The application of segment axial density profiles to a human body inertia model

The intention of this study was to construct segment density profiles and compare segment inertias calculated when uniform densities and profile densities are used in a mathematical model. Axial densities from computerized tomography (CT) slices for the body segments of a sample of Chinese females (Zheng et al., Proceedings of the Beijing Asian Games Scientific Congress, 1990) were used to form profiles which could be employed in body segment models. Polynomials based on proportion of segment length were fitted to the reported mean slice densities. These profiles were then used with five widely divergent samples (n = 10); young adult females, young adult males, infants, male children and elderly adults. The mathematical model used is based on an assumption that all segments can be represented by stacked elliptic cylinders. The results show that when the profile densities were substituted for average cadaver densities the increase in the estimated total body mass was less than 0.85%. For the individual segments, use of the profile rather than average density increased the average segment mass estimate by up to 2.7%. The centres of mass and the principal moments were affected by the variations in density along the axis as well as the magnitudes, by up to 0.54 and 3.8%, respectively. Although the effects of using the profiles appear to be small the differences for individual samples, segments and parameters ranged up to 22.5%. It is not possible to decide if average or profile densities produce more accurate estimates of inertia, but the profile allows for axial variation in density and is therefore recommended.