Wearing a sports bra that's too tight may impact your workout, study finds

Choosing what to wear to exercise can be a matter of fashion, comfort or convenience, but new research suggests that women may want to pay special attention to one particular aspect of their workout wear: the sports bra.

When a sports bra is too tight during exercise, it can impair breathing and impact energy and performance, according to a study published in the National Library of Medicine.

The study, which was funded by the Lululemon Athletica athletic apparel retailer and conducted by the University of British Columbia, involved nine highly trained female runners with normal lung function. The athletes ran on treadmills while wearing custom sports bras with lower bands that could be adjusted to loose, tight or standard sizes selected by the runners. Throughout the study, the subjects adjusted the tightness of the sports bras differently for varying treadmill workout sessions.

Researchers tracked the runners' breathing and lung function to determine how the pressure of the sports bra's underband on the rib cage can impact respiratory function and the mechanical work of breathing during exercise. 

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They found that sports bras that were too tight caused the runners to take a few extra breaths each minute, and that the breaths were more shallow. When the bands were looser, the women took less frequent breaths that were deeper, and they ran more efficiently. Overall, the looser sports bras boosted comfort and running performance. 

"In the current study, loosening the underband pressure resulted in a decreased work of breathing, changed the ventilatory breathing pattern to deeper, less frequent breaths, and decreased submaximal oxygen uptake (improved running economy)," the study authors wrote. "Our findings suggest sports bra underbands can impair breathing mechanics during exercise and influence whole-body metabolic rate."

In the study, switching a tight bra to a looser one had an average 1.3 percent improvement in running economy. Running economy is a cornerstone of physiological performance in distance running, according to Runner's World. It deals with how efficiently you use oxygen, which can determine how long you can maintain your running pace. While a 1.3 percent improvement may seem like a small figure, it's nothing to sniff at when it comes to distance running. Lead study author Shalaya Kipp told the Washington Post that a 2 percent change in running economy would translate to a three-minute improvement for a three-hour marathon runner.

“People ask, ‘What sports bra should I wear?’ I say, ‘Wear one that is correctly fitted,’” Kipp told the Washington Post. “That’s probably the biggest thing that would help.”

How to choose the best-fitting sports bra

When it comes to choosing the best sports bra for running or other exercise, the study results suggest that women should reach for pieces that are not too tight on the ribs. But there are numerous other important factors, including breast support. A 2022 study found that greater breast support was linked with reduced oxygen consumption and increased running economy in women with varying bust sizes.

Purchasing a sports bra with adjustable features, such as bands and straps, is one way to ensure that the bra can fit an athlete's support and comfort needs. 

“[Sports bra] features really depend on a runner’s support needs,” Catherine Moloznik, vice president of purchasing and merchandising for running retailer Fleet Feet, told Condé Nast Traveler. “All runners are built differently, so having the ability to adjust the band size and straps are helpful. The less movement the bra has, the less likely you’ll experience chafing.”

Chafing, and overall discomfort, can also become an issue when customers choose the wrong sports bra size. Moloznik suggests going to a local running specialty store to determine the correct size and purchase a sports bra, since women's sports bra size can differ from their daily bra size.

Several companies, including Nike and REI, also offer online guides to finding your best sports bra fit.