By Tanith Carey for the Daily Mail. Breasts have always been a source of fascination - for both men and women. And as a smart bra launches - which includes skin sensors to measure size, breathing and heart-rate - science is finally shining a light on their charms. From how breasts move to why you should wear a bra in the swimming pool, here's the surprising science behind your breasts - and how to keep them healthy and well-supported. With each step a woman takes, her breasts move an average of 9 cm - not just up and down, but side to side. With each step a woman takes, her breasts move an average of 9 cm - not just up and down, but also sideways - in a figure of eight motion.