Semitendinosus

Origin, Insertion, Action, Innervation

Origin: Ischial tuberosity (shared common tendon with the long head of biceps femoris)
Insertion: Proximal medial surface of the tibial shaft (pes anserine), posterior to the insertions of sartorius and gracilis
Action:
Primary: Flexion of the knee
Extension of the hip (two-joint muscle)
Internal (medial) rotation of the knee when the knee is flexed
Posterior pelvic tilt (when the femur is fixed)
Innervation: Tibial division of the sciatic nerve (L5, S1, S2)

Palpation Guide

Client position: Prone with the knee slightly flexed.
Landmark sequence:

Locate the ischial tuberosity — semitendinosus originates here, sharing a common tendon with the biceps femoris long head.
Follow the muscle belly distally along the posteromedial thigh. Semitendinosus lies superficial and medial to semimembranosus in the distal half of the thigh.
At the posterior knee, locate the medial border of the popliteal fossa. The prominent, cord-like tendon running from the posteromedial thigh toward the medial tibia is semitendinosus. It is the most superficial and most easily palpable tendon on the medial side of the knee.
Follow the tendon distally to the pes anserine insertion on the proximal medial tibia, approximately 5–7 cm below the medial joint line.

Tissue feel: The proximal muscle belly is thick and fleshy, blending with the biceps femoris long head at the shared ischial origin. In the mid-thigh, the muscle belly transitions into a long, cord-like tendon that is narrow, round, and easily rolled under the fingers. This tendon is the most distinctive palpation feature of semitendinosus — it feels like a firm rope on the medial posterior thigh.
Confirmation test: With the client prone, ask for resisted knee flexion with the foot internally rotated (turned inward). Internal rotation biases the medial hamstrings. You should feel the cord-like semitendinosus tendon become taut at the medial popliteal fossa.
Common errors:
Confusing with semimembranosus — semimembranosus is deeper and broader, lying directly beneath semitendinosus in the distal thigh. If the tendon you feel is broad and flat rather than cord-like, you may be on semimembranosus.
Confusing with gracilis — gracilis lies anterior and medial to semitendinosus and also inserts at the pes anserine. Gracilis is thinner and contracts with hip adduction rather than knee flexion.

Trigger Point Referral

Common TrP locations: (1) Proximal, near the ischial tuberosity in the shared origin region, and (2) mid-belly, at the center of the posteromedial thigh.
Referral pattern: Pain in the lower buttock, the posteromedial thigh, and the medial knee (pes anserine region). The proximal TrP refers up to the gluteal fold and ischial tuberosity.
Clinical significance: Semitendinosus TrP referral to the pes anserine area mimics pes anserine bursitis. If a client reports medial knee pain at the anteromedial tibial condyle (the pes anserine insertion) and the bursa is not inflamed, check semitendinosus TrPs — the muscle's referral pattern terminates exactly at its insertion site.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [Semitendinosus at TriggerPoints.net](http://www.triggerpoints.net/muscle/semitendinosus).

Clinical Notes

Common conditions:

Relevant to conditions/hamstring-strain — semitendinosus is the second most commonly strained hamstring (after biceps femoris), particularly at the proximal musculotendinous junction. Strains are common in sprinters and sports requiring rapid hip flexion with knee extension.
Key structure in pes anserine bursitis differential — the semitendinosus tendon overlies the pes anserine bursa. Chronic friction from a hypertonic or shortened semitendinosus can irritate the underlying bursa, producing medial knee pain at the anteromedial tibial condyle.
Its tendon is harvested for ACL reconstruction (hamstring autograft) — post-surgical semitendinosus weakness affects knee flexion strength and internal rotation control. Relevant to post-ACL rehabilitation considerations.
Relevant to conditions/sciatica differential — semitendinosus TrP referral down the posteromedial thigh can contribute to posterior leg pain that is attributed to sciatic nerve compression.

What you'll typically find:

Like all hamstrings, semitendinosus is frequently tight in the sedentary population. The medial hamstrings are often tighter than biceps femoris, contributing to a bias toward internal rotation of the flexed knee.
The proximal ischial attachment is a common site of tenderness, shared with the biceps femoris long head. Differentiating proximal hamstring pain between the individual muscles requires resisted testing with internal rotation (medial hamstrings) versus external rotation (biceps femoris).
In clients with medial knee pain, the pes anserine insertion is often tender. The question is whether the tenderness is in the semitendinosus tendon, the sartorius tendon, the gracilis tendon, or the underlying bursa. Resisted knee flexion with internal rotation (semitendinosus), resisted hip flexion with external rotation (sartorius), and resisted adduction (gracilis) help differentiate.

Treatment effects:

The long, cord-like tendon responds well to longitudinal stripping and cross-fiber work at the distal medial thigh. The tendon can be "rolled" under the fingers like a guitar string.
The proximal belly responds to sustained compression and longitudinal stripping along the posteromedial thigh. Same cautions as all hamstrings — warm the tissue before deep work.
Post-treatment hamstring stretching (SLR position) is essential, with emphasis on internal rotation (turn the foot inward during the stretch) to bias the medial hamstrings.

Cautions:

The popliteal artery and tibial nerve lie deep in the popliteal fossa, medial to the semitendinosus tendon. Avoid deep sustained pressure in the center of the popliteal fossa.
Post-ACL surgery (hamstring autograft), the semitendinosus tendon may have been harvested. Treatment to the medial posterior thigh should be adjusted — the tendon may be absent or regenerating.

Clinical pearl:

The pes anserine is the Swiss Army knife of the medial knee — three muscles from three different groups (sartorius from the hip flexors, gracilis from the adductors, semitendinosus from the hamstrings) all converge on the same point. When a client has medial knee pain at the pes anserine, you have three suspects. Test each one individually: knee flexion with internal rotation (semitendinosus), hip flexion-abduction-external rotation (sartorius), hip adduction (gracilis). The one that reproduces the pain is your primary target.

Assessment

Resisted knee flexion with internal rotation:

Client prone. Ask for resisted knee flexion with the foot turned inward (internally rotating the tibia). This biases the medial hamstrings (semitendinosus and semimembranosus). Pain at the ischial tuberosity or along the posteromedial thigh implicates the medial hamstrings.

Active knee extension test / SLR (hamstring length):

Tests hamstring group length collectively. See anatomy/muscles/biceps-femoris Assessment section for details.

Muscle Groups

Hamstrings (anatomical):

Pes anserine (anatomical):

anatomy/muscles/sartorius
anatomy/muscles/gracilis
Semitendinosus (this article)

Knee flexors (functional):

Tibial nerve group (innervation):

anatomy/muscles/biceps-femoris (long head)
Semitendinosus (this article)
anatomy/muscles/semimembranosus

Related Muscles

Hamstring group:

anatomy/muscles/biceps-femoris — lateral hamstring; shares ischial tuberosity origin
anatomy/muscles/semimembranosus — deeper medial hamstring; lies directly beneath semitendinosus

Pes anserine group:

anatomy/muscles/sartorius — most anterior pes anserine insertion
anatomy/muscles/gracilis — middle pes anserine insertion; the only adductor crossing the knee

Antagonists (knee extension):

anatomy/muscles/rectus-femoris — two-joint quadriceps
anatomy/muscles/vastus-medialis — medial patellar stabilizer

Key Takeaways

The cord-like distal tendon is the most distinctive palpation landmark of the medial hamstrings and forms the superficial medial border of the popliteal fossa.
Medial knee pain at the pes anserine — differentiate semitendinosus, sartorius, and gracilis with specific resisted tests.
The semitendinosus tendon is harvested for ACL reconstruction — relevant to post-surgical rehabilitation.
TrP referral to the pes anserine mimics bursitis — check the muscle before assuming the bursa.