This main objective of this study was to introduce and investigate the concept of load of perceptual indifference (LPI) for assessment of load heaviness in manual lifting tasks. The loads of perceptual indifference were defined as those box weights which would result in the same values of subjective compatibility scores for a given pair of perceptual categories of load heaviness. At the point of indifference, the loads are perceived as to be acceptable, safe or not-too-heavy with an equal strength as the loads judged to be too-heavy for continuous lifting. The linguistic magnitude estimation (LME) method (Karwowski, 1990) was used for experimental and modeling purposes. This allowed to develop a quantitative model for the human assessment of four categories of lifted loads of interest. The results indicate that the lack of cognitive benchmark introduces inconsistency in subjects perception of load acceptability and safety compared to the concept of to-heavy load. In order to overcome this problem, a new research approach to manual lifting tasks is needed, based on the integration of cognitive engineering, active psychophysics and ecological approach.
