Here's how to ensure the largest muscle in your body isn't also the laziest.
The Science Behind Your Booty
I'm alone with a handsome young doctor who has his hands down my shorts, but neither of us is too happy about it. Joseph Herrera, a doctor of osteopathic medicine, furrows his brow as he struggles to untangle a 48-inch lead attached to an electrode that he has taped to my butt. The electrode is slick with conductivity gel and he's holding it in place as he attaches the other end of the wire to the electromyography (EMG) machine, a hulking piece of equipment that measures electrical activity in your muscles. Dr. Herrera, the director of sports medicine at the Mount Sinai School of Medicine's rehabilitation department in New York City, has agreed to help me figure out what, if anything, is going on with my glutes when I'm not explicitly working them out. Which is almost never. And it shows. Or rather, doesn't show on my unremarkable, flat butt.
On Dr. Herrera's signal, I mimic myriad daily moves, including walking, jogging at catch-a-bus speed, crouching to pursue a toddler, and shifting my weight from hip to hip, as I do waiting in line at the ATM. With each movement, the line on the machine's screen gives a pathetic little jump; the readout looks like a lie detector test minus the lies. Dr. Herrera shrugs and says that this is normal no matter what shape butt you have. "Your glute muscles are typically not that engaged throughout the day," he explains. "The quads do a lot of the heavy lifting."
Then I step up onto a stair and the screen's readout shows a violent spike. Finally, my gluteus maximus is doing what it was born -- or rather, evolved -- to do: keeping me upright as I stride, especially during more balance-challenging hikes up stairs.
Among the things that differentiate us from our knuckle-dragging primate ancestors are not only our big brains but our big butts -- or, as Stephanie P. Marango, MD, a physician and anatomy expert in New York City, puts it, our well-developed gluteus maximus. "That ability to be bipedal is a huge deal," Dr. Marango says. "And it's this muscle that is really doing a major component of that."
It's ironic, then, that the largest muscle in our body, which has given humans their signature upright strut, spends most of the day metaphorically sitting on its butt.
Anything Butt Normal
While we obsess about shaping the ideal butt, the multibillion-dollar jeans industry and even the government have been hoping to define exactly what the typical one looks like. For years the rule of thumb for chairs was that seats be at least 18 inches wide -- to fit 95 percent of female fannies, because our hips outspan men's -- or about three inches wider than this magazine when it's open. (Go ahead, sit on it to see how you stack up.) Data confirmed that our collective backside is indeed spreading.
When 3-D scanning technology was developed, the air force, along with a group of automotive engineers and apparel companies, was the first to use it, conducting the CAESAR (Civilian American and European Surface Anthropometry Resource) study, published in 2002. Thanks to CAESAR, "you can get a digital replica of a whole butt in 3-D," says Kathleen Robinette, PhD, an anthropologist at the U.S. Air Force research laboratory at Wright-Patterson Air Force Base in Ohio and director of CAESAR research. "You can prepare a before-and-after scan, and see how apparel affects shape." The armed forces used such data to reconfigure women's uniforms, adding "women's" cuts -- that is, a wider hip and butt area -- to cover many more of their female soldiers without resorting to costly alterations.
The apparel industry subsequently pulled together with the U.S. Department of Commerce to conduct the Size USA study, using different 3-D scanning with a much larger, 10,000-person sample that included a wider range of body mass indexes, ages, and ethnic backgrounds.
From these studies, we've learned a lot about the average American female butt -- mainly that there's no such thing.
The Size USA study found that 86 percent of women 26 to 55 years old who are between five foot two and five foot seven and weigh less than 160 pounds have a seat circumference of 37 to 43 inches at the widest point. Those numbers, however, should be taken with a grain of salt, Robinette says. "As soon as you start trying to make an amalgam, an 'average' female body, you lose touch with reality," she explains. In other words, you can have a statistically average butt size, but your weight, height, or waist measurement is unlikely to be likewise average. A short woman with a so-called average butt size would have a proportionately large bottom, whereas a tall, heavy woman with an average butt size would have a proportionately small one.
In other words, shopping for your dream butt in celebrity magazines is a bad idea.