A biomechanical model of a simple lifting motion is presented in which the forces and moments of force acting about the L4-L5 vertebral articulation were determined from kinetic and kinematic data. Derivation of the forces due to motion permitted a unique method for estimating intervertebral stress during each phase of the lift and provided a logical extension to an earlier static model of intervertebral stress presented by Troup (13). The intervertebral force was found to have a maximum value of over 400 kg during the initial phase of the lift. By partitioning this force into its shearing and compressional components, it was possible to determine quantitatively the lumbar stress involved in lifting. The values are discussed briefly in relation to the aetiology of back injuries and some of the predictions and applications of the dynamic model are also considered.