An anti-thrust seating system and low profile, adjustable back system primarily intended for use in a wheelchair. The anti-thrust seating system includes a base seating member with a forward section to support the user's thighs and a rearward section to support the user's buttocks including the ischial tuberosities. The upper surfaces of the forward and rearward sections meet to form an edge extending across the base seating member. The upper surface of the rearward section has two portions which form a first step face or anti-thrust barrier to the user's ischial tuberosities. The seating system further includes at least one separate step means that can be removably positioned adjacent and rearwardly of the first step face wherein the separate step means has a second step face. The separate step means also has an upper surface which serves to effectively extend the thigh supporting surface or shelf rearwardly. The adjustable back system has first and second relatively rigid shell members with a back cushion removably attached to the second or forward shell member. In their retracted positions, portions of both the first and second shell members and cushion extend between and along the back posts of the wheelchair and very little of the back extends forward of the back posts.