Hamstrings

Introduction

Introduction to the Hamstrings

The hamstrings are a group of three large muscles located in the posterior compartment of the thigh: the biceps femoris (long head), semitendinosus, and semimembranosus. The short head of biceps femoris, though not a true hamstring by embryological origin, is functionally included. These muscles share a common origin from the ischial tuberosity (except the short head of biceps femoris), cross both the hip and knee joints, and are innervated by the tibial division of the sciatic nerve (with the short head of biceps femoris receiving innervation from the common peroneal division). The hamstrings are powerful flexors of the knee and extensors of the hip, playing a vital role in walking, running, jumping, and maintaining an upright posture. They are the most commonly injured muscle group in sprinting athletes and are frequently used as graft sources for ligament reconstructions.

Study

1. General Characteristics of the Hamstrings

The hamstrings meet four defining criteria: (1) they originate from the ischial tuberosity, (2) they cross both the hip and knee joints, (3) they are innervated by the tibial portion of the sciatic nerve, and (4) they are flexors of the knee and extensors of the hip. Only the long head of biceps femoris, semitendinosus, and semimembranosus satisfy all four criteria; the short head of biceps femoris is excluded because it arises from the femur and does not cross the hip joint. Nevertheless, it is considered part of the hamstring group functionally.

2. Common Origin and Individual Attachments

The long head of biceps femoris and the semitendinosus share a common conjoint tendon that originates from the upper medial facet of the ischial tuberosity. The semimembranosus originates from the upper lateral facet of the same tuberosity, deep to the conjoint tendon. The short head of biceps femoris arises from the lateral lip of the linea aspera and the lateral supracondylar line of the femur. The distal attachments are: biceps femoris – head of the fibula; semitendinosus – pes anserinus on the medial tibia; semimembranosus – posterior aspect of the medial tibial condyle, with expansions to the oblique popliteal ligament, posterior capsule, and medial meniscus.

3. Nerve Supply

All three true hamstrings (long head of biceps femoris, semitendinosus, and semimembranosus) are innervated by the tibial division of the sciatic nerve (L5, S1, S2). The short head of biceps femoris is innervated by the common peroneal division of the sciatic nerve (L5, S1, S2). This dual innervation within the biceps femoris is unique and clinically significant in nerve conduction studies and localisation of sciatic nerve lesions.

4. Blood Supply

The hamstrings receive their blood supply from the inferior gluteal artery (proximal portions), perforating branches of the profunda femoris artery (mid‑belly), and the popliteal artery (distal portions). This segmental vascular pattern allows for surgical mobilisation and use of the muscles as flaps.

5. Actions and Functional Roles

Collectively, the hamstrings flex the knee joint and extend the hip joint. The biceps femoris is also a lateral rotator of the flexed knee; the semitendinosus and semimembranosus are medial rotators. During the gait cycle, the hamstrings are active from late swing phase (decelerating knee extension) through loading response (controlling hip flexion and extending the hip). They also stabilise the pelvis in the sagittal plane. In sprinting, they undergo powerful eccentric contractions, making them vulnerable to injury.

6. Clinical Significance

Hamstring strains are among the most common sports injuries, typically occurring at the proximal myotendinous junction during eccentric overload. The biceps femoris long head is the most frequently injured. Proximal hamstring avulsions from the ischial tuberosity can be partial or complete and may require surgical reattachment. Chronic proximal hamstring tendinopathy causes deep buttock and posterior thigh pain. The hamstring tendons, particularly the semitendinosus and gracilis, are commonly harvested for autografts in anterior cruciate ligament (ACL) and other ligament reconstructions. Tight hamstrings contribute to low back pain and are assessed with the popliteal angle. In neurological conditions such as cerebral palsy, hamstring spasticity leads to crouch gait and knee flexion contractures, often treated by surgical lengthening.

Summary

Summary of the Hamstrings

The hamstrings (biceps femoris, semitendinosus, semimembranosus) are powerful flexors of the knee and extensors of the hip, arising from the ischial tuberosity and innervated primarily by the tibial nerve (L5–S2). The short head of biceps femoris is a functional part but has a different origin and innervation. The hamstrings are essential for locomotion, commonly injured in sports, and used as tendon grafts for ligament reconstruction.