SEGMENTAL INERTIAL PARAMETERS OF THE HUMAN TRUNK AS DETERMINED FROM COMPUTED-TOMOGRAPHY

This study used computed tomography (CT) imaging to determine in vivo mass, center of mass (CM), and moments of inertia (Icm) about the CM o f discrete segments of the human torso. Four subjects, two males and t wo females, underwent serial transverse CT scans that were collected a t l-cm intervals for the full length of the trunk. The pixel intensity values of transverse images were correlated to tissue densities, ther eby allowing trunk section mass properties to be calculated. The perce ntage of body mass observed by vertebral levels ranged from 1.1% at Tl to 2.6% at L5. The masses of the upper, middle, and lower trunk segme nts as percentages of body mass were estimated to be 18.5, 12.2, and 1 0.7%, respectively. The whole trunk mass was estimated to comprise 41. 6% of the total body mass. Transverse vertebral CM values were found t o lie anterior to their respective vertebral centroids by up to 5.0 cm in the lower thoracic region. For the upper, middle, and lower trunk segments, the average CM positions were found to be 25.9, 62.5, and 86 .9% of the distance from the superior to inferior ends of the trunk. T he upper and middle trunk CMs corresponded to approximately 4.0 cm ant erior to T7/T8 vertebral centroid levels and 1.0 cm anterior to L3/L4 vertebral centroid levels, respectively. For the whole trunk, the CM w as 52.7% of the distance from the xiphoid process and approximately 2. 0 cm anterior to L1/L2 vertebral centroid levels. Variations in CM and Icm values were observed between subject, but these were within the r ange of previous reports of body segment parameters. Differences from previous studies were attributable to variations in boundary definitio ns, measurement techniques, population groups, and body states (live v ersus cadaver) examined. The disparity between previous findings and t he findings of this study emphasizes the need to better define the seg mental properties of the trunk so that improved biomechanical represen tation of the body can be achieved.