The effect of inertial factors on spinal stress when lifting.

Biomechanical models used tor the evaluation ol spinal stress have mostly been static: they reveal the postural stress caused by gravity but ignore the in€rtial forces and torques induced by the accelerations of body segments and the load handled. In this study both static and dynamic models for del€rmining lumbosac-ral compression were used, and the resultS f rom different models wer€ compared. Twenty subiects litted a 15 kg box from a 10 cm high shelf to knuckle height with tour lifting t€chniqu€s: the conventional back and leg litts and two 'kinetic' lifts with which the subiects gave kingtic energy €ither to thC horizontally moving load or to the verticalty moving body belore litting the load itself vertically. The results showed that thc inertial tactors increass the spinal load considerably. Comparison between thc litting techniqucs showed that the dynamic pcak str€ss was clearly smaller in thc leg lilt then in th€ back lift. although the static peak slress was smajler in the back lift. Th€ LS/Sl<ompression x tims intcgral, describing th€ total strsss of a tift, was smallesl in the back litt with both models. Introductlon Lumbosacral compression induced by manual materials handling has been evaluated using biomechanical models based on the analysis of forces and torques acting on the musculoskeletal system of a human body. postural stress due to gravity, but ignoring the inertial forces and torques induced by the accelerations of the load and body segments. Ayoub and El-Bassoussi (1978) introduced a dynamic element in their model by using a mathematical simulation of acceleration. A simple dynamic sagittal plane model employing direct recording of the movements of the body, thc acceleration of the load and the force at the feet was presentcd by Lcskincn et al. (1983). The aim of this paper is to describe the effects of dynamic factors on the spinal stress when lifting.