The crutch-walker skill level is related to the the penalty received to movement. As the skill develops, the character moves at speeds closer to their ambulatory movement rate up to Legendary+5 rank, at which movement rate is no longer impaired while a crutch is held. The quality or type of crutch is not related to outcome. It is possible to gain experience in this skill in adventure mode by holding a crutch in either hand.
Crutch-walkers are the natural byproducts of a fortress' health care industry.