Structures of seating systems for human users are disclosed. In one embodiment, the structure comprises: a frame; a seat structure supported by the frame; and a back rest structure supported by the frame. The seat structure comprises a thigh support region and a pelvic support region where the pelvic support region defines a pelvic well for receiving an ischial tuberosity of the user and also provides a fulcrum to assist in rotating the pelvis of the user. The back rest structure comprises a gluteal panel for supporting a gluteal mass of the user and a posterior superior iliac spine (PSIS) panel disposed above the gluteal panel. The PSIS panel is adjustable to cause movement of a PSIS of the user and cooperate with the pelvic well to cause rotation of the pelvis of the user about the fulcrum provided by the pelvic well.