Advances in functional soft material muscles (i.e., actuators) that contract, expand, or rotate when triggered with an external stimulus are necessary to realize the future of new robotic assemblies with superior biologically relevant functions. Current research efforts are focused on synthesizing new soft materials to mimic natural muscles from a performance perspective, but neglect the impact of chemical composition and structure, which are core features for the exceptional actuation properties of human muscles. Recently, a new class of fiber actuators have been reported that contract or rotate when triggered by heat or hydration. The fibers, termed “strain crystallized actuators” (SCAs), are produced by combining solution-phase block copolymer self-assembly and strain-programmed crystallization. The strained fibers consist of highly aligned nanoscale structures with alternating crystalline and amorphous domains, resembling the ordered and striated pattern of mammalian skeletal muscles. The presentation will first cover the necessary macromolecular parameters for creating hydrogel fibers using amphiphilic block copolymers and then the nanoscale self-assembly mechanism during fiber straining that gives rise to the actuation properties. The versatility and recyclability of the polymer fibers, combined with the facile fabrication method, opens new avenues for creating soft actuators.