The hamstring muscle group is primarily responsible for knee flexion.
The hamstring muscle group, located on the posterior (back) of the thigh, consists of three main muscles:
- Semitendinosus: Located on the medial (inner) side of the thigh.
- Semimembranosus: Also located on the medial side of the thigh, deep to the semitendinosus.
- Biceps Femoris: Located on the lateral (outer) side of the thigh. It has two heads: a long head and a short head.
These muscles all originate from the ischial tuberosity (a part of the pelvis) except for the short head of the biceps femoris. They cross the knee joint posteriorly and insert onto the tibia and fibula. This positioning allows them to effectively flex (bend) the knee.
While the hamstrings are the primary knee flexors, other muscles also assist in knee flexion, though to a lesser degree. These include:
- Gastrocnemius: A calf muscle that also crosses the knee joint.
- Popliteus: A small muscle located at the back of the knee.
- Sartorius: A long, thin muscle that runs across the front of the thigh.
- Gracilis: A medial thigh muscle.
However, the hamstring group bears the major responsibility for this action.
