hard chair. It shares a common tendon of origin with the long head of the biceps femoris.
It inserts at the pes anserinus, a shared attachment point on the upper medial surface of the tibia. The pes anserinus is composed of the semitendinosus, gracilis, and sartorius tendons. This location places the muscle in a good position to medially rotate the tibia when the knee is bent.
The tibial division of the sciatic nerve supplies the semitendinosus muscle, usually through branches from the L5, S1, and S2 nerve roots. Blood reaches the muscle through perforating branches of the profunda femoris artery and contributions from the medial circumflex femoral artery.
The table below compares the three hamstring muscles for quick reference.
| Muscle | Origin | Insertion | Main Actions |
|---|---|---|---|
| Semitendinosus | Ischial tuberosity | Pes anserinus on the medial tibia | Hip extension, knee flexion, medial rotation of the tibia |
| Semimembranosus | Ischial tuberosity | Posterior surface of the medial tibial condyle | Hip extension, knee flexion, medial rotation of the tibia |
| Biceps femoris long head | Ischial tuberosity | Fibular head | Hip extension, knee flexion, lateral rotation of the tibia |
The semitendinosus muscle contributes to movement at both the hip and the knee. It also helps control the pelvis during standing and walking.
When the thigh is moving backward, such as when you push off to walk or run, the semitendinosus contracts to extend the hip. This action is especially powerful during sprinting and stair climbing.
At the knee, the semitendinosus bends the knee by pulling the lower leg toward the thigh. It works together with the other hamstring muscles to produce strong knee flexion during running and jumping.
When the knee is bent, the semitendinosus rotates the tibia inward. This motion is important for balancing the rotational forces placed on the knee during cutting and pivoting sports.
The semitendinosus is a two-joint muscle, which means it acts on both the hip and the knee. This dual role makes it highly active during compound movements but also prone to strain when stretched too quickly.
Finding the semitendinosus muscle is easier than many people think. With the knee slightly bent, you can feel its tendon on the inner and back side of the knee, just above the joint line.
To test the semitendinosus in a clinical setting, the patient lies face down and bends the knee against resistance while the foot is turned slightly inward. Resistance applied to the ankle should produce a visible contraction of the muscle belly in the posterior thigh.
Hamstring strains are among the most frequent sports injuries, and the semitendinosus muscle is often involved. These injuries typically happen during explosive movements like sprinting, kicking, or changing direction.
A mild strain causes pain and tightness but little loss of strength. A more severe strain can tear the muscle fibers and produce bruising, swelling, and a noticeable gap in the muscle contour. In very rare cases, the tendon can avulse from the ischial tuberosity with a piece of bone attached.
Because it crosses both the hip and knee, the semitendinosus is at risk when the hip is flexed and the knee is extended at the same time. That position places a rapid stretch on the muscle while it is still trying to generate force.
A semitendinosus strain is not the same as a pulled hamstring. It is a specific injury to one of the three hamstring muscles, and it needs specific treatment and rehabilitation.
The semitendinosus tendon is a common choice for anterior cruciate ligament reconstruction. Surgeons harvest the tendon through a small incision on the inner side of the knee, leaving the muscle belly itself intact. The tendon is then folded to create a strong graft that replaces the torn ACL.
One advantage of using the semitendinosus tendon is that it can be harvested with minimal damage to the surrounding tissue. Another advantage is that the remaining hamstring muscles, especially the semimembranosus, can compensate for some of the lost knee flexion strength.
Keeping the semitendinosus muscle strong and flexible is important for anyone who runs, plays sports, or simply wants healthy knees and hips. A balanced training program should include both strengthening and stretching exercises.
Nordic hamstring curls are one of the best exercises for the semitendinosus because they train eccentric strength, which protects the muscle from future injuries. Hip thrusts and Romanian deadlifts also target the hamstring group effectively.
The semitendinosus muscle is a major player in hip extension, knee flexion, and rotational control of the lower leg. Its long tendon makes it anatomically distinct and surgically valuable.
The semitendinosus muscle may not be the largest muscle in the body, but it plays an outsized role in everyday movement and sports performance. Understanding its anatomy helps you appreciate why it gets injured, how it is tested, and why surgeons value it for knee reconstruction. Whether you are a clinician, a patient, or someone who simply wants to prevent hamstring problems, knowing how this muscle works is a solid foundation for better decisions.
The semitendinosus muscle is one of the three muscles in the hamstring group. It is located in the back of the thigh and helps extend the hip and flex the knee.
It originates from the ischial tuberosity, which is the bony part of the pelvis you sit on. It inserts at the pes anserinus on the upper medial surface of the tibia.
The tibial part of the sciatic nerve innervates the semitendinosus muscle. This typically involves nerve fibers from the L5, S1, and S2 spinal nerve roots.
The semitendinosus extends the hip, flexes the knee, and medially rotates the tibia when the knee is bent. It also helps stabilize the pelvis during walking.
Surgeons use the semitendinosus tendon because it is long, strong, and can be harvested with relatively little damage. It is often combined with the gracilis tendon to create a durable ACL graft.
You may feel sudden pain in the back of the thigh, especially during sprinting or kicking. Swelling, bruising, and weakness during knee flexion are also common signs.
Pain is usually a sharp pull or deep ache in the inner and back portion of the thigh. It may worsen when you bend your knee against resistance or sit for long periods.
Mild strains may improve within one to two weeks with proper rest and rehabilitation. More severe tears can take several months, especially if surgery is needed.
Exercises like Nordic hamstring curls, Romanian deadlifts, and single-leg hip bridges are excellent choices. Focus on controlled movements and gradual increases in difficulty.
The semitendinosus has a long tendon and inserts at the pes anserinus, while the semimembranosus has a broader flat tendon and inserts higher on the back of the tibia. Both perform similar actions but have different anatomical shapes and surgical uses.
