The aim of this paper is to develop a method to determine the material characteristics of bras that could limit breast motion during exercise. A single participant ran on a treadmill at 10 km h−1 wearing either a sports bra, an everyday bra or no bra. The relative motion between the suprasternal notch and the breast was recorded using a passive marker system at 200 Hz and was modelled as forced damped-harmonic motion with a linear spring and damper, with the driving force provided by the suprasternal notch. The spring and damper values were found by matching the model to the experimental data. It was found that both the damping and stiffness values increased with the use of an everyday bra, and increased further still with the use of a sports bra. The stiffness parameter, however, was shown to be the most important criterion for minimisation of the breast motion. The model predicted that an increase in breast mass from 100 to 700 g (a 32A-cup to a 32F-cup) increased the vertical motion of the unsupported breast by around 70% when running and 30% when walking. This was reduced with an everyday bra and further reduced with the high stiffness sports bra. Although predictions were sensible, the model requires further verification with a cohort of participants.