The effects of box size, frequency and extended horizontal reach on maximum acceptable weights of lifting

In the development of our present manual materials handling (MMH) guidelines (Ergonomics 34 (1991) 1197), the assumption was made that the effects of frequency on maximum acceptable weights (MAWS) of lifting with a large box (hand distance. 38 em from chest) were similar to frequency effects on MAWs of lifting with a small box (hand distance, 17 em from chest). The first purpose of the present experiment was to investigate this assumption. The second purpose was to study the effects of extended horizontal reach lifting (hand distance, 48 cm from chest) on MAWs as a confirmation of the results of a previous study on this variable. Lastly, we studied the effects of high frequency (20 lifts/ min) on MAWS of lifting. Eight male industrial workers performed 15 variations of lifting using our psychophysical methodology. As expected, the results revealed that MAWs of lifting with the large box was significantly effected by frequency. Frequency factors based on the 1 lift/min task illustrated less change to higher frequencies ( > 1 lift/min) and more change to slower frequencies (< 1 lift/min) as compared to lifting with the small box. It was concluded that our existing guidelines present a conservative estimate of lifting large boxes in the spectrum of frequencies studied. The results also verified the extreme effects of lifting with an extended horizontal reach and quantified the effects of the 20 lifts/min lifting frequency.