Trunk marker sets and the subsequent calculation of trunk and breast kinematics during treadmill running

Female participants present a unique challenge as the design of the bra used to support the breasts occludes the correct positioning of many recommended trunk marker sets. This study aimed to compare the effect of two existing and one new trunk marker set on the calculation of trunk and breast kinematics. Twelve females had markers placed on their trunk and right nipple; these markers were tracked using infrared cameras during five running gait cycles and used to define three trunk calculation methods: Trunk 1: suprasternal notch, right and left ribs; Trunk 2: supersternal notch, processus xiphoideus, 7th cervical and 8th thoracic spinous process; Trunk 3: Trunk 2 plus a marker 33% from the suprasternal notch to the processus xiphoideus, and another 50% between the 7th cervical and 8th thoracic spinous process. Trunk segment capture success, segment origin instability, segmental residual, trunk kinematics, and breast range of motion (relative to the trunk segment), were calculated for each trunk segment. Segment capture success varied from 88% (Trunk 1) to 100% (Trunk 2 and 3). Segment origin instability ranged from 0.2 cm (Trunk 2 and 3) to 1.5 cm (Trunk 1). Maximum trunk extension differed by 7° and breast range of motion varied by 41% (anterioposterior), 54% (mediolateral), and 21% (superioinferior) between trunk calculation methods. The selection of marker set used to construct the trunk segment is critical before recommending improvements to bra design to improve breast support. The Trunk 3 marker set is recommended for subsequent breast research.