An artificial muscle (10) comprises a chamber (12) having a flexible wall of plastics material, a device for heating and/or cooling a low-boiling-point fluid contained in, or in communication with, the chamber so that at least some of the fluid changes state between liquid and gaseous so that the force in the chamber on the flexible wall changes and/or the flexible wall moves to change the volume of the chamber, and a tendon (14) which at least partly encircles the chamber over an extent that varies with variations in the chamber volume for transmitting force between the flexible wall and a load. The use of a low-boiling-point fluid operating at around its boiling point enables plastics materials to be used without thermal damage, and the use of plastics materials enables low production costs. The use of a flexible wall to convey an effort to a load avoids the need for sliding parts and can provide advantages such as reduced friction and ease of sealing. The use of a low-boiling-point fluid operating at around its boiling point also enables large changes in pressure to be achieved with relatively small changes in temperature, so that the muscle can operate over a relatively small temperature range. The use of a low-boiling-point fluid operating at around its boiling point also reduces the risk of personal injury. The tendon arrangement provides less of a restriction on the operating range of the muscle than a McKibben type arrangement.