The effect of inertial factors on spinal stress when lifting.

Abstract

Biomechanical models used for the evaluation of spinal stress have mostly been static: they reveal the postural stress caused by gravity but ignore the inertial forces and torques induced by the accelerations of body segments and the load handled. In this study both static and dynamic models for determining lumbosacral compression were used, and the results from different models were compared. Twenty subjects lifted a 15 kg box from a 10 cm high shelf to knuckle height with four lifting techniques: the conventional back and leg lifts and two kinetic' lifts with which the subjects gave kinetic energy either to the horizontally moving load or to the vertically moving body before lifting the load itself vertically. The results showed that the inertial factors increase the spinal load considerably. Comparison between the lifting techniques showed that the dynamic peak stress was clearly smaller in the leg lift than in the back lift, although the static peak stress was smaller in the back lift. The L5/S1-compression × time integral, describing the total stress of a lift, was smallest in the back lift with both models.