Dynamic factors and force-weighting corrections influence estimates of cumulative vertebral joint compression

Abstract

Estimates of cumulative load are typically based on a direct integration of instantaneous compressive loads. Recent evidence suggests that higher loads as well as smaller rest to work durations should receive greater importance in the calculation of cumulative compressive load, especially if insight into potential injury mechanisms is sought. The purpose of this investigation was to determine the effects that a novel weighting factor protocol would have on cumulative compressive load estimates using three different biomechanical models applied to a repetitive lifting task. Bilateral electromyographic activity of the external abdominal obliques and lumbar erector spinae, as well as unilateral kinematics of the upper body were collected from eight male and eight female participants while they performed a 5-min repetitive lifting/lowering task at a rate of one lift/lower every 6 seconds. Two-dimensional reaction L5/S1 joint kinetics were calculated using each of a static, quasi-static and dynamic linked segment model; muscular contributions to L5/S1 compression were computed using a single equivalent muscle model. Unweighted and weighted cumulative compression was calculated for each model type. Application of the weighting factor increased the estimates of cumulative compression for both the male and female participants. Since the males in this study loaded their spines to a relatively higher percentage of their predicted compressive strength when lifting/lowering, the effects of weighting instantaneous compressive loads had a greater influence on cumulative load magnitudes in males. A consistent finding between genders was that estimates of cumulative compressive joint loads from the dynamic model were most affected by applying weighting factors, as the inclusion of inertial information in the calculations lead to greater predicted compressive forces relative to estimated compressive strength. These findings suggest that biomechanical modelling assumptions made and the gender of the participants tested could influence interpretations drawn from studies using force-weighting approaches to understand potential injury mechanisms associated with cumulative exposures. Future work is needed to explore more thoroughly the influence of different rest to work durations on weighted cumulative load estimates.