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This study aimed to investigate 1) the prevalence and magnitude of breast movement asymmetry, 2) the interaction between static and dynamic breast asymmetry during running and 3) the influence of sports bras on breast asymmetry. Nipple position data were collected from 167 female participants whilst treadmill running and then from a sub-group of twelve participants running in different bra conditions. Breast movement asymmetry was present in 74% of participant during running, with greater resultant static breast position asymmetry for participants that displayed asymmetry whilst running. Asymmetry was most commonly caused (65 - 80%) by greater movement of the left than right breast. Sports bras reduced asymmetry prevalence to as few as 17% of participants in the antero-posterior direction but only 58% in the infero-superior direction.