The effects of box size, vertical distance, and height on lowering tasks

Abstract

The most frequent and expensive cause category of compensable loss is manual material handling (MMH). In an attempt to minimize these losses, refinement of existing MMH guidelines is a component of redesigning high-risk MMH jobs. In the development of our present MMH guidelines (Snook and Ciriello, Ergonomics 34 (1991) 1197), maximum acceptable weights (MAWs) of lowering were assumed to respond similarly to the parameters of lifting. Also, MAWs of lifting various vertical distances with a large box were assumed to respond the same as lifting various vertical distances with a small box. The purpose of this experiment was to investigate the effects of vertical distance and box size on MAWs of lifting and lowering and the effects of height on MAWs of lowering. The effects of a four component combination task on MAW were also investigated. Eight male industrial workers performed 27 variations of lifting, lowering, pushing, pulling, and carrying. A psychophysical methodology was used whereby the subjects were asked to select a workload they could sustain for eight hours without “straining themselves or without becoming unusually tired, weakened, overheated or out of breath.” The results revealed that MAWs of lowering were not affected by distance of lowering (except for the 25 cm vertical distance), height of lowering or box size. The results also indicated that MAWs of lifting large boxes were not significantly affected by vertical distance of lift. MAW of a four component combination task was significantly reduced compared to individual components. It was concluded that our existing guidelines present a conservative estimate of the variables studied except for the 25 cm lowering task and the combination task. In these cases, recommendations are given for adjustment of MAW. Relevance to industry Lowering is the second most common MMH task but has been given considerably less attention in research as compared to lifting. This study investigated select variables of lowering in an effort to improve estimation of MAWs for ergonomic redesign.