The hamstring is a group of muscles located on the posterior (back) of the thigh. This muscle complex extends from the pelvis down to the lower leg bones, crossing both the hip and knee joints. The term “hamstring” is a collective name for three distinct muscles. Understanding their specific attachment points, known as insertions, is foundational to grasping how these muscles produce movement and why they are prone to injury.
The Three Muscles of the Hamstring Group
The hamstring complex consists of three separate muscles: the semimembranosus, the semitendinosus, and the biceps femoris. The semimembranosus and semitendinosus are referred to as the medial hamstrings, positioned toward the inner thigh.
The biceps femoris is the lateral (outer) hamstring muscle. It is structurally unique because it has two separate heads—a long head and a short head—which merge before reaching the knee.
Defining the Proximal Attachment (Origin)
The majority of the hamstring muscles share a common starting point, or origin, high up on the pelvis. This shared anchor point is the ischial tuberosity, often known as the “sit bone.” The semitendinosus, semimembranosus, and the long head of the biceps femoris all originate here.
This high attachment allows the muscles to span two major joints (the hip and the knee). This biarticular structure enables the main actions of the hamstrings: extending the hip and flexing the knee. The short head of the biceps femoris is the sole exception; it originates lower down on the shaft of the femur (thigh bone) and therefore only acts on the knee.
Detailed Distal Attachments (Insertion Points)
The distal attachment, or insertion, is where the muscle tendons connect to the bones of the lower leg, the tibia and the fibula. The three hamstring muscles diverge at the knee to insert at separate locations, each contributing to different rotational functions.
The lateral muscle, the biceps femoris, inserts onto the fibula, the smaller bone on the outer side of the lower leg. The tendon of the biceps femoris primarily wraps around and inserts onto the head of the fibula. This outer-side attachment allows the biceps femoris to facilitate lateral rotation (external turning) of the lower leg when the knee is bent.
The two medial hamstrings, the semitendinosus and the semimembranosus, both insert onto the tibia, the large bone on the inner side of the lower leg. The semitendinosus tendon attaches to the upper, medial surface of the tibia. This insertion site is part of a conjoined tendon structure called the pes anserinus.
The pes anserinus translates to “goose’s foot” and is formed by the tendons of the semitendinosus, the sartorius, and the gracilis muscles. This complex insertion provides dynamic stability to the inner side of the knee joint.
Deep to the semitendinosus is the semimembranosus, which has a broad insertion. Its tendon attaches to the posterior and medial aspect of the medial condyle of the tibia. This deeper attachment helps stabilize the posterior capsule of the knee joint.
The combined insertions of the semitendinosus and semimembranosus on the inner side of the tibia allow them to medially rotate (internally turn) the lower leg when the knee is flexed. The distinct medial and lateral insertions allow for the complex rotational control of the knee joint.
Common Injuries at the Insertion Sites
The points where the hamstring tendons attach to the lower leg bones are frequent locations for specific types of injury. One common issue is distal hamstring tendinopathy, which involves irritation and degeneration of the tendon tissue just above the knee joint. This condition typically occurs from overuse or repetitive strain where the tendons rub against the bone near their insertion.
On the inner side of the knee, inflammation can affect the pes anserinus insertion site, leading to a localized condition known as pes anserine bursitis. This involves irritation of the bursa situated beneath the conjoined tendons on the upper, medial tibia.
Acute injuries can also occur at these distal sites, most commonly involving an avulsion, where the tendon tears away from its bony insertion. These distal avulsion injuries most frequently involve the biceps femoris tendon, pulling away from the head of the fibula.
While mid-thigh muscle strains are the most common hamstring injury, problems directly at the insertion points are often more severe or chronic. Understanding the precise anatomy of where the hamstring attaches is important for correctly diagnosing and treating these localized tendon and bone injuries.