Introduction/Purpose This study investigated the effect of breast size on running economy, breast displacement, total body center of mass excursion, trunk angular velocity, and exercise-induced breast pain at different running velocities. Methods Fifteen female recreational runners with a small breast size (volume range: 90-338 mL per breast) and 15 female recreational runners with a medium-large breast size (volume range: 351-1029 mL per breast) were matched for age and body mass index. Kinematic and oxygen consumption (VO2) data were collected while participants completed an 8-min treadmill protocol at two incremental velocities (8 and 10 km·h-1). Results Running economy was not affected by breast volume. Multiplanar breast displacement was significantly greater for larger breast volumes, despite participants wearing a high-support sports bra. A higher breast volume was also associated with less vertical center of mass excursion at velocity of 10 km·h-1 and higher trunk lateral flexion angular velocity at foot flat. Conclusions Women with large breast volumes experience significantly more breast motion compared with their smaller breasted counterparts during running, despite the presence of a high-support sports bra. Although increased breast motion as a result of breast size did not translate to differences in running economy, it did alter upper body biomechanical factors known to influence running economy—total body center of mass vertical excursion and peak angular velocity of the trunk in the frontal plane. Future research should explore what effect excessive trunk motion has on the mechanics of the lower limb during running among women across the breast size spectrum, as well as how this may influence neuromuscular control and coordination between the upper and the lower limb during running.
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