An anti-thrust seating system (1) and low profile, adjustable back system (2) primarily intented for use in a wheelchair (3). The anti-thrust seating system includes a base seating member (7) with a forward section (15) to support the user's thighs and a rearward section (17) to support the user's buttocks including the ischial tuberosities. The upper surfaces of the forward and rearward sections meet to form an edge (33) extending across the base seating member (7). The upper surface of the rearward section has two portions which form a first step face or anti-thrust barrier (29) to the user's ischial tuberosities. The seating further includes at least one separate step means (41) that can be removably positioned adjacent and rearwardly of the first step face (29) wherein the separate step means has a second step face (29'). The separate step means also has an upper surface (21') which serves to effectively extend the thigh supporting surface or shelf rearwardly. The adjustable back system has first and second relatively rigid shell members (6, 8) with a back cushion (10) removably attached to the second or forward shell member (8).