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Researchers from the University of Portsmouth in England and other schools recently attached reflective markers to the breasts of a group of female runners and had the women jog along a track while wearing various types of bras or forgoing breast support. The researchers charted the trajectories of the women’s breasts, using infrared cameras. The track was also equipped with a force platform to measure the force of each runner’s foot strikes.

Many women have long wondered whether breast movement, especially a lot of it, can affect running form. This was the first experiment to formally put that question to a practical test. What the researchers found was that breast sway did, in fact, have a significant effect on the women’s running. When the runners were braless, their strides changed; they landed more heavily, with more of the impact force moving through the inside of their feet. This alteration in stride seemed to be related to “significantly higher amounts of breast movement in that direction,” said Jenny White, a doctoral candidate at the University of Portsmouth and the study’s lead author. As the breasts swung from side to side, so, in effect, the researchers hypothesized, did the women’s body weights. The implications of this finding are disquieting. “Higher forces exerted by the foot when running indicate a higher intensity of stress for a runner,” Ms. White said, “which has potential to increase physiological demand.” The extra forces also, over time, can “lead to the development of stress-related injuries.” Jiggle may make running both more difficult and injurious than it needs to be.

For years, scientists (most of them women) studying breast movement during sports have struggled for respect. A 2007 report about the work being done in the field of breast biomechanics at the University of Portsmouth was titled, rather defensively, “Bouncing Breasts: A Credible Area of Scientific Research.” Some people (a k a men) may have considered breasts to be simple things, not requiring such high-tech attention. But a raft of new studies has established, convincingly, that breasts are more mobile and less manageable than most people once believed.

Researchers at the Portsmouth lab, for instance, recently completed a series of experiments that delineate just how breasts move during activity. Instead of merely bouncing up and down, it turns out that breasts arc through a complicated figure-8 pattern when a woman runs or walks. Few sports bras are designed to accommodate breasts’ side-to-side or lateral sway.

In fact, one of the most telling recent studies, from the University of Wollongong in Australia, published last month in the journal Medicine & Science in Sports & Exercise, concluded that the most effective style of sports bra, particularly for women who wear a D-cup bra or larger, does not yet exist, at least in stores. Typical sports bras fall into two varieties: they either cradle each breast in individual cups, a style known as encapsulation, or they smash the bosom against the chest, using compression. In most studies, encapsulation bras reduce up-and-down breast bounce best, particularly for large-breasted women, but are rated the least comfortable bra, which matters. Breast discomfort and embarrassment keep many women from exercising.

In the new Australian study, an experimental bra combined compression of the bosom, generally rated the most comfortable type of breast support during exercise, with elevation of each individual breast, achieved using small foam pads tucked into the bra. This design was not simply a mash-up of an encapsulation and a compression bra, a style some bra manufacturers already offer (and many enterprising female athletes have been cobbling together on their own for years, by layering one type of bra over the other). In this case, the breasts remained uncupped, but were held up slightly by the pads, “elevating the low point of breast displacement dynamically,” according to one researcher. During an earlier study, the researchers had noted that when large-breasted women ran in deep water wearing only a crop-top-style singlet, they felt little breast discomfort. The water buoyed their breasts. In the Australian researchers’ experimental sports bra, the foam pads served the same purpose. Large-breasted women who wore the experimental bra and a series of other bras while running reported that by a wide margin, the experimental bra was the most comfortable. It also effectively reduced breast motion not only vertically but also from side to side.

Unfortunately, no such foam-padded crop-top-style compression bra is yet on the market. So barring creating your own with a tight crop top and some pads, the best advice that scientists currently can offer to women hoping to corral their breasts during exercise is more commonsensical than high-tech. Find a sports bra that “feels supportive,” Ms. White said. That advice may “sound obvious,” she added, “but many bras are marketed as a sports bra” but are instead “just a fashionable crop top that has very stretchy material and that would not provide a lot of support.” To test support, jump “up and down in the changing room and assess how much movement occurs.” The chest band should be “firmer than an everyday bra, but should not dig into your skin.” All in all, if the bra “is uncomfortable, then this is probably not the bra for you.”