Effect of shape of back support adjustment on shear force applied to buttocks when tilt-in-space and reclining functions are combined in wheelchairs.

This study aimed to investigate the effect of the shape of the back support adjustment on the shear force applied to the buttocks when tilt-in-space and reclining functions are combined in wheelchairs. Fourteen healthy adult men were included in the study. The force plate was used to measure the parallel force as shear force. The measurement posture, leaning against the back support of an experimental chair, was a comfortable sitting posture. The tilt-in-space angle was set to 15°. The back support was inclined at increasing angles, starting from the upright position (IUP), proceeding to a fully reclined position (FRP), and returning to the upright position (RUP). The experimental conditions were as follows: adjusting the back-support shape (aBS) and non-adjusting the back support shape (non-aBS). Positive values indicate a parallel force applied to the buttocks posteriorly. The average values in the aBS condition were 3.4 ± 2.3, 13.6 ± 2.2, and −7.1 ± 2.4% body weight in the IUP, FRP, and RUP, respectively. The average values in the non-aBS condition were 3.8 ± 2.5, 11.4 ± 2.1, and −6.2 ± 3.1% body weight in the IUP, FRP, and RUP, respectively. There were significant differences between the two conditions in FRP (p < 0.01). These findings suggest that the shape of the back support adjustment function increased the shear force applied to the buttocks posteriorly when the back support was inclined backwards using both the tilt-in-space and reclining functions. When utilizing both the tilt-in-space and reclining functions to incline the back support, the shear force applied to the buttocks is greatly affected by the shape of the back support. The shape of back support adjustment is a function that can stabilize elderly persons' sitting posture, but it may increase the external force applied to the buttocks and back. [ABSTRACT FROM AUTHOR]