You can only hope to contain them

But nowhere do breasts pose more of a liability than in the world of elite women's gymnastics, where any hint of a curve can mean early retirement. "Look at missiles that shoot into the air, batons that twirl -- they're straight up and down," says Joan Ryan, author of the 1995 expose of gymnastics and figure skating, Little Girls in Pretty Boxes. In order to stay stick straight, elite gymnasts undereat and overtrain, which delays menstruation. "You can't afford to have a woman's body and compete at the highest level," Ryan says.

To keep competitors from reaching puberty, coaches would push away bread baskets at the table and riffle through their belongings to sniff out hidden treats, says Dominique Moceanu, who was, at 14, the youngest, teensiest competitor on the 1996 gold medal USA Olympic team. "The sport pushes us to be breastless little girls as long as possible," she says. But though breasts were forbidden, privately "we longed for them."

Laying off the carbs may do the trick for preteens, but most adult athletes can't starve their boobs out of existence. So every year, some competitors head to the Marina del Rey, Calif., office of Dr. Grant Stevens in pursuit of a streamlined frame.

Stevens, a plastic surgeon with backswept blond hair and a boyish face he maintains through injections of Botox and Restylane, is known as the inventor of a scalpel-free procedure that leaves women multiple cup sizes (and up to $15K) lighter with minimal recovery time. The doctor says he's treated volleyball players, golfers, ballet dancers and assorted Olympians, though he won't name names. (He trains his lasers on men as well, because nothing calls their abilities into question like a pair of man boobs.) But many of his patients have already lost out on the years of weightless chests needed to reach the highest levels of competition. At the size they walk in with, Stevens says, "they would never get to be a pro athlete."

Not all athletes agree that large breasts constitute a competitive disadvantage. In 2009 then-18-year-old Romanian tennis player Simona Halep announced she was having her breasts surgically reduced from a 34DD to a 34C, saying they were slowing her reaction time and causing back pain. Upon hearing about Halep's plan, retired South African beach volleyball player Alena Schurkova took the opportunity to launch a big-boob-pride campaign. "If she does this, it sends out the message that girls with big boobs can't play sports, and that is just wrong," Schurkova said. "I am 32E, and I have never found them to be a problem. I could be double what I have" -- 6 pounds per boob! -- "and I would still be okay to perform."

Maybe so, but Halep's downsizing appears to have paid off: Before she went under the knife, she was ranked around 250; by 2012, she'd cracked the top 50.

WHEN KATHERINE SWITZER became the first woman to don a bib at the Boston Marathon in 1967, science was unprepared to grapple with the female frame in motion. Critics warned her that the repetitive movement could cause her breasts to atrophy and her uterus to drop out of her vagina. (She ran the race in a flimsy fashion bra under a T-shirt and sweatshirt.) The sports bra wasn't even invented until 10 years later, when a group of women sewed two jock straps together and slung them over their shoulders. (An early version of the original Jogbra is now preserved behind glass at the Smithsonian.)

The advent of the sports bra "was like the birth control of the women's sports revolution," Switzer says. Still, for the next 10-plus years, scientists stayed out of athletes' efforts to make their breasts stay put. Finally, in 1990, Oregon State University researcher LaJean Lawson invited female subjects onto a treadmill and filmed the results in the first-ever study of breast movement. Today, labs have sprung up in the U.K., Australia and Hong Kong to study breast biomechanics -- and deliver the results to bra manufacturers seeking to develop cutting-edge solutions.

At Britain's University of Portsmouth sits a laboratory outfitted with black floors, black curtains and a treadmill surrounded by infrared cameras aimed directly below a subject's neck. Here, Jenny White, a lecturer in the school's sport and exercise science department, invites women to take off their shirts, outfit their breasts and torso with reflective markers, step onto the treadmill and break into a jog. On a set of monitors, White and her group of female researchers track 3-D images of the migrating dots in an attempt to better understand how breasts move through space. Her research has confirmed that size does matter: As breasts get bigger, they accelerate quicker, move faster and bounce higher. What she doesn't know -- yet -- is whether these speedy breasts really slow athletes down.

Part of the problem is that, 23 years after Lawson's seminal study, data collection is limited to relatively sluggish treadmill jaunts. "We can't take them to the park to do a decathlon," White says. It's easy to get a group of women to run at the same low speed. It's almost impossible to get them all to jump to the same height, swing a racket at the same trajectory, punch with the same power or run at a world-record pace. And while breasts are all built from the same basic elements, the proportions and densities of the tissues vary among individuals; they fluctuate throughout the month; they transform in puberty, pregnancy, motherhood and menopause. "It makes our job quite difficult," she says.

The research does reveal the self-selection process by which some women end up on the court while others -- disproportionally, those with bigger breasts -- are relegated to the stands. Hormones could play a part: "Studies suggest that curvier women may have higher estrogen levels, while higher testosterone levels are associated with more competitiveness and aggression," says Florence Williams, author of Breasts: A Natural and Unnatural History. "So it's possible that if you have more estrogen, you might be somewhat less inclined to compete." Other factors include the pain and embarrassment associated with larger breasts in motion. Deirdre McGhee, a senior lecturer at Breast Research Australia, has been studying breast support and bra fit for the past decade -- and watching young athletes drop out as their breasts pop up. "They're embarrassed. They don't want to talk about it. And so they stop," McGhee says. "They just don't move."

McGhee counsels women to engage in physical activity that puts less of a strain on their breasts. But as the breasts get bigger, the field narrows. Busty ballet dancers are transferred to hip-hop. Postpubescent gymnasts get put on the rings. Runners are instructed to play in the water instead.

If all else fails: yoga.

THE PHYSICAL AND social barriers that come with a larger cup size mean that the Schurkovas and Haleps of the world stand out. Nothing appears to be weighing Serena Williams down on the court, but her measurements represent such an outlier that when Caroline Wozniacki stuffed her tank top and skirt with towels at a Brazilian exhibition match last year, everyone knew which great she was ridiculing. Serena took the impression in jest, dismissing charges that it was racist. (Apparently, Wozniacki's temporary augmentation didn't weigh her down either; she won the point.)

But even when an athlete's breasts aren't notably large -- and no matter how expertly she works to contain them -- she still must contend with oglers who fixate on her peaks instead of her performance. When Halep announced her plans for surgery, more than 1,400 men signed a petition begging her to stay busty. Water polo matches are so notorious for nipple slips that bloggers hover over the pause button in hopes of glimpsing an areola. And in the rare case that a breast is on full display, all hell can break loose. Even as Carmouche was threatening to break her neck, Rousey felt as if her falling bra was a life-or-death situation too. If she failed to get a grip, "I'd be morbidly embarrassed," she says.

Nebiat Habtemariam can relate. At the 1997 world championships, the 18-year-old Eritrean runner suffered the longest wardrobe malfunction of all time during a qualifying heat for the women's 5,000-meter run. Lacking her own gear, Habtemariam asked to borrow another runner's red singlet for the race. What she failed to borrow was a sports bra. She spent her 18 minutes on the track with one breast perpetually in view. She didn't leave her hotel room for the rest of the week.

But the run of shame wasn't the end of Habtemariam's story. She kept running -- in two more world championships, three Olympic Games and countless other competitions. Last year she was the third woman to finish the Milano City Marathon, her lime-green and blue sports bra securely in place. It was further confirmation that the world's best athletes are those who have managed to transcend the limits -- and the addendums -- to the human body. Or as Rousey put it about her one-two punch of neutralizing Carmouche and her little black bra at the same time: "Multitasking!"

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