Flexible supporting membrane (8) for positioning and supporting knees and lower legs of a patient being lifted by a mobile standing and raising aid (1). The membrane (8) is made from a stretchable material formed into a sheet that adjusts itself based on the pressure applied by the patient's knees. Moreover, it is shaped to cover the knees and shins of the patient and comprises: a top edge (28) adapted to be located in vicinity of the knees of the patient when in use； a bottom edge (29) adapted to be located in vicinity of the patient's ankles when in use； and a first and second lateral edge (26, 27) extending between the bottom edge (29) and the top edge (28). The flexible supporting membrane (8) is characterized in that it has a central section (19) and two peripheral sections (20, 21) extending along the direction defined by the lateral edges (26, 27) between the top edge (28) and the bottom edge (29) of the sheet； and in that the peripheral sections (20, 21) 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； and in that the central section (19) is stiffer than the two adjacent peripheral sections (20, 21), such that during use of the flexible supporting membrane (8) the central section (19) deforms less than the peripheral sections (20, 21) and acts as a barrier keeping the knees separated from each other in their respective depression.
