A three-dimensional mathematical model for predicting spinal joint force distribution during manual liftings

Objective. A three-dimensional dynamic mathematical model was developed to discover what loads are imposed on the lumbar structures by performance of asymmetric manual liftings. Design. An external model was used to estimate the intersegmental resultant forces and moments at the L₅/S₁ joint in this dynamic biomechanical model. Using an optimization algorithm, an internal model then distributed the intersegmental resultants to forces of muscle, disc, facet joints, and ligaments. Background. To study the relation between large loads and low-back disorders, many biomechanical models have been developed. Most of the models were two-dimensional models discussed with symmetric activities. Some three-dimensional biomechanical models were static models or only included limited elements of the disc and muscles in the model. Methods. A healthy young male subject was asked to perform asymmetric lift with bent knees. Dynamic data of body motion and ground reaction forces were monitored, and the EMG of six muscles were recorded simultaneously. A Newtonian equation was used to calculate the joint intersegmental resultant forces and moments. In the internal model, three components of the disc force, eight muscle forces, two ligament forces and two facet joint forces were computed. Results. The correlation between the reaction moments from the upper and lower models of the external part were generally above 0.94, and the root mean square differences were below 19 Nm. In this internal model, the maximal disc compression was close to the data showed on the literature, and the estimation of muscle forces corresponded to the EMG activities. Conclusions. A three-dimensional biomechanical model has been developed and evaluated to estimate the spinal joint force distribution during asymmetric manual lifting activities. Relevance. The cause of most low-back disorders is unknown, but circumstantial evidence points to the involvement of large loads on the lumbar spinal structures. The knowledge of reaction forces and moments in relation to postures are helpful in understanding the mechanisms of low back pain. Such prediction of segmental forces in spinal joint structure will be applied to the optimization of occupational performance and safety considerations.