Association between spinal loads and the psychophysical determination of maximum acceptable force during pushing tasks

Abstract

The objective of this study was to investigate potential associations between an individual's psychophysical maximum acceptable force (MAF) during pushing tasks and biomechanical tissue loads within the lumbar spine. Ten subjects (eight males, two females) pushed a cart with an unknown weight at one push every two minute for a distance of 3.9 m. Two independent variables were investigated, cart control and handle orientation while evaluating their association with the MAF. Dependent variables of hand force and tissue loads for each MAF determination and preceding push trial were assessed using a validated, electromyography-assisted biomechanical model that calculated spinal load distribution throughout the lumbar spine. Results showed no association between spinal loads and the MAF. Only hand forces were associated with the MAF. Therefore, MAFs may be dependent upon tactile sensations from the hands, not the loads on the spine and thus may be unrelated to risk of low back injury. Practitioner Summary: Pushing tasks have become common in manual materials handling (MMH) and these tasks impose different tissue loads compared to lifting tasks. Industry has commonly used the psychophysical tables for job assent and decision of MMH tasks. However, due to the biomechanical complexity of pushing tasks, psychophysics may be misinterpreting risk.

Why cumulative loading calculated using non-weighted integration may not be suitable for assessing physical stress of the lower back: an empirical investigation of strain during lifting and lowering tasks

“…Therefore, before weighting factors for an alternative calculation method can be determined empirically, further research is needed on the question of which indicator represents the bottleneck for specific types of physical work activities, and should be used to calculate cumulative loading. Spearman's correlation coefficients show the greatest correlation between load intensity and both rate of HR change (.386) and RPE (.460), although the RPE may to some extent be influenced by the use of handheld weights, since Le et al (2012) have shown that psychophysical parameters when handling loads depend on tactile sensations of the hands. Due to the periodic nature of the loading conditions shown in Figure 4, a doubling of the area under the loading curve leads to a doubling of the duration of the lifting/lowering task.…”