Psychophysical capacity modeling of individual and combined manual materials handling activities

Most psychophysical studies in the field of MMH involve only with single MMH activities, i.e., lifting, lowering, carrying, holding, pushing, or pulling. The combination of two or more activities (e.g., lifting a box, then carrying the box; or carrying a box, then lowering the box) has never been examined. These kinds of combined activities are prevalent in industry and our daily life. The objectives of this study were: 1) To develop models for both individual and combined MMH activities, and 2) To determine the relationship between individual and combined MMH activities.

Three types of strength tests (isometric, isokinetic, and isoinertial) using four testing machines (static strength testing machine, Mini-Gym, Cybex, and incremental weight lifting machine) were conducted using 12 male subjects. The capacities for four individual MMH activities and three combined MMH activities were tested psychophysically under three frequency conditions: one time maximum, one handling per minute, and six handlings per minute. In all experimental conditions, the psychophysically determined capacities were the maximum acceptable workloads for a one hour work periods. As a result the mean capacities determined in this study were higher than those reported for eight hour work periods.

Second order polynomial regression models for individual MMH capacities were developed using isoinertial 6 foot maximum strength or static back strength. Combined MMH capacities models were developed using the following three methods: a limiting individual MMH capacity, isoinertial 6 foot maximum strength, and fuzzy sets theory. The advantages and disadvantages of different models were compared.

The isoinertial 6 foot maximum lift strength was selected because it was a simple, economical, easily applied, and representative test. The models developed in this study provide information about the relationship between a person's strength and his capacity for MMH activities, and about the relationship between individual and combined MMH activities.