Comparative effects of posture on pressure and shear at the bodyseat interface

Abstract-This study considers the effects of seated posture and body orientation on the pressure-distribution and surface shear (tangential) forces acting at the body-seat interface. Nine postures typically assumed by wheel-chair users were studied. Comparisons were made within and between two study groups, made up of 12 subjects with spinal cord injuries (SCI) and 10 nondisabled subjects. Both interface pressure and the surface shear were measured simultaneously in each of nine reproduc-ible, seated postures. The same seat cushion was used for all trials. The Oxford Pressure Monitor, a pneumatic cell device, was used to measure and record the interface pressures. Instrumentation for measuring and recording the surface shear force was constructed specifically for the study. Analysis consisted of statistically comparing changes in pressure values and shear forces derived from eight sitting postures with reference to values recorded in a defined neutral sitting posture. The pressure-distribu-tion findings suggest that in the postures studied SCl subjects have maximum pressures that are higher than nondisabled subjects in all postures, ranging from 6 % to 46 % depending on the posture. Maximum pressures can be reduced by postural changes: forward flexion to 50°,-9%; backrest recline to 120°,- 12%; and, full body tilt,- 11 %. On average, the §GI group members have peak pressure gradients (PPG) that are 1.5 to 2.5 greater than the nondisabled group. The maximum reduction in PPG occurred at backrest recline of 120°,- 18%. Tangential shear force acts at the body-seat interface in all nine postures studied. Extrapolation of results suggests that full-body tilt to approximately 25 " reduces the surface shear force to near zero. In contrast, a backrest-Address all correspondence and reprint requests to: Douglas A