Motor equivalence and structure of variance: multi-muscle postural synergies in Parkinson’s disease

Falaki, Ali; Huang, Xuemei; Lewis, Mechelle M.; Latash, Mark L.

doi:10.1007/s00221-017-4971-y

Cited by 34 publications

(32 citation statements)

References 59 publications

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“…From classical statistics with folded distributions (Leone et al 1961), inter-trial standard deviations within each of these spaces are expected to be proportional to ME and nME, respectively. Indeed, the data presented here have confirmed the expected correlations between VUCM and ME, corroborating two recent studies utilizing cyclical whole-body tasks analyzed at comparable action phases (Falaki et al 2017; Furmanek et al 2018). However, in this study, there was no significant correlation between VORT and nME.…”

Section: Discussionsupporting

confidence: 91%

“…The results suggest that the two sets of indices of synergic action, V UCM /V ORT and ME/nME, are not redundant, potentially reflecting different neural control processes. The earlier results (Falaki et al 2017; Furmanek et al 2018) may be related to the cyclical nature of the used tasks, which may be viewed as a particular case of steady-state action. In discrete actions (see also Mattos et al 2011), the two groups of indices may behave differently and supply complementary information.…”

Section: Discussionmentioning

confidence: 93%

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Multi-finger synergies and the muscular apparatus of the hand

et al. 2018

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We explored whether the synergic control of the hand during multi-finger force production tasks depends on the hand muscles involved. Healthy subjects performed accurate force production tasks and targeted force pulses while pressing against loops positioned at the level of fingertips, middle phalanges, and proximal phalanges. This varied the involvement of the extrinsic and intrinsic finger flexors. The framework of the uncontrolled manifold (UCM) hypothesis was used to analyze the structure of inter-trial variance, motor equivalence, and anticipatory synergy adjustments prior to the force pulse in the spaces of finger forces and finger modes (hypothetical finger-specific control signals). Subjects showed larger maximal force magnitudes at the proximal site of force production. There were synergies stabilizing total force during steady-state phases across all three sites of force production; no differences were seen across the sites in indices of structure of variance, motor equivalence, or anticipatory synergy adjustments. Indices of variance, which did not affect the task (within the UCM), correlated with motor equivalent motion between the steady states prior to and after the force pulse; in contrast, variance affecting task performance did not correlate with non-motor equivalent motion. The observations are discussed within the framework of hierarchical control with referent coordinates for salient effectors at each level. The findings suggest that multi-finger synergies are defined at the level of abundant transformation between the low-dimensional hand level and higher-dimensional finger level while being relatively immune to transformations between the finger level and muscle level. The results also support the scheme of control with two classes of neural variables that define referent coordinates and gains in back-coupling loops between hierarchical control levels.

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Section: Discussionsupporting

confidence: 91%

Section: Discussionmentioning

confidence: 93%

Multi-finger synergies and the muscular apparatus of the hand

et al. 2018

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“…In particular, a drop in the index of stability of salient performance variables, such as total force produced by a set of digits, has been documented for multidigit tasks in patients with Parkinson's disease (Jo et al 2015;Park et al 2012), multisystem brain atrophy (Park et al 2013), and multiple sclerosis (Jo et al 2017). A drop in stability of the center of pressure coordinate, analyzed within the space of activations of major muscle groups, has been reported in patients with Parkinson's disease (Falaki et al 2016(Falaki et al , 2017.…”

Section: Coactivation Within the Equilibrium-point Hypothesismentioning

confidence: 99%

Muscle coactivation: definitions, mechanisms, and functions

Latash

2018

Journal of Neurophysiology

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The phenomenon of agonist-antagonist muscle coactivation is discussed with respect to its consequences for movement mechanics (such as increasing joint apparent stiffness, facilitating faster movements, and effects on action stability), implication for movement optimization, and involvement of different neurophysiological structures. Effects of coactivation on movement stability are ambiguous and depend on the effector representing a kinematic chain with a fixed origin or free origin. Furthermore, coactivation is discussed within the framework of the equilibrium-point hypothesis and the idea of hierarchical control with spatial referent coordinates. Relations of muscle coactivation to changes in one of the basic commands, the c-command, are discussed and illustrated. A hypothesis is suggested that agonist-antagonist coactivation reflects a deliberate neural control strategy to preserve effector-level control and avoid making it degenerate and facing the necessity to control at the level of signals to individual muscles. This strategy, in particular, allows stabilizing motor actions by covaried adjustments in spaces of control variables. This hypothesis is able to account for higher levels of coactivation in young healthy persons performing challenging tasks and across various populations with movement impairments.

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“…A number of studies explored multi-muscle posture-stabilizing synergies (defined as neural mechanisms stabilizing salient performance variables, Latash 2008) within the framework of the UCM hypothesis (Krishnamoorthy et al 2003b; Wang et al 2006; Klous et al 2010; Krishnan et al 2010; Falaki et al 2017; Piscitelli et al 2017). Those studies addressed steady-state whole-body actions, such as voluntary sway, and preparation to an action or to a predictable perturbation (early postural adjustments and anticipatory postural adjustments).…”

Section: Introductionmentioning

confidence: 99%

Stability of vertical posture explored with unexpected mechanical perturbations: synergy indices and motor equivalence

2018

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We explored the relations between indices of mechanical stability of vertical posture and synergy indices under unexpected perturbations. The main hypotheses predicted higher posture-stabilizing synergy indices and higher mechanical indices of center of pressure stability during perturbations perceived by subjects as less challenging. Healthy subjects stood on a force platform and held in fully extended arms a bar attached to two loads acting downward and upward. One of the loads was unexpectedly released by the experimenter causing a postural perturbations. In different series, subjects either knew or did not know which of the two loads would be released. Forward perturbations were perceived as more challenging and accompanied by co-activation patterns among the main agonist-antagonist pairs. Backward perturbation led to reciprocal muscle activation patterns and was accompanied by indices of mechanical stability and of posture-stabilizing synergy which indicated higher stability. Changes in synergy indices were observed as early as 50-100 ms following the perturbation reflecting involuntary mechanisms. In contrast, predictability of perturbation direction had weak or no effect on mechanical and synergy indices of stability. These observations are interpreted within a hierarchical scheme of synergic control of motor tasks and a hypothesis on the control of movements with shifts of referent coordinates. The findings show direct correspondence between stability indices based on mechanics and on the analysis of multi-muscle synergies. They suggest that involuntary posture-stabilizing mechanisms show synergic organization. They also show that predictability of perturbation direction has strong effects on anticipatory postural adjustment but not corrective adjustments. We offer an interpretation of co-activation patterns that questions their contribution to postural stability.

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Motor equivalence and structure of variance: multi-muscle postural synergies in Parkinson’s disease

Cited by 34 publications

References 59 publications

Multi-finger synergies and the muscular apparatus of the hand

Multi-finger synergies and the muscular apparatus of the hand

Muscle coactivation: definitions, mechanisms, and functions

Stability of vertical posture explored with unexpected mechanical perturbations: synergy indices and motor equivalence

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