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The cooperative operation of artificial molecular motors and switches has been amplified in polymer-based approaches that have led to versatile motion at the macroscale. As these active, shape-shifting polymers have become ever more sophisticated in their morphing capabilities, a major remaining challenge is to encode muscle-like mechanical adaptability during their operation and to explore its molecular origin. Here, we describe the mechanical adaptability of materials in which the light-induced action of molecular switches modifies the intrinsic interfacial tension, in a phase heterogeneous design featuring a liquid crystal polymer network swollen by a liquid crystal. When the swelling creates sufficient interfacial tension, light triggers an unprecedented and reversible photo-stiffening, analogous to myosin-powered muscle fibers. These mechanoadaptive materials adjust their stiffness to the task they must perform, also while they move, and display muscle-like behaviour that might contribute significantly to the development of human-friendly and soft robotics.