Accurate determination of lumbar spinal loads in ‘real world’ manual handling (MH) tasks is an important step in reducing the incidence of lower back pain (LBP). In this paper we describe some of the developments that have occurred in spinal modelling between a recently released MH management program known as Watbak (University of Waterloo, 1999) and the first commercially available spinal modelling package known as 3-Dimensional Static Strength Prediction Program (3DSSPP) (University of Michigan, 1994). Particular emphasis is placed on the calculation and interpretation of lumbar shear forces and its implications for lifting mechanics.

3DSSPP is a static single equivalent muscle force vector model, which allows users to estimate L5/S1 spinal loads associated with ‘real world’ manual handling (MH) tasks as a function of hand load, kinematic and anthropometric characteristics. Like 3DSSPP, Watbak provides estimates of lumbar spinal loads via a single equivalent muscle force vector model assuming that inertial forces acting on the hand load and body segments are negligible, except compression and shear forces are calculated at the L4/L5 level. The main difference between 3DSSPP and Watbak is in the way in which the anatomy of the lumbar spinal muscles and ligaments are represented during MH performed with flexed and lordotic lumbar spinal curvatures. In particular, there are substantial differences between lumbar shear force estimates from 3DSSPP and Watbak due to differences in the anatomical assumptions.

McGill and Norman (1987) reported that the lumbar extensor musculature is aligned oblique to the lumbar spine when the lumbar lordosis is maintained, and is aligned parallel to the vertebral column at L4/L5 during full flexion. Therefore, Watbak incorporates a posterior shear force from the lumbar extensors during lordosis and no shear force during lumbar flexion. Although Watbak assumes that no posterior shear force is produced by the lumbar extensors when the lumbar spine is flexed, the model does incorporate the anterior shear force generated by the obliquely aligned spinal ligaments (McGill, 1988). Shear force experienced at L4/L5 is subsequently calculated by adding any anterior shear force produced by the ligaments to the reaction shear force and subtracting any posterior shear force generated by the lumbar extensors. Shear force limits have been also set for the first time in the Watbak model based on research by McGill et al. (1998). Due to its improved representation of the anatomy of the lumbar spine, Watbak can be used to provide new information on the loading patterns experienced while performing MH tasks with different postures.