The hamstrings are a group of three individual muscles in the back of the thigh responsible for a range of movement in the hips and knees. The human body consists of three basic types of muscles, and all of the hamstring muscles fall under the same category.
Types
The hamstrings consist of three muscles -- the semitendinosus, the semimembranosus and the biceps femorus. Each of these muscles attaches to the hip at the top and to the knee at the bottom. The muscles wrap around the back of the femur -- the thigh bone -- in various overlapping directions.
Skeletal Muscle
Each of the three hamstring muscles is classified as skeletal muscle. Skeletal muscle is the most common form of muscle tissue in the body. It is so named because it refers to muscles that are attached to and control the bones in the skeleton. All skeletal muscle is controlled by a conscious act of brain function -- as opposed to cardiac muscle, for example, which contracts involuntarily. Skeletal muscle is also referred to as striated muscle because its composition gives it a striped appearance.
Fast-Twitch Muscle
The hamstring muscles are composed primarily of fast-twitch muscle fibers. Fast-twitch fibers are responsible for quick, strong, explosive movements. This is in contrast to slow-twitch muscle fibers, which are responsible for slower movements carried out over longer periods of time. Each muscle in the body has a balance of each type of fiber, but in some muscles, like the hamstrings, that balance is weighted more heavily toward one type or the other.
Function
The biceps femoris helps bend the knee and extend the hip behind you. It is also responsible for your ability to rotate your foot outward at the hip and knee joints. The semimembranosus and semitendinosus also function in bending the knee and extending the hip. They cooperate to perform the opposite rotational movement of the biceps femoris. The semimembranosus and semitendinosus rotate the foot inward with motion at the hips and knees.
