Posterior Compartment of the Thigh

Introduction and Fascial Framework

• Gray's Anatomy for Students, p. 686-687

Muscles

1. Biceps Femoris

• Insertion: Both heads converge to form a palpable tendon that inserts into the lateral surface of the head of the fibula. Extensions blend with the fibular collateral ligament and lateral knee ligaments.
• Actions: Flexes the leg at the knee. The long head also extends and laterally rotates the thigh at the hip. With the knee partly flexed, the biceps femoris laterally rotates the leg at the knee.
• Innervation: Long head - tibial division of the sciatic nerve (L5, S1, S2); Short head - common fibular division of the sciatic nerve (L5, S1, S2).

2. Semitendinosus

• Origin: Inferomedial part of the upper area of the ischial tuberosity, shared with the long head of biceps femoris via a conjoined tendon.
• Morphology: The spindle-shaped muscle belly ends in the lower half of the thigh, giving way to a long cord-like tendon. In about 90% of individuals a tendinous inscription called the raphe divides it into two distinct parts, making it functionally a digastric muscle.
• Insertion: The tendon curves around the medial condyle of the tibia and inserts into the medial surface of the proximal tibia, posterior to the tendons of gracilis and sartorius, as part of the pes anserinus.
• Actions: Flexes the leg at the knee; extends the thigh at the hip; medially rotates both the thigh at the hip and the leg at the knee.
• Innervation: Tibial division of the sciatic nerve (L5, S1, S2) - via two distinct branches, the lower arising in common with the nerve to semimembranosus.

3. Semimembranosus

• Origin: Superolateral impression on the ischial tuberosity. Notably, the semimembranosus tendon originates on the superolateral aspect of the ischial tuberosity and is conjoint with the BF-L/semitendinosus common tendon for only 2-5 cm before separating. The proximal tendon is elongated (mean length ~9 cm) and hemi-pennate in morphology.
• Insertion: Primarily into the groove and adjacent bone on the medial and posterior surfaces of the medial tibial condyle. Expansions from the tendon insert into:
  • The oblique popliteal ligament (reinforces the posterior capsule of the knee)
  • The posterior oblique ligament (provides medial knee stability)
  • The medial meniscus (the semimembranosus tendon retracts the medial meniscus posteriorly during knee flexion)
• Actions: Flexes the leg at the knee; extends the thigh at the hip; medially rotates the thigh at the hip and the leg at the knee. In knee extension, the semimembranosus tendon resists valgus forces; in knee flexion, it resists external rotation.
• Innervation: Tibial division of the sciatic nerve (L5, S1, S2) - single branch from the tibial division.

Summary Table: Muscles of the Posterior Compartment

Proximal Attachments: The Ischial Tuberosity

• The long head of biceps femoris and the semitendinosus share a conjoined tendon from the inferomedial part of the ischial tuberosity. This conjoined tendon divides into two separate tendons at a mean distance of 9-10 cm from the ischial tuberosity.
• The semimembranosus originates from the superolateral aspect of the ischial tuberosity, anterior and medial to the semitendinosus in the proximal thigh.
• The gluteus maximus covers the ischial tuberosity and 4-6 cm of the proximal hamstring muscles.
• A fibrotic retinaculum covers the proximal hamstring tendons, compartmentalizing the sciatic nerve and posterior femoral cutaneous nerve. This retinaculum can compress these structures, producing symptoms.

• Imaging Anatomy, Vol. 3, p. 281-283

Nerve Supply

Course in the Posterior Thigh

• The sciatic nerve enters the thigh from the gluteal region, passing deep to the gluteus maximus.
• In the posterior compartment, it lies on the adductor magnus muscle and is crossed by the long head of the biceps femoris muscle.
• It descends vertically in the midline of the posterior thigh.
• Proximal to the knee (and sometimes already within the pelvis), it divides into its two terminal branches:
  • Tibial nerve - continues through the popliteal fossa into the posterior compartment of the leg
  • Common fibular nerve - winds around the neck of the fibula into the anterior and lateral compartments of the leg

Muscular Branches

• Tibial division supplies: long head of biceps femoris, semitendinosus, semimembranosus, and the hamstring portion of adductor magnus.
• Common fibular division supplies: short head of biceps femoris only (in the thigh).

Posterior Cutaneous Nerve of the Thigh

Blood Supply

Perforating Arteries

• Three perforating arteries branch from the profunda femoris as it descends anterior to the adductor brevis.
• All three perforate through the adductor magnus near its attachment to the linea aspera to enter the posterior compartment.
• Within the compartment, each vessel has ascending and descending branches that interconnect to form a longitudinal channel (cruciate anastomosis superiorly, anastomosis with popliteal artery branches inferiorly).
• This anastomotic chain provides the principal blood supply to the posterior thigh musculature and also contributes to collateral circulation around the hip and knee.

Additional Supply

• The inferior gluteal artery (from the internal iliac) contributes to supply of the proximal hamstring muscles.
• Venous drainage mirrors the arterial supply via perforating veins that drain into the profunda femoris vein.
• Gray's Anatomy for Students, p. 703

Applied Anatomy: Hamstring Injuries

Mechanisms and Patterns

• Injury most commonly occurs at the proximal musculotendinous junction or at the ischial tuberosity origin during eccentric contraction (hamstring lengthening while contracting, e.g., late swing phase of sprinting).
• Range of injury from minor muscle strain to complete avulsion of the conjoined proximal tendon.
• The semitendinosus and biceps femoris long head are most frequently injured at their common origin.
• A hamstring avulsion involves tearing of the proximal tendons from the ischial tuberosity, visualized on coronal MRI as tendon retraction with surrounding hemorrhage/edema.

Clinical Features

• Sudden posterior thigh pain ("pop") during sprinting or forced hip flexion with knee extension.
• Tenderness over the ischial tuberosity or along the posterior thigh.
• Weakness of knee flexion and hip extension.
• Bruising may track distally over days.

Imaging

• MRI is the modality of choice, demonstrating the grade of injury (partial vs. complete), degree of retraction, and involvement of specific tendons.
• A coronal MRI can clearly show hamstring avulsion injury and tendon retraction.

Pes Anserinus

Relations and Clinical Relevance

Popliteal Fossa Boundaries

• Superomedial boundary: Semimembranosus and semitendinosus
• Superolateral boundary: Biceps femoris

Sciatic Nerve Injury

• Weakness of all hamstrings (loss of knee flexion and hip extension)
• Loss of all sensation and motor function below the knee (supplied by tibial and common fibular nerves)
• Preserved medial thigh sensation (obturator nerve) and hip flexion/knee extension (femoral nerve)

Summary

Video Lectures: Posterior Compartment of the Thigh

1. YouTube - Muscles of the Thigh (Posterior Compartment)

2. Kenhub (Free/Paid)

3. TeachMeAnatomy (Podcast/Audio version)

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