Baecheol Han

Date of Graduation


Document Type



The lifting motion with feet positioned in the middle between the origin and destination of a load (middle twist) has rarely been studied. The effects of asymmetry for the middle twist and turning were examined in detail up to 180{dollar}\\sp\\circ{dollar} of task angle in this research. The purpose of this study was to investigate the effects of asymmetry on a worker's lifting capacities by employing the psychophysical and biomechanical approaches. Ten male college students lifted a box from pallet height (15cm) to conveyor height (75cm). In the psychophysical experiment, three types of asymmetric lifting tasks were studied: turning, middle twist, and twist. At 90{dollar}\\sp\\circ{dollar} of task angle, the middle twist lifting capacity was the greatest. At 180{dollar}\\sp\\circ{dollar} of task angle, the lifting capacities for the turning and middle twist were not significantly different. The measured lifting reduction for the 90{dollar}\\sp\\circ{dollar} twist (16.1%) was about half of the reduction (30%) which would be calculated by the NIOSH lifting equation. The maximum acceptable weights for turning at 90{dollar}\\sp\\circ{dollar} and 180{dollar}\\sp\\circ{dollar} decreased by 13.4% and 14.9%, respectively in the present study although the reduction is zero according to the 1991 NIOSH lifting equation. Also, the reduction was not proportional to the angle of asymmetry in the turning and middle twist lifts in the present study. Therefore, it is suggested that three lifting types be designated: the turning, middle twist, and twist. The L5/S1 compression force generated by the maximum weight a subject selected at a frequency of 1 lift/minute was estimated using a three dimensional biomechanical model. Based on the results of the dynamic analysis, the biomechanical lifting limit could be higher than the psychophysical lifting limit for lifting tasks at a frequency of 1 or more lifts/minutes. The current asymmetric multiplier (AM) set by NIOSH is inappropriate based on this research. A new more comprehensive AM employing the three lifting types is suggested. The new AM would estimate the lifting capacities appropriately.