The structure indicated is the quadratus femoris muscle.

The quadratus femoris muscle is one of the muscles of the gluteal region. The muscles in the gluteal region are divided into a superficial and deep group. There are four muscles of the superficial group:

1. Gluteus maximus
2. Gluteus medius
3. Gluteus minimus
4. Tensor fasciae latae

There are 5 muscles in the deep group:

1. Gemellus superior
2. Gemellus inferior
3. Quadratus femoris
4. Piriformis
5. Obturator internus

The deep muscles of the thigh are much smaller than the superficial group of muscles and generally serve to laterally rotate the femur at the hip joint.

The quadratus femoris muscle is the most inferiorly located muscle in the deep gluteal region of muscles and is named according to its approximately rectangular shape.

Origin: Lateral margin of obturator ring (above ischial tuberosity)

Insertion: Quadrate tubercle and intertrochanteric crest of proximal posterior femur

Action: Lateral rotation of hip. Assists with adduction of hip

Innervation: Nerve to quadratus femoris

Learn more about the anatomy of the thigh and gluteal muscles in this tutorial.