Is it so bad for us really? Sports bra designers, engineers, and breast health experts weigh in

By Kylie Gilbert

"I can totally just run in my yoga bra, right?" you've probably mused at least once. Well, we have an answer for you in just one word: that would be a big fat "no".

We tapped the authorities in breast health and sports bra mechanics-including designers, engineers, and researchers-to give us the low-down on what actually happens to our breasts while we run, the long-term damage that can come from not having the right support, and what to really look for when sports bra shopping to make sure we are as protected (and stylish!) as possible.

Breast Anatomy 101

The need for the proper sports bra all comes down to our basic anatomy, explains Joanna Scurr, Ph.D., who heads the University of Portsmouth's Research Group in Breast Health, a group internationally renowned for their research on the biomechanics of the breast, and works with brands like Under Armour on sports bra development. There are no muscles within the breast (the pectoris major and minor sit behind our breasts) so all of our natural support comes from our skin and the Cooper's ligaments, which are between the inner side of the breast skin and the pectoral muscles. These ligaments are extremely thin (the thickness of a piece of paper) and delicate and are weaved throughout the breast like a spider web, Scurr explains. And they're not meant to provide support (we know, seems like quite the oversight!) but rather to protect our glandular tissue. (Want to know more? See 7 Things You Didn't now About Your Boobs.)

What's the Damage?

When you're running, your breasts move not just up and down (your bounce factor) but also side to side and in and out, in a pattern that looks much like the infinity symbol (or a side figure 8) explains Laura O'Shea, a sports technology engineer and senior researcher at Progressive Sports Technologies at Loughborough University, who conducts biomechanical testing focused on 3D breast motion for brands including Sweaty Betty.

"When exercising, the natural tendency of our breasts is to move independently of each other, up to about 8 inches from where they lay at rest," explains Kate Williams, Senior Director of Women's Design at Under Armour, who works closely with Scurr to test and design the brand's sports bras. "That's a lot of movement." Um, you're not kidding!

In the short-term, not wearing a supportive enough bra during this motion can result in breast pain and discomfort as well as back and shoulder pain, but if you're consistently running without adequate support, you run the risk of irreversible tearing of the breast tissue plus stretching of the skin and those Cooper's ligaments, which has been linked to breast sagging, O'Shea explains.

Does Size Matter?

Although it might seem as though smaller chested women need less support than their larger-chested friends, choosing the right sports bra actually isn't based on size, since research shows that, even if you're a AA, the breasts move in that same figure 8 motion, explain O'Shea and Lisa Ndukwe, senior designer for Sweaty Betty.

Larger breasts are heavier breasts, and therefore have the potential to cause more damage, Scurr explains, but there's research that suggests women with smaller breasts may have weaker natural support within their breasts (i.e. skin and ligaments), meaning they need just as much support from the right sports bra as larger-breasted woman. Not to mention, breast pain can affect women of all sizes equally, since size actually isn't the key factor but rather our hormonal cycle, she adds.

Bottom line: Whether you're an A cup or a G cup, you'll get just as much benefit from a supportive sports bra. (Check out The Best Sports Bras for Small Boobs.)

Fit Is King

At this point, we've probably made the case that a high-impact bra is necessary for running to minimize all that scary-sounding damage. But more than anything, the right bra comes down to fit.

"We work with manufacturers to develop the best products in the world, but if they're not worn in the correct size they won't work optimally," says Scurr. What's more, "what fits one person who is a 34D might not fit another person who is a 34D," she explains, since fit depends on a myriad of factors such as the position of the breast and the shape of the chest wall and shoulders.

So forget the numbers on the tape measure, and check these five key areas according to Scurr:

1. Underband: This is the foundation of any bra and an appropriate fit is crucial. There should be no more than five centimeters (or about two inches) give in the underband, and it should be level all the way around the body.

2. Shoulder strap: You shouldn't be able to pull them up any more than five centimeters (almost two inches).

3. Cup: No breast tissue should be spilling out of the cup or be compressed by the cup.

4. Underwire: You don't want it sitting on any breast tissue (especially under the arm)

5. The center point: If you're wearing a sports bra that encapsulates each breast separately, it needs to sit flat on your chest (i.e. no space between your bra and body). If not, it means your cups are too small.

And Sarah Barber, a garment technician for Sweaty Betty, offers up a few other factors to look out for when sports bra shopping:

1. Compression, which helps to reduce free movement of the breast tissue, and/or encapsulation (these look more like everyday bras and encapsulate each breast separately), which holds the breast in place to prevent movement. (A combination of both, as seen in designs like the Sweaty Betty Ultra run bra or Under Armour's high-impact bra, is the best you can get.)

2. Coverage of the upper chest, which helps to prevent upwards motion, as well as a firm hem band to prevent downward motion.

3. Coverage of the sides of the breast tissue, which is very important to reduce the movement sideways.

4. A firmer fabric made with minimal stretch to help minimize too much movement.

And some things to avoid: Very stretchy straps or fabric, since this will counteract the rest of the bra and allow the bust to move up and down, and any sports bra that's too revealing, as this generally means there's less protection against movement.

The good news? As brands like Under Armour and Sweaty Betty and more continue to team up with universities studying the latest in breast health research to design their sports bras, incredible style, performance, and protection in one product is becoming more attainable than ever. "Avoid compromising on any aspect of your bra. Fit, mobility, breathability, comfort and looking good…these are all important and achievable," says Williams.

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