Effect of asymmetrical buttock pressure on forces exerted in buttock movement when using a reclining chair.

[Purpose] To clarify the effect of asymmetrical buttock pressure on the shear forces exerted on a buttock. [Participants and Methods] Sixteen healthy adult males participated in this study. A cushion 0 or 2 cm high was placed on the left side of the seat for all participants. The 0- and 2-cm height conditions were called "without difference condition" and "difference condition", respectively. The back support was inclined at increasing angles, starting at the upright position, to a fully reclined position, and back to the upright position. [Results] With the "difference condition", the force on the left buttock was 147.4% body weight and that on the right buttock was 105.6% body weight. In contrast, with the "without difference condition", there was no significant difference in the force on the left buttock and right buttock in terms of percent body weight. [Conclusion] Our results suggest that asymmetrical buttock pressure while in the sitting position causes a difference in shear force exerted on the left and right buttocks when using a reclining chair.