A seat with biomechanical articulation for a person seated in the seat includes a seat portion extending generally horizontally, a back portion extending generally upwardly from the seat portion, a frame interconnecting the seat portion and the back portion, the seat portion having a seat member pivotally connected to the frame, and a structure for supporting a pelvis of a person seated on the seat member and moving in response to movement of the seat member by the person seated in the seat. The frame may be replaced by the frame of an existing seat such as a wheelchair frame.