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 multiadjustable 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.

A parametric investigation on seat/occupant contact forces and their relationship with initially perceived discomfort using a configurable seat

“…A small but significant effect of interaction between A_SP and A_SB was also observed (Figure 3), showing that a more reclined seat pan lowered discomfort much more with a more reclined seat back. Self-selected seat-pan angles were on average respectively 5.9° (±3.1) and 6.7° (±3.0) for A_SB=10° and 20° (Wang et al, 2018c), quite close to 5°. No significant difference in discomfort was observed between A_SP=5° and A_SP=PR ( Figure 2a).…”

“…One of our initial working hypotheses was that people prefer a seat-pan angle that minimizes the shear force at the seat pan for a given backrest angle. This was verified in Wang et al (2018c) by comparing the shear forces of the conditions involving a self-selected seat-pan angle (A_SP=PR) with those using two imposed seat-pan angles (A_SP=0° and 5°) under the reference configurations (Table 7). Both A_SP and A_SB significantly affected shear force, which increased with A_SB and decreased with A_SP.…”

Effects of seat parameters and sitters’ anthropometric dimensions on seat profile and optimal compressed seat pan surface

“…One of our initial working hypotheses was that people prefer a seat-pan angle that minimizes the shear force at the seat pan for a given backrest angle. This was verified in Wang et al (2018c) by comparing the shear forces of the conditions involving a self-selected seat-pan angle (A_SP=PR) with those using two imposed seat-pan angles (A_SP=0° and 5°) under the reference configurations ( Table 7).…”