Load heaviness and perceived weight lifted: Implications of human cognition for setting design limits in manual lifting tasks

This paper examines interrelationships between the cognitive concepts of load heaviness, perceived weight lifted, and the size-weight illusion effect, and their importance for setting load limits in manual lifting tasks. Results of three related studies are presented, including: (1) a questionnaire survey of the perceived relationship between physical weights and subjective perception of load heaviness for a large subject population; (2) modeling of the above relationship using the fuzzy linguistic approach; and (3) laboratory examination of the perceived weights and load heaviness for five pre-weighted boxes. Thirty subjects who participated in the questionnaire survey, were asked to provide estimates of the perceived weights and corresponding linguistic classification of load heaviness, while lifting five pre-weighted boxes. The subjects had previously gone through another phase of the project where they had determined their maximum acceptable weight of lifting using the psychophysical approach. The perceived weights were not significantly different from the physical weights for the ‘light’ to ‘moderate’ categories of load heaviness. Furthermore, the concept of recommended weight limit (RWL) fell in the ‘light’ load category for which the subjects were good judges of load heaviness. The results of this study provide support for setting the lifting limits based on the RWL concept designed to reduce the risk of low back injury.