Bras support and shape a woman’s breast in everyday life, during activities, pregnancy, or during the healing process after an illness. In all these cases, a good fit is important to ensure the comfort and function of the bra. One way to describe the comfort of a bra is to analyse the movement of the bust relative to the moving body. In this study, breast movement relative to body movement was analysed. Using a 4D scanner, a woman was scanned while running on a treadmill. Different bras were worn by the woman, and the treadmill speed was varied to values of 5 kmh, 7.5 kmh, and 10 kmh. A method for investigating breast movement has been developed. The amplitudes of the main frequencies of the movements were compared and the amplification factor was calculated. The relationships between treadmill speed, movement frequency, and breast movement were investigated. It has been shown that any tested bra reduces breast movement. Differences in bra support were observed, especially at higher treadmill speeds. A method for comparing different bra models with a 4D scanner was successfully developed.