The gluteus maximus is the muscle in the human body that makes up the majority of the buttocks. It is one of three gluteal muscles, which also includes the gluteus medius and the gluteus minimus. Not only is it the largest muscle of the gluteal group, it is the largest muscle in the body. It extends from the top of the illium, the largest pelvis bone, and goes down to the groin area by the upper thigh.
Its main action is to extend the thigh at the hip to assist in lateral rotation of the upper leg. It also serves as an extender of the pelvis that goes to the thigh. The gluteus maximus helps support the trunk of the body; since this is so demanding on the muscle, many people consider the the gluteus maximus to be the strongest muscle in the human body.
In addition to the muscle mass, there are other components inside the gluteus maximus. There are are three bursae, which are small sacs filled with fluid to help prevent the muscle from scraping against the bone. There is also the large inferior gluteal nerve, which is vital to the proper performance of the muscle, as it communicates actions made by the muscle to the brain. The major superior and inferior gluteal arteries are also contained within the large leg muscle.
The gluteus muscles are often referred to as "glutes" by trainers and athletes, and are usually the focus of workout programs that claim to be able to tone and define the buttocks. Since the gluteus maximus is the largest muscle of the group, many toning and strength building exercises are focused on this muscle in particular. Squats, lunges and various kinds of presses can all help define and strengthen the muscle. Running, rowing and nearly all other physical activities that require the use of abdominals and buttocks use the muscle as well.
The muscle is so often used in day-to-day life, exercises and sports, so injuries to it are fairly common. When the muscle is damaged, a person's posture can be severely affected, usually resulting in slouching. An injury to the muscle also makes the action of raising or lowering the body extremely painful, and sitting down often doesn't help to alleviate the pain. It can actually make the pain much worse because it adds pressure to the surface of the muscle. Injuries to the gluteus maximus can most often be repaired completely with rest and limited movement. Straining the muscle during an injury may make it worse.