Compressive loads on the lumbar spine during lifting: 4D WATBAK versus inverse dynamics calculations

M H Cole,P N Grimshaw

doi:10.1533/abbi.2004.0029

Abstract

Numerous two- and three-dimensional biomechanical models exist for the purpose of assessing the stresses placed on the lumbar spine during the performance of a manual material handling task. More recently, researchers have utilised their knowledge to develop specific computer-based models that can be applied in an occupational setting; an example of which is 4D WATBAK. The model used by 4D WATBAK bases its predications on static calculations and it is assumed that these static loads reasonably depict the actual dynamic loads acting on the lumbar spine. Consequently, it was the purpose of this research to assess the agreement between the static predictions made by 4D WATBAK and those from a comparable dynamic model. Six individuals were asked to perform a series of five lifting tasks, which ranged from lifting 2.5 kg to 22.5 kg and were designed to replicate the lifting component of the Work Capacity Assessment Test used within Australia. A single perpendicularly placed video camera was used to film each performance in the sagittal plane. The resultant two-dimensional kinematic data were input into the 4D WATBAK software and a dynamic biomechanical model to quantify the compression forces acting at the L4/L5 intervertebral joint. Results of this study indicated that as the mass of the load increased from 2.5 kg to 22.5 kg, the static compression forces calculated by 4D WATBAK became increasingly less than those calculated using the dynamic model (mean difference ranged from 22.0% for 2.5 kg to 42.9% for 22.5 kg). This study suggested that, for research purposes, a validated three-dimensional dynamic model should be employed when a task becomes complex and when a more accurate indication of spinal compression or shear force is required. Additionally, although it is clear that 4D WATBAK is particularly suited to industrial applications, it is suggested that the limitations of such modelling tools be carefully considered when task-risk and employee safety are concerned.

Highlights

Over the past three decades, several two- and threedimensional biomechanical models have been devised to quantify the various components of vertebral loading during a variety of manual handling tasks (Chaffin and Baker 1970; de Looze et al 1992; Kingma et al 1996)
The reader is cautioned as to the interpretation of the statistical significance of these results (Table 2). The results of this investigation show that the static compression forces (4D WATBAK) underestimate the dynamic compression forces in all lifts from the 2.5 kg lift (x = 22.0 ± 4.1%) to the 22.5 kg lift (x = 42.9 ± 7.1%)
It is clear from the results that the 4D WATBAK modelling software underestimated the lumbar compression forces when compared to the values from inverse dynamics calculations

Summary

Introduction

Over the past three decades, several two- and threedimensional biomechanical models have been devised to quantify the various components of vertebral loading during a variety of manual handling tasks (Chaffin and Baker 1970; de Looze et al 1992; Kingma et al 1996). One of the underlying assumptions of the 4D WATBAK model is that a static analysis of human lifting reasonably estimates the magnitude of the moments, joint reaction forces and lumbar compression and shear forces during dynamic movements (Norman et al 1999). The implications of this assumption could effectively cause the risk associated with a task to be underestimated, as it has previously been argued that a static assessment of dynamic movement patterns does not represent the dynamic spinal loads (Freivalds et al 1984; McGill and Norman 1985)

Methods

Results

Conclusion