Introduction

The semitendinosus muscle is a member of the posterior component of the thigh which also includes the biceps femoris and the semimembranosus muscles. These three muscles together are collectively referred to as the hamstring muscle complex which serves as hip extensors and knee flexors that are integral to gait and running.[1][2] Injuries to the hamstring muscle complex are commonly seen in athletes and are usually brought on through explosive or high speed running. 

Structure and Function

The semitendinosus, just medial to the biceps femoris, arises from three locations. The medial border of the ischial tuberosity, the medial border of the proximal tendon of the long head of the biceps femoris, and the proximal aponeurosis.[3] Distal from the ischial tuberosity, the semitendinosus becomes fusiform until it reaches below the middle of the thigh. Here it transitions into a long tendon that also serves as the medial border of the popliteal fossa.[4] The tendon then curves over the pes anserine bursa, which is superficial to the medial collateral ligament and inserts into the superior anteromedial surface of the tibia. The sartorius muscle and the gracilis muscle also terminate at this common insertion point to make the pes anserinus. The attachment of the semitendinosus is most posterior behind the tendinous insertions of the sartorius and gracilis muscles.[5][6][7]

The semitendinosus muscle collectively with the other two muscles of the posterior compartment of the thigh work to extend at the hip and flex at the knee. The semitendinosus muscle, in particular, has the added functionality of assisting the popliteus muscle in rotating the leg internally. 

Embryology

The gastrulation phase of the embryo gives rise to three germ layers: the ectoderm, endoderm, and mesoderm. The paraxial mesoderm specifically develops into striated skeletal muscle and the skeleton except for the skull.[8] Therefore, the bony insertions of the semitendinosus muscle and the muscle itself derive from the mesoderm. As a whole, the lower limb begins to form towards the end of the fourth week of the embryonic period. The limb continuously develops and becomes well-differentiated towards the eighth week as the embryo transitions to the fetal period.

Blood Supply and Lymphatics

The semitendinosus muscle largely receives vascular supply from the profunda femoris artery, also known as the deep artery of the thigh, is the largest branch of the femoral artery. Specifically, the perforating branches of the profunda femoris artery are responsible for the vascular supply of the muscle.[9][10] Similarly, vascular drainage of the semitendinosus muscle is by the perforating veins of the profunda femoris vein which drains into the femoral vein.[11]

Nerves

The tibial nerve, an offshoot of the sciatic nerve, innervates the semitendinosus muscle. Proximally, the sacral plexus branches into the sciatic nerve which involve nerve roots from L4 to S2. The sciatic nerve then travels through the posterior compartment which branches into the common fibular nerve and the tibial nerve. The tibial nerve, either prior to or after its separation from the common fibular nerve, innervates the semitendinosus and all other muscles of the posterior compartment of the thigh except for the short head of the biceps femoris.[12]

Muscles

The semitendinosus muscle accompanies the biceps femoris muscle and the semimembranosus muscle in the posterior compartment of the thigh. These three muscles compose the hamstring muscle complex. The semimembranosus muscle lies deeper to the semitendinosus muscle and is the most medial muscle of the posterior compartment of the thigh. The semimembranosus muscle originates from the superolateral aspect of the ischial tuberosity and inserts into the medial tibial condyle, posterior oblique ligament, and the posterior joint capsule and arcuate ligament.  The biceps femoris muscle is the most lateral muscle in the posterior compartment with the long head originating from the ischial tuberosity and the short head rising from the lateral lip of the linea aspera of the femur. The biceps femoris then inserts into the head of the fibula and lateral condyle of the tibia.

Physiologic Variants

Case studies have reported anatomical variations of the semitendinosus muscle. One such case was during an anterior cruciate ligament reconstructive surgery where during graft harvesting, the semitendinosus muscle had given off an additional tendinous attachment. Along with the original insertion of the semitendinosus muscle, the muscle gave off an extra tendinous slip that was attached the gracilis tendon at its insertion point.[13] The gracilis also followed suit with an additional tendinous insertion into the semitendinosus tendon. Therefore, there was a “double” pes anserinus. There have also been reports of the semitendinosus tendon inserting into the crural fascia of the leg rather than the superior anteromedial tibia.[14]

Surgical Considerations

The semitendinosus and gracilis muscle tendons are commonly used as an alternative to the bone-patellar-tendon-bone graft (BPTB) for ACL reconstruction. When grafting for ACL reconstruction, the sartorius fascia is incised to expose the underlying semitendinosus and gracilis tendon. These two tendons are not adherent to the bone outside of the attachment sites which allows differentiation of the tendons from the medial collateral ligament which lies deep to the semitendinosus and gracilis muscles. As compared to the BPTB graft, the diameter of the hamstring graft is predetermined by the original diameter of the semitendinosus and gracilis tendons. In the BPTB graft, the diameter of the graft can be determined by the surgeon during the harvesting of the graft. Unlike the hamstring autograft, the BPTB graft includes bone which is believed to improve graft incorporation.[15][16]

The BPTB graft is the most common autograft used in the United States and has originally been considered the gold-standard graft for ACL reconstruction. However, there is a debate about which autograft is superior. Hamstring autografts have benefits of decreased anterior knee pain, osteoarthritis, and donor site morbidity. The benefits of the BPTB autograft include faster graft incorporation, decreased risk of rupture, and a larger proportion of patients returning to preinjury activity levels. Despite BTBP having a lower failure rate, both graft choices have low failure rates. Therefore, utilizing the semitendinosus and gracilis tendons as an autograft remain a reasonable option for reconstruction and its use should be evaluated on a case by case basis.[17] MRI studies have also shown that the semitendinosus tendon has also been found to regenerate in 75% of patients after harvesting with the remaining 25% of patients having an increase in the size of the semimembranosus muscle as compensation. Outside the common use of the semitendinosus and gracilis tendon grafts in ACL reconstruction, these tendons have also found use in repair of massive rotator cuff tears that have already undergone retraction, degeneration, and muscle atrophy.[18]

Clinical Significance

Hamstring injuries are among the commonest muscle injuries seen in athletes and correlate with significant time away from the sport. Injuries typically occur during rapid acceleration or high-speed running. The biceps femoris generally is the most commonly injured muscle of the posterior compartment; however, the semitendinosus muscle has also been implicated in hamstring strains.[19]