Both with respect to the aspect of pressure sores and of comfort, the inclination of backrest and seat are, amongst other factors, important design criteria. In this study the combination of seal and backrest inclination which reduces shear forces on the seat in passive seating forms the centre of attention. A biomechanical model was developed to predict these combinations and a new measurement apparatus was used for verification of the model on 10 healthy subjects (age 24.4 S.D. 2.1 yr, height 1.77 S.D. 0.08 m, mass 66.3 S.D. 11 Kg). For chairs it was found that when little shear is accepted, a fixed inclination between seat and backrest can be chosen between 90° and 95°. For beds a parabolic relationship was found between seat and backrest inclination with a maximum seat inclination of 20° at a backrest inclination of 50°. When lying with the knees bent to a position with equal inclination of thighs and shanks, the model predicts a shear force on the seat that shoves the person into the bed for every combination of seat and backrest inclination.

Introduction. The aim of the study was to compare a new anti-decubitus mattress with other low-cost mattresses. Methods. Interface pressure measurements were made in supine and in sitting position. Results. The new mattress has significant lower interface pressures than other low-cost mattresses in both positions, and that there is no significant difference with an expensive anti-decubitus bed like the air-fluidized type. Measurements of shear between body and cover show that the shear is far below 10 kPa. This limit is known as a limit above which shear may play an important role in the causation of decubitus ulcers. Conclusion. The goals of the research have been achieved: the development of a low-cost anti-decubitus mattress with, in supine position, interface pressures comparable to the well-known and expensive beds; in sitting position interface pressures are also below the capillary pressure of 4.2 kPa.

Summary. Since the early studies on decubitus many authors expect significant influence of shear on decubitus risk. In the study described here skin oxygen tension at the sacrum was measured with combined pressure and shear stress on a young healthy population (mean age 25‐5 years, SD 3–4 years). Cut‐off pressure is defined as the level of external pressure exerted on the skin at which the skin oxygen tension is 1–3 kPa. At this level ischaemia of the skin can be expected (Bar, 1988). A mean cut‐off pressure of 11‐6 kPa was found when no shear stress was applied vs. a significantly lower mean cut‐off pressure of 8‐7 kPa with a shear stress of 3‐1 kPa. No significant relationship was found between cut‐off pressure and systolic blood pressure, diastolic blood pressure, skin thickness at the sacrum, percentage of fat and skin oxygen tension in the unloaded situation.,

An anti-decubitus mattress is introduced. Primary requirements were low costs, low pressure and shear in lying and sitting, favourable heat and humidity exchange and adaptation to body weight. The different media a mattress may consist of, are described. After analysis of physical properties air has been chosen because of equally distributed pressure along the body contour. Details of the construction of the mattress and the control system are described. A market research revealed that the price of the complete system has to be lower than Dfl 5000 (approximately US$ 3000). The present article describes how one of the main goals of the research has been achieved: a low-cost mattress. In part 2 pressure and shear are analysed in lying and sitting and a comparison is made with cheap as well as expensive anti-decubitus mattresses.

The risk of pressure sores is a common problem to wheelchair users. A great variety of wheelchair cushions are produced to reduce this risk. To evaluate wheelchair cushions in most studies the pressure under the ischial tuberosities is measured in a static situation with the subjects seated in one posture. However wheelchair users do interact with their environment and during the day they will drive on different surfaces adopting different postures, leading to continuous pressure variations under the buttocks. Therefore in this study, the pressure under the buttocks of nine spinal cord injured subjects was measured continuously during wheelchair driving, while they were sitting on a standard foam cushion (thickness 5 cm) or a Jay Active® cushion. The results of dynamic pressure measurements correlate with three classes of tissue response under the buttocks (Bar, 1988): severe, moderate and mild. The results from the data showed that a 5 cm thick foam cushion produced only severe responses, while a Jay Active® cushion showed mild responses most of the time.

By means of continuous recording of the positions of the trunk and head, an investigation was carried out to examine the effect of the Erasmus office-desk - a desk with a 10° inclination - on sitting posture whilst reading and writing, sitting in an adjustable chair at an adjustable table. The posture assumed by a person who is doing mixed reading and writing work at a table with the Erasmus office-desk is more upright than the posture of the same person doing the same work without the desk. This applies for both the head and the trunk. The average position of the trunk was changed from 26·0° to 18·2° when working with the desk placed on the table. The moment of force caused by gravity is proportional to the sine of the angle. Therefore the change of angle results in a decrease of the average moment of force on the back at L5-S1 of 29%. The largest individual decrease found was 86%. The average change of the position of the head was changed from 38·5° to 29·6°, resulting in an average decrease of the moment of force, on C7-T1, of 21%. The largest individual decrease was 51%. Lateroflexion of the back was less often assumed when the subjects were using the desk. The posture assumed by the subjects when they were doing writing work only was generally more upright when working at a desk with a 10° inclination than when working at a flat desk. Marked differences were found between the postures of the subjects. The change of posture while using the desk also showed marked differences for each subject individually.