AXIAL STIFFNESS OF HUMAN LUMBAR MOTION SEGMENTS, FORCE DEPENDENCE

This paper addresses the axial stiffness of human lumbar motion segmen ts while subjected to moderate loads. Impacts in axial direction were applied to Functional Spinal Units while they were subjected to weight s acting as static pre-load. Accelerations were recorded proximal and distal of the FSU. The transfer function and the resonant frequency we re calculated from this data. The stiffness was calculated from the re sonant frequency and the load. A simple non-linear model was fitted to the data and a linear relationship was found between stiffness square d and force. The non-linear component in the model strongly affected t he stiffness within the chosen load range. The present model may allow in vivo dynamic force determination with improved accuracy, e.g. in e xperiments where accelerometers have been fixated to pins inserted int o the spinous processes of lumbar vertebrae if the static force is kno wn. (C) 1998 Elsevier Science Ltd. All rights reserved.