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I believe you have not read the article entirely. The principle is not to twist the line to contract the muscle, that is made with an actuator, the principle is to apply a current to a mechanically twisted fiber to contract it and produce a movement.

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My mistake! From the PM article: "Also, by blending in conductive wire or wrapping the muscle with a light-absorbing coating, the researchers can control the muscles' movements with electricity and light instead of direct heat."

So these are heat-driven actuators (and potentially electricity or light driven). Very cool! Thanks for the clarification. (Also, this isn't at all evident from the Science abstract. I will read full paper tomorrow when I can pierce the paywall.)

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