Biomechanical stresses during asymmetric lifting: a dynamic three-dimensional approach

The objectives of this study were (1) to establish a three-dimensional dynamic biomechanical model to determine the mechanical stresses on the musculoskeletal system which occur during asymmetric lifting, (2) to examine the difference between symmetric and asymmetric liftings in terms of the loads on the lower spine, and (3) to examine the effects of weights on spinal loads for both symmetric and asymmetric lifting.

To accomplish this, a dynamic biomechanical model was developed and designed to study the mechanical stresses imposed on the human body. The model estimated the forces and moments acting on each major joint center, as well as the L5/S1 disc. The dynamic effects of body segment motions and trunk rotation were accounted for in the model. A linear programming optimization technique was used to solve the indeterminate system of internal trunk muscle forces and, in turn, estimated the compressive and shear forces acting on L5/S1.

A laboratory experiment was conducted utilizing male subjects who performed a sagittal and two asymmetric lifting tasks consisting of a floor to knuckle height lift using three different weights. The "ExpertVision" motion analysis system was used to track a set of predefined body landmarks and to describe the body motion. This model was validated through the comparison between the model computed force components at ground level and the force components measured by the "Kistler" force platform.

A presentation routine was designed to examine the output from the model either in graphical or alphanumerical forms. The biomechanical model, the linear programming module and this presentation routine were constructed and integrated as a package running under a MS-DOS operated microcomputer.

Results indicated that the L.P. technique was unable to explain the behavior of trunk muscles under the dynamic experimental situation. The effects of weights on spinal loads showed that heavier weight produced higher stresses than lower weights did. The effects of lifting types--symmetric and asymmetric--on the spinal loads showed that the symmetric lifting yielded higher compression than the asymmetric lifting. On the other hand, the asymmetric lifting created higher antero-posterial and lateral shear forces than the symmetric lifting. Results also indicated that the current biomechanical model predicted significant lateral shear force for symmetric lifting which was assumed to be negligible by most sagittal models.