The word hamstring originates from the old English word Hamm, which means thigh. The term commonly refers to a group of thigh muscles and the respective tendons. It is thought that they are now known as hamstrings due to the rod like stringy appearance of the tendons.
The hamstring muscles are situated at the back of the thighs. They start at the ischium, the sitting bones at the base of the pelvis. They span the thighs and cross the hip and knee joints. Three muscles make up the hamstring group, the semimembranosus is situated along the innermost part of the thigh, and the semitendinosus sits alongside it and towards the center. The biceps femoris extends along the center of the back of the thigh, with the lower portion reaching to the outside of the leg at the knee joint.
Some tendons attach muscles to each other, and others, including the hamstring tendons connect muscles to bone.
The hamstring tendons are situated at the inferior end of the muscles. The semimembranosus and the semitendinosus tendons cross the knee joint and attach to the tibia, commonly known as the shin bone. The bicep femoris tendon attaches to the head of the fibula, the outer bone of the lower leg.
Hamstring tendons comprise of soft collagenous tissue and they transfer the force generated by the muscles along to the bones. They are responsible for knee flexion, moving the foot towards the buttocks. When the muscles contract they pull on the tendons which then pull on the bone, and this action results in movement. The hamstring muscles and tendons play a role in knee extension, moving the foot away from the buttocks. However, the quadriceps, the muscles at the front of the upper thigh are mostly responsible for this action.
Most hamstring injuries occur within the muscles, or where the muscles connect to the tendons. The most debilitating of hamstring injuries strike in the tendons which can sometimes completely tear away from the bone. On occasion trauma may be so severe that a piece of bone breaks away with the tendon, and this is referred to as an avulsion injury or fracture.
Many knee injuries involve the ligaments, which incidentally connect bones. When the anterior cruciate ligament (ACL) within the knee is torn or injured to the extent that surgery is required, a piece of hamstring tendon is taken to replace the damaged ligament. It is usually the semitendinosus tendon which is taken for this type of surgery. This however, does not mean that ligaments are more important than tendons. Without the tendons the hamstring muscles would not be able to link to the bones. The necessary force would not be generated and therefore movement pertaining to the lower leg would be impossible.