A psychophysical study of the effects of load and frequency upon selection of workload in repetitive lifting

Abstract

Human capabilities in manual materials-handling activities may be influenced by a number of task variables. The psychophysical method of Snook (1978) was used to determine the effects of load and lifting frequency upon workloads selected by eight male soldiers. The subjects repeatedly lifted and lowered a box (38 × 55 × 24 cm) to and from a platform set at 40% stature for 10 minutes and either adjusted the load, or the lifting frequency, or a combination of both variables in order to select a workload which they considered to be a maximum acceptable workload (MAWL) for a work-period of one hour. When the lifting frequency was adjusted, the mean MAWL was 94·5 kg m min ⁻¹ (S.D. 28·4). This was significantly greater (P ≤ 0·05) by 27% than when the load alone was adjusted (74·3 kg m min ⁻¹, S. D. 23·3). Control of both variables together produced a MAWL of 76·5 kg m min ⁻¹ (S. D. 24·1) which was similar to that selected when the load alone was adjusted. When the soldiers lifted and lowered their selected workloads for a one-hour work-period, the average heart rate was 101 beats min ⁻¹ and the mean oxygen cost was 23·3% of their maximal oxygen uptake (determined by cycle ergometry). The work intensity was subjectively assessed as ‘Fairly Light’ using the Rating of Perceived Exertion scale (Borg 1971). In spite of significant differences between the MAWLs selected by the adjustment of the two variables, the physiological measurements and subjective assessments made whilst lifting these workloads were not significantly different. This suggests that other factors may influence an individual's perception of MAWL. The results in this study may have been affected by limitations imposed by the experimental design. However, they do suggest that the frequency of lifting could be a more appropriate variable to manipulate in the self-selection of workloads.