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The goal of the current study was to measure the processing demands on the stages of information processing with internal and external foci of attention. Participants completed simple and two-choice reaction time tasks with internal and external foci of attention. Donders' subtraction method was used to isolate the cumulative duration of stages unique to simple and choice reaction time tasks. Mean reaction time was comparable with internal and external foci of attention in simple and two-choice reaction time tasks. These results suggest that processing demands were comparable with internal and external foci of attention. We hypothesize that there was not a processing advantage for an external focus in simple reaction time because the required movements had low movement complexity. [ABSTRACT FROM AUTHOR]

The ability to coordinate finger forces to dexterously perform tasks develops in children as they grow older. Following brain injury, either developmental (as in cerebral palsy-CP) or acquired (as in traumatic brain injury--TBI), this developmental trajectory will likely be impaired. In this study, we compared finger coordination in a group of children aged 4-12 with CP and TBI to a group of typically developing children using an isometric pressing task. As expected, deficits were observed in functional tests (Jebsen Taylor test of hand function, Box and Block test) for both groups, and children in both groups performed the pressing task less well than the control group. However, differing results were observed between the CP and TBI groups when using the uncontrolled manifold hypothesis to look at the synergy index. This index measures the relative amount of "good" (does not affect the outcome measure) and "bad" (does affect the outcome measure) variability, where in this case the outcome measure is the total force produced by the fingers. While children with CP were more variable in their performance, their synergy index was not significantly different from typically developing children, suggesting the development of compensatory strategies. In contrast, the children following TBI showed performance that got worse as a function of age (i.e., the older children with TBI performed worse than the younger children with TBI). These differences between the groups may be a result of different areas of brain injury typically observed in CP and TBI, and the different amount of time that has passed since the injury. [ABSTRACT FROM AUTHOR]

The ability to coordinate finger forces to dexterously perform tasks develops in children as they grow older. Following brain injury, either developmental (as in cerebral palsy–CP) or acquired (as in traumatic brain injury—TBI), this developmental trajectory will likely be impaired. In this study, we compared finger coordination in a group of children aged 4–12 with CP and TBI to a group of typically developing children using an isometric pressing task. As expected, deficits were observed in functional tests (Jebsen Taylor test of hand function, Box and Block test) for both groups, and children in both groups performed the pressing task less well than the control group. However, differing results were observed between the CP and TBI groups when using the uncontrolled manifold hypothesis to look at the synergy index. This index measures the relative amount of “good” (does not affect the outcome measure) and “bad” (does affect the outcome measure) variability, where in this case the outcome measure is the total force produced by the fingers. While children with CP were more variable in their performance, their synergy index was not significantly different from typically developing children, suggesting the development of compensatory strategies. In contrast, the children following TBI showed performance that got worse as a function of age (i.e., the older children with TBI performed worse than the younger children with TBI). These differences between the groups may be a result of different areas of brain injury typically observed in CP and TBI, and the different amount of time that has passed since the injury.

Behavioral stability partially depends on the variability of net outcomes by means of the co-varied adjustment of individual elements such as multi-finger forces. The properties of cyclic actions affect stability and variability of the performance as well as the activation of the prefrontal cortex that is an origin of subcortical structure for the coordinative actions. Little research has been done on the issue of the relationship between stability and neuronal response. The purpose of the study was to investigate the changes in the neural response, particularly at the prefrontal cortex, to the frequencies of isometric cyclic finger force production. The main experimental task was to produce finger forces while matching the produced force to sine-wave templates as accurately as possible. Also, the hemodynamics responses of the prefrontal cortex, including oxy-hemoglobin concentration (ΔHbO) and the functional connectivity, were measured using functional near-infrared spectroscopy. The frequency conditions comprised 0.1, 1, and 2 Hz. The uncontrolled manifold (UCM) approach was applied to compute synergy indices in time-series. The relative phase (RP), the coefficient of variation (CV) of the peak and trough force values were computed as the indices of performance accuracy. The statistical parametric mapping (SPM) was implemented to compare the synergy indices of three frequency conditions in time-series. A less accurate performance in the high-frequency condition was caused not by the RP, but mainly by the inconsistent peak force values (CV; p < 0.01, η p 2 = 0.90). The SPM analysis revealed that the synergy indices were larger in the low-frequency than in high-frequency conditions. Further, the ΔHbO remained unchanged under all frequency conditions, while the functional connectivity decreased with an increase in the frequency of cyclic force production. The current results suggested that the concurrent activation of the prefrontal region mainly depends on the frequency of cyclic force production, which was associated with the strength of stability indices and performance errors. The current study is the first work to uncover the effect of frequency on the multi-finger synergies as to the hemodynamic response in the prefrontal cortex, which possibly provides a clue of the neural mechanism of synergy formation and its changes. [ABSTRACT FROM AUTHOR]

Behavioral stability partially depends on the variability of net outcomes by means of the co-varied adjustment of individual elements such as multi-finger forces. The properties of cyclic actions affect stability and variability of the performance as well as the activation of the prefrontal cortex that is an origin of subcortical structure for the coordinative actions. Little research has been done on the issue of the relationship between stability and neuronal response. The purpose of the study was to investigate the changes in the neural response, particularly at the prefrontal cortex, to the frequencies of isometric cyclic finger force production. The main experimental task was to produce finger forces while matching the produced force to sine-wave templates as accurately as possible. Also, the hemodynamics responses of the prefrontal cortex, including oxy-hemoglobin concentration (ΔHbO) and the functional connectivity, were measured using functional near-infrared spectroscopy. The frequency conditions comprised 0.1, 1, and 2 Hz. The uncontrolled manifold (UCM) approach was applied to compute synergy indices in time-series. The relative phase (RP), the coefficient of variation (CV) of the peak and trough force values were computed as the indices of performance accuracy. The statistical parametric mapping (SPM) was implemented to compare the synergy indices of three frequency conditions in time-series. A less accurate performance in the high-frequency condition was caused not by the RP, but mainly by the inconsistent peak force values (CV; p < 0.01, ηp2 = 0.90). The SPM analysis revealed that the synergy indices were larger in the low-frequency than in high-frequency conditions. Further, the ΔHbO remained unchanged under all frequency conditions, while the functional connectivity decreased with an increase in the frequency of cyclic force production. The current results suggested that the concurrent activation of the prefrontal region mainly depends on the frequency of cyclic force production, which was associated with the strength of stability indices and performance errors. The current study is the first work to uncover the effect of frequency on the multi-finger synergies as to the hemodynamic response in the prefrontal cortex, which possibly provides a clue of the neural mechanism of synergy formation and its changes.

The target article presents a review of the neural control of the human hand. The review emphasizes the physical approach to motor control. It focuses on such concepts as equilibrium-point control, control with referent body configurations, uncontrolled manifold hypothesis, principle of abundance, hierarchical control, multidigit synergies, and anticipatory synergy adjustments. Changes in aspects of the hand neural control with age and neurological disorder are discussed. The target article is followed by six commentaries written by Alexander Aruin, Kelly Cole, Monica Perez, Robert Sainburg, Marco Sanello, and Wei Zhang. [ABSTRACT FROM AUTHOR]

Background: Dysfunction in peripheral and neural structure with spastic cerebral palsy (CP) causes impaired performance and stability of various behaviors. Recent progress of quantification methods for the stability properties, which is based on the uncontrolled manifold hypothesis, has been applied to various neurological disorders. A prior study revealed that the ability for purposeful regulation of stability properties is weakened with CP during finger and hand actions. Successive regulation of stability properties is crucial for human locomotion; therefore, it is imperative to quantify the changes in the intersegmental coordination as to the stable performance in CP individuals during gait.Research Question: We hypothesized that (1) Spastic CP group will show smaller step length and gait velocity with larger variability, and (2) Spastic CP group will show no changes in average stability indices for both the COM and head position stabilization, while the smaller difference between stable and unstable posture during the gait cycle.Methods: Whole-body kinematic data during walking were collected from CP and control subjects. Step length, velocity, and coefficient of variation (CV) were calculated as spatiotemporal parameters. We quantified the intersegmental stability index in time-series during gait for the stabilization of the whole-body COM and head position.Results: The CP subjects showed smaller step length and velocity with larger CV than the controls. However, the CP group showed a significantly less difference in the stability indices between the single- and double-limb support phases as compared to the controls for both the COM and head position stabilization.Significance: Present study is the first to document the quantification of changing intersegmental stability in the spastic CP during locomotion. The dysfunction of intentional modulation of stability properties in CP individuals may be a more common problem, which is not limited to a specific body effector. [ABSTRACT FROM AUTHOR]

We tested a prediction that equifinality at the task level may be accompanied by violations of equifinality within the redundant set of elemental variables. Seated subjects grasped a handle, and occupied an initial arm configuration against a bias force produced by a robot. The robot applied a smooth, transient change in the force (perturbation). The subjects were instructed "not to intervene voluntarily" with hand deviations. After the robot force returned to the bias value, the hand returned close to the original position and orientation. Analysis of across-trials variance in the joint configuration space confirmed that most variance of the difference between the initial and final states was compatible with unchanged values of both hand position and orientation. These results provide direct support for the theoretical scheme that combines the ideas of synergies and control with referent configurations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The notion that human activity can be characterised in terms of dynamic systems is a well-established alternative to motor schema approaches. Key to a dynamic systems approach is the idea that a system seeks to achieve stable states in the face of perturbation. While such an approach can apply to physical activity, it can be challenging to accept that dynamic systems also describe cognitive activity. In this paper, we argue that creativity, which could be construed as a 'cognitive' activity par excellence, arises from the dynamic systems involved in jewellery making. Knowing whether an action has been completed to a 'good' standard is a significant issue in considering acts in creative disciplines. When making a piece of jewellery, there a several criteria which can define 'good'. These are not only the aesthetics of the finished piece but also the impact of earlier actions on subsequent ones. This suggests that the manner in which an action is coordinated is influenced by the criteria by which the product is judged. We see these criteria as indicating states for the system, e.g. in terms of a space of 'good' outcomes and a complementary space of 'bad' outcomes. The skill of the craftworker is to navigate this space of available states in such a way as to minimise risk, effort and other costs and maximise benefit and quality of the outcome. In terms of postphenomonology, this paper explores Ihde's human-technology relations and relates these to the concepts developed here. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 re- moved. 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 [ABSTRACT FROM AUTHOR]

The aim of this study was to discover finger interaction indices during single-finger ramp tasks and multi-finger coordination during a steady state force production in two directions, flexion, and extension. Furthermore, the indices of anticipatory adjustment of elemental variables (i.e., finger forces) prior to a quick pulse force production were quantified. It is currently unknown whether the organization and anticipatory modulation of stability properties are affected by force directions and strengths of in multi-finger actions. We expected to observe a smaller finger independency and larger indices of multi-finger coordination during extension than during flexion due to both neural and peripheral differences between the finger flexion and extension actions. We also examined the indices of the anticipatory adjustment between different force direction conditions. The anticipatory adjustment could be a neural process, which may be affected by the properties of the muscles and by the direction of the motions. The maximal voluntary contraction (MVC) force was larger for flexion than for extension, which confirmed the fact that the strength of finger flexor muscles (e.g., flexor digitorum profundus) was larger than that of finger extensor (e.g., extensor digitorum). The analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify the motor synergy of elemental variables by decomposing two sources of variances across repetitive trials, which identifies the variances in the uncontrolled manifold (VUCM) and that are orthogonal to the UCM (VORT). The presence of motor synergy and its strength were quantified by the relative amount of VUCM and VORT. The strength of motor synergies at the steady state was larger in the extension condition, which suggests that the stability property (i.e., multi-finger synergies) may be a direction specific quantity. However, the results for the existence of anticipatory adjustment; however, no difference between the directional conditions suggests that feed-forward synergy adjustment (changes in the stability property) may be at least independent of the magnitude of the task-specific apparent performance variables and its direction (e.g., flexion and extension forces).

Manipulating objects with the hands requires the accurate production of resultant forces including shear forces; effective control of these shear forces also requires the production of internal forces normal to the surface of the object(s) being manipulated. In the present study, we investigated multi-finger synergies stabilizing shear and normal components of force, as well as drifts in both components of force, during isometric pressing tasks requiring a specific magnitude of shear force production. We hypothesized that shear and normal forces would evolve similarly in time and also show similar stability properties as assessed by the decomposition of inter-trial variance within the uncontrolled manifold hypothesis. Healthy subjects were required to accurately produce total shear and total normal forces with four fingers of the hand during a steady-state force task (with and without visual feedback) and a self-paced force pulse task. The two force components showed similar time profiles during both shear force pulse production and unintentional drift induced by turning the visual feedback off. Only the explicitly instructed components of force, however, were stabilized with multi-finger synergies. No force-stabilizing synergies and no anticipatory synergy adjustments were seen for the normal force in shear force production trials. These unexpected qualitative differences in the control of the two force components-which are produced by some of the same muscles and show high degree of temporal coupling-are interpreted within the theory of control with referent coordinates for salient variables. These observations suggest the existence of two classes of neural variables: one that translates into shifts of referent coordinates and defines changes in magnitude of salient variables, and the other controlling gains in back-coupling loops that define stability of the salient variables. Only the former are shared between the explicit and implicit task components. [ABSTRACT FROM AUTHOR]

Behavior of biological systems is based on basic physical laws, common across inanimate and living systems, and currently unknown physical laws that are specific for living systems. Living systems are able to unite basic laws of physics into chains and clusters leading to new stable and pervasive relations among variables (new physical laws) involving new parameters and to modify these parameters in a purposeful way. Examples of such laws are presented starting from the tonic stretch reflex. Further, the idea of control with referent coordinates is formulated and merged with the idea of hierarchical control and the principle of abundance. The notion of controlled stability of behaviors is linked to the idea of structured variability, which is a common feature across living systems and actions. The explanatory and predictive power of this approach is illustrated with respect to the control of both intentional and unintentional movements, the phenomena of equifinality and its violations, preparation to quick actions, development of motor skills, changes with aging and neurological disorders, and perception. [ABSTRACT FROM AUTHOR]

We explored the origins of unintentional changes in performance during accurate force production in isometric conditions seen after turning visual feedback off. The idea of control with referent spatial coordinates suggests that these phenomena could result from drifts of the referent coordinate for the effector. Subjects performed accurate force/moment production tasks by pressing with the fingers of a hand on force sensors. Turning the visual feedback off resulted in slow drifts of both total force and total moment to lower magnitudes of these variables; these drifts were more pronounced in the right hand of the right-handed subjects. Drifts in individual finger forces could be in different direction; in particular, fingers that produced moments of force against the required total moment showed an increase in their forces. The force/moment drift was associated with a drop in the index of synergy stabilizing performance under visual feedback. The drifts in directions that changed performance (non-motor equivalent) and in directions that did not (motor equivalent) were of about the same magnitude. The results suggest that control with referent coordinates is associated with drifts of those referent coordinates toward the corresponding actual coordinates of the hand, a reflection of the natural tendency of physical systems to move toward a minimum of potential energy. The interaction between drifts of the hand referent coordinate and referent orientation leads to counterdirectional drifts in individual finger forces. The results also demonstrate that the sensory information used to create multifinger synergies is necessary for their presence over the task duration. [ABSTRACT FROM AUTHOR]

The central nervous system (CNS) is believed to use the abundant degrees of freedom of muscles and joints to stabilize a particular task variable important for task success, such as footpath during walking. Stroke survivors often demonstrate impaired balance and high incidences of falls due to increased footpath variability during walking. In the current study, we use the uncontrolled manifold (UCM) approach to investigate the role of motor abundance in stabilizing footpath during swing phase in healthy individuals and stroke survivors. Twelve stroke survivors and their age- and gender-matched controls walked over-ground at self-selected speed, while electromyographic and kinematic data were collected. UCM analysis partitioned the variance of muscle groups (modes) across gait cycles into 'good variance' (i.e., muscle mode variance leading to a consistent or stable footpath) or 'bad variance' (i.e., muscle mode variance resulting in an inconsistent footpath). Both groups had a significantly greater 'good' than 'bad' variance, suggesting that footpath is an important task variable stabilized by the CNS during walking. The relative variance difference that reflects normalized difference between 'good' and 'bad' variance was not significantly different between groups. However, significant differences in muscle mode structure and muscle mode activation timing were observed between the two groups. Our results suggest that though the mode structure and activation timing are altered, stroke survivors may retain their ability to explore the redundancy within the neuromotor system and utilize it to stabilize the footpath. [ABSTRACT FROM AUTHOR]

We use an approach rooted in the recent theory of synergies to analyze possible co-variation between two hypothetical control variables involved in finger force production based on the equilibrium-point (EP) hypothesis. These control variables are the referent coordinate ( R ) and apparent stiffness ( C ) of the finger. We tested a hypothesis that inter-trial co-variation in the { R ; C } space during repeated, accurate force production trials stabilizes the fingertip force. This was expected to correspond to a relatively low amount of inter-trial variability affecting force and a high amount of variability keeping the force unchanged. We used the “inverse piano” apparatus to apply small and smooth positional perturbations to fingers during force production tasks. Across trials, R and C showed strong co-variation with the data points lying close to a hyperbolic curve. Hyperbolic regressions accounted for over 99% of the variance in the { R ; C } space. Another analysis was conducted by randomizing the original { R ; C } data sets and creating surrogate data sets that were then used to compute predicted force values. The surrogate sets always showed much higher force variance compared to the actual data, thus reinforcing the conclusion that finger force control was organized in the { R ; C } space, as predicted by the EP hypothesis, and involved co-variation in that space stabilizing total force. [ABSTRACT FROM AUTHOR]

Our main goal was to test a hypothesis that transient changes in performance of a steady-state task would result in motor equivalence. We also estimated effects of visual feedback on the amount of reorganization of motor elements. Healthy subjects performed two variations of a four-finger pressing task requiring accurate production of total pressing force (F TOT ) and total moment of force (M TOT ). In the Jumping-Target task, a sequence of target jumps required transient changes in either F TOT or M TOT . In the Step-Perturbation task, the index finger was lifted by 1 cm for 0.5 s leading to a change in both F TOT and M TOT . Visual feedback could have been frozen for one of these two variables in both tasks. Deviations in the space of finger modes (hypothetical commands to individual fingers) were quantified in directions of unchanged F TOT and M TOT (motor equivalent – ME) and in directions that changed F TOT and M TOT (non-motor equivalence – nME). Both the ME and nME components increased when the performance changed. After transient target jumps leading to the same combination of F TOT and M TOT , the changes in finger modes had a large residual ME component with only a very small nME component. Without visual feedback, an increase in the nME component was observed without consistent changes in the ME component. Results from the Step-Perturbation task were qualitatively similar. These findings suggest that both external perturbations and purposeful changes in performance trigger a reorganization of elements of an abundant system, leading to large ME change. These results are consistent with the principle of motor abundance corroborating the idea that a family of solutions is facilitated to stabilize values of important performance variables. [ABSTRACT FROM AUTHOR]

The authors explored task-specific stability during accurate multifinger force production tasks with different numbers of instructed fingers. Subjects performed steady-state isometric force production tasks and were instructed not to interfere voluntarily with transient lifting-and-lowering perturbations applied to the index finger. The main results were (a) intertrial variance in the space of finger modes at steady states was larger within the subspace that had no effect on the total force (the uncontrolled manifold [UCM]); (b) perturbations caused large deviations of finger modes within the UCM (motor equivalence); and (c) deviations caused by the perturbation showed larger variance within the UCM. No significant effects of the number of task fingers were noted in any of the 3 indicators. The results are discussed within the frameworks of the UCM and referent configuration hypotheses. The authors conclude, in particular, that all the tasks were effectively 4-finger tasks with different involvement of task and nontask fingers. [ABSTRACT FROM AUTHOR]

The minimum intervention principle and the uncontrolled manifold hypothesis state that our nervous system only responds to force perturbations and sensorimotor noise if they affect task success. This idea has been tested in muscle and joint coordinate frames and more recently using workspace redundancy (e.g., reaching to large targets). However, reaching studies typically involve spatial and or temporal constraints. Constrained reaches represent a small proportion of movements we perform daily and may limit the emergence of natural behavior. Using more relaxed constraints, we conducted two reaching experiments to test the hypothesis that humans respond to task-relevant forces and ignore task-irrelevant forces. We found that participants responded to both task-relevant and -irrelevant forces. Interestingly, participants experiencing a task-irrelevant force, which simply pushed them into a different area of a large target and had no bearing on task success, changed their movement trajectory prior to being perturbed. These movement trajectory changes did not counteract the task-irrelevant perturbations, as shown in previous research, but rather were made into new areas of the workspace. A possible explanation for this behavior change is that participants were engaging in active exploration. Our data have implications for current models and theories on the control of biological motion. [ABSTRACT FROM AUTHOR]

We tested predictions of a hierarchical scheme on the control of natural movements with referent body configurations. Subjects occupied an initial hand position against a bias force generated by a HapticMaster robot. A smooth force perturbation was applied to the hand consisting of an increase in the bias force, keeping it at a new level for 5 s, and decreasing it back to the bias value. When the force returned to the bias value, the arm stopped at a position different from the initial one interpreted as an involuntary movement. We then asked subjects to make voluntary movements to targets corresponding to the measured end-position of the unintentional movements. No target for hand orientation was used. The joint configuration variance was compared between intentional and unintentional movements within the framework of the uncontrolled manifold hypothesis. Our central hypothesis was that both unintentional and intentional movements would be characterized by structure of joint configuration variance reflecting task-specific stability of salient performance variables, such as hand position and orientation. The analysis confirmed that most variance at the final steady states was compatible with unchanged values of both hand position and orientation following both intentional and unintentional movements. We interpret unintentional movements as consequences of back-coupling between the actual and referent configurations at the task level. The results suggested that both intentional and unintentional movements resulted from shifts of the body referent configuration produced intentionally or as a result of the hypothesized back-coupling. Inter-trial variance signature reflects similar task-specific stability properties of the system following both types of movements, intentional and unintentional. [ABSTRACT FROM AUTHOR]

We explored stability of multi-finger cyclical accurate force production action by analysis of responses to small perturbations applied to one of the fingers and inter-cycle analysis of variance. Healthy subjects performed two versions of the cyclical task, with and without an explicit target. The 'inverse piano' apparatus was used to lift/lower a finger by 1 cm over 0.5 s; the subjects were always instructed to perform the task as accurate as they could at all times. Deviations in the spaces of finger forces and modes (hypothetical commands to individual fingers) were quantified in directions that did not change total force (motor equivalent) and in directions that changed the total force (non-motor equivalent). Motor equivalent deviations started immediately with the perturbation and increased progressively with time. After a sequence of lifting-lowering perturbations leading to the initial conditions, motor equivalent deviations were dominating. These phenomena were less pronounced for analysis performed with respect to the total moment of force with respect to an axis parallel to the forearm/hand. Analysis of inter-cycle variance showed consistently higher variance in a subspace that did not change the total force as compared to the variance that affected total force. We interpret the results as reflections of task-specific stability of the redundant multi-finger system. Large motor equivalent deviations suggest that reactions of the neuromotor system to a perturbation involve large changes in neural commands that do not affect salient performance variables, even during actions with the purpose to correct those salient variables. Consistency of the analyses of motor equivalence and variance analysis provides additional support for the idea of task-specific stability ensured at a neural level. [ABSTRACT FROM AUTHOR]

We explored the multi-digit synergies and hand performance in object manipulations and pressing tasks in patients with early-stage Parkinson's disease (PD) and healthy controls. Synergies were defined as inter-trials co-variation patterns among forces/moments produced by individual digits that stabilized a resultant mechanical variable. The subjects performed three main tasks: pressing (steady-state force production followed by a force pulse into the target), prehension (manipulation of a handheld instrumented handle imitating the action of taking a sip from a glass), and functional object manipulation (moving a glass with water as quickly and accurately as possible along a chain of targets). The PD patients were slower compared to controls in all three tasks. Patients showed smaller synergy indices in the pressing and prehension tasks. In the prehension tasks, patients showed elevated grip force at steady states with smaller grip force modulation during the handle motion. PD patients showed smaller feed-forward synergy adjustments in preparation to the quick action in the pressing and (to a smaller degree) prehension tasks. Synergy indices correlated with the time index of performance in the functional glass-with-water task, whereas none of the indices correlated with the Unified PD Rating Scale part III-motor scores. We interpret the results as pointing at an important role of subcortical structures in motor synergies and their feed-forward adjustments to action. [ABSTRACT FROM AUTHOR]

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-andlowering) 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. [ABSTRACT FROM AUTHOR]

We tested several predictions of a recent theory that combines the ideas of control with referent configurations, hierarchical control, and the uncontrolled manifold (UCM) hypothesis. In particular, we tested a hypothesis that unintentional changes in hand coordinate can happen following a long-lasting transient perturbation. The subjects grasped a handle with the right hand, occupied an initial position against a bias force produced by the HapticMaster robot, and then tried not to react to changes in the robot-produced force. Changes in the force were smooth and transient; they always ended with the same force as the bias force. The force-change amplitude and the time the force was kept at the new level (dwell time) varied across conditions. After the transient force change was over, the handle rested in a position that differed significantly from the initial position. The amplitude of this unintentional movement increased with the amplitude of transient force change and with the dwell time. In the new position, the across-trials joint configuration variance was mostly confined to a subspace compatible with the average handle coordinate and orientation (the UCMs for these variables). We view these results as the first experimental support for the hypothesis on back-coupling between the referent and actual body configurations during multi-joint actions. The results suggest that even under the instruction 'not to react to transient force changes,' the subjects may be unable to prevent unintentional drift of the referent configuration. The structure of joint configuration variance after such movements was similar to that in earlier reports on joint configuration variance after intentional movements. We conclude that the intentional and unintentional movements are products of a single neural system that can lead to intentional and unintentional shifts of the referent body configuration. [ABSTRACT FROM AUTHOR]

We used robot-generated perturbations applied during position-holding tasks to explore stability of induced unintentional movements in a multidimensional space of muscle activations. Healthy subjects held the handle of a robot against a constant bias force and were instructed not to interfere with hand movements produced by changes in the external force. Transient force changes were applied leading to handle displacement away from the initial position and then back toward the initial position. Intertrial variance in the space of muscle modes (eigenvectors in the muscle activations space) was quantified within two subspaces, corresponding to unchanged handle coordinate and to changes in the handle coordinate. Most variance was confined to the former subspace in each of the three phases of movement, the initial steady state, the intermediate position, and the final steady state. The same result was found when the changes in muscle activation were analyzed between the initial and final steady states. Changes in the dwell time between the perturbation force application and removal led to different final hand locations undershooting the initial position. The magnitude of the undershot scaled with the dwell time, while the structure of variance in the muscle activation space did not depend on the dwell time. We conclude that stability of the hand coordinate is ensured during both intentional and unintentional actions via similar mechanisms. Relative equifinality in the external space after transient perturbations may be associated with varying states in the redundant space of muscle activations. The results fit a hierarchical scheme for the control of voluntary movements with referent configurations and redundant mapping between the levels of the hierarchy. [ABSTRACT FROM AUTHOR]

Abstract: Background: We applied the idea of synergies and the framework of the uncontrolled manifold hypothesis to explore the effects of dopamine replacement therapy on finger interaction and coordination in patients with early-stage Parkinson's disease (PD). Methods: Eight patients performed single-finger and multi-finger force production tasks with both the dominant and non-dominant hand before (off-drug) and after (on-drug) taking their dopaminergic medications. Synergy indices were defined as co-varied adjustments of commands to fingers that stabilized the total force produced by the hand. Results: PD patients showed significantly lower maximal finger forces off-drug compared to the on-drug condition, while indices of finger individuation (enslaving) were unchanged. The synergy indices were weaker during steady-state force production off-drug compared to on-drug. Anticipatory adjustments of synergies prior to the quick force pulse initiation were delayed and reduced off-drug as compared to the on-drug condition. These drug effects were observed in both the symptomatic and asymptomatic hands of the patients whose symptoms were limited to one side of the body. Conclusions: The study demonstrates dopaminergic modulation of motor coordination in PD and supports that the analysis of different components of multi-finger synergies offers a set of indices sensitive to the effects of dopamine replacement therapy in early-stage PD. The results suggest an important role of the basal ganglia in synergy formation and in feed-forward synergy adjustments. Future studies using these methods may yield more objective, quantitative biomarker(s) of motor coordination impairments in PD, and better understanding of subcortical involvement in the neural control of natural actions. [Copyright &y& Elsevier]

We explored the role of the basal ganglia in two components of multi-finger synergies by testing a group of patients with early-stage Parkinson's disease and a group of healthy controls. Synergies were defined as co-varied adjustments of commands to individual fingers that reduced variance of the total force and moment of force. The framework of the uncontrolled manifold hypothesis was used to quantify such co-variation patterns, while average performance across repetitive trials (sharing patterns) was analyzed using the analytical inverse optimization (ANIO) approach. The subjects performed four-finger pressing tasks that involved the accurate production of combinations of the total force and total moment of force and also repetitive trials at two selected combinations of the total force and moment. The ANIO approach revealed significantly larger deviations of the experimental data planes from an optimal plane for the patients compared to the control subjects. The synergy indices computed for total force stabilization were significantly higher in the control subjects compared to the patients; this was not true for synergy indices computed for moment of force stabilization. The differences in the synergy indices were due to the larger amount of variance that affected total force in the patients, while the amount of variance that did not affect total force was comparable between the groups. We conclude that the basal ganglia play an important role in both components of synergies reflecting optimization of the sharing patterns and stability of performance with respect to functionally important variables. [ABSTRACT FROM AUTHOR]

This study joined two approaches to motor control. The first approach comes from cognitive psychology and is based on the idea that goal postures and movements are chosen to satisfy task-specific constraints. The second approach comes from the principle of motor abundance and is based on the idea that control of apparently redundant systems is associated with the creation of multi-element synergies stabilizing important performance variables. The first approach has been tested by relying on psychophysical ratings of comfort. The second approach has been tested by estimating variance along different directions in the space of elemental variables such as joint postures. The two approaches were joined here. Standing subjects performed series of movements in which they brought a hand-held pointer to each of four targets oriented within a frontal plane, close to or far from the body. The subjects were asked to rate the comfort of the final postures, and the variance of their joint configurations during the steady state following pointing was quantified with respect to pointer endpoint position and pointer orientation. The subjects showed consistent patterns of comfort ratings among the targets, and all movements were characterized by multi-joint synergies stabilizing both pointer endpoint position and orientation. Contrary to what was expected, less comfortable postures had higher joint configuration variance than did more comfortable postures without major changes in the synergy indices. Multi-joint synergies stabilized the pointer position and orientation similarly across a range of comfortable/uncomfortable postures. The results are interpreted in terms conducive to the two theoretical frameworks underlying this work, one focusing on comfort ratings reflecting mean postures adopted for different targets and the other focusing on indices of joint configuration variance. [ABSTRACT FROM AUTHOR]

In this study, we tested several hypotheses related to changes in finger interaction and multifinger synergies during multifinger force production tasks in Parkinson's disease. Ten patients with Parkinson's disease, mostly early stage, and 11 healthy control subjects participated in the study. Synergies were defined as covaried adjustment of commands to fingers that stabilized the total force produced by the hand. Both Parkinson's disease patients and control subjects performed accurate isometric force production tasks with the fingers of both the dominant and nondominant hands. The Parkinson's disease patients showed significantly lower maximal finger forces and higher unintended force production (enslaving). These observations suggest that changes in supraspinal control have a major effect on finger individuation. The synergy indexes in the patients were weaker in both steady-state and cyclic force production tasks compared with the controls. These indexes also were stronger in the left (nondominant) hand in support of the dynamic-dominance hypothesis. Half of the patients could not perform the cyclic task at the highest frequency (2 Hz). Anticipatory adjustments of synergies prior to a quick force pulse production were delayed and reduced in the patients compared with the controls. Similar differences were observed between the asymptomatic hands of the patients with symptoms limited to one side of the body and matched hands of control subjects. Our study demonstrates that the elusive changes in motor coordination in Parkinson's disease can be quantified objectively, even in patients at a relatively early stage of the disease. The results suggest an important role of the basal ganglia in synergy formation and demonstrate a previously unknown component of impaired feedforward control in Parkinson's disease reflected in the reduced and delayed anticipatory synergy adjustments. [ABSTRACT FROM AUTHOR]

We used two methods to address two aspects of multi-finger synergies and their changes after fatigue of the index finger. Analytical inverse optimization (ANIO) was used to identify cost functions and corresponding spaces of optimal solutions over a broad range of task parameters. Analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify co-variation of finger forces across repetitive trials that helped reduce variability of (stabilized) performance variables produced by all the fingers together. Subjects produced steady-state levels of total force and moment of force simultaneously as accurately as possible by pressing with the four fingers of the right hand. Both before and during fatigue, the subjects performed single trials for many force-moment combinations covering a broad range; the data were used for the ANIO analysis. Multiple trials were performed at two force-moment combinations; these data were used for analysis within the UCM hypothesis. Fatigue was induced by 1-min maximal voluntary contraction exercise by the index finger. Principal component (PC) analysis showed that the first two PCs explained over 90% of the total variance both before and during fatigue. Hence, experimental observations formed a plane in the four-dimensional finger force space both before and during fatigue conditions. Based on this finding, quadratic cost functions with linear terms were estimated from the experimental data. The dihedral angle between the plane of optimal solutions and the plane of experimental observations ( D) was very small (a few degrees); it increased during fatigue. There was an increase in fatigue of the coefficient at the quadratic term for the index finger force balanced by a drop in the coefficients for the ring and middle fingers. Within each finger pair (index-middle and ring-little), the contribution of the 'central' fingers to moment production increased during fatigue. An index of antagonist moment production dropped with fatigue. Fatigue led to higher co-variation indices during pronation tasks (index finger is an agonist) but opposite effects during supination tasks. The results suggest that adaptive changes in co-variation indices that help stabilize performance may depend on the role of the fatigued element, agonist or antagonist. [ABSTRACT FROM AUTHOR]

We used the framework of the uncontrolled manifold hypothesis to explore the relations between anticipatory synergy adjustments (ASAs) and anticipatory postural adjustments (APAs) during feedforward control of vertical posture. ASAs represent a drop in the index of a multimuscle-mode synergy stabilizing the coordinate of the center of pressure in preparation to an action. ASAs reflect early changes of an index of covariation among variables reflecting muscle activation, whereas APAs reflect early changes in muscle activation levels averaged across trials. The assumed purpose of ASAs is to modify stability of performance variables, whereas the purpose of APAs is to change magnitudes of those variables. We hypothesized that ASAs would be seen before APAs and that this finding would be consistent with regard to the muscle-mode composition defined on the basis of different tasks and phases of action. Subjects performed a voluntary body sway task and a quick, bilateral shoulder flexion task under self-paced and reaction time conditions. Surface muscle activity of 12 leg and trunk muscles was analyzed to identify sets of 4 muscle modes for each task and for different phases within the shoulder flexion task. Variance components in the muscle-mode space and indexes of multimuscle-mode synergy stabilizing shift of the center of pressure were computed. ASAs were seen ∼ 100-150 ms prior to the task initiation, before APAs. The results were consistent with respect to different sets of muscle modes defined over the two tasks and different shoulder flexion phases. We conclude that the preparation for a self-triggered postural perturbation is associated with two types of anticipatory adjustments, ASAs and APAs. They reflect different feedforward processes within the hypothetical hierarchical control scheme, resulting in changes in patterns of covariation of elemental variables and in their patterns averaged across trials, respectively. The results show that synergies quantified using dissimilar sets of muscle modes show similar feedforward changes in preparation to action. [ABSTRACT FROM AUTHOR]

Two approaches to motor redundancy, optimization and the principle of abundance, seem incompatible. The former predicts a single, optimal solution for each task, while the latter assumes that families of equivalent solutions are used. We explored the two approaches using a four-finger pressing task with the requirement to produce certain combination of total normal force and a linear combination of normal forces that approximated the total moment of force in static conditions. In the first set of trials, many force-moment combinations were used. Principal component (PC) analysis showed that over 90% of finger force variance was accounted for by the first two PCs. The analytical inverse optimization (ANIO) approach was applied to these data resulting in quadratic cost functions with linear terms. Optimal solutions formed a hyperplane ('optimal plane') in the four-dimensional finger force space. In the second set of trials, only four force-moment combinations were used with multiple repetitions. Finger force variance within each force-moment combination in the second set was analyzed within the uncontrolled manifold (UCM) hypothesis. Most finger force variance was confined to a hyperplane (the UCM) compatible with the required force-moment values. We conclude that there is no absolute optimal behavior, and the ANIO yields the best fit to a family of optimal solutions that differ across trials. The difference in the force-producing capabilities of the fingers and in their moment arms may lead to deviations of the 'optimal plane' from the subspace orthogonal to the UCM. We suggest that the ANIO and UCM approaches may be complementary in the analysis of motor variability in redundant systems. [ABSTRACT FROM AUTHOR]

In a multifinger cyclic force production task, the finger force variance measured across trials can be decomposed into two components, one that affects the combined force output (“bad variance”) and one that does not (“good variance”). Previous studies have found similar time patterns of “bad variance” and force rate leading to an approximately linear relationship between them. Based on this finding and a recently developed model of multifinger force production, we expected the “bad variance” during cyclic force production to increase monotonically with the rate of force change, both within a cycle and across trials at different frequencies. Alternatively, “bad variance” could show a dependence on task frequency, not on actual force derivative values. Healthy subjects were required to produce cyclic force patterns to prescribed targets by pressing on unidimensional force sensors, at a frequency set by a metronome. The task was performed with only the index finger, and with all four fingers. In the task with all four fingers, the “good variance” increased approximately linearly with an increase in the force magnitude. The “bad variance” showed within-a-cycle modulation similar to that of the force rate. However, an increase in the frequency did not lead to an increase in the “bad variance” that could be expected based on the natural relationships between action frequency and the rate of force change modulation. The results have been interpreted in the framework of an earlier model of multifinger force production where “bad variance” is a result of variance of the timing parameter. The unexpected lack of modulation of the “bad variance” with frequency suggests a drop in variance of the timing parameter with increased frequency. This mechanism may serve to maintain a constant acceptable level of variance under different conditions. [ABSTRACT FROM AUTHOR]

Abstract: We studied multi-joint coordination during tasks of transporting real and imaginary objects with two arms. One of the arms was unexpectedly arrested in one-third of trials performed. In the absence of perturbation, multi-joint synergies stabilizing the distance between the arms early and late in the movement were seen in both conditions and even were stronger in the imaginary object condition. However, quick adjustments in the non-perturbed arm were seen only in the real object condition, whereas the non-perturbed arm did not react to the perturbation in the imaginary object condition. We conclude that tactile information is important for the central nervous system to quickly respond to perturbations in bimanual tasks. The results underscore potential differences between stability in the absence of external perturbations that may be ensured by setting a reference aperture between the hands and stability that requires adjustments in this reference aperture following a major perturbation. [Copyright &y& Elsevier]

Abstract: In this study, a hierarchical structure is proposed to model human movement control during sit-to-stand transfer. At the highest level the desired movement is planned. Then, the task to be performed is decomposed to its constitutive sub-tasks. To decompose the sit-to-stand movement, the spatial trajectory of the body center of mass is automatically approximated by partially linearized trajectories. Each linearized part defines a sub-task. At the second level, corresponding to each sub-task a module is developed that learns to control the movement during the performance of that sub-task. Since the procedure of decomposition is performed automatically, the number of modules and assessment of suitable data to train the modules are also determined automatically. This feature is one of the main differences between the proposed structure and the MOdular Selection And Identification for Control (MOSAIC) structure [M. Haruno, D.M. Wolpert, M. Kawato, MOSAIC model for sensorimotor learning and control, Neural Computation 13 (2001) 2201–2220.]. Our proposed model is in conformity with the recent physiological and neurobehavioral findings and provides a framework for examining a given movement under different conditions. [Copyright &y& Elsevier]

We used the idea of hierarchical control to study multi-muscle synergies during a whole-body sway task performed by a standing person. Within this view, at the lower level of the hierarchy, muscles are united into groups (M-modes). At the higher level, gains at the M-modes are co-varied by the controller in a task-specific way to ensure low variability of important physical variables. In particular, we hypothesized that (1) the composition of M-modes could adjust and (2) an index of M-mode co-variation would become weaker in more challenging conditions. Subjects were required to perform a whole-body sway at 0.5 Hz paced by a metronome. They performed the task with eyes open and closed, while standing on both feet or on one foot only, with and without vibration applied to the Achilles tendons. Integrated indices of muscle activation were subjected to principal component analysis to identify M-modes. An increase in the task complexity led to an increase in the number of principal components that contained significantly loaded indices of muscle activation from 3 to 5. Hence, in more challenging tasks, the controller manipulated a larger number of variables. Multiple regression analysis was used to define the Jacobian of the system mapping small changes in M-mode gains onto shifts of the center of pressure (COP) in the anterior–posterior direction. Further, the variance in the M-mode space across sway cycles was partitioned into two components, one that did not affect an average across cycles COP coordinate and the other that did (good and bad variance, respectively). Under all conditions, the subjects showed substantially more good variance than bad variance interpreted as a multi-M-mode synergy stabilizing the COP trajectory. An index of the strength of the synergy was comparable across all conditions, and there was no modulation of this index over the sway cycle. Hence, our first hypothesis that the composition of M-modes could adjust under challenging conditions has been confirmed while the second hypothesis stating that the index of M-mode co-variation would become weaker in more challenging conditions has been falsified. We interpret the observations as suggesting that adjustments at the lower level of the hierarchy—in the M-mode composition—allowed the subjects to maintain a comparable level of stabilization of the COP trajectory in more challenging tasks. The findings support the (at least) two-level hierarchical control scheme of whole-body movements. [ABSTRACT FROM AUTHOR]

We considered a hypothetical two-level hierarchy participating in the control of vertical posture. The framework of the uncontrolled manifold (UCM) hypothesis was used to explore the muscle groupings (M-modes) and multi-M-mode synergies involved in the stabilization of a time profile of the shear force in the anterior–posterior direction. Standing subjects were asked to produce pulses of shear force into a target using visual feedback while trying to minimize the shift of the center of pressure (COP). Principal component analysis applied to integrated muscle activation indices identified three M-modes. The composition of the M-modes was similar across subjects and the two directions of the shear force pulse. It differed from the composition of M-modes described in earlier studies of more natural actions associated with large COP shifts. Further, the trial-to-trial M-mode variance was partitioned into two components: one component that does not affect a particular performance variable ( V UCM), and its orthogonal component ( V ORT). We argued that there is a multi-M-mode synergy stabilizing this particular performance variable if V UCM is higher than V ORT. Overall, we found a multi-M-mode synergy stabilizing both shear force and COP coordinate. For the shear force, this synergy was strong for the backward force pulses and nonsignificant for the forward pulses. An opposite result was found for the COP coordinate: the synergy was stronger for the forward force pulses. The study shows that M-mode composition can change in a task-specific way and that two different performance variables can be stabilized using the same set of elemental variables (M-modes). The different dependences of the Δ V indices for the shear force and COP coordinate on the force pulse direction supports applicability of the principle of superposition (separate controllers for different performance variables) to the control of different mechanical variables in postural tasks. The M-mode composition allows a natural mechanical interpretation. [ABSTRACT FROM AUTHOR]

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., (Copyright © 2017 the American Physiological Society.)

The aim of this study was to discover finger interaction indices during single-finger ramp tasks and multi-finger coordination during a steady state force production in two directions, flexion, and extension. Furthermore, the indices of anticipatory adjustment of elemental variables (i.e., finger forces) prior to a quick pulse force production were quantified. It is currently unknown whether the organization and anticipatory modulation of stability properties are affected by force directions and strengths of in multi-finger actions. We expected to observe a smaller finger independency and larger indices of multi-finger coordination during extension than during flexion due to both neural and peripheral differences between the finger flexion and extension actions. We also examined the indices of the anticipatory adjustment between different force direction conditions. The anticipatory adjustment could be a neural process, which may be affected by the properties of the muscles and by the direction of the motions. The maximal voluntary contraction (MVC) force was larger for flexion than for extension, which confirmed the fact that the strength of finger flexor muscles (e.g., flexor digitorum profundus) was larger than that of finger extensor (e.g., extensor digitorum). The analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify the motor synergy of elemental variables by decomposing two sources of variances across repetitive trials, which identifies the variances in the uncontrolled manifold ( V UCM ) and that are orthogonal to the UCM (V ORT ). The presence of motor synergy and its strength were quantified by the relative amount of V UCM and V ORT . The strength of motor synergies at the steady state was larger in the extension condition, which suggests that the stability property (i.e., multi-finger synergies) may be a direction specific quantity. However, the results for the existence of anticipatory adjustment; however, no difference between the directional conditions suggests that feed-forward synergy adjustment (changes in the stability property) may be at least independent of the magnitude of the task-specific apparent performance variables and its direction (e.g., flexion and extension forces).