A method for optimizing the geometry of a seat membrane, the method comprising the steps of: (a) forming a three-dimensional map of a contact body portion of a subject seated in a predetermined posture by dividing one or more defined reference planes into transverse bands and (b) dividing each of the bands into cells and calculating an average displacement of the cells from the one or more reference planes; the method further characterized by (c) defining positions of frame members (30, 31) supporting each end of the seat membrane relative to each of the bands; (d) identifying cells of tangency (40, 41) from the positions of the frame members (30, 31) to the bands and calculating the distance (D5,D6) from each of the positions of the frame members to their the cells of tangency; (e) determining positions of the seat membrane at extreme ends of areas that bridge areas of concavity on the surface of the subject's body for each of the bands and calculating the bridging distance of supporting bands across areas of concavity; (f) calculating the lengths (L8,L9) of the supporting bands in contact with the body; (g) calculating the total length under load of each supporting band; (h) calculating an average pressure multiplier for each supporting band by dividing the sum of the distances obtained in step (d) by the sum of the displacement of the cells of tangency from the frame members; (i) calculating a desired total perpendicular-to-plane load for each supporting band as the sum of the pressures measured by a pressure measurement mat along the supporting band when the mat and the subject are supported by a freely accommodating pad; (j) calculating a relaxed length of each supporting band when not installed on the frame members; (k) calculating an optimum relaxed length of each supporting band when not installed on the frame members based on the frame position and seat membrane elasticity to define a relaxed shape of the seat membrane when not installed on the frame members; and (l) optimize seat membrane elasticity to accommodate multiple subjects in dependence upon the variation of relaxed length of each supporting band. (a) forming a three-dimensional map of a contact body portion of a subject seated in a predetermined posture by dividing one or more defined reference planes into transverse bands and (b) dividing each of the bands into cells and calculating an average displacement of the cells from the one or more reference planes; the method further characterized by (c) defining positions of frame members (30, 31) supporting each end of the seat membrane relative to each of the bands; (d) identifying cells of tangency (40, 41) from the positions of the frame members (30, 31) to the bands and calculating the distance (D5,D6) from each of the positions of the frame members to their the cells of tangency; (e) determining positions of the seat membrane at extreme ends of areas that bridge areas of concavity on the surface of the subject's body for each of the bands and calculating the bridging distance of supporting bands across areas of concavity; (f) calculating the lengths (L8,L9) of the supporting bands in contact with the body; (g) calculating the total length under load of each supporting band; (h) calculating an average pressure multiplier for each supporting band by dividing the sum of the distances obtained in step (d) by the sum of the displacement of the cells of tangency from the frame members; (i) calculating a desired total perpendicular-to-plane load for each supporting band as the sum of the pressures measured by a pressure measurement mat along the supporting band when the mat and the subject are supported by a freely accommodating pad; (j) calculating a relaxed length of each supporting band when not installed on the frame members; (k) calculating an optimum relaxed length of each supporting band when not installed on the frame members based on the frame position and seat membrane elasticity to define a relaxed shape of the seat membrane when not installed on the frame members; and (l) optimize seat membrane elasticity to accommodate multiple subjects in dependence upon the variation of relaxed length of each supporting band.