Metallic Muscles, developed by our team, are monolithic macroscopic bodies of nanoporous metal. By reversibly varying the capillary forces at the interface between the solid and a gas or liquid in the pore space, one can induce reversible elastic strain of the entire macroscopic sample, with amplitudes up to and exceeding 1%. The performance suggests possible applications as stiff, large strain actuators. The strain can be controlled by electric signals or – similar to muscle activity in biological systems – it can be prompted by direct conversion of chemical energy into mechanical work.