The structure indicated is the gluteus maximus.

The gluteus maximus 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 gluteus maximus muscle is the largest of the gluteal muscles, and is also the largest muscle in the human body.

Origin: Posterior surface of ilium, posterior superior iliac crest, posterior surface of sacrum and coccyx

Insertion: Primarily to fascia lata at iliotibial tract. Upper posterior surface of femur at gluteal tuberosity.

Action: Hip extension, lateral (external) rotation, abduction (middle and upper fibres)

Innervation: Inferior gluteal nerve

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