A mathematical lifting model of the lumbar spine

The objective of this study was to develop a comprehensive mathematical lifting model which takes into account both task and personal factors. The outcome measure of this model was compared to that of the 1981 and 1991 NIOSH lifting equations as well as the model reported by Drury and Pfeil (Int J Prod Res 13:137, 1975). The model was developed in two stages. In the first stage, the complete model was derived from the psychophysical data. In the second stage, the discounting factors of the various parameters included in the model were tested and adjusted using the physiological and biomechanical data. The rationale for building the model in two stages was based upon the hypothesis proposed and tested by Karwowski (Proc Ergon Soc Conf 95, 1983), which, combining the acceptability of physiological and biomechanical stresses, should lead to an overall measure of lifting task acceptability--namely, the acceptability of psychophysical stress. The following input parameters were incorporated into the model: (a) horizontal distance in the sagittal plane, (b) vertical distance between the origin of lift and the floor, (c) vertical travel distance, (d) lifting frequency, (e) task duration, (f) twisting angle, (g) coupling, (h) heat stress, (i) gender, (j) age, (k) body weight, and (l) percentage of worker population. The lifting capacity values predicted from the present study are consistently lower than the values generated from prior research. This can largely be explained by the incorporation of new biomechanical and physiological guidelines for lifting tasks.