For competitive swimmers with a physical disability, the ideal stroke for crawl swimming has not yet been clarified, and it may be different from that of able-bodied swimmers because of differences in body structure. The objective of this study was to solve computationally the fastest arm stroke of crawl swimming for swimmers with bilateral transfemoral amputation, and to investigate the features of the optimal stroke. The optimal stroke was obtained by the optimization method with Particle Swarm Optimization (PSO) and the swimming human simulation model (SWUM). In this method, the design variables were the joint angles of the upper limbs, and the objective function was to maximize swimming speed. The optimal stroke was obtained for each stroke cycle of 0.8 s to 1.5 s with increments of 0.1 s. In addition, it was also obtained for several palmar flexion angles of the wrist at the catch phase. The fastest stroke was obtained when the stroke cycle was 1.0 s and the palmar flexion angle was 35 degrees. For short stroke cycles, the optimal stroke was the motion pulling and pushing the water near the mid-line of the trunk. For long stroke cycles, the optimal stroke was the motion pushing the water toward the outside of the trunk. In both strokes, the water was pulled and pushed shallowly. Possible reason for these features was that the optimal strokes were the solutions to reduce the torque which sinks the lower body and to stabilize the posture.
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