A rotational energy (1) typically for use in a fall arrest system has a coiler (5), a length of plastically deformable strip (4) and a deformer structure (3,3c). The plastically deformable strip has a first end (4a) attached to the coiler (5) and a second free end (4b) and extends past the deformer structure at a position between the first and second ends. Relative rotation of the coiler member and deformer structure (3,3c) causes the strip to be drawn past the deformer structure, plastically deforming the strip and winding the strip coil form about the coiler member.