When you stand up from a chair, you are extending your thigh at your hip joint. The main extensors of the thigh are the gluteus maximus, the adductor magnus and the hamstring muscles. Several other muscles also help to extend the thigh, including some of the other gluteal and adductor muscles.
The gluteus maximus muscle is the most powerful extensor of the thigh. It has a broad origin, from the rim of the back of the pelvis, the sides of the sacrum and the tailbone, as well as several pelvic ligaments. It inserts into the back of the thigh bone, or femur, as well as into the iliotibial band, a sheet of tough connective tissue that runs down the side of the thigh. When it contracts, it pulls the femur backward, extending the thigh.
The hamstrings are also important extensors of the thigh. There are three hamstring muscles. All attach to the pelvis at the sitting bone, or ischial tuberosity. The biceps femoris inserts onto the top of a long bone on the outer shin called the fibula. The semimembranosus and the semitendinosus insert onto the inner upper part of the shin bone, or tibia. Because they attach below the knee, the hamstrings have a dual role, flexing the knee joint as well as extending the thigh.
The adductor magnus muscle, located on the inner thigh, has two parts, or heads. The front, or anterior, part originates on the bottom of the pelvis and inserts along the length of the back of the femur. The posterior head runs from the ischial tuberosity, very close to where the hamstrings originate, to the lower part of the femur. The posterior head is a strong extensor of the thigh, with a similar action to the hamstrings.
Several other muscles, such as the anterior part of the adductor magnus, secondarily extend the thigh. In fact, if the femur starts in a flexed position, most of the adductor muscles can help to extend the thigh. The gluteus medius muscle, located on the side of the pelvis, runs from the upper rim of the pelvis to the outer thigh bone. It primarily helps to stabilize the pelvis, but its posterior fibers can assist with extending the thigh.