Hamstring Muscles - Short Note

Overview

The hamstring muscles are a group of three muscles occupying the posterior compartment of the thigh. They are so named because their long tendons are prominent behind the knee ("ham" = back of thigh, "string" = tendon). They are biarticular - crossing both the hip and knee joints.

Posterior compartment of the thigh showing hamstring muscles

Fig. 6.63 - Muscles of the Posterior Compartment of the Thigh (Posterior view) — Gray's Anatomy for Students

The Three Hamstring Muscles

1. Biceps Femoris

Two heads - long head and short head
Long head origin: Inferomedial part of the upper area of the ischial tuberosity (with semitendinosus)
Short head origin: Lateral lip of the linea aspera on the femoral shaft
Insertion: Head of the fibula (lateral side of knee)
Innervation: Long head - tibial division of sciatic nerve; short head - common fibular division of sciatic nerve (L5, S1, S2)
Action: Flexes knee; long head also extends hip and laterally rotates thigh. With knee partly flexed, laterally rotates the leg

2. Semitendinosus

Origin: Inferomedial part of the upper area of the ischial tuberosity (shared with long head of biceps femoris)
Insertion: Medial surface of proximal tibia, as part of the pes anserinus (along with gracilis and sartorius)
Innervation: Tibial division of sciatic nerve (L5, S1, S2)
Action: Flexes knee, extends hip, medially rotates thigh at hip and leg at knee
Note: The spindle-shaped belly ends midthigh, continuing as a long cord-like tendon

3. Semimembranosus

Origin: Superolateral impression on the ischial tuberosity
Insertion: Groove and adjacent bone on medial and posterior surface of the medial tibial condyle; expansions contribute to ligaments and fascia around the knee
Innervation: Tibial division of sciatic nerve (L5, S1, S2)
Action: Flexes knee, extends hip, medially rotates thigh and leg (with semitendinosus)
Note: Lies deep to semitendinosus; its tendon is flat and membranous proximally - hence the name

Summary Table

Muscle	Origin	Insertion	Nerve	Key Actions
Biceps femoris (long head)	Ischial tuberosity	Head of fibula	Sciatic - tibial division (L5, S1, S2)	Flexes knee; extends + laterally rotates hip
Biceps femoris (short head)	Lateral lip, linea aspera	Head of fibula	Sciatic - common fibular division (L5, S1, S2)	Flexes knee
Semitendinosus	Ischial tuberosity	Medial tibia (pes anserinus)	Sciatic - tibial division (L5, S1, S2)	Flexes knee; extends + medially rotates hip
Semimembranosus	Ischial tuberosity	Medial tibial condyle	Sciatic - tibial division (L5, S1, S2)	Flexes knee; extends + medially rotates hip

Nerve Supply

All three muscles are innervated by the sciatic nerve (L5, S1, S2). The tibial division supplies all three plus the long head of biceps femoris, while the short head of biceps femoris is the only hamstring supplied by the common fibular (peroneal) division.

Blood Supply

The hamstrings are supplied primarily by the perforating branches of the profunda femoris artery and the inferior gluteal artery (to the proximal portions).

Functions

Knee flexion - primary action; all three muscles
Hip extension - two-joint muscles extend the hip (e.g., rising from a chair, running)
Rotation - semitendinosus and semimembranosus medially rotate; biceps femoris laterally rotates the leg at the knee
Pelvic control - stabilize the pelvis during gait

Clinical Relevance - Hamstring Injuries

Hamstring injuries are among the most common musculoskeletal injuries in athletes, particularly in sports requiring high power and speed (sprinting, track and field, football). - Gray's Anatomy for Students, p. 701

Types of injury:

Mild strain - micro-tears within the muscle belly; focal fluid on MRI/ultrasound
Partial tear - more muscle fibers involved
Complete tear/rupture - at the musculotendinous junction (most common site in adults)
Proximal avulsion - avulsion of the ischial tuberosity; common in adolescents during sudden hip flexion because the ischial apophysis is the weakest element in this age group

Mechanism: Typically occurs during sudden acceleration/deceleration, rapid change in direction, or the late swing phase of running (when the hamstring contracts eccentrically to decelerate the leg).

Imaging: Both ultrasound and MRI can assess the injury. MRI provides extent of injury and prognostic information (risk of re-tear, loss of function).

Pes Anserinus - The combined insertion of sartorius, gracilis, and semitendinosus at the medial tibia is a clinically important landmark for surgical harvesting (e.g., hamstring autograft in ACL reconstruction).

Sources: Gray's Anatomy for Students, pp. 699-702; Campbell's Operative Orthopaedics 15th Ed 2026

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