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Sitting Biomechanics Laboratory
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Evaluating Sitting Load using the Established Buttock-Thigh Sitting FEM: Part I—Off-Loading Sitting Posture
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Off-Loading Sitting Posture It was confirmed that this Off-Loading concept, by lowering down the back portion of the seat pan, significantly reduced interface pressure on the buttocks. This finding was, however, only on the superficial pressure mapping, which did not provide clues of mechanical responses within the deep tissue such as in fat and muscle. Since it is likely that different tissues have different susceptibility to damage under mechanical compression, it is necessary to investigate whether the Off-Loading posture can benefit deep tissues. This study assessed how the pressure relief provided by partially removing the ischial support, i.e. using the Off-Loading posture, will effect the mechanical stress within deep soft tissues. The objective of this investigation was, therefore, to evaluate the Off-Loading concept using a 3D FE model which has been developed and validated in our previous study. It was expected that the FE analysis may help predict reliably how the skin, fat and muscle in the buttock-thigh region mechanically respond differently to the sitting load, as compared with a regular sitting posture. Thirty five individuals (41.3±12.1years; 72.1±12.6kg; 168.0±8.5cm) with no history of neuromuscular disorders were used to obtain the actual interface pressure distribution between the seat cushion and the buttock-thigh. Recordings were obtained when the subjects sat in the Normal and Off-Loading postures. |
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Effect of altered sitting posture on seating strain The change pattern of the compressive strain on the muscle induced by Off-Loading posture was in general comparable with that of the displacement for the entire buttock-thigh structure. The compressive strain on the muscle closer to the IT was decreased from 6.9±0.7% (N=3) in Normal posture to 0.3±1.3% (N=3) for the Off-Loading posture (Fig. 3). On the other hand, the compressive strain on the center of the thigh region was increased from 0.2±0.0% in Normal posture to 1.1±0.6% in Off-Loading posture. Anterior-posterior compressive strain (%) of the muscle layer in sagittal plane in a portion covered 15mm thickness buttock-thigh from 7.5mm medial to the ischial tuberosity to 7.5mm lateral to it. The large compressive strain in the Normal sitting posture, which was seen underneath the ischial tuberosity, was shifted toward the thigh with a largely decreased compressive strain in the Off-Loading posture. |
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Predicted Effect of altered Sitting Posture on Stresses Near Bony Prominences The predicted compressive stress and von Mises stress distributions for the entire buttock-thigh structure are shown. It was seen that, in Normal posture, the high-pressure and stress region was at the location right beneath the bony prominence of ischial tuberosity. However, this high-stress region was relocated to the thighs with a reduction of magnitude in the Off-Loading posture. A) Internal compressive stress and B) von Mises stress distributions for the entire structure (top) and a group of muscles (Minimus, Medius and Maximus Gluteus) below ischial tuberosity of the buttock-thigh (bottom) predicted for both the Normal and Off-Loading postures. |
Last updated
May 25, 2008 by Makhsous.
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