Abstract: This study presents an assessment system of human body discomfort index based on muscle hardness experiments— Assessment system based on muscle hardness experiments (ASMH), which is a guideline of the steering layout design. First of all, a group of experiments are carried out to find out the impact of human body static postures on muscle hardness index (MHI). In the process, relative muscle hardness value (MHV) are measured by muscle hardness gauge so that the variation rule of MHV in different operating postures is obtained. Based on the physiological feature that muscle hardness may result in muscle discomfort in human body, muscle discomfort index (MDI) is defined through the normalization, and weighting of MHV. Then concerning the motions of two and three degree-freedom joints, different calculation methods for the overall MDI are adopted. Accordingly, ASMH is proposed, which can provide more objective ergonomic suggestions. At last, through the comparison between ASMH and rapid upper limb assessment (RULA) which is widely adopted in engineering, it is proved that the accuracy and applicability of ASMH are improved. The range of score increases to 12 and the rank increases to 6. Therefore, ASMH is more effective in ergonomic design.

Key words: Discomfort assessment system, Muscle hardness, Static operating postures, Steering layout design