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Mark L. Latash

The article offers a way to unite three recent developments in the field of motor control and coordination: (1) The notion of synergies is introduced based on the principle of motor abundance; (2) The uncontrolled manifold hypothesis is described as offering a computational framework to identify and quantify synergies; and (3) The equilibrium-point hypothesis is described for a single muscle, single joint, and multijoint systems. Merging these concepts into a single coherent scheme requires focusing on control variables rather than performance variables. The principle of minimal final action is formulated as the guiding principle within the referent configuration hypothesis. Motor actions are associated with setting two types of variables by a controller, those that ultimately define average performance patterns and those that define associated synergies. Predictions of the suggested scheme are reviewed, such as the phenomenon of anticipatory synergy adjustments, quick actions without changes in synergies, atypical synergies, and changes in synergies with practice. A few models are briefly reviewed.

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David P. Black, Michael A. Riley and Christopher K. McCord

The authors conducted two experiments that served as a test bed for applying the recently developed uncontrolled manifold (UCM) approach to rhythmic motor coordination, which has been extensively investigated from a coordination dynamics perspective. The results of two experiments, one investigating withinperson and one investigating between-persons rhythmic movement coordination, identified synergistic behaviors in both of those types of coordination. Stronger synergies were identified for in-phase than antiphase coordination, at the endpoints of the movement cycles compared with the midpoints, for movement frequencies closer to the intrinsic frequency of the coordinated limbs, and for within-person coordination. Frequency detuning did not weaken the strength of interlimb rhythmic coordination synergies. The results suggest the synergistic behavior captured by the UCM analysis may be identifiable with the strength of coupling between the coordinated limbs. The UCM analysis appears to distinguish coordination parameters that affect coupling strength from parameters that weaken coordination attractors.

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Mark L. Latash, John P. Scholz and Gregor Schöner

Driven by recent empirical studies, we offer a new understanding of the degrees of freedom problem, and propose a refined concept of synergy as a neural organization that ensures a one-to-many mapping of variables providing for both stability of important performance variables and flexibility of motor patterns to deal with possible perturbations and/or secondary tasks. Empirical evidence is reviewed, including a discussion of the operationalization of stability/flexibility through the method of the uncontrolled manifold. We show how this concept establishes links between the various accounts for how movement is organized in redundant effector systems.

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Alexander W. Hooke, Sohit Karol, Jaebum Park, Yoon Hyuk Kim and Jae Kun Shim

The purpose of this study was to investigate central nervous system (CNS) strategies for controlling multifinger forces during a circle-drawing task. Subjects drew 30 concentric, discontinuous clockwise and counter clockwise circles, at self and experimenter-set paces. The three-dimensional trajectory of the pen’s center of mass and the three-dimensional forces and moments of force at each contact between the hand and the pen were recorded. Uncontrolled Manifold Analysis was used to quantify the synergies between pen-hand contact forces in radial, tangential and vertical directions. Results showed that synergies in the radial and tangential components were significantly stronger than in the vertical component. Synergies in the clockwise direction were significantly stronger than the counterclockwise direction in the radial and vertical components. Pace was found to be insignificant under any condition.

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Stacey L. Gorniak, Marcos Duarte and Mark L. Latash

We explored possible effects of negative covariation among finger forces in multifinger accurate force production tasks on the classical Fitts’s speed-accuracy trade-off. Healthy subjects performed cyclic force changes between pairs of targets “as quickly and accurately as possible.” Tasks with two force amplitudes and six ratios of force amplitude to target size were performed by each of the four fingers of the right hand and four finger combinations. There was a close to linear relation between movement time and the log-transformed ratio of target amplitude to target size across all finger combinations. There was a close to linear relation between standard deviation of force amplitude and movement time. There were no differences between the performance of either of the two “radial” fingers (index and middle) and the multifinger tasks. The “ulnar” fingers (little and ring) showed higher indices of variability and longer movement times as compared with both “radial” fingers and multifinger combinations. We conclude that potential effects of the negative covariation and also of the task-sharing across a set of fingers are counterbalanced by an increase in individual finger force variability in multifinger tasks as compared with single-finger tasks. The results speak in favor of a feed-forward model of multifinger synergies. They corroborate a hypothesis that multifinger synergies are created not to improve overall accuracy, but to allow the system larger flexibility, for example to deal with unexpected perturbations and concomitant tasks.

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Afshin Samani and Mathias Kristiansen

A paramount question in the field of human motor control is how the central nervous system handles “motor redundancy” ( Bernstein, 1967 ). Modular control of muscles is a possible explanation for the problem, and modules are often referred to as muscle synergies ( Chvatal & Ting, 2012 ). According

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Mark L. Latash

basket, which defines success at the task. The UCM-based approach would try to link salient performance variables to those that describe coordination of body-level variables such as joint rotations. Synergies: Definition, Role, and Indices Since the seminal works by Hughlings Jackson ( 1889 ) and

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Fariba Hasanbarani and Mark L. Latash

variable (orthogonal to the UCM, V ORT ). The inequality V UCM  >  V ORT has been viewed as a sign of a synergy at the level of EVs stabilizing the salient PV. Most studies used the framework of the UCM hypothesis to explore relatively artificial laboratory tasks or well-practiced tasks, such as walking

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Mark L. Latash

(elemental variables) at a selected level of analysis into groups that are controlled by one higher level variable per group. Various terms have been used to address groups of elemental variables, including modes, modules, factors, and synergies; sometimes, these terms are used as synonyms of primitives. In

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Alex Knowles, Tristan L. Wallhead and Tucker Readdy

examined this synergy. Although larger scale intervention studies have shown that physical education interventions can increase children’s leisure-time PA (e.g. SPARK; Sallis, McKenzie, & Conway, 2003 ), our understanding of the mechanisms by which this synergy and transfer occurs remains limited