The invention is an assembly of a plurality of relatively movable jointed mbers such as robotic arm segments and a mechanical muscle connected between two of the segments. The muscle has a longitudinally inflexible, radially expandable sleeve containing a pressurizable bladder of elastic material. The bladder enlarges diametrically when pressurized and bulges the wall of the sleeve outward. The sleeve thereupon contracts axially to compensate for taken up by the sleeve's outward bulge. A terminus at either end of the sleeve connects the sleeve to different segments of a jointed arm, so that the sleeve's axial contraction effects movement of one segment relative to the other.