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.,

Knowledge of the interaction of forces between persons and the bed in which they lie or the seat on which they are sitting, provides an insight into the loading of their muscles, bones and soft tissue. To determine the total forces on the body-supporting surfaces (backrest, seat pan, foot rest) resolved in components perpendicular and parallel to these surfaces a new instrument has been developed, with which the forces perpendicular and parallel to three different freely adjustable body-supporting surfaces can be registered. During the first measurements the forces on a bed were measured when a person sits in a bed with the backrest at an angle of 45° to the horizontal and the mattress horizontal. The measurements on a healthy population (mean mass = 77 kg, sd = 11 kg) showed an accuracy of ± 10 N. In this position the mean shear force on the seat pan was 97 N.