3D whole body scanning to determine mass properties of legs

We describe a novel method of measuring the mass properties of the limbs, specifically legs. We use the method to obtain the mass and centre of mass of the legs which enables us to calculate the leg joint moments from measurements of ground reaction forces using force plates. The data are obtained by using a 3D whole body scanner to obtain a data set representing the surface of both legs. The bones are significantly denser than the soft tissue so their mass is calculated. Textbook values for the densities of bone and soft tissue are used. The actual bones are approximated by stretching appropriate bone shapes to fit the X-ray of the subject. Numerical integration is then used to obtain the mass and centre of mass of the limb. The system is fast and reliable and allows an individual's mass properties to be measured rather than relying upon population surveys which may be biased, particularly, when the subject is atypical by being disabled. Paraplegics can be measured in the scanner using a modified Oswestry standing frame. When compared with a water displacement method, for 10 legs the errors in the total leg volume using this method are less than 1% and in the location of the centre of mass are less than 4%.