The sit-to-stand (STS) movement is performed throughout the day, and providing handrails is one method of making the STS movement easy. However, designers may have determined the installation position of handrails using intuition and trial and error. The aim of this study is to determine the optimum position and orientation of handrails by minimizing the quantified physical load of the STS movement. Twelve university students participated, and eight electromyograms (EMGs), namely, of the brachioradialis, flexor carpi ulnaris, extensor carpi radialis longus, latissimus dorsi, right and left rectus femoris, and right and left tibialis anterior, were recorded. Observations with handrails at various tilt angles and forward distances from the edge of the seat were analyzed for the optimization. The total physical load (TPL) function was formulated as the weighted sum of the EMGs. The weight coefficients were determined by maximizing the correlation coefficient between the measured subjective scores and the TPL function values. The result shows that the handrail installation position significantly affects all of the EMGs except those of the right and left rectus femoris. The weight coefficients of the TPL function are positive for the upper limb muscles, whereas they are zero for the lower limb muscles. The handrail position for multiple users was formulated to minimize the TPL function, and hence the optimum position was determined.
Physical workload reduction is a significant factor in product design. However, experimental bioinstrumentation measurements involve substantial time and costs. This study proposed a simulation based ergonomic design method with digital human modeling (DHM) to accomplish efficient ergonomic product design. DHM simulation was applied to evaluate the joint moment ratios (JMRs). The product design for physical workload reduction was formulated as a minimization of the average and maximum JMRs to determine the optimal solution. The proposed method was applied to a problem of designing the forward distance of a handrail to support the sit-to-stand (STS) movement. The STS motion, the force exerted on the handrail, and the subjective perceived workload were measured for nine subjects. The STS motions and exerted force of the DHMs were predicted from the measured data, and physical workload simulation was performed with multiple DHMs to reflect anthropometric diversity. The response surfaces of the average and maximum JMRs were predicted as functions of the forward distance, and Pareto frontiers of each DHM condition were determined. The findings revealed that there were no trade-offs between the average and maximum JMRs, and that the optimal forward distance was in the range of 345-400 mm. (Key words: Digital human modeling, Joint moment, Multi-objective optimization, Response surface methodology) 1
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