A fuzzy sets based model of the interaction between stresses involved in manual lifting tasks

The primary objectives of this research were:

1. Development and testing of a mathematical, fuzzy sets based model for the acceptability of the stresses involved in lifting activity.
2. Evaluation of the hypothesis that a combination of the acceptability of the biomechanical and physiological stresses leads to an overall measure of lifting task acceptability, namely the acceptability of the psychophysical stress.
3. Development of a general criterion for establishing maximum permissible weights of load to be lifted safely.
4. Comparison and evaluation of existing recommendations versus this proposed general criterion.

The group of acceptable weights of load that can be lifted safely was defined as a class with no sharp transition from membership to nonmembershlp. The acceptability measure of the stresses were associated with fuzziness, rather than randomness.

Measures of acceptability were expressed by membership functions which describe the degree to which the stresses were acceptable for the human operator (with respect to the chosen design criteria). A synergistic effect was used as the basis to combine the biomechanical and physiological stresses into one category.

The psychophysical methodology was used to carry out an experiment with nine male subjects to provide the data needed for the model development.

An iterative procedure was used to generate conditions for membership functions which resulted in the minimum (or near-minimum) mean value of the Hamming distance between the acceptability measures of these stresses. The combined stress was then compared with the psychophysical one using the similarity measure between them. The conditions under which the acceptability of the psychophysical stress were similar to the acceptability of the combined stress were found. The points of reference for the biomechanical stress where the hypothesis holds true were also deterniined. A general criterion for the acceptability of the lifting task was proposed as one which considers the effect of both biomechanical and physiological stresses on the human operator's performance during manual lifting activities.