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Nicholas Stergiou, Jenny A. Kent, and Denise McGrath

An optimal level of variability enables us to interact adaptively and safely to a continuously changing environment, where often our movements must be adjusted in a matter of milliseconds. A large body of research exists that demonstrates natural variability in healthy gait (along with variability in other, healthy biological signals such as heart rate) and a loss of this variability in aging and injury, as well as in a variety of neurodegenerative and physiological disorders. We submit that this field of research is now in pressing need of an innovative “next step” that goes beyond the many descriptive studies that characterize levels of variability in various patient populations. We need to devise novel therapies that will harness the existing knowledge on biological variability and create new possibilities for those in the grip of disease. We also propose that the nature of the specific physiological limitation present in the neuromuscular apparatus may be less important in the physiological complexity framework than the control mechanisms adopted by the older individual in the coordination of the available degrees of freedom. The theoretical underpinnings of this framework suggest that interventions designed to restore healthy system dynamics may optimize functional improvements in older adults. We submit that interventions based on the restoration of optimal variability and movement complexity could potentially be applied across a range of diseases or dysfunctions as it addresses the adaptability and coordination of available degrees of freedom, regardless of the internal constraints of the individual.

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Nicholas Stergiou, Yawen Yu, and Anastasia Kyvelidou

Movement variability is considered essential to typical motor development. However, multiple theoretical perspectives and measurement tools have limited interpretation of the importance of movement variability in biological systems. The complementary use of linear and nonlinear measures have recently allowed for the evaluation of not only the magnitude of variability but also the temporal structure of variability. As a result, the theoretical model of optimal movement variability was introduced. The model suggests that the development of healthy and highly adaptable systems relies on the achievement of an optimal state of variability. Alternatively, abnormal development may be characterized by a narrow range of behaviors, some of which may be rigid, inflexible, and highly predictable or, on the contrary, random, unfocused, and unpredictable. In the present review, this theoretical model is described as it relates to motor development in infancy and specifically the development of sitting posture.

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Joshua Haworth, Anastasia Kyvelidou, Wayne Fisher, and Nicholas Stergiou

Children with autism spectrum disorder tend to have little interest in the presence, actions, and motives of other persons. In addition, these children tend to present with a limited and overly redundant movement repertoire, often expressing hyperfixation and aversion to novelty. We explore whether this is related to a more fundamental lack of appreciation for various temporal dynamics, including periodic, chaotic, and aperiodic motion structures. Seven children with ASD (age, gender, and height matched with children without ASD) were asked to stand and watch the motion of a visual stimulus displayed on a large (55") video monitor. Gaze and posture movements were recorded and assessed using cross recurrence quantification analysis for qualities of coordination, including rate and duration of bouts of coordination. Results showed that children with ASD do not express an affinity to chaotic motion of the stimulus in the same way as children without ASD. We contend that this indifference to chaotic motion is foundational to their general disinterest in biological motion.