Chests: They do nothing to help with running. They cross the road and get in the way – they need their own equipment and in some cases cause discomfort.

In a 2013 study of runners at the London Marathon, 32 percent said they experienced occasional pain in their chests. Of those, 17 percent sometimes curtailed their training because of breast pain.

But the news isn’t all bad. Researchers continue to study breast movement during exercise, bra technology is improving all the time, and evidence is mounting that running is one of the best things you can do to protect yourself from breast cancer.

Here’s what scientists know — and runners should — about taking care of their breasts while running

The body is not naturally nice to the breasts

“Depending on the size, it can be pretty heavy,” says Dr. Andrea Schiffel, director of the cancer rehabilitation program at the Mayo Clinic in Rochester, Minnesota. “The body doesn’t support them well. There is not much to keep them stable and frozen.” Just your skin and some laces.

Other parts of the body are more fortunate. “If you think of something like the abdominal fascia, it’s incredibly powerful,” Scheffel says. “You can run and not shake your insides, because we have a strong fibrous envelope. But this is not true for breasts. They basically have no support. However, they do have pain receptors. And when the limited elements of support are extended, it hurts.” Therefore, a sports bra should be used.

They move more than you think

Michelle Norris, senior research associate in the Department of Sport and Exercise Science at the University of Portsmouth, studies breast movement and tests breast support products such as a sports bra in the lab.

To do this, she and her colleagues have women run on a treadmill topless (bless these ladies), then in low- or high-support bras. They use 3D motion capture to look at the breasts’ range of motion.

“We have some very willing participants — and we owe them a lot,” Norris says.

In the lab, Norris and her colleagues found that breasts move in an eight shape. Not just up and down — that vertical movement is what most runners think of — but side to side, forward, and backward as well. “[The breast] is just a lump of tissue, not a muscle,” Norris says. “It is not a rigid structure. It can move in all three dimensions when we run.” And she is.

When you add all this movement in three flats together, the breasts — unsupported — move about 15cm while running, Norris says. (Different modulus indicates different numbers, depending on the cup size of the group being tested.) About 50 percent of the movement is in the vertical position, then 25 percent is side-to-side movement, and the other 25 percent is forward-backward motion.

Regardless, that’s about six inches of movement with each step.

A good sports bra is a must

With all this movement, runners need support from a sports bra.

Shop carefully, experts urge. Try lots of different bras. Get the best fit possible from a sports bra. Find a running specialty store with a knowledgeable female sales representative. Buy a high support running bra.

But the news isn’t all bad. Researchers continue to study breast movement during exercise, bra technology is improving all the time, and evidence is mounting that running is one of the best things you can do to protect yourself from breast cancer with a sports bra.

Some women prefer compression, while others prefer packaging models. Did not matter. The best bra is the one you’re happy to wear.

“It’s all about bra fit here,” Norris says. “It is one of the most important aspects of any woman in sports. We always say it has to fit you very well and put you at ease.”

If you get a sports bra with a hook in the back, you should be able to wear it in the loosest buckle. When the washing is done and becomes looser, you can use tighter grades.

Your speed does not affect movement

Here’s what Norris, (a runner herself), surprised from the research in her lab: No matter how fast you’re going, your breasts move the same amount. “I used to think that the faster I ran, the more my breasts moved. That’s not really the way things are,” she says. “If you’re running at 10kph (saddle up around 10:00), they’re actually moving with maximum displacement. If you’re running 14 kilometers per hour (about 7:00 pace), they won’t move any further.”

The lesson for runners: Don’t think if you’re going to run a long slow run, you can use a less supportive bra. You need a high supportive bra all the time.

Are they shrinking?

Athletes new to a running program often notice a dramatic reduction in breast size. What is happening?

In essence, Norris says, running doesn’t make your breasts smaller. But the breasts are made up of fat and fibrous tissue. “So if a person trains, eats well, and generally reduces body fat, it is reasonable to believe that they can also reduce breast size because they decrease the fat in their chests,” she says. “It works more like reducing total body fat rather than just reducing areas.”

Pain must be taken seriously

There are different types of breast pain – most of them can be easily explained. But you should not ignore it. “Breast pain is pain,” Scheffel says.

A lot of breast pain during exercise is caused by insufficient support – a sports bra falling off during training. For women with large breasts, Schiffel says, it can be difficult to find the right support from a sports bra.

Many women experience excessive sensitivity in their breasts in the days leading up to their period. It is highly variable from person to person. If the pain is severe enough, you may want to choose a lower impact exercise. “I realize this too will pass, and while you’re sensitive, take it easy,” Scheffel says.

If you want to get strong, it’s OK to use an over-the-counter anti-inflammatory such as ibuprofen. And watch your salt intake, which can lead you to retain water during perimenopause, which increases swelling in your breasts, she says.

Any unusual pain, see a doctor. “A small-breasted woman has new pain in the onset that’s hard to explain and more worrying,” Scheffel says. “You’ll probably see a doctor.”

Annoyance can kill your motivation

Recent research from the Portsmouth Laboratory shows that breast comfort affects a woman’s desire to run. It can be hard enough to get motivated to pull yourself out the door for a few miles. Without realizing it, you may be letting your breasts ruin your workout plans. “If you pick up the wrong bra, it won’t go,” Norris says.

Breast cancer occurs in runners less frequently than in sedentary women

Scheffel says recent studies have demonstrated a clear link between exercise and lower rates of breast cancer.

“Primary prevention means that it prevents you from getting it in the first place,” she says. “Secondary prevention means it prevents it from coming back. For breast cancer, there is very strong evidence for primary and secondary prevention through exercise.”

A 2014 study showed that running beats walking in breast cancer survival rates. Vigorous exercise was better than moderate exercise for the women in the study.

All the more reason to go to the workout—with a good sports bra, of course.