Female athletes and active women benefit greatly from wearing well-fitting sports bras to provide essential support and control during dynamic movements like running. Understanding how bra components like straps, neckline and underband affect breast motion is crucial for designing sports bras that reduce discomfort and maintain stability. However, evaluating sports bra performance with human participants faces challenges in recruitment and consistency. To address this, this study introduces a soft manikin system that accurately simulates and measures 3D breast movements during running. This system offers a novel method for objectively assessing sports bra effectiveness in controlling breast motion. By incorporating deformable breast forms into the manikin, researchers can consistently evaluate how bra features impact breast movement. The soft manikin’s breast displacement values align well with experimental results in the medial-lateral and vertical directions. This innovative approach provides valuable data for designing activewear bras, enabling informed adjustments to enhance breast motion control during physical activities.Content