Gert Kwakkel1, Carel G M Meskers2. 1. Department of Rehabilitation Medicine, MOVE Research Institute Amsterdam, Vrije Universiteit Medical Center, 1007 MB, Amsterdam, Netherlands; Amsterdam Rehabilitation Research Centre Reade Amsterdam, Netherlands. Electronic address: g.kwakkel@vumc.nl. 2. Department of Rehabilitation Medicine, MOVE Research Institute Amsterdam, Vrije Universiteit Medical Center, 1007 MB, Amsterdam, Netherlands.