Soft materials are known for their compliance, adaptivity and dexterity. They have been well-pursued to create artificial muscles for robotic applications, however, at the cost of load bearing, bit rates and energy efficiency compared with their rigid body counterparts. Despite significant efforts to improve their performance, very few can match the biological muscles, achieving fast speed, high elasticity, and high power density simultaneously. Here, we self-assemble composite fibers by wet-spinning graphene oxide (GO) nanosheets in their lyotropic liquid crystal (LLC) phase mixed with conducting polymers and depleting agent, poly(ethylene glycol), followed by chemical reduction of GO. The resulting reduced GO (rGO) nanosheets are highly aligned and closely packed, which is essential to their high mechanical strength and toughness and actuation behaviors. The composite fiber exhibits fast (80 ms) and reversible bending via electrostatic repulsion between the rGO sheets. When the fibers are plied with nylon yarns, our actuators can achieve 75 J/kg work capacity and 924 W/kg power density without diminishing the bending strain and efficiency up to 10,000 cycles.