The Uniform Density Assumption: Its Effect upon the Estimation of Body Segment Inertial Parameters

Abstract

The effect of the uniform density assumption upon estimation of body segment inertial parameters was examined by employing directly measured, CT-derived, and cadaver-derived density values. Sectional and average density values for the right leg segments of a patient 29 years of age and a cadaver (65 years) were obtained with a GE 9800 computed tomography scanner using dual energy radiographic factors of 80 kV, 200 mAs, and 140 kV, 200 mAs. Careful sectioning of the cadaver leg following these scans permitted mass and density measurements to be directly performed. The results for both legs showed marked variation in cross-section density values throughout their lengths, which highlighted the limitations of the assumption of uniform segment density. The effect of employing this assumption was tested using a series of inertial parameter estimation strategies by means of mathematical modeling. Adoption of the uniform density assumption when estimating inertial parameters of the human leg segment was shown to produce only minor errors. However, greater errors were shown to be caused by inaccurate estimates of segment volume.