The drive for ease of articulation and repercussions for morphology and historical linguistics
Sign languages exhibit a drive for ease of articulation in the movement parameter by reducing active effort via several mechanisms, including reducing the number of articulators, the number of articulations, and the mass that is moved. They also exhibit the drive by reducing reactive effort -- which is the effort to resist incidental movement of a body part caused by movement in another body part; for example, the swinging of an arm can cause the torso to rock or spin. Avoiding signs that would call for reactive effort turns out to have an effect on the shape of the lexicon. Finally, in reconstructing the history of a sign language, recognition of biomechanical methods for reducing effort allows one to develop standards for judging whether the movement parameter of signs in different languages can be judged "the same" or not.