Non-linear dynamics, fractals, periodic oscillations, bifurcations, chaos, and other terminologies have been used to describe human biological systems in the literature for a few decades. The eight manuscripts included in this special issue discussed the historical background, selected theoretical frames, exemplar concepts, as well as some current applications in the field of non-linear dynamical systems. For example, in the first paper in the series, van Emmerik and colleagues1 have tried, very successfully in my opinion, to discuss the differences and commonalities among the terms of “variability”, “stability”, and “complexity”, and also how they all related to human biological system's adaptability to human movement. Along this line, Williams III and his group2 have tried to compare the results approximate entropy, a non-linear measure of variability and its structure, and stability measured by the reaction of upright standing posture to external perturbation. These discussions can help us clarify some fundamental and important concepts when we investigate non-linearity in human movement dynamics.
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