scholarly journals What Do Synergies Do? Effects of Secondary Constraints on Multidigit Synergies in Accurate Force-Production Tasks

2008 ◽  
Vol 99 (2) ◽  
pp. 500-513 ◽  
Author(s):  
Wei Zhang ◽  
John P. Scholz ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Stable Condition ◽  
Force Production ◽  
Principal Component ◽  
Additional Constraint ◽  
Uncontrolled Manifold ◽  
Total Force ◽  
Principle Of Superposition ◽  
Mode Space ◽  
Young Subjects ◽  
Similar Solutions

We used the framework of the uncontrolled manifold (UCM) hypothesis to explore changes in the structure of variability in multifinger force-production tasks when a secondary task was introduced. Healthy young subjects produced several levels of the total force by pressing with the four fingers of the hand on force sensors. The frame with the sensors rested on the table ( Stable condition) or on a narrow supporting beam ( Unstable conditions) that could be placed between different finger pairs. Most variance in the finger mode space was compatible with a fixed value of the total force across all conditions, whereas the patterns of sharing of the total force among the fingers were condition dependent. Moment of force was stabilized only in the Unstable conditions. The finger mode data were projected onto the UCM computed for the total force and subjected to principal component (PC) analysis. Two PCs accounted for >90% of the variance. The directions of the PC vectors varied across subjects in the Stable condition, whereas two “default” PCs were observed under the Unstable conditions. These observations show that different persons coordinate their fingers differently in force-production tasks. They converge on similar solutions when an additional constraint is introduced. The use of variable solutions allows avoiding a loss in accuracy of performance when the same elements get involved in another task. Our results suggest a mechanism underlying the principle of superposition suggested in a variety of human and robotic studies.

scholarly journals Age-related changes in the control of finger force vectors

2010 ◽  
Vol 109 (6) ◽  
pp. 1827-1841 ◽  
Author(s):  
Shweta Kapur ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Neural Control ◽  
Healthy Aging ◽  
Force Production ◽  
Uncontrolled Manifold ◽  
Total Force ◽  
Finger Force ◽  
Healthy Elderly ◽  
Age Related ◽  
Young Subjects ◽  
Force Vectors

We explored changes in finger interaction in the process of healthy aging as a window into neural control strategies of natural movements. In particular, we quantified the amount of force produced by noninstructed fingers in different directions, the amount of force produced by the instructed finger orthogonally to the task direction, and the strength of multifinger synergies stabilizing the total force magnitude and direction during accurate force production. Healthy elderly participants performed accurate isometric force production tasks in five directions by individual fingers and by all four fingers acting together. Their data were compared with a dataset obtained in a similar earlier study of young subjects. Finger force vectors were measured using six-component force/torque sensors. Multifinger synergies were quantified using the framework of the uncontrolled manifold hypothesis. The elderly participants produced lower force magnitudes by noninstructed fingers and higher force magnitudes by instructed fingers in nontask directions. They showed strong synergies stabilizing the magnitude and direction of the total force vector. However, the synergy indexes were significantly lower than those observed in the earlier study of young subjects. The results are consistent with an earlier hypothesis of preferential weakening of intrinsic hand muscles with age. We interpret the findings as a shift in motor control from synergic to element-based, which may be causally linked to the documented progressive neuronal death at different levels of the neural axis.

scholarly journals Equifinality and its violations in a redundant system: multifinger accurate force production

2013 ◽  
Vol 110 (8) ◽  
pp. 1965-1973 ◽  
Author(s):  
Luke Wilhelm ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Force Production ◽  
Uncontrolled Manifold ◽  
Total Force ◽  
Redundant System ◽  
Uncontrolled Manifold Hypothesis ◽  
Final State ◽  
Performance Variables ◽  
Force Mode ◽  
Finger Forces ◽  
The Individual

We explored a hypothesis that transient perturbations applied to a redundant system result in equifinality in the space of task-related performance variables but not in the space of elemental variables. The subjects pressed with four fingers and produced an accurate constant total force level. The “inverse piano” device was used to lift and lower one of the fingers smoothly. The subjects were instructed “not to intervene voluntarily” with possible force changes. Analysis was performed in spaces of finger forces and finger modes (hypothetical neural commands to fingers) as elemental variables. Lifting a finger led to an increase in its force and a decrease in the forces of the other three fingers; the total force increased. Lowering the finger back led to a drop in the force of the perturbed finger. At the final state, the sum of the variances of finger forces/modes computed across repetitive trials was significantly higher than the variance of the total force/mode. Most variance of the individual finger force/mode changes between the preperturbation and postperturbation states was compatible with constant total force. We conclude that a transient perturbation applied to a redundant system leads to relatively small variance in the task-related performance variable (equifinality), whereas in the space of elemental variables much more variance occurs that does not lead to total force changes. We interpret the results within a general theoretical scheme that incorporates the ideas of hierarchically organized control, control with referent configurations, synergic control, and the uncontrolled manifold hypothesis.

scholarly journals Age-related changes in multifinger synergies in accurate moment of force production tasks

2007 ◽  
Vol 102 (4) ◽  
pp. 1490-1501 ◽  
Author(s):  
Halla Olafsdottir ◽  
Wei Zhang ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Hand Function ◽  
The Elderly ◽  
Time Profile ◽  
Uncontrolled Manifold ◽  
Elderly Subjects ◽  
Total Force ◽  
Dominant Hand ◽  
Total Moment ◽  
Age Related ◽  
Young Subjects

The purpose of this investigation was to document and quantify age-related differences in the coordination of fingers during a task that required production of an accurate time profile of the total moment of force by the four fingers of a hand. We hypothesized that elderly subjects would show a decreased ability to stabilize a time profile of the total moment of force, leading to larger indexes of moment variability compared with young subjects. The subjects followed a trapezoidal template on a computer screen by producing a time profile of the total moment of force while pressing down on force sensors with the four fingers of the right (dominant) hand. To quantify synergies, we used the framework of the uncontrolled manifold hypothesis. The elderly subjects produced larger total force, larger variance of both total force and total moment of force, and larger involvement of fingers that produced moment of force against the required moment direction (antagonist moment). This was particularly prominent during supination efforts. Young subjects showed covariation of commands to fingers across trials that stabilized the moment of total force (moment-stabilizing synergy), while elderly subjects failed to do so. Both subject groups showed similar indexes of covariation of commands to the fingers that stabilized the time profile of the total force. The lack of moment-stabilizing synergies may be causally related to the documented impairment of hand function with age.

scholarly journals Stability of multifinger action in different state spaces

2014 ◽  
Vol 112 (12) ◽  
pp. 3209-3218 ◽  
Author(s):  
Sasha Reschechtko ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Force Production ◽  
Total Force ◽  
Force Output ◽  
Large Magnitude ◽  
State Spaces ◽  
Theoretical Scheme ◽  
Lower Stability ◽  
Mode Space ◽  
Finger Forces

We investigated stability of action by a multifinger system with three methods: analysis of intertrial variance, application of transient perturbations, and analysis of the system's motion in different state spaces. The “inverse piano” device was used to apply transient (lifting-and-lowering) perturbations to individual fingers during single- and two-finger accurate force production tasks. In each trial, the perturbation was applied either to a finger explicitly involved in the task or one that was not. We hypothesized that, in one-finger tasks, task-specific stability would be observed in the redundant space of finger forces but not in the nonredundant space of finger modes (commands to explicitly involved fingers). In two-finger tasks, we expected that perturbations applied to a nontask finger would not contribute to task-specific stability in mode space. In contrast to our expectations, analyses in both force and mode spaces showed lower stability in directions that did not change total force output compared with directions that did cause changes in total force. In addition, the transient perturbations led to a significant increase in the enslaving index. We consider these results within a theoretical scheme of control with referent body configurations organized hierarchically, using multiple few-to-many mappings organized in a synergic way. The observed volatility of enslaving, greater equifinality of total force compared with elemental variables, and large magnitude of motor equivalent motion in both force and mode spaces provide support for the concept of task-specific stability of performance and the existence of multiple neural loops, which ensure this stability.

Effects of age and gender on finger coordination in MVC and submaximal force-matching tasks

2003 ◽  
Vol 94 (1) ◽  
pp. 259-270 ◽  
Author(s):  
Minoru Shinohara ◽  
Sheng Li ◽  
Ning Kang ◽  
Vladimir M. Zatsiorsky ◽  
Mark L. Latash
Keyword(s):  
Age Groups ◽  
Force Production ◽  
Voluntary Contraction ◽  
Total Force ◽  
Age And Gender ◽  
Force Matching ◽  
Single Finger ◽  
The Difference ◽  
Young Subjects ◽  
And Gender

The objective of the study is to examine the effects of age and gender on finger coordination. Twelve young (24 ± 8 yr; 6 men and 6 women) and 12 elderly (75 ± 5 yr; 6 men and 6 women) subjects performed single-finger maximal contraction [maximal voluntary contraction (MVC)], four-finger MVC, and four-finger ramp force production tasks by pressing on individual force transducers. A drop in the force of individual fingers during four-finger MVC tasks compared with single-finger MVC tasks (force deficit) was larger, whereas unintended force production by other fingers during single-finger MVC tasks (enslaving) was smaller, in elderly than in young subjects and in women than in men. Force deficit was smaller and enslaving was larger in subjects with higher peak force. During the ramp task, the difference between the variance of total force and the sum of variances of individual forces showed a logarithmic relation to the level of total force, across all subject groups. These findings suggest that indexes of finger coordination scale with force-generating capabilities across gender and age groups.

scholarly journals Stability of hand force production. I. Hand level control variables and multifinger synergies

2017 ◽  
Vol 118 (6) ◽  
pp. 3152-3164 ◽  
Author(s):  
Sasha Reschechtko ◽  
Mark L. Latash
Keyword(s):  
Visual Feedback ◽  
Neural Control ◽  
Control Variable ◽  
Force Production ◽  
Uncontrolled Manifold ◽  
Total Force ◽  
Level Control ◽  
Control Variables ◽  
Finger Forces ◽  
Hand Action

We combined the theory of neural control of movement with referent coordinates and the uncontrolled manifold hypothesis to explore synergies stabilizing the hand action in accurate four-finger pressing tasks. In particular, we tested a hypothesis on two classes of synergies, those among the four fingers and those within a pair of control variables, stabilizing hand action under visual feedback and disappearing without visual feedback. Subjects performed four-finger total force and moment production tasks under visual feedback; the feedback was later partially or completely removed. The “inverse piano” device was used to lift and lower the fingers smoothly at the beginning and at the end of each trial. These data were used to compute pairs of hypothetical control variables. Intertrial analysis of variance within the finger force space was used to quantify multifinger synergies stabilizing both force and moment. A data permutation method was used to quantify synergies among control variables. Under visual feedback, synergies in the spaces of finger forces and hypothetical control variables were found to stabilize total force. Without visual feedback, the subjects showed a force drift to lower magnitudes and a moment drift toward pronation. This was accompanied by disappearance of the four-finger synergies and strong attenuation of the control variable synergies. The indexes of the two types of synergies correlated with each other. The findings are interpreted within the scheme with multiple levels of abundant variables. NEW & NOTEWORTHY We extended the idea of hierarchical control with referent spatial coordinates for the effectors and explored two types of synergies stabilizing multifinger force production tasks. We observed synergies among finger forces and synergies between hypothetical control variables that stabilized performance under visual feedback but failed to stabilize it after visual feedback had been removed. Indexes of two types of synergies correlated with each other. The data suggest the existence of multiple mechanisms stabilizing motor actions.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

2010 ◽  
Vol 103 (1) ◽  
pp. 278-289 ◽  
Author(s):  
W. S. Yu ◽  
H. van Duinen ◽  
S. C. Gandevia
Keyword(s):  
Control System ◽  
Healthy Subjects ◽  
Neural Control ◽  
Force Production ◽  
Total Force ◽  
Neural Drive ◽  
Antagonist Muscles ◽  
Hand Performance ◽  
The Difference ◽  
Flexion And Extension

In humans, hand performance has evolved from a crude multidigit grasp to skilled individuated finger movements. However, control of the fingers is not completely independent. Although musculotendinous factors can limit independent movements, constraints in supraspinal control are more important. Most previous studies examined either flexion or extension of the digits. We studied differences in voluntary force production by the five digits, in both flexion and extension tasks. Eleven healthy subjects were instructed either to maximally flex or extend their digits, in all single- and multidigit combinations. They received visual feedback of total force produced by “instructed” digits and had to ignore “noninstructed” digits. Despite attempts to maximally flex or extend instructed digits, subjects rarely generated their “maximal” force, resulting in a “force deficit,” and produced forces with noninstructed digits (“enslavement”). Subjects performed differently in flexion and extension tasks. Enslavement was greater in extension than in flexion tasks ( P = 0.019), whereas the force deficit in multidigit tasks was smaller in extension ( P = 0.035). The difference between flexion and extension in the relationships between the enslavement and force deficit suggests a difference in balance of spillover of neural drive to agonists acting on neighboring digits and focal neural drive to antagonist muscles. An increase in drive to antagonists would lead to more individualized movements. The pattern of force production matches the daily use of the digits. These results reveal a neural control system that preferentially lifts fingers together by extension but allows an individual digit to flex so that the finger pads can explore and grasp.

scholarly journals Case Studies in Neuroscience: The central and somatosensory contributions to finger interdependence and coordination: lessons from a study of a “deafferented person”

2019 ◽  
Vol 121 (6) ◽  
pp. 2083-2087 ◽  
Author(s):  
Cristian Cuadra ◽  
Ali Falaki ◽  
Robert Sainburg ◽  
Fabrice R. Sarlegna ◽  
Mark L. Latash
Keyword(s):  
Peripheral Neuropathy ◽  
Equilibrium Point ◽  
Force Production ◽  
Total Force ◽  
Equilibrium Point Hypothesis ◽  
Somatosensory Feedback ◽  
Central Origin ◽  
Neural Origin ◽  
Zero Values ◽  
Large Fiber

We tested finger force interdependence and multifinger force-stabilizing synergies in a patient with large-fiber peripheral neuropathy (“deafferented person”). The subject performed a range of tasks involving accurate force production with one finger and with four fingers. In one-finger tasks, nontask fingers showed unintentional force production (enslaving) with an atypical pattern: very large indices for the lateral (index and little) fingers and relatively small indices for the central (middle and ring) fingers. Indices of multifinger synergies stabilizing total force and of anticipatory synergy adjustments in preparation to quick force pulses were similar to those in age-matched control females. During constant force production, removing visual feedback led to a slow force drift to lower values (by ~25% over 15 s). The results support the idea of a neural origin of enslaving and suggest that the patterns observed in the deafferented person were reorganized based on everyday manipulation tasks. The lack of significant changes in the synergy index shows that synergic control can be organized in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis and suggest that force drift results from an unintentional drift of the control variables to muscles toward zero values. NEW & NOTEWORTHY We demonstrate atypical patterns of finger enslaving and unchanged force-stabilizing synergies in a person with large-fiber peripheral neuropathy. The results speak strongly in favor of central origin of enslaving and its reorganization based on everyday manipulation tasks. The data show that synergic control can be implemented in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis.

scholarly journals Stability of hand force production. II. Ascending and descending synergies

2018 ◽  
Vol 120 (3) ◽  
pp. 1045-1060 ◽  
Author(s):  
Sasha Reschechtko ◽  
Mark L. Latash
Keyword(s):  
Visual Feedback ◽  
Neural Control ◽  
Uncontrolled Manifold ◽  
Total Force ◽  
Level Control ◽  
Control Variables ◽  
Specific Level ◽  
Referent Coordinate ◽  
Apparent Stiffness ◽  
Different Levels

We combined the theory of neural control of movement with referent coordinates and the uncontrolled manifold hypothesis to investigate multifinger coordination. We tested hypotheses related to stabilization of performance by covarying control variables, translated into apparent stiffness and referent coordinate, at different levels of an assumed hierarchy of control. Subjects produced an accurate combination of total force and total moment of force with the four fingers under visual feedback on both variables and after feedback was partly or completely removed. The “inverse piano” device was used to estimate control variables. We observed strong synergies in the space of hypothetical control variables that stabilized total force and moment of force, as well as weaker synergies stabilizing individual finger forces; whereas the former were attenuated by alteration of visual feedback, the latter were much less affected. In addition, we investigated the organization of “ascending synergies” stabilizing task-level control variables by covaried adjustments of finger-level control variables. We observed intertrial covariation of individual fingers’ referent coordinates that stabilized hand-level referent coordinate, but we observed no such covariation for apparent stiffness. The observations suggest the existence of both descending and ascending synergies in a hierarchical control system. They confirm a trade-off between synergies at different levels of control and corroborate the hypothesis on specialization of different fingers for the control of force and moment. The results provide strong evidence for the importance of central back-coupling loops in ensuring stability of action.NEW & NOTEWORTHY We expand analysis of action in the space of hypothetical control variables to hierarchically organized multieffector systems. We also introduce the novel concept of ascending synergies, which reflect covariation of control variables to individual effectors (fingers) that stabilize task-specific control variables at a hierarchically higher, task-specific level (hand).

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