An assembly includes a seatback defining an occupant-seating area. The seatback has a frame and a rear trim cover. The frame is between the occupant-seating area and the rear trim cover. The assembly includes a seat bottom extending from the seatback below the occupant-seating area. The assembly includes a deformable cell between the frame and the rear trim cover. The deformable cell extends from a first end proximate the frame to a second end proximate the rear trim cover. The deformable cell has a contraction between the first end and the second end.