All motors require energy, and you mentioned that heat maybe problematic because of heat loss. So you might be interested in photochemistry. These reactions can be made to be highly photon (i.e. energy) efficient and directional.
A promising technology for artificial muscles are those based off the light-driven molecular motors such as the Feringa rotary motors. For example, the Feringa motor can be utilized to produce "Macroscopic contraction of a gel induced by the integrated motion of light-driven molecular motors" . This is almost exactly how a muscle fiber works. Essentially, there are three components: a) the motor which turns unidirectionally with light and heat, b) two polymer strands which are connected to the opposite ends of the motor, and c) the modulator which binds together the end of the polymer. Therefore, when the motor spins it causes the polymer strands to wrap around each other and to contract. Furthermore, when you want to uncontract/uncoil the strands you open the modulator with a different wavelength of light.
Another great example are "optomechanical devices" which can produce macroscopic motion. My favorite is the "Photoinduced peeling of crystals". See this
Photoinduced peeling a molecular crystal on Chemistry World
Another author to look at for optomechanics is the Professor Garcia-Garibay, he has been interested in crystalline artificial muscles. For example, in reference  is a picture similar to this so it deserves a citation.
Finally, rotaxane is a mechanically interlocked molecular architecture that can be used to produce macroscopic motion similar to a muscle. These typically operate by changing the protonation states. But this action can be performed by either chemical or photochemical inputs (e.g. a photoacid!)
The operation of a molecular shuttle on Wikipedia
- Ke, C. A Light-Powered Clockwork. Nat. Nanotechnol. 2017, 12 (6), 504–506. https://doi.org/10.1038/nnano.2017.44.
- Tong, F.; Al-Haidar, M.; Zhu, L.; Al-Kaysi, R. O.; Bardeen, C. J. Photoinduced Peeling of Molecular Crystals. Chem. Commun. 2019, 55 (26), 3709–3712. https://doi.org/10.1039/c8cc10051a.
- Vogelsberg, C. S.; Garcia-Garibay, M. A. Crystalline Molecular Machines: Function, Phase Order, Dimensionality, and Composition. Chem. Soc. Rev. 2012, 41 (5), 1892–1910. https://doi.org/10.1039/c1cs15197e.