Semitendinosus Anatomy

Semitendinosus is the superficial and posterior of the two medial hamstrings, and its name describes its architecture. The belly occupies the proximal half of the thigh and gives way at about the mid-thigh to a long, round, cord-like tendon that runs almost half the length of the thigh. A long, uniform, tubular free tendon with a single distal attachment is exactly what makes it the most useful autograft in the knee.

The raphe. An oblique tendinous raphe across the mid-belly divides the muscle into a proximal and a distal portion, and each portion has its own motor branch from the tibial division of the sciatic nerve. That is why a partial denervation can weaken one half of the muscle without abolishing its function, and it is a favourite question in a nerve-injury viva.

The pes anserinus. Semitendinosus ends with sartorius and gracilis in the pes anserinus, and the order is the same counted anterior to posterior or superficial to deep: sartorius, gracilis, semitendinosus (Say Grace before Tea). The three also look different. Sartorius, the most anterior and superficial, is a broad fascial sheet rather than a cord; gracilis, in the middle, is a thin, flat, ribbon-like tendon; semitendinosus, the most posterior and deepest, is a thick, round cord.

Three tendons, three nerves. They share one insertion but are supplied by three different nerves from two different plexus divisions: sartorius by the femoral nerve (L2-L4), gracilis by the obturator nerve (L2-L4) and semitendinosus by the tibial division of the sciatic nerve (L5-S2). This is the classic pes anserinus question.

AttachmentsInnervationBlood SupplyRelations

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Origin. Semitendinosus arises from the posteromedial aspect of the ischial tuberosity through the conjoint tendon it shares with the long head of biceps femoris. The semimembranosus footprint lies anterolateral to it, contiguous with it but distinct.

Why the shared tendon matters. Semitendinosus and biceps long head avulse together because they share a tendon. A "two-tendon avulsion" therefore usually means the conjoint tendon plus a partial semimembranosus, and a "three-tendon avulsion" means the conjoint tendon has separated from semimembranosus as well.

The raphe in the belly. It crosses the muscle at about the level of the mid-thigh, running from proximal-medial to distal-lateral. At open dissection or on an MRI cross-section it is the reliable feature that distinguishes semitendinosus from gracilis and semimembranosus.

Insertion. The tendon becomes free at roughly the junction of the middle and distal thirds of the thigh, runs distally and anteriorly around the medial femoral condyle, and inserts into the superomedial surface of the tibia as the deepest layer of the pes anserinus, the conjoined insertion resembling a goose's foot. The footprint lies on the anteromedial tibia roughly 2-3 cm distal to the medial joint line, just medial to the tibial tuberosity and beneath the sartorius fascia.

The layers at the pes. From superficial to deep: sartorius fascia, gracilis, semitendinosus, the pes anserine bursa, the superficial MCL, then bone. The bursa interposed between the tendons and the ligament is the anatomical basis of pes anserine bursitis.

The harvestable tendon. A standard harvest typically gives 24-30 cm of usable tendon, longer than the gracilis at about 20-26 cm. Distally the tendon sends an expansion into the crural fascia; more proximally, and critically, one or more fascial bands pass to the medial head of gastrocnemius.

Two motor branches. Semitendinosus is unusual in receiving two separate branches from the tibial division, one entering proximal to the tendinous raphe and one distal to it. Both arise from the medial side of the tibial division and enter the anterior (deep) surface of the muscle in the proximal and middle thirds of the thigh.

Localising a sciatic lesion. Every hamstring except the short head of biceps femoris takes the tibial division. Preserved short-head function with denervated semitendinosus, semimembranosus and biceps long head places the lesion in the tibial division; foot drop with normal hamstrings points instead to a common peroneal lesion at the fibular neck, not a sciatic lesion.

Harvest does not denervate the muscle. The motor branches enter well proximal to the harvest field. That is why the belly survives, retracts proximally and often regenerates a new distal tendon.

Arterial supply. The conjoint origin is supplied by the inferior gluteal and medial circumflex femoral arteries. The dominant supply is to the belly, from the first and second perforating branches of the profunda femoris, entering its deep surface in the proximal and middle thirds. The distal tendon has small branches from the superior medial genicular and descending genicular arteries, plus the periosteal supply at the pes insertion.

A tendon, not a flap. The vascular pattern is Mathes-Nahai type IV (segmental), which is why semitendinosus is a poor pedicled flap donor. Its value is as a free tendon graft.

After harvest. Harvest divides the distal tendon and its small distal vessels, but the belly keeps its perforator supply and remains viable. The retracted belly re-scars to the posteromedial capsule and adjacent fascia, and a regenerated tendon commonly forms along the scar. Bleeding at harvest usually comes from the saphenous branches and the crural fascial vessels rather than from a named artery.

In the posterior thigh. Superficial to semitendinosus lie the deep fascia of the thigh, gluteus maximus proximally and the posterior femoral cutaneous nerve in the subcutaneous plane. Semimembranosus lies deep to it for most of the thigh. Lateral is the long head of biceps femoris, its partner at the origin, with the sciatic nerve deep to it; medial are gracilis and adductor magnus.

The crossing at the knee. Proximally semitendinosus is superficial to semimembranosus. Distally it swings anteriorly around the medial femoral condyle to reach the pes, so at the tibia it ends up anterior to semimembranosus.

Around the harvest field. Immediately deep and posterior to the field lies the medial head of gastrocnemius, tethered to the tendon by the fascial bands. Anterior to it, the infrapatellar branch of the saphenous nerve runs transversely across the sartorial fascia. The structures at risk are tabulated under Surgical Relevance.

The fascial bands to medial gastrocnemius. This is the single most examinable technical hazard of hamstring harvest. An accessory insertion, a fascial expansion, runs from the deep surface of the tendon posteriorly and proximally toward the medial head of gastrocnemius, arising a few centimetres proximal to the tibial insertion; cadaveric work found it in 77 per cent of knees. The tendon is also ensheathed in a dense investing fascial layer that impedes stripping.

Actions. At the hip, extension, with a minor contribution to adduction through its medial line of pull. At the knee, flexion, which is the dominant action, and internal rotation of the flexed tibia. With gracilis and sartorius it contributes to the medial dynamic sling, which resists tibial external rotation and valgus.

A two-joint muscle. Semitendinosus is biarticular, so it is placed on maximal stretch by hip flexion combined with knee extension. That is the late-swing position in sprinting, and the mechanism of hamstring strain.

Moment arms. The knee flexion moment arm rises from extension to about 30-50 degrees and is broadly maintained thereafter. The tibial internal rotation moment arm rises steeply with flexion, so the rotatory contribution is negligible near extension and substantial at 90 degrees, which is also why the post-harvest deficit is deep-flexion specific.

Synergists and antagonists. It works with semimembranosus, biceps femoris long head, gracilis, sartorius, popliteus and gastrocnemius. It is opposed by the quadriceps at the knee, by iliopsoas and rectus femoris at the hip, and by biceps femoris for tibial rotation.

Why it protects an ACL graft. This is the examiner's favourite paradox: we harvest a structure that protects the ligament we are reconstructing. The medial hamstrings pull the tibia posteriorly on the femur, the same direction as the ACL's restraint of anterior tibial translation, so hamstring contraction unloads the ACL and the ACL graft. That is the basis of hamstring-dominant rehabilitation after reconstruction and of the hamstring-strengthening core of neuromuscular ACL prevention programmes.

Rotation and valgus. Internal rotation of the tibia opposes the anterolateral rotatory subluxation of the pivot shift, and as a pes tendon semitendinosus is a secondary restraint to valgus.

What harvest costs. Harvest removes part of that protection. Immediately afterwards knee flexion strength is reduced, most measurably at deep flexion, while hip extension power is essentially maintained: hip extension weakness is measurable in some studies but rarely functionally important, because semimembranosus and biceps compensate. At 6-12 months overall knee flexion torque is close to the contralateral side at mid-range, but a deficit at deep flexion, greater than 70-90 degrees, persists for years.

When the deficit matters. The counterargument is that the tendon often regenerates and that the functional deficit at everyday flexion angles is small, but regeneration cannot be assumed. The deficit does matter in sports demanding force at deep knee flexion, such as wrestling, sprinting from a deep start position and the deep-flexion positions of some martial arts, and in patients who will need the tendon later for a different reconstruction.

Palpation. With the patient prone and the knee flexed to 90 degrees against resistance, the most prominent, most posterior, round cord at the posteromedial knee is semitendinosus. Immediately anterior and medial to it is the thinner, flatter ribbon of gracilis, and deep and slightly lateral to it lies the broad, blunt semimembranosus.

Marking before harvest. Ask a supine patient to flex the knee against resistance with the hip flexed and externally rotated, the "figure-of-four" position. Gracilis and semitendinosus then stand out as two distinct cords beneath the sartorial fascia and can be marked on the skin. The pes insertion is palpable on the anteromedial tibia, with the pes anserine bursa just deep to it.

Isolating the medial hamstrings. Resisted prone knee flexion with the tibia internally rotated preferentially loads semitendinosus and semimembranosus; with the tibia externally rotated it biases biceps femoris.

Grading. Manual grading uses the standard 0-5 scale, but after harvest the deficit is angle-specific, so a manual test at 45 degrees will miss it. Test at 90 degrees or more, or use isokinetic testing where available. For return to sport, functional grading matters more than manual grading: single-leg bridge repetitions, Nordic hamstring performance and hop-test symmetry.

Absence is not weakness. After harvest the cord is absent on palpation, but the muscle still contracts, and a regenerated tendon may reappear over the following year or two.

Pain and numbness at the pes. Pain over the pes after ACL reconstruction is more often incisional or infrapatellar branch neuropathic pain than true pes bursitis. Numbness in a patch below and lateral to the incision is the infrapatellar branch, not the saphenous trunk.

Premature graft amputation. The classic pitfall: the stripper follows an undivided fascial band to the medial gastrocnemius and transects the tendon early, leaving an unusably short graft. Reported incidence varies widely between series but is consistently higher for surgeons early in their learning curve, and it is more common when the harvest is done through too small an incision or with the knee inadequately flexed. Prevention is the answer the examiner wants: an adequate incision, adequate knee flexion, and steps 6 and 7 of the harvest.

When it happens anyway. Always consent for, and have available, an alternative graft. Then:

• Measure what you have. A short graft can often still be used by increasing the number of strands or with adjustable-loop suspensory fixation at both ends, which needs less graft in the tunnels
• Harvest the gracilis as well if it has not already been taken
• Convert to a quadriceps tendon or bone-patellar tendon-bone autograft, or use an allograft; this is why consent for an ACL reconstruction should always mention alternative graft sources
• Recover the retracted proximal stump only if it is safely accessible, and do not chase it blindly into the posterior thigh

Infrapatellar branch of the saphenous nerve. By far the commonest injury, and a mandatory consent point: numbness over the anterolateral proximal leg is common enough to be expected, not exceptional. The branch crosses the field transversely, from posteromedial to anterolateral, deep to or through the sartorial fascia, so a vertical incision crosses it at right angles and cuts it; an oblique incision parallel to its course substantially reduces the risk. The result is numbness and hypersensitivity over the anterolateral proximal leg and the lateral aspect of the incision, and occasionally a painful neuroma requiring excision and burial in muscle.

Saphenous nerve trunk. Deeper, running with the great saphenous vein under sartorius. Injury numbs the medial leg to the medial malleolus; it is far less common than a branch injury but far more troublesome and more disabling.

Popliteal vessels and tibial nerve. Injury is the rare catastrophic complication of the harvest, extremely rare but entirely preventable. The bundle is only at risk if a stripper is driven blindly posteriorly and laterally, and the injury is caused by a stripper driven posterolaterally with the knee extended; knee flexion of 90 degrees moves the neurovascular bundle away from the harvest axis and is the primary protection.

Great saphenous vein. Bleeding and, rarely, thrombophlebitis.

Donor-site morbidity. The reproducible, angle-specific deficit is deep-flexion weakness, so counsel athletes whose sport demands force in deep flexion. The altered muscle architecture is described under Regeneration. Harvest also reduces future graft availability, a real problem in the young patient facing revision or a multiligament reconstruction.

Graft problems on the back table. Slippage comes from inadequate whip-stitching; secure both ends before tensioning.

The wound. A subcutaneous haematoma in the posteromedial calf and thigh, tracking distally from the harvest bed, is a recognised nuisance complication; closing the sartorial fascia and careful haemostasis of the crural fascial vessels reduce it. A painful scar and hypersensitivity at the pes also occur. Infection at the donor site is uncommon but carries the same significance as an intra-articular infection, because the tunnels communicate.

Preventing each.

Structures at risk. Each structure in the harvest field has a landmark and a way to protect it.

Approaches that meet the tendon. In the medial approach to the knee and the posteromedial corner, the pes tendons are retracted distally or reflected to expose the superficial MCL and the posteromedial capsule beneath. For medial and posteromedial tibial plateau fixation, the pes insertion is elevated with a periosteal cuff and repaired at closure. Medial parapatellar and subvastus arthrotomies for knee arthroplasty do not disturb the pes, but the incision may cross the infrapatellar branch and produce the familiar lateral incisional numbness.

ACL reconstruction is the commonest indication for harvest worldwide, using a doubled semitendinosus-gracilis construct or a quadrupled or quintupled semitendinosus alone. In paediatric and adolescent ACL reconstruction, a physeal-sparing or transphyseal soft-tissue graft avoids the bone plug of a patellar tendon graft crossing the physis.

MCL and posteromedial corner. Semitendinosus is frequently left attached distally at the pes and routed proximally to a femoral tunnel at the sMCL isometric point. The intact insertion gives immediate distal fixation and preserves the tendon's own blood supply at the tibia.

MPFL. Most units use gracilis, but semitendinosus works well when a larger graft is needed or the gracilis is inadequate. The femoral attachment is the Schöttle point, and the graft is tensioned at 30 degrees of flexion with minimal tension, because over-tensioning an MPFL graft causes medial patellofemoral overload.

Elsewhere. The tendon is used in:

• PCL and multiligament knee reconstruction
• The posterolateral corner, for the fibular-based Larson sling or the anatomical popliteofibular and fibular collateral ligament reconstruction
• Ankle lateral ligament reconstruction and, in some techniques, spring ligament and deltoid reconstruction
• Chronic patellar and Achilles tendon reconstruction, as an augmentation graft

Choosing between graft sources. Each graft trades one set of problems for another.

Registry Signals on Graft Choice

• National ligament registries (Scandinavian registries, the UK National Ligament Registry, and the Australian and New Zealand registries) consistently report that hamstring autograft is the most commonly used graft in most countries, while showing a higher revision rate for hamstring autograft than for bone-patellar tendon-bone autograft, particularly in young patients.
• The counterpoint repeatedly made in the same datasets is that patellar tendon grafts carry more anterior knee and kneeling pain, so the choice is a trade-off rather than a hierarchy — and that graft diameter and technique are at least as influential as graft type.
• Registries also demonstrate that allograft has a substantially higher failure rate in young, high-demand patients, which is the strongest practice signal against allograft in that group.

Variation in Described Technique

Anatomical Variation Across Populations

• Tendon length and cross-sectional area vary with body size, and mean harvested semitendinosus length is reported as shorter in some East and South Asian cohorts than in Northern European cohorts. This is clinically important because a shorter tendon limits the number of strands and therefore the achievable graft diameter.
• The consequence in practice is that units serving smaller-statured populations more often need to quadruple or quintuple the semitendinosus, harvest the gracilis routinely, or plan a quadriceps tendon graft to reach an adequate diameter.
• The fascial bands to the medial gastrocnemius are described as present in essentially all specimens across populations; their number varies from one to three.

High- and Limited-Resource Practice

• Well-resourced settings: pre-operative MRI estimation of tendon cross-sectional area, adjustable-loop suspensory fixation permitting short grafts, and ready availability of alternative autografts and allografts.
• Limited-resource settings: hamstring autograft is the graft of choice worldwide precisely because it needs only a tendon stripper and a whip-stitch, no bone plugs, no allograft bank and minimal implant cost. Interference screw or suspensory fixation with a simple button is entirely adequate. The technical priorities are unchanged: an adequately sized incision, release of the fascial bands, and a knee at 90 degrees.