Michelle Norris, senior research associate in the department of sport and exercise science at the University of Portsmouth in the U.K., studies breast movement and tests breast support products in the lab. 

To do so, she and her colleagues have women running on a treadmill bare breasted (bless those ladies), and then in low or high support bras. They use 3-D motion capture to look at the range of movement of the breasts. 

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

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

When you add all that movement in three planes together, Norris says, breasts—unsupported—move about 15 centimeters during running. (Different labs report different numbers, depending on the cup size of the cohort they’re testing.) About 50 percent of the movement is in the vertical, and then 25 percent is side to side movement, and the other 25 percent is anterior-posterior motion.

Regardless, that’s about six inches of motion with every stride.