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Richard E.A. Van Emmerik, Michael T. Rosenstein, William J. McDermott, and Joseph Hamill

Nonlinear dynamics and dynamical systems approaches and methodologies are increasingly being implemented in biomechanics and human movement research. Based on the early insights of Nicolai Bernstein (1967), a significantly different outlook on the movement control “problem” over the last few decades has emerged. From a focus on relatively simple movements has arisen a research focus with the primary goal to study movement in context, allowing the complexity of patterns to emerge. The approach taken is that the control of multiple degrees-of-freedom systems is not necessarily more difficult or complex than that of systems only comprising a few degrees of freedom. Complex patterns and dynamics might not require complex control structures. In this paper we present a tutorial overview of the mathematical underpinnings of nonlinear dynamics and some of its basic analysis tools. This should provide the reader with a basic level of understanding about the mathematical principles and concepts underlying pattern stability and change. This will be followed by an overview of dynamical systems approaches in the study of human movement. Finally, we discuss recent progress in the application of nonlinear dynamical techniques to the study of human locomotion, with particular focus on relative phase techniques for the assessment of coordination.

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Daniel P. Ferris and Bryan R. Schlink

Robotic exoskeletons and bionic prostheses have moved from science fiction to science reality in the last decade. These robotic devices for assisting human movement are now technically feasible given recent advancements in robotic actuators, sensors, and computer processors. However, despite the ability to build robotic hardware that is wearable by humans, we still do not have optimal controllers to allow humans to move with coordination and grace in synergy with the robotic devices. We consider the history of robotic exoskeletons and bionic limb prostheses to provide a better assessment of the roadblocks that have been overcome and to gauge the roadblocks that still remain. There is a strong need for kinesiologists to work with engineers to better assess the performance of robotic movement assistance devices. In addition, the identification of new performance metrics that can objectively assess multiple dimensions of human performance with robotic exoskeletons and bionic prostheses would aid in moving the field forward. We discuss potential control approaches for these robotic devices, with a preference for incorporating feedforward neural signals from human users to provide a wider repertoire of discrete and adaptive rhythmic movements.

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Julie Vaughan-Graham, Kara Patterson, Karl Zabjek, and Cheryl A. Cott

Human movement is a complex phenomenon presenting considerable challenges regarding how it is systematically and consistently described and investigated both from clinical and research perspectives ( Harbourne & Stergiou, 2009 ; Latash, Levin, Scholz, & Schöner, 2010 ; Levin, Liebermann, Parmet

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Paul G. Taylor, Michael Small, Kwee-Yum Lee, Raul Landeo, Damien M. O’Meara, and Emma L. Millett

Entropy is an effective tool for investigation of human movement variability. However, before applying entropy, it can be beneficial to employ analyses to confirm that observed data are not solely the result of stochastic processes. This can be achieved by contrasting observed data with that produced using surrogate methods. Unlike continuous movement, no appropriate method has been applied to discrete human movement. This article proposes a novel surrogate method for discrete movement data, outlining the processes for determining its critical values. The proposed technique reliably generated surrogates for discrete joint angle time series, destroying fine-scale dynamics of the observed signal, while maintaining macro structural characteristics. Comparison of entropy estimates indicated observed signals had greater regularity than surrogates and were not only the result of stochastic but also deterministic processes. The proposed surrogate method is both a valid and reliable technique to investigate determinism in other discrete human movement time series.

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Fabien Cignetti, Sébastien Caudron, Marianne Vaugoyeau, and Christine Assaiante

There is evidence that adolescence is a critical period in development, most likely involving important modifications of the body schema and of the sensorimotor representations. The present study addressed this issue, by investigating the differences between adolescents and adults regarding the integration of proprioceptive information at both perceptual and postural levels and the visual recognition of human movement. Proprioceptive integration was examined using muscle-tendon vibration that evoked either a postural response or an illusory sensation of movement. The ability to recognize human movement was investigated from a paradigm where the participants had to discern between human movements performed with and without gravity. The study produced three main findings. First, the adolescents had larger postural responses to tendon vibrations than the adults, with visual information enabling them to reduce this exaggerated postural reaction. Second, the adolescents had a greater illusory perception of movement compared with the adults. Third, the adolescents had the same perceptual ability as adults in the human movement perception task. In conclusion, we were able to highlight notable differences between adolescents and young adults, which confirms the late maturation of multisensory integration for postural control and the privileged visual contribution to postural control.

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Andy Roosen, Matthew T.G. Pain, and Mickaël Begon

Much research is ongoing into improving the accuracy of functional algorithms to determine joint centers (JC), but there has been limited testing using human movement data. This paper is in three parts: Part 1, errors in determining JCs from real human movement data using the SCoRE method; Part 2, variability of marker combinations during a punch; Part 3, variability in the JC due to reconstruction. Results indicate determining the JC of the shoulder or elbow with a triad of markers per segment with an accuracy greater than 20 mm is unlikely. Part 2 suggests conducting a pilot study with abundant markers to obtain triads, which are most stable due to differences of 300–400% in variability between triads. Variability due to the choice of reference frame for reconstruction during the punch ranged from 2.5 to 13.8 mm for the shoulder and 1.5 to 21.1 mm for the elbow. It would appear more pertinent to enhance the practical methods in situ than to further improve theoretical accuracy of functional methods.

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Landon B. Lempke, Jeonghoon Oh, Rachel S. Johnson, Julianne D. Schmidt, and Robert C. Lynall

human movement. Dual-task differences among musculoskeletal-injured and concussed individuals on gait and balance tasks after initial injury are concerning. However, it is unknown if these dual-task deficits are present, or even amplified, under more sport-like, functional movements. Growing research

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Anantha Narayanan, Farzanah Desai, Tom Stewart, Scott Duncan, and Lisa Mackay

different behaviors impact health separately, but an emerging paradigm— time-use epidemiology —has prompted researchers to examine the interactions among these behaviors across complete (24 h) days. 4 For this to occur, uninterrupted measurement of human movement behavior is required. Advancements in both

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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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Kelley K. Pettee Gabriel, James R. Morrow Jr., and Anne-Lorraine T. Woolsey

Context:

The selection of the most psychometrically appropriate self-report tool(s) to measure specific physical activity constructs has been a challenge for researchers, public health practitioners, and clinicians, alike. The lack of a reasonable gold standard measure and inconsistent use of established and evolving terminology have contributed to these challenges. The variation of self-report measures and quality of the derived summary estimates could be attributed to the absence of a standardized conceptual framework for physical activity.

Objective:

To present a conceptual framework for physical activity as a complex and multidimensional behavior that differentiates behavioral and physiological constructs of human movement.

Process:

The development of a conceptual framework can provide the basic foundation from which to standardize definitions, guide design and development of self-report measures, and provide consistency during instrument selection.

Conclusions:

Based on our proposed conceptual framework for physical activity, we suggest that physical activity is more clearly defined as the behavior that involves human movement, resulting in physiological attributes including increased energy expenditure and improved physical fitness. Utilization of the proposed conceptual framework can result in better instrument choices and consistency in methods used to assess physical activity and sedentary behaviors across research and public health practice.