Meet your Butt Muscles

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- Your Behind Defined

Meet the three main muscles that make up your rear view.

1. Gluteus minimus
The smallest of the glute muscles lies directly under the gluteus medius.

2. Gluteus medius
This pork chop-shaped muscle sits near the outside of your pelvis.

3. Gluteus maximus
True to its name, the maximus is the biggest muscle in your body.

A few weeks after my experiment with Dr. Herrera, I am at the gross anatomy lab of the Mount Sinai Medical Center. On a blindingly white table lie pieces of various cadavers' lower extremities, including glute muscles on a tray. Without manually and with chemicals so that medical students may examine just the muscle or connective tissue -- there are only brownish pink slabs of fibers. No matter whether our butts are perfectly rounded or flat and droopy, they all look more or less the same on the inside.
fat on them -- it's stripped off 

Your glutes are made up of three main muscles: the maximus, the biggest portion of your behind; the pork chop-shaped medius near the top of your hips; and the minimus, which is tucked beneath the two aforementioned muscles.

The gluteus maximus gets all the attention, but the medius does just as important a job. Along with the minimus, "it's responsible for stabilizing your pelvis when you walk or anytime you're off balance," Dr. Herrera says. Without it you would lurch from side to side like a drunken sailor as you lifted your feet.

Because everyone's gluteal muscles attach at the same points on their skeleton -- the maximus runs diagonally from the top of the pelvis to the femur and iliotibial band on the outside of the upper thigh -- if you have a tall pelvis, "you may have a longer, squarer shape to your posterior," says Kimberly Topp, PhD, chair of the department of physical therapy at the University of California, San Francisco. "With a wide pelvis, you may have a more horizontal orientation of the muscle." If your back is a bit more curved, your buns may appear more lifted. You can work on your glutes and change their size and shape (more on that later), but some people start off with the nicely rounded gluteal muscles that inspire pop songs, while many of us do not.

So just how much of our butt shape is predetermined by genetic roulette? Up to 70 percent of the body's overall shape, and therefore your rear's, is genetic, Dr. Marango says. "The rest is going to be influenced by nutrition, exercise, sleep, posture -- anything outside your genetic code." You can thank our gender for the fact that women generally have more posterior padding than men, and thank Mom and Dad for where that padding tends to congregate.

But no matter what shape butt you start out with, it will morph later in life. At the same body mass index, says Matthew P. Reed, PhD, a research associate professor at the University of Michigan in Ann Arbor who studies body shape, a woman of 50 has a flatter butt profile than a woman half her age because shifting hormone levels postmenopause signal the body to store fat in the belly rather than the buns. "You'll see less fat at the side of the butt and more up at the iliac crest at the top of the pelvis," he says. "That's why pants don't fit the same way as you get older."

The good news? Even with a little less padding, you can give gravity a good run for its money by getting your butt muscles firm and keeping them that way.