During running, it is difficult to control breast motion and change torso motion or vice versa within empirical data collection. This study investigated how different levels of breast support (and consequently breast motion) influence torso motion, breast forces, lumbar and thoracic spinal moments during running, using a computer simulation model. A subject-specific female full body musculoskeletal model with an articulated thoracolumbar spine and sliding joints between the breasts and torso to enable breast motion was customised for this study. One female (bra size 34DD) had 59 markers attached to anatomical locations and ran over three force platforms at a self-selected speed (3.15–3.40 m/s) in three breast support conditions (no bra, everyday bra and sports bra). An ‘extreme’ bra condition was simulated during the modelling process by eliminating all breast motion relative to the torso. Two categories of simulations were run, investigating 1) how different breast support garments affect torso motion, breast and spinal moments; and 2) how changes in torso motion affect breast and spinal moments. Key findings suggest that peak lumbar and thoracic spine moments demonstrate changes (> 0.05 Nm/kg) between bra conditions due to changes in running gait kinematics. Additionally, eliminating breast motion relative to the torso, but using the same input running gait kinematics, increased (> 0.05 Nm/kg) lumbar joint moments. Therefore, it is possible that bras aimed at preventing relative motion between the torso and breasts may increase internal loading within the spine.