An Evaluation of the NIOSH Guidelines for Manual Lifting, with Special Reference to Horizontal Distance

A laboratory study was conducted to determine the percentage of subjects capable of lifting loads at the maximum permissible limit (MPL) and the corresponding energy expenditure, heart rate, compressive force on the L5/S1 disc, and ratings of perceived exertion. A second objective was to compare MPLs based on measured horizontal distances with those based on the rule of thumb proposed in the National Institute for Occupational Safety and Health (NIOSH) guide (H=15+w/2 cm). Thirteen male subjects were required to lift three different boxes (25, 38, and 51 cm wide) at four different frequencies (0.2,3,6, and 9 lifts/min) and at two heights (floor level to bench height [0.8 m] and bench height to 1.5 m). Each lift was performed for 1 hr, and horizontal distances for all 13 subjects were measured at the origin of the lifts. MPLs were computed for all lifting tasks. A psychophysical method was used to determine the maximum acceptable weight of a lift. This weight, however, was not allowed to exceed the corresponding MPL. Maximum acceptable weights for all 13 subjects were equal to the MPLS. Of 13 subjects, 9 indicated that they could lift weights significantly greater than the MPLS. MPLs for the bench to 1.5-m height were significantly greater than the corresponding MPLs for the floor to 0.8-m height. The mean heart rate for 1-hr lifting tasks ranged from 104 to 114 beats/min. The estimated compressive force on the L5/S1 disc ranged from 340 to 598 kg. The mean ratings of perceived exertion for floor level to bench height and bench height to 1.5-m lifts were 12 and 9.6 for the back and 9.3 and 13 for the shoulder, respectively. An important finding was that there were large differences in MPLs between subjects for a given lifting task caused by differences in horizontal distance. There appears to be a lack of agreement between the guideline criteria and the measured response variables when the MPLs were computed using measured H values. It is concluded that there is considerable variation in the measured H distance among the subjects and that this distance is much greater than the rule of thumb proposed in the guide (H=w/2+15 cm). This results in different maximum permissible limits for each subject and raises the issue of measuring the H versus estimating it from the rule of thumb. It is suggested that more specific instructions and extreme care are needed if horizontal distance is to be measured.