A flexible supporting membrane is made from a stretchable material formed into a sheet that adjusts itself based on the pressure applied by a patient's knees. The membrane includes: a top edge adapted to be located in a vicinity of the knees of the patient when in use； a bottom edge adapted to be located in a vicinity of the patient's ankles when in use； and a first and second lateral edge extending between the bottom edge and the top edge. The flexible supporting membrane further includes a central section and two peripheral sections extending along the direction defined by the lateral edges between the top edge and the bottom edge of the sheet. The peripheral sections are adapted for each supporting one of the two knees of a patient by yielding to the pressure exerted by the knees and thus forming a knee cradling depression.
