Three muscles comprise "the hamstrings." These posterior leg muscles originate at the bottom of the pelvis from a bony process called the ischial tuberosity. There is one ischial tuberosity on each side of the pelvis, and they are often referred to as, "the sit bones." Two of the hamstrings insert into the top of the tibia; the other inserts into the fibula.
Where the Hamstrings Are
Orient yourself looking at the back of a left leg. On the most left, or lateral side, is the biceps femoris. The biceps femoris is what protrudes from the middle of the back of the thigh, particularly when the hips and knees are flexed. The biceps femoris has two origins: the long head originates from the ischial tuberosity, and the short head originates more medially, from the linea aspera, near the head of the femur. Moving inward toward the midline of the body, the semitendinosus and semimembranosus are the two smaller and more medial hamstrings, respectively. A diagram of the hamstrings may be found on projectswole.com. A link is in the Resources section.
What Hamstrings Do
The hamstrings are multi-joint muscles, spanning the hip and the knee. When the hamstrings shorten, the knee bends. Conversely, the hamstrings also lengthen to flex the hip. The paradox of flexing and extending these joints simultaneously, such as when you sit in a chair, is called, "Lombard's paradox."
The muscle fiber types of the hamstrings are a mix of both fast and slow-twitch fibers, meaning the hamstrings can work in an explosive burst of power, for example in the high jump. The hamstrings also are able to contract repeatedly for an extended time, such as in running.
The Name Hamstrings
Being "hamstrung" refers to the immobility of a pig's hips when hung from the back of its hind legs to be cured for meat. A hog hook was inserted through the hind leg tendons of the pig near the origin of the hamstrings. If you have ever had a hamstring injury, you can sympathize with the pig. Typically, hamstring pulls or tears occur near the proximal end of the femur, and are very painful. Injuries to a muscle near the bony attachment are slow to heal because the muscle becomes mostly tendon at the origin and insertion. Tendons have less blood supply than does muscle, so the process of inflammation and healing happens slowly.
Injury Prevention
The song lyric, "the thigh bone's connected to the shin bone..." is not far from the truth. Hamstring injuries are often caused by irritation in the lower back or hips. A tight or weak lower back affects the hamstrings. Conversely, tight hamstrings can exacerbate lower back injuries when flexing the spine without support. For example, a grandmother ruptures an intervertebral disk when picking up her grandchild. She should have bent her knees and kept her spine in neutral, much like a squat. An untreated hamstring injury can cause changes in walking or running gait, which often emerge as an injury elsewhere.
Unsupport Forward Flexion
People at the gym or running trail often bend at the waist to touch their toes, attempting to stretch their hamstrings. "Unsupported forward flexion" can lead to the intervertebral disk injury grandmother had. Standing hip flexion, even with the spine in neutral, is not the best hamstring stretch. When you bend over, your hamstrings actually contract to keep you from falling forward onto your head.
