The Effects of Box Weight,Size,and Handle Coupling on Spine Loading during Depalletizing Operations

Abstract

It is widely known that the order selection task in food distribution centers places the worker at risk of occupationally-related low back disorders (LBDs). One approach to controlling this risk consists of manipulating the characteristics of the object or box to be handled in the food distribution center. However, it is currently unknown what effect these changes to the box characteristics would have on the loading of the spine and the subsequent risk of low back disorder. Hence, the objectives of this study were to determine the change in spine loading at L5/S1 associated with selecting boxes that varied as a function of: 1) weight (40, 50, and 60 lbs), 2) size (2681 or 1584 cu. in.), and 3) the existence of handles or hand holds. In addition, these variables were explored as a function of where the box was on a pallet. Ten experienced order selectors were recruited from a local food distribution center and were evaluated as they selected boxes of different characteristics from a slot (bin) on to a pallet jack. Workers were monitored for their trunk motion characteristics as well as the EMG activity of 10 trunk muscles as they performed the task. The kinematic and EMG information was used as inputs to an EMG-assisted model that was used to predict the three-dimensional spine loadings that occurred during the task. The results indicated that conditions where a worker must reach to a low level of the pallet significantly increases spinal load. Thus, spinal loads were significantly great and only of a magnitude that would be expected to lead to low back disorders when workers lifted form the lowest layer of the pallet. Handles had the affect of reducing the spinal loading by an amount that was equivalent to a reduction in box weight of about 10 pounds. This effort has also facilitated our basic understanding as to why spine loading increases under the various conditions studied in this experiment. Nearly all differences in spinal loading can be explained by a corresponding difference in coactivation of the trunk musculature. This in turn significantly increase the synergistic forces supplied by each muscle to the spine and resulted in an increase in spinal loading.