Changes in ground reaction forces that result from different breast support conditions may have implications for sports performance and transmission of forces through the skeleton. The aim of this investigation was to compare kinetic variables and breast motion in a no-bra, everyday-bra and two sports-bra conditions. Following ethical approval, eight female participants with D-cup breasts had retro-reflective markers placed on the left and right nipples, anterior superior iliac spines and clavicles. Five calibrated ProReflex infra-red cameras (100 Hz; Qualisys) measured 3-D displacement of markers and synchronised kinetic data were collected using a force platform (500 Hz, Kistler 9281CA). A repeated measures one-way ANOVA revealed a significantly higher medial impact force in the no-bra condition (0.15 times body weight) compared with the compression sports-bra condition (0.12 times body weight) (F = 3.64 (3,21), p = 0.03). Findings suggest that inadequate breast support affects a female's running kinetics, which may have negative physiological consequences on sports performance.