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Posterior thigh muscles (hamstrings)

Origins, insertions, innervation and functions of the muscles of the posterior thigh
Posterior thigh muscles (Hamstring muscles)

The hamstring muscles, or simply the hamstrings, are a group of three long muscles located in the posterior compartment of the thigh, shaping up the surface anatomy of this region. These muscles are the biceps femoris, semimembranosus and semitendinosus muscles. 

The hamstrings are closely related to each other as they share a common origin point, and they all attach to the proximal parts of the tibia and fibula. They are innervated by the tibial and common fibular (peroneal) divisions of the sciatic nerve (L4-S3)

All the hamstring muscles cross the hip and knee joints and act upon them. The primary function of the hamstrings is to flex the knee joint and extend the hip, enabling some of the essential lower limb activities such as walking, running, and climbing. The hamstrings have an important stabilizing function as well; they are inactive when the bodyweight is equally distributed between both lower limbs in a standing position. However, when a person starts tilting forward, these muscles activate and counteract the tilting movement in order to stabilize the hip joint and prevent falling. Also, due to the location of their insertions, the hamstrings act together with the collateral ligaments to stabilize the knee joint. 

This article will introduce you to the anatomy and function of the hamstring muscles.

Key facts about the hamstring muscles
Definition and function The hamstrings are the muscles of the posterior thigh that are primarily in charge of knee flexion and thigh extension
Muscles Biceps femoris, semimembranosus, semitendinosus
Innervation Tibial and common fibular (peroneal) divisions of the sciatic nerve
Primary functions Flexion of leg, extension of thigh, stabilization of knee and hip joints
  1. Biceps femoris muscle
  2. Semimembranosus muscle
  3. Semitendinosus muscle
  4. Clinical notes
    1. Strains
    2. Avulsions
  5. Sources
+ Show all

Biceps femoris muscle

Biceps femoris is a two-headed muscle that runs superficially through the posterior thigh. Each head of biceps femoris has its own origin and supply, but they share the common insertion. The long head of biceps femoris arises from the ischial tuberosity and sacrotuberous ligament. The short head arises from the lateral lip of the linea aspera and lateral supracondylar line of femur. The muscle fibers from both heads converge and form the conjoint aponeurotic sheet that inserts onto the head of fibula. 

The long head of biceps femoris is innervated by the tibial division of sciatic nerve, while the short head receives the nerve supply from the common fibular division of sciatic nerve. 

The long head of biceps femoris acts upon two joints (hip and knee), while the short head acts only on the knee joint. Firstly, the long head contributes to the extension of the hip. This action is particularly strong when the flexed trunk is being raised to the erect position. Both heads of biceps femoris contribute to the flexion of the leg in the knee joint. Additionally, biceps femoris can act as the external rotator of the leg when the knee is semi flexed or internal rotator of the thigh if the foot is fixed on the surface. 

Start with the anatomy of the hip and thigh muscles by exploring our videos, quizzes, labeled diagrams, and articles.

Semimembranosus muscle

Semimembranosus is a fusiform muscle of the posterior thigh. It runs deep to semitendinosus and medial to biceps femoris. Semimembranosus originates from the ischial tuberosity of pelvis and inserts into the medial condyle of tibia.

Are you feeling a bit overwhelmed? Learn the attachments, innervations and functions of the hamstring muscles faster and easier with our muscle charts!

Semimembranosus is innervated by the tibial division of the sciatic nerve. The blood supply for this muscle comes from the femoral and popliteal arteries. 

Like the other muscles from this group, semimembranosus also acts on both knee and hip joints. Its functions mainly depend on the initial position of the lower extremity. When the foot is firmly placed on the ground, semimembranosus extends the hip, bringing the trunk into an erect position. In contrast, when the foot is elevated, semimembranosus flexes and internally rotates the leg. Additionally, when the hip is in full extension, semimembranosus causes internal rotation of the hip

Semitendinosus muscle

Semitendinosus is a long, fusiform muscle that runs in the posterior thigh, medially to biceps femoris. More specifically, it extends from the ischial tuberosity of bony pelvis to the proximal end of the tibia below medial condyle, where it inserts via the pes anserinus

Semitendinosus receives its innervation through the tibial division of sciatic nerve. It is vascularized by the deep femoral (profunda femoris) and medial circumflex femoral arteries. 

Similar to semimembranosus, the actions of semitendinosus on hip and knee joints mainly depend on the body position. When the trunk is erect, semitendinosus acts as an internal rotator of the thigh. While on the other hand, when the trunk is flexed anteriorly, it extends the thigh. 

At the knee joint, semitendinosus primarily flexes the leg and stabilizes the knee joint. Additionally, when the knee is semiflexed, it contributes to the internal rotation of the leg. 

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