Psychophysical and physiological study of one-handed and two-handed combined tasks

Most studies in manual material handling (MMH) have paid attention to single MMH activities – lifting, lowering, carrying, holding, pushing or pulling and have ignored combined activities. Also, most studies have involved two-handed (symmetric) MMH activities rather than one-handed MMH activities. Very few studies have reported information on workers' capacities for combinations of one-handed MMH activities (e.g. lifting a box, then carrying the box, and lowering the box). These kinds of combined activities are common in industry and in our daily lives. The objective of this study was to utilize the psychophysical approach to examine the combinations of lifting, carrying and lowering activities from a simulated industrial task involving one-handed and two-handed combined tasks, and to develop mathematical models for combined tasks. Ten male students served as subjects for the study. Maximum acceptable weight workload capacities for 1h work periods for one-handed and two-handed combined tasks (lifting a box from floor-to-knuckle height, carrying the box for 4.3m, and lowering the box from knuckle-to-floor height) were determined psychophysically under three frequencies: six handlings per minute, one handling per minute, and one handling per 5min. In addition to maximum acceptable weight, heart rate, ratings of perceived exertion (RPE) values and tasking time were also measured as response variables. Combined MMH capacity models for one-hand and two-handed tasks were developed. The advantages and disadvantages of different models are discussed.