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Wenchong Du, Anna Barnett and Kate Wilmut

Perception and action are tightly coupled, and previous studies have demonstrated that action experience can improve perceptual judgment. We investigated whether this improvement in perceptual judgment could be attributed to knowledge regarding movement variability being gained during action experience. Fifteen adults made perceptual judgments regarding the passability of a series of aperture sizes. These judgments were made both before and after walking through the same set of apertures (action experience). When considering the group as a whole, perceptual judgment did not change after action experience. However, when splitting the group into those with low and high preaction perceptual judgments, only those with low perceptual judgments showed an improvement in perceptual judgment following action experience, which could be explained in part by movement variability during the approach. These data demonstrate that action informs perception and that this allows adults to account for movement variability when making perceptual judgments regarding action capabilities.

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Andrew D. Nordin and Janet S. Dufek

Intra-individual trial-to-trial variability is inherent in human movement. 1 – 4 A variety of methods exist to examine movement variability, with greater variable inclusion and time series analyses each considered essential advances in understanding movement control. 5 , 6 Movement coordination

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Bryan C. Heiderscheit

The purpose of this paper is to discuss the role of variability in human movement, with emphasis on locomotion variability. Based on the assessment of stride characteristics, movement variability has been associated with reduced gait stability and unsteadiness. However, based on the measure of joint coordination during locomotion, variability has been suggested to provide a source of adaptation. Therefore, it would appear that the assessment of movement coordination from either the task outcome (i.e., stride characteristics) or the joint coordination patterns provide distinctly opposing views of variability. This paper will discuss the use of the variability measures, specifically joint coordination variability, from a clinical perspective. Investigations will be presented in which a reduction in joint coordination variability has been associated with pathology. Finally, the clinical implications of these measures as well as treatment suggestions are discussed.

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Paul G. Taylor, Raul Landeo and Jennifer Coogan

The purpose of this study was to explore movement variability of throwing arm and ball release parameters during the water polo shot and to compare variability between successful (hit) and unsuccessful (miss) outcomes. Seven injury free, subelite, females completed 10 trials of the 5 m water polo penalty shot. Intraindividual coefficient of variation percentage (CV%) values were calculated for elbow and wrist angular displacement, wrist linear velocity and ball release parameters (height, angle and velocity). Coordination variability (elbow/wrist angular displacement) was calculated as the CV% of the mean cross-correlation coefficient. Elbow and wrist displacement variability decreased to 80% of throwing time then increased toward release. Wrist linear velocity variability reduced toward release. Individual CV% values ranged between 1.6% and 23.5% (all trials), 0.4% and 20.6% (hit), and 0.4% and 27.1% (miss). Ball release height and velocity variability were low (< 12%; all trials) whereas release angle variability was high (>27%; all trials). Cross-correlation results were inconclusive. Roles of the elbow and wrist in production of stable ball release height and velocity and control of the highly variable release angle in the water polo shot are discussed and suggested for further study. Optimal levels of variability warrant future investigation.

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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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Alessia Longo and Ruud Meulenbroek

Movement repetition and thus monotony is a common factor underlying many manual motor tasks such as typing, texting, and handwriting. However, each movement is unique and repetition necessarily implies variation. Movement variability has recently received growing attention in motor control studies

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Anne-Marie Heugas and Isabelle A. Siegler

, the mechanisms by which individuals switch gait are not yet fully understood. Furthermore, most studies have mainly focused either on metabolic, movement variability, or perceived exertion responses related with gait transition. Indeed, only a few have observed the relationship between these various

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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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Jia Yi Chow, Keith Davids, Chris Button and Robert Rein

From a nonlinear dynamics perspective, presence of movement variability before a change in preferred movement patterns is hypothesized to afford the necessary adaptability and flexibility for seeking novel functional behaviors. In this study, four novice participants practiced a discrete multiarticular movement for 12 sessions over 4 weeks. Cluster analysis procedures revealed how changes between preferred movement patterns were affected with and without the presence of variability in movement clusters before a defined change. Performance improved in all participants as a function of practice. Participants typically showed evidence of change between preferred movement clusters and higher variability in the use of movement clusters within a session. However, increasing variability in movement clusters was not always accompanied by transition from one preferred movement cluster to another. In summary, it was observed that intentional and informational constraints play an important role in influencing the specific pathway of change for individual learners as they search for new preferred movement patterns.

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William M. Land, Gershon Tenenbaum, Paul Ward and Christian Marquardt

Attunement to visual information has been suggested to mediate the performance advantage associated with adopting an external focus of attention (e.g., Al-Abood, Bennett, Moreno Hernandez, Ashford, & Davids, 2002; Magill, 1998). We tested this hypothesis by examining the extent to which online visual information underpins the external focus advantage. The study examined skilled golfers on a putting task under one of three attentional focus conditions: control (no instructions), irrelevant (tone counting), and external (movement effect focus), with either full or occluded vision. In addition to task performance, the effect of attentional focus and vision on between-trial movement variability was examined. We found a significant advantage for an external focus of attention in the absence of vision. The results of the movement variability analysis further indicated that external focus was not mediated by the online use of vision. We discuss these findings in the context of traditional cognitive perspectives to external focus effects.