Development and evaluation of a scalable and deformable geometric model of the human torso.

A model is developed to provide a geometric representation of the human spine including thoracic and lumbar motion segments, lumbar muscles, ribs, sternum, sacrum, and pelvis. An existing model was modified in order to allow for scaling using standard anthropometric measures, deformation to specific 3-dimensional postures using surface markers, and incorporation of muscle length-tension and motion segment passive bending properties. Experiments were performed to evaluate the accuracy of model postural predictions. Analysis of surface marker displacements demonstrated that the thoracic spine deforms only minimally over a range of flexion, extension, and lateral bending torso postures, suggesting that it can be treated as essentially rigid during low-weight lifting over the range of passive flexibility. Locations of bony landmarks were accurately reproduced (mean errors 2.9-6.8 mm) as were several body dimensions (mean differences 2.6-15.4 mm). It is concluded that linear scaling to subject-specific anthropometry and the use of specific surface markers provides an accurate and direct technique for describing spinal geometry. Predicted passive spinal moments were found to be comparable to those required to support body weight in different extreme postures. It is recommended that data obtained from this type of model be incorporated in future investigations of spinal loading.