Forearm extension for use in an extension arm

A reinforced channel (104), for use in an extension arm that mounts a device to a support, comprises a U-shaped body (130) with a pair of spaced apart sidewalls (182) and a bottom wall (180) therebetween, each sidewall having an upper edge opposing the bottom wall. A rib assembly within the body between the sidewalls overlies the bottom wall, and comprises a plurality of first ribs (400) projecting upwardly from the bottom wall transversely from one of the sidewalls to the other of the sidewalls and a plurality of second ribs (402) projecting upwardly from the bottom wall at an oblique angle to the sidewalls. The rib assembly extends from the bottom wall to a location below the upper edge of each sidewall and forms, within the body, a passageway extending from its first end to its second end between the upper edge of each sidewall and the rib assembly.