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The classical psychophysical approach to setting limits in manual lifting tasks is discussed in view of experimental procedures used, non-linearity of human perception of load heaviness, and the related experimental outcomes. The results of two studies investigating the human assessment of load acceptability and safety are presented. The first study compares the classical concept of the maximum acceptable weight of lift (MAWL) to the alternative concept of the maximum safe weight of lift (MSWL). The second study utilizes the linguistic magnitude estimation (LME) method to mathematically model human assessment of four categories of lifted loads, including the concepts of acceptable, safe, not-too-heavy, and too-heavy loads for continuous lifting. It is shown that the concepts of the lifted load acceptability and safety are non-linear, and can be modelled with great accuracy using the third degree polynomials. This study also introduces and investigates the concept of the load indifference in assessment of load heaviness, and shows that lack of a cognitive benchmark introduces inconsistency in subjects' perception of load acceptability and safety compared to the concept of too-heavy load for continuous lifting. It is concluded that a new research approach to manual lifting tasks based on cognitive engineering is needed to improve the quality of research methodologies currently utilized in this field. This unexplored area of research should lead to greater understanding of human capacities and limitations in manual lifting tasks in the context of cultural and linguistic anthropology.
Karwowski et al. (Fri,) studied this question.