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The influence of the distance between the backrest of a chair and the position of the pelvis on the maximum pressure on the ischium and estimated shear force

Purpose. For some people with leg disorders, the alternative to becoming bedridden is sitting in a wheelchair for long periods of time. The purpose of this study was to investigate the relation between the backrest of a chair and the position of the pelvis on the maximum pressure at the ischium and the estimated shear force encountered while sitting in a chair. Methods. Ten healthy males aged 22.4 ± 2.3 years without pathologies in the leg and/or trunk participated in this study. The subjects were instructed to sit in a chair and the pressure and shear forces were measured under three sitting conditions: In the standard position, a 5 cm forward position and a 10 cm forward position. The effects of the positions of the pelvis on the maximum pressure at the ischium were measured by an instrument for measuring pressure distribution and the shear forces were estimated using an experimental model proposed by us. Results. Comparisons were made among the three sitting positions; the range of displacement of the maximum pressure point was not significant. The estimated shear force on the seat was 15.5 ± 12.4 N in the standard position, 34.4 ± 8.5 N in the 5-cm forward position, and 53.2 ± 16.7 N in the 10 cm forward position. There were significant differences among the three values (p < 0.01). Conclusions. Displacing the pelvis forward and leaning against the backrest tends to increase the shear force and raise the risk of decubitus ulcers.