Predictive models for setting safe limits in manual materials handling

Most current data on manual handling performance are in the form of single-factor studies using the criterion of maximum weight liftable and measuring its variation with experimental conditions. This does not always help the manager attempting to set safe handling limits as his/her application will involve many of the factors. A recent paper by Drury and Pfeil showed how the task-factors of height of lift and awkwardness of object could be combined with the age and sex of the materials handler to predict performance. This paper is an extension of these methods to incorporate more data on the effects of lifting height, introduce a more general measure of awkwardness and modify the model to cover extended periods of lifting rather than a single maximum exertion. It is shown that the criterion of duration of lifting at constant weight leads to the same results as the criterion of maximum weight lifted. Other findings are that the height of the object at both the beginning and the end of the lift are required to specify performance, and that carrying performance can be treated as an extension of static lifting performance.