Abstract: Previous electrochemically-powered yarn muscles cannot be usefully operated between extreme negative and extreme positive potentials, since strokes during electron injection and during hole injection partially cancel because they are in the same direction. Unipolar-stroke carbon nanotube yarn muscles are described in which muscle strokes are additive between extreme negative and extreme positive potentials, and stroke increases with potential scan rate. These electrochemical artificial muscles include an electrically conducting twisted or coiled yarn and a material that dramatically shifts the potential of zero charge of the electrochemically actuated yarn.