Does preferred seat pan inclination minimize shear force?

Abstract

Past biomechanical studies on seated postures showed that effects of seat parameters, such as seat pan angle, back angle and friction coefficient, on muscle activities, shear force between buttocks and seat and spinal loads are complex. Reducing all these biomechanical loads at the same time may not be possible. Lowered muscle activation may require higher frictional shear force. It is interesting to investigate how people behave compared to biomechanical simulations. In this paper, the question whether sitters prefer a seat pan angle for reducing shear force was investigated using the data collected from a multi-adjustable experimental seat. Two imposed seat pan angles (A_SP = 0°, 5°) and one self-selected were tested for two backrest angles (A_SB = 10°, 20°, from the vertical). A flat seat pan surface was used. Other seat parameters such as seat height, length and position of three back supports were defined with respect to each participant’s anthropometry. As expected, results showed that shear force increased with backrest recline and decreased with seat pan recline. No significant difference in self-selected seat pan angle was found between two backrest angles. An average of 6.2° (±3°) was observed. The lowest shear was observed for the condition of self-selected seat pan angle, supporting the idea that seat pan should be oriented to minimize shear force. However, self-selected angle did not completely remove the shear. A zero shear would require a more reduced trunk-thigh angle, suggesting a minimum trunk-thigh angle should also be maintained.