The research showed that a too-tight sports bra affected the women’s breathing, causing them to take more shallow breaths and a few extra breaths per minute.Additionally, when the band was loosened, the runners consumed less oxygen at a given speed. This meant they had better running economy, which is the amount of oxygen consumed for a given speed.

Experts recommend that runners wear a sports bra that offers adequate support but isn’t too tight around the rib cage.

“People ask, ‘What sports bra should I wear?’ I say, ‘Wear one that is correctly fitted,’” said Shalaya Kipp, the lead author of the study who is now a postdoctoral fellow at the Mayo Clinic in Minnesota. “That’s probably the biggest thing that would help.”

How bra tightness affects breathing and running efficiency

Kipp, a 2012 Olympian in the steeplechase, also participated in the study as one of the nine subjects. The runners wore compression bras designed for women with bra sizes ranging from 30 to 34, which represents the band size around the rib cage, and a B or C cup.

The authors hypothesized that as the bra was tightened around the rib cage, respiratory or lung function may become compromised. “A decline in lung function makes breathing harder, which is especially critical during exercise or daily physical activity,” Kipp said.

To measure that, the researchers inserted a catheter that goes up the nasal passage and down into the esophagus.

“It was quite invasive,” Kipp said. “It's the hardest experimental protocol I've ever had someone do.”

The runners completed various treadmill tests measuring running economy and adjusted the tightness of the sports bra each time.The modified version of Lululemon’s Energy sports bras were customized so that the underband tightness could be adjusted around the rib cage without participants taking off the bra.

Overall the researchers found that the runners took deeper, less frequent breaths and ran more efficiently when the underband was loosened — all of which can affect running performance and comfort.

Dharini Bhammar, an assistant professor in internal medicine at Ohio State University, called it “mechanistically a very strong study.”

“No one’s looked at underband tightness separate from total breast support, and no one has done work of breathing with sports bras, because that’s a really difficult measurement,” said Bhammar, whose research focuses on respiratory physiology. “So that’s a really cool, kind of novelty in the study, which makes it special.”

How the findings apply to runners

Going from a tight to looser underband had, on average, a 1.3 percent improvement in running economy, the study found. While thatmay not seem like a lot, it’s enough to affect running performance. Kipp said that a 2 percent change in running economy would translate to a three-minute improvement for a three-hour marathon runner.

Olympic distance runner Kim Conley, who was not part of the study, said she had never given the tightness of her sports bra and breathing much thought before, but Kipp’s study has changed that.

“For an elite runner, that’s significant, especially as qualifying standards get harder and harder,” Conley said. “Everyone is looking for even a 1 percent gain. I think it’s significant and something I’ll think about now.”

Laura Hernandez, a 25-year-old Boston resident, is a trail runner who is training for her first road marathon this year. She runs at a pace of about eight to nine minutes per mile and said that she has been “shocked” that sports bras don’t come up in conversations about running gear more often. “It’s just as essential as a pair of sneakers,” she said.

She said that she tends to wear tight sports bras because she’s worried about chafing. “But now that I’ve seen this study, I will be thinking about where’s the line between not too loose where it’s chafing, but not too tight where it’s hindering my breathing in any way,” she said. “I think I had only looked at one side of that line.”

There’s also a difference between how a sports bra feels at rest and while exercising. Kipp noted that most women pick a sports bra based on the former. She recommends trying on a sports bra the same way you do a running shoe. “Get your ventilation up,” Kipp said. “Do some deep breaths in the changing room.”

Don’t forget about support

The study focused on the tightness of the band around the rib cage, not overall breast support.Other studies that included women with different bust sizes have found that greater breast supporthelped runners and is associated with reduced oxygen consumption and greater running economy.

Douglas Powell, the director of the Breast Biomechanics Research Center at the University of Memphis, said that individuals with larger breasts benefit more from the high-support sports bras. He was the co-author of a 2022 study that found running economy improves as you increase sports bra support.

In that study, 14 participants ran on a treadmill in different sports bra conditions. The researchers recruited recreational runners who had bust sizes ranging from B to DD cups.

They found that, on average, there was approximately a 7 percent improvement in running economy when participants ran in a high-support sports bra.

“We believe that by reducing breast motion, we reduce that jiggle effect and make running more efficient, and that that contributes to improved economy,” he said.

Powell called the new study’s data “interesting” and said it doesn’t contradict the conclusions from his own research because it focuses on different aspects of sports bra design. “I would take away that we need to start considering band tightness as a factor to pay attention to,” he said.

Hailey Fong, the lead author in the 2022 study and a doctoral student researching sports bra biomechanics at the University of Memphis, said that the participants in the studies are very different, and welcomes more research in the area.

“I think the biggest thing to take from all of these papers and all of this research that we’re doing is that support matters, the sports bra you’re wearing matters,” she said.

Do you have a fitness question? Email YourMove@washpost.com and we may answer your question in a future column.