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ABSTRACT: This is an exploratory study of the accurate endpoint force vector production by the human arm in isometric conditions. We formulated three common-sense hypotheses and falsified them in the experiment. The subjects (n = 10) exerted static forces on the handle in eight directions in a horizontal plane for 25 s. The forces were of 4 magnitude levels (10, 20, 30 and 40 % of individual maximal voluntary contractions). The torsion moment on the handle (grasp moment) was not specified in the instruction. The two force components and the grasp moment were recorded, and the shoulder, elbow, and wrist joint torques were computed. The following main facts were observed: (a) While the grasp moment was not prescribed by the instruction, it was always produced. The moment magnitude and direction depended on the instructed force magnitude and direction. (b) The within-trial angular variability of the exerted force vector (angular precision) did not depend on the target force magnitude (a small negative correlation was observed). (c) Across the target force directions, the variability of the exerted force magnitude and directional variability exhibited opposite trends: In the directions where the variability of force magnitude was maximal, the directional variability was minimal and vice versa. (d) The time profiles of joint torques in the trials were always positively correlated, even for the force directions where flexion torque was produced at one joint and extension torque was produced at the other joint. (e) The correlations between the grasp moment and the wrist torque were negative across the tasks and positive within the individual trials. (f) In static serial kinematic chains, the pattern of the joint torques distribution could not be explained by an optimization cost function additive with respect to the torques. Plans for several future experiments have been suggested.
Experimental Brain Research 11/2012; 223(2):159-75. · 2.39 Impact Factor
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ABSTRACT: We investigated the effect of fatigue produced by timed maximal
voluntary contraction (MVC) of the index finger of one of the hands
on performance in MVC and accurate cyclic force production tasks
in right-handed subjects. Based on earlier studies, we hypothesized
that fatigue would produce an increase in the indices of forcestabilizing
synergies in both hands as well as between the hands
in two-hand tasks. Synergies were defined as co-varied adjustments
of commands to fingers (modes) across cycles that stabilized total
force. Fatigue caused a significant reduction in the MVC of the exercised
as well as the non-exercised hand. Indices of finger enslaving
(lack of individuation) increased with fatigue in both hands,
although the increase was significant in the exercised hand only.
In contrast to the significant effects of fatigue on MVC forces performed
by the non-exercised hand, there were no comparable transfer
effects on the root mean square errors during accurate force
production. During one-hand tasks, both hands showed high indices
of force-stabilizing synergies. These indices were larger in the left
hand. Fatigue led to a general increase in synergy indices. Exercise
by the left hand had stronger effects on synergy indices seen in both
hands. Exercise by the right hand showed ipsilateral effects only.
Smaller effects of fatigue were observed on accuracy of performance
of the force-down segments of the force cycles compared to the
force-up segments. For the bimanual tasks, synergies were analyzed
at two hierarchical levels, two-hand (four-finger) and withina-
hand (two-finger). An increase in the synergy index with fatigue
was observed at the lower (two-finger) level of the hierarchy only.
We interpret the lack of effects of fatigue at the upper (two-hand)
level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed
within the dynamic dominance hypothesis.
Human Movement Science 01/2012; 31(6):1379-1398. · 1.77 Impact Factor
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ABSTRACT: We studied the coordination of arm movements in standing persons who performed an out-of-phase arm-swinging task while stepping in place or while standing. The subjects were instructed to stop one of the arms in response to an auditory signal while trying to keep the rest of the movement pattern unchanged. A significant increase was observed in the amplitude of the arm that continued swinging under both the stepping and standing conditions. This increase was similar between the right and left arms. A dynamic model was developed including two coupled nonlinear van der Pol oscillators. We assumed that stopping an arm did not eliminate the coupling but introduced a new constraint. Within the model, superposition of two factors, a command to stop the ongoing movement of one arm and the coupling between the two oscillators, has been able to account for the observed effects. The model makes predictions for future experiments.
Motor control 04/2011; 15(2):206-20. · 1.53 Impact Factor
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ABSTRACT: When one finger changes its force, other fingers of the hand can show unintended force changes in the same direction (enslaving) and in the opposite direction (error compensation). We tested a hypothesis that externally imposed changes in finger force predominantly lead to error compensation effects in other fingers thus stabilizing the total force. A novel device, the "inverse piano", was used to impose controlled displacements to one of the fingers over different magnitudes and at different rates. Subjects (n=10) pressed with four fingers at a constant force level and then one of the fingers was unexpectedly raised. The subjects were instructed not to interfere with possible changes in the finger forces. Raising a finger caused an increase in its force and a drop in the force of the other three fingers. Overall, total force showed a small increase. Larger force drops were seen in neighbors of the raised finger (proximity effect). The results showed that multi-finger force stabilizing synergies dominate during involuntary reactions to externally imposed finger force changes. Within the referent configuration hypothesis, the data suggest that the instruction "not to interfere" leads to adjustments of the referent coordinates of all the individual fingers.
Human movement science 03/2011; 30(3):446-58. · 2.15 Impact Factor
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ABSTRACT: Two types of finger interaction are characterized by positive co-variation (enslaving) or negative co-variation (error compensation) of finger forces. Enslaving reflects mechanical and neural connections among fingers, while error compensation results from synergic control of fingers to stabilize their net output. Involuntary and voluntary force changes by a finger were used to explore these patterns. We hypothesized that synergic mechanisms will dominate during involuntary force changes, while enslaving will dominate during voluntary finger force changes. Subjects pressed with all four fingers to match a target force that was 10% of their maximum voluntary contraction (MVC). One of the fingers was unexpectedly raised 5.0 mm at a speed of 30.0 mm/s. During finger raising the subject was instructed "not to intervene voluntarily". After the finger was passively lifted and a new steady-state achieved, subjects pressed down with the lifted finger, producing a pulse of force voluntarily. The data were analyzed in terms of finger forces and finger modes (hypothetical commands to fingers reflecting their intended involvement). The target finger showed an increase in force during both phases. In the involuntary phase, the target finger force changes ranged between 10.71 ± 1.89% MVC (I-finger) and 16.60 ± 2.26% MVC (L-finger). Generally, non-target fingers displayed a force decrease with a maximum amplitude of -1.49 ± 0.43% MVC (L-finger). Thus, during the involuntary phase, error compensation was observed--non-lifted fingers showed a decrease in force (as well as in mode magnitude). During the voluntary phase, enslaving was observed--non-target fingers showed an increase in force and only minor changes in mode magnitude. The average change in force of non-target fingers ranged from 21.83 ± 4.47% MVC for R-finger (M-finger task) to 0.71 ± 1.10% MVC for L-finger (I-finger task). The average change in mode of non-target fingers was between -7.34 ± 19.27% MVC for R-finger (L-finger task) and 7.10 ± 1.38% MVC for M-finger (I-finger task). We discuss a range of factors affecting force changes, from purely mechanical effects of finger passive lifting to neural synergic adjustments of commands to individual fingers. The data fit a recently suggested scheme that merges the equilibrium-point hypothesis (control with referent configurations) with the idea of hierarchical synergic control of multi-element systems.
Experimental Brain Research 02/2011; 208(3):423-35. · 2.39 Impact Factor
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M L Latash
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ABSTRACT: Persons with Down syndrome are frequently described as 'clumsy'. The recent progress in the development of quantitative approaches to motor synergies has allowed researchers to move towards an understanding of 'clumsiness' at the level of underlying control mechanisms. This progress has also offered an opportunity to quantify changes in motor synergies that accompany improvement in the performance of motor tasks. Previous studies of our group have shown, in particular, that persons both with and without Down syndrome are able to show improvements in indices of their multi-finger synergies in tasks that require accurate production of finger forces. In particular, 3 days of practice has been shown to lead to significant improvements in indices of multi-finger synergies that stabilize the time patterns of the total force produced by the fingers of a hand. Persons with Down syndrome showed a qualitative change in their synergies that failed to stabilize the total force altogether prior to practice and became able to do so after practice. In addition, the studies have also shown that variable practice is more beneficial for the improvement of motor synergies than blocked practice. I would like to draw an optimistic conclusion that persons with Down syndrome are not inherently 'clumsy', but have a vast potential for an improvement of their motor performance. The current state of the area of motor control allows researchers and practitioners to tap into these reserves, and to use quantitative indices of changes in motor synergies with practice to optimize motor performance of these individuals.
Journal of Intellectual Disability Research 01/2008; 51(Pt 12):962-71. · 1.88 Impact Factor
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ABSTRACT: Two phenomena previously observed in multi-finger static maximum voluntary contraction (MVC) tasks—(1) force deficit and (2) enslaving—were compared with the force patterns produced during sub-maximal dynamic tasks. A new tool, the inverse piano, was designed to measure the finger forces during the sub-maximal dynamic tasks. During the dynamic experiments, the keys of the IP elevated according to a computer run program. Subjects (n = 9) were instructed to press down the elevated keys as fast as they can. All finger combinations were tested (totally 15). Force deficit was not observed for the dynamic tasks. Two aspects of the enslaving effects (EE) for the dynamic task were found to be similar with the MVC task: the EEs were relatively large (as much as 47.4% of the maximum force produced) and nearly symmetrical. Proximity effects and occlusion were not prevalent in the experiment where the key combination was known prior to key activation. In the case where the key combination was unknown and randomly chosen, proximity effects were retained, but occlusion was not observed. Inter-finger connection matrices (IFM) calculated for both the MVC and dynamic tasks further stressed the dissimilarity between the force patterns used to complete the tasks.
Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2006 14th Symposium on; 04/2006
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ABSTRACT: To evaluate postural performance and head stabilization of patients with chronic neck pain.
A single-blind comparative group study.
Patients with work-related chronic neck pain (n = 9), with chronic whiplash associated disorders (n = 9) and healthy subjects (n = 16).
During quiet standing in different conditions (e.g. 1 and 2 feet standing, tandem standing, and open and closed eyes) the sway areas and the ability to maintain the postures were measured. The maximal peak-to-peak displacement of the centre of pressure and the head translation were analysed during predictable and unpredictable postural perturbations.
Patients with chronic neck pain, in particular those with whiplash-associated disorders, showed larger sway areas and reduced ability to successfully execute more challenging balance tasks. They also displayed larger sway areas and reduced head stability during perturbations.
The results show that disturbances of postural control in chronic neck pain are dependent on the aetiology, and that it is possible to quantify characteristic postural disturbances in different neck pain conditions. It is suggested that the dissimilarities in postural performance are a reflection of different degrees of disturbances of the proprioceptive input to the central nervous system and/or of the central processing of such input.
Journal of Rehabilitation Medicine 10/2003; 35(5):229-35. · 2.05 Impact Factor
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ABSTRACT: We studied the coordination of forces and moments exerted by individual digits in static tasks that required balancing an external load and torque. Subjects ( n=10) stabilized a handle with an attachment that allowed for change of external torque. Thumb position and handle width systematically varied among the trials. Each subject performed 63 tasks (7 torque values x 3 thumb locations x 3 widths). Forces and moments exerted by the digit tips on the object were recorded. Although direction and magnitude of finger forces varied among subjects, each subject used a similar multidigit synergy: a single eigenvalue accounted for 95.2-98.5% of the total variance. When task parameters were varied, regular conjoint digital force changes (prehension synergies) were observed. Synergies represent preferential solutions used by the subjects to satisfy mechanical requirements of the tasks. In particular, chain effects in force adjustments to changes in the handle geometry were documented. An increased handle width induced the following effects: (a). tangential forces remained unchanged, (b). the same tangential forces produced a larger moment T (t), (c). the increased T (t) was compensated by a smaller moment of the normal forces T(n), and (d). normal finger forces were rearranged to generate a smaller moment. Torque control is a core component of prehension synergies. Observed prehension synergies are only mechanically necessitated in part. The data support a theory of hierarchical organization of prehension synergies.
Experimental Brain Research 02/2003; 148(1):77-87. · 2.39 Impact Factor
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ABSTRACT: A comprehensive study of patterns of finger forces during one-hand and two-hand multifinger maximal force production trials was performed with particular emphasis on differences between tasks involving symmetrical and asymmetrical finger groups (symmetrical and asymmetrical tasks). Twelve healthy right-handed subjects performed maximal voluntary force production tasks with different finger combinations. Force deficit (FD) for a finger group within a hand was defined as a drop in peak force in a multifinger task as compared to the sum of individual finger peak forces in single-finger tasks. FD showed a dependence on both the number of fingers within the hand and the number of fingers in the other hand. An additional drop in peak finger forces was seen in two-hand tests (bilateral deficit, BD). BD summed over two hands was independent of the number of fingers involved in the two-hand tasks, but dependent on the distribution of fingers between the two hands. BD for a hand was larger for tasks involving fewer fingers within the hand and more fingers in the other hand. It was higher for asymmetrical tasks than for symmetrical tasks. The difference between asymmetrical and symmetrical tasks was due to the different behavior of asymmetrically involved fingers. FD was larger for asymmetrical master (explicitly involved) fingers, while forces produced involuntarily by asymmetrical slave (explicitly non-involved) fingers were larger. These differences brought down the total moment produced by both hands in the frontal plane. FD and BD are phenomena of different origin whose effects sum up. The observations have led to further development of a previously proposed double-representation, mirror-image (DoReMi) hypothesis and refinement of the neural network underlying the two-hand finger interaction.
Experimental Brain Research 01/2002; 141(4):530-40. · 2.39 Impact Factor
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ABSTRACT: The framework of the uncontrolled manifold hypothesis (UCM hypothesis) was applied to the analysis of the structure of finger force variability during oscillatory force production tasks. Subjects produced cycles of force with one, two (index and middle), or three (index, middle, and ring) fingers acting in parallel against force sensors mounted inside a small frame. The frame could be placed on the top of a table (stable conditions) or on a 4-mm-wide supporting surface (unstable conditions). Subjects were less variable when they used two fingers than when using one finger; adding the third finger did not change indices of variability of the performance. Components of finger force variance that did (VUN) or did not (VCOMP) change the value of a particular functional variable were computed for two control hypotheses: (1) at each time, the subjects tried to stabilize the total value of force (force-control); and (2), at each time, the subjects tried to stabilize the total moment produced with respect to an axis parallel to the hand/forearm (moment-control). Most subjects showed selective stabilization of moment and destabilization of force throughout most of the force cycle, in both stable and unstable conditions. The shapes of VUN and VCOMP suggested a possibility of selective compensation of timing errors across fingers within force cycles. One subject showed different relations between VUN and VCOMP, suggesting that these relations did in fact reflect particular central strategies of solving the tasks. The UCM method is applicable to force production tasks. It allows the comparison of control hypotheses in a quantitative way and unveils central strategies of control of redundant motor systems. Within this approach, redundancy (rather, abundance) is not a problem but an inherent part of a solution for natural motor tasks.
Experimental Brain Research 12/2001; 141(2):153-65. · 2.39 Impact Factor
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ABSTRACT: The role of the intrinsic finger flexor muscles was investigated during finger flexion tasks. A suspension system was used to measure isometric finger forces when the point of force application varied along fingers in a distal-proximal direction. Two biomechanical models, with consideration of extensor mechanism Extensor Mechanism Model (EMM) and without consideration of extensor mechanism Flexor Model (FM), were used to calculate forces of extrinsic and intrinsic finger flexors. When the point of force application was at the distal phalanx, the extrinsic flexor muscles flexor digitorum profundus, FDP, and flexor digitorum superficialis, FDS, accounted for over 80% of the summed force of all flexors, and therefore were the major contributors to the joint flexion at the distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) joints. When the point of force application was at the DIP joint, the FDS accounted for more than 70% of the total force of all flexors, and was the major contributor to the PIP and MCP joint flexion. When the force of application was at the PIP joint, the intrinsic muscle group was the major contributor for MCP flexion, accounting for more than 70% of the combined force of all flexors. The results suggest that the effects of the extensor mechanism on the flexors are relatively small when the location of force application is distal to the PIP joint. When the external force is applied proximally to the PIP joint, the extensor mechanism has large influence on force production of all flexors. The current study provides an experimental protocol and biomechanical models that allow estimation of the effects of extensor mechanism on both the extrinsic and intrinsic flexors in various loading conditions, as well as differentiating the contribution of the intrinsic and extrinsic finger flexors during isometric flexion.
Journal of Biomechanics 09/2001; 34(8):1097-102. · 2.43 Impact Factor
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ABSTRACT: (1) To study differences in the generation of anticipatory postural adjustments (APAs) in arm and trunk/leg muscles prior to catching a load released either by the subject him-/herself or by the experimenter. (2) To study the importance of different mechanical characteristics of the load at impact for the generation of APAs prior to load catching.
Standing subjects were asked to catch loads dropped onto the left hand from different heights either by the experimenter or by the subject's right hand. The load mass and release height were manipulated to keep either the mass or the momentum of the load at impact constant. APAs were quantified with integral electromyographic indices.
APAs were observed in leg, trunk and arm muscles prior to load impact for both self- and experimenter-release trials. Kinetic energy showed higher correlations with the magnitude of APA than momentum, but only in experimenter-release trials.
Subjects can generate APAs in both arm and trunk/leg muscles in the absence of an explicit voluntary action. The relative importance of kinetic energy and momentum for defining the magnitude of APAs can reflect the difference in the sources of information used to prepare for the forthcoming perturbation during self- and experimenter-released load catch.
Clinical Neurophysiology 08/2001; 112(7):1250-65. · 3.41 Impact Factor
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ABSTRACT: We studied the effects of fatigue, induced by a 60-s maximal isometric force production with the index finger, on multi-finger coordination and force production by the other fingers of the hand. Finger forces were measured during single- and multi-finger maximal voluntary force production (MVC) at two sites, the middle of the distal or the middle of the proximal phalanges. Two fatiguing exercises involving force production by the index finger were used, one at the distal phalanx and the other at the proximal phalanx. The MVC of the index finger dropped by about 33% when it was produced at the site involved in the fatiguing exercise. In addition, large transfer effects of fatigue were observed across sites of force application and across fingers. Force deficit increased under fatigue, especially due to a drop in the recruitment of the index finger. Under fatigue, the index finger was less enslaved during force production by other fingers. During multi-finger tasks, the percentage of total force produced by the index finger was significantly reduced after the fatiguing exercise. The principle of minimization of secondary moments was violated under fatigue. We suggest that the most impaired (fatigued) finger shows less interaction with other fingers or, in other words, is being progressively removed from the multi-finger synergy. Some of the observed changes in finger coordination suggest effects of fatigue at a central (neural) level.
Experimental Brain Research 07/2001; 138(3):322-9. · 2.39 Impact Factor
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ABSTRACT: The main purpose of the present study has been to find an answer to the question: Can the subject generate anticipatory postural adjustments (APAs) when a predictable postural perturbation occurs in the absence of a voluntary action? Answering this question would allow us to distinguish between two competing hypotheses on the relation between APAs and voluntary movements. One hypothesis considers both APAsigma and voluntary "focal" movements different peripheral patterns associated with a single control process, while the alternative hypothesis considers them outcomes of two parallel control processes. Healthy subjects performed series of loading and unloading trials that included: (1) catching a falling load onto another load held in extended hands; (2) catching a falling load onto a tray attached to the trunk; (3) allowing a falling load to hit another load out of the extended hands, causing an unloading; and (4) releasing a load held in extended hands by a voluntary shoulder movement. In series 1, precautions were taken to avoid possible small hand movements prior to the impact of the falling load. Available visual information on the trajectory of the falling load was manipulated. In all conditions, except when the subject's eyes were closed, APAs were seen with patterns that were adequate for counteracting expected perturbations. Quantitative electromyographic indices of APAs depended on the availability of visual information and particular methods of introducing postural perturbations despite the fact that the magnitude of the perturbation was always the same. Our findings support a hypothesis that control processes resulting in APAs can be different from control processes associated with focal voluntary movements.
Experimental Brain Research 07/2001; 138(4):458-66. · 2.39 Impact Factor
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ABSTRACT: We studied the role of different leg and trunk muscle groups in the generation of anticipatory postural adjustments (APAs) prior to lateral and rotational perturbations associated with predictable and self-triggered postural perturbations during standing. Postural perturbations were induced by a variety of manipulations including catching and releasing a load with the right hand extended either in front of the body or to the right side, performing bilateral fast shoulder movements in different directions, and applying brief force pulses with a hand against the wall. Perturbations in a frontal plane ("lateral perturbations") were associated with significant asymmetries in APAs seen in the right and left distal (soleus and tibialis anterior) muscles; these asymmetries dependent on the direction of the perturbation. Rotational perturbations about the vertical axis of the body generated by fast movements of the two shoulders in the opposite directions were also associated with direction-dependent asymmetries in the APAs in soleus muscles. However, rotational perturbations generated by an off-body-midline force pulse application were accompanied by direction-dependent asymmetries in proximal muscle groups, but not in the distal muscles. We conclude that muscles controlling the ankle joint play an important role in the compensation of lateral and rotational perturbations. The abundance of muscles participating in maintaining vertical posture allows the control system to use different task-dependent strategies during the generation of APAs in anticipation of rotational perturbation.
Journal of Electromyography and Kinesiology 03/2001; 11(1):39-51. · 1.97 Impact Factor
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ABSTRACT: Human voluntary movements face a problem of kinematic redundancy. We study a planar bimanual task, when one hand moves a target and the other hand moves a pointer that must reach the target. We hypothesized that the stabilized task variable was the vectorial difference of the pointer tip and the target. The 6D state space was defined by “joint configuration vectors” whose elements were intersegmental joint angles. The subjects repeated the movements 15 times, and the movements were recorded by a movement analysis system. Then, the subjects practised the movements (300 trials). After practice 15 trials were recorded again. We computed the variance of the joint configurations before and after practice. Six joint rotations affected the 2D task variable. The uncontrolled manifold (UCM) corresponding to this variable is 4D, while the subspace of the state space that is orthogonal (ORT) to the UCM is 2D. The variance within the UCM was larger than in the ORT conforming to the UCM hypothesis. After practice the joint variance decreased and the drop in the component of variance that did not affect the task variable was larger than the drop of the other component. Thus, practice lead to more stable time courses of the task variable and of the corresponding joint configuration
Neural Networks, 2001. Proceedings. IJCNN '01. International Joint Conference on; 02/2001
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ABSTRACT: The question of degrees of freedom in the control of multijoint movement is posed as the problem of discovering how the motor control system constrains the many possible combinations of joint postures to stabilize task-dependent essential variables. Success at a task can be achieved, in principle, by always adopting a particular joint combination. In contrast, we propose a more selective control strategy: variations of the joint configuration that leave the values of essential task variables unchanged are predicted to be less controlled (i.e., stabilized to a lesser degree) than joint configuration changes that shift the values of the task variables. Our experimental task involved shooting with a laser pistol at a target under four conditions. The seven joint angles of the arm were obtained from the recorded positions of markers on the limb segments. The joint configurations observed at each point in normalized time were analyzed with respect to trial-to-trial variability. Different hypotheses about relevant task variables were used to define sets of joint configurations ("uncontrolled manifolds" or UCMs) that, if realized, would leave essential task variables unchanged. The variability of joint configurations was decomposed into components lying parallel to those sets and components lying in their complement. The orientation of the gun's barrel relative to a vector pointing from the gun to the target was the task variable most successful at showing a difference between the two components of joint variability. This variable determines success at the task. Throughout the movement, not only while the gun was pointing at the target, fluctuations of joint configuration that affected this variable were much reduced compared with fluctuations that did not affect this variable. The UCM principle applied to relative gun orientation thus captures the structure of the motor control system across different parts of joint configuration space as the movement evolves in time. This suggests a specific control strategy in which changes of joint configuration that are irrelevant to success at the task are selectively released from control. By contrast, constraints representing an invariant spatial position of the gun or of the arm's center of mass structured joint configuration variability in the early and mid-portion of the movement trajectory, but not at the time of shooting. This specific control strategy is not trivial, because a target can be hit successfully also by controlling irrelevant directions in joint space equally to relevant ones. The results indicate that the method can be successfully used to determine the structure of coordination in joint space that underlies the control of the essential variables for a given task.
Experimental Brain Research 01/2001; 135(3):382-404. · 2.39 Impact Factor
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ABSTRACT: Human voluntary movements face a problem of kinematic redundancy. We study a planar bimanual task, when one hand moves a target and the other hand moves a pointer that must reach the target. We hypothesized that the stabilized task variable was the vectorial difference of the pointer tip and the target. The 6D state space was defined by “joint configuration vectors” whose elements were intersegmental joint angles. The subjects repeated the movements 15 times, and the movements were recorded by a movement analysis system. Then, the subjects practised the movements (300 trials). After practice 15 trials were recorded again. We computed the variance of the joint configurations before and after practice. Six joint rotations affected the 2D task variable. The uncontrolled manifold (UCM) corresponding to this variable is 4D, while the subspace of the state space that is orthogonal (ORT) to the UCM is 2D. The variance within the UCM was larger than in the ORT conforming to the UCM hypothesis. After practice the joint variance decreased and the drop in the component of variance that did not affect the task variable was larger than the drop of the other component. Thus, practice lead to more stable time courses of the task variable and of the corresponding joint configuration
01/2001; 2.
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M L Latash
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ABSTRACT: Within this study, we tested a hypothesis that common organization of central commands to the elbow and the wrist joints within a two-joint synergy is associated with a similar organization of pre-programmed corrections to unexpected changes in the external conditions.
The subjects (n = 7) performed series of very fast movements or isometric contractions against a pad at the level of the palm or at the level of the forearm. Some trials within a series of movements were unexpectedly blocked at the initial position leading to an isometric contraction, while some trials within a series of isometric contractions were unexpectedly released leading to a movement. Movement kinematic and electromyographic (EMG) patterns were analyzed.
In cases of unexpectedly blocked or unexpectedly released trials, differences in the EMG patterns between perturbed and unperturbed trials were seen at latencies between 50 and 80 ms. Two patterns were observed representing interindividual differences among subjects. One included reciprocal changes in the flexor-extensor pairs controlling both joints. The other pattern included reciprocal changes in the wrist flexor-extensor pair and unidirectional changes in the EMGs of biceps and triceps. The patterns were reproducible within each individual subject across tasks and conditions. In particular, the same pattern in the wrist flexor-extensor pair was seen when the pad was applied to the palm and when it was applied to the forearm, despite the fact that early wrist joint deviations were in opposite directions.
It is argued that the observed early EMG changes may be unrelated to local joint kinematics and stretch reflexes, and represent consequences of control patterns for fast corrective movements that are organized with respect to the motion of the endpoint of the limb similarly to the organization of voluntary movements. Within this framework, the organization of joints of a multi-joint limb into a synergy implies a simultaneous, automatic organization of pre-programmed reactions into a similarly organized synergy.
Clinical Neurophysiology 07/2000; 111(6):975-87. · 3.41 Impact Factor
