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The high cost of powerful,large-stroke,high-stress artificial muscle fibers and wires has combined with typical performance problems,like low-cycle-life under high load,hysteretic behavior,and low efficiencies,to restrict applications.We here demonstrate that inexpensive,highstrength polymer fibers used for fishing line and sewing thread can be easily transformed by non-conventional,extreme twist-insertion processes to provide fast,long-life tensile and torsional muscles that contract by over 49%and lift 100 times heavier loads than the same length and weight natural muscle.These muscles,which are normally thermally actuated,can be powered electrically using subvolt voltages,photonically,chemically,or by harvesting chemical or thermal energy to provide over 2 times higher specific work than competing nickel-titanium actuator wires.A single muscle fiber weighing 600 mg,made by coiling an 860 μm diameter nylon fishing line,can reversibly lift a kilogram weight 3 cm,and accomplish this once per second when powered by a 75°C temperature change.By weaving or braiding these fibers,we demonstrate textiles that open and close in response to temperature,which might lead to clothing that changes porosity to provide protection and comfort.Use of these muscles for harvesting thermal energy and for window shutters that help conserve energy is demonstrated.