A vehicle seat has an energy-absorbing device connected to a transverse support member of the seat frame. The device has a plurality of rib members configured to plastically deform in response to an impact between an object and the device, thereby absorbing energy from the object. The device has a shell portion with a pair of intersecting side walls having a generally U-shaped profile defining a recess, and the rib members are connected to each of the side walls within the recess. The shell portion is one piece, and the shell portion and rib members are formed of molded plastic. Each rib member is equally spaced within the recess.