A force transmitting frame may have a length greater than a width. Stiffnesses of first and second end portions of the force transmitting frame may be greater than a stiffness of a central area of the force transmitting frame in a longitudinal direction of the force transmitting frame. The force transmitting frame may include: an inner frame configured to support one side of a user; and/or an outer frame of which first and second end portions are fixed to first and second end portions of the inner frame, and of which a central portion is not fixed to a central portion of the inner frame. The central portion of the outer frame may be configured to slide with respect to the central portion of the inner frame.