Improving finger coordination in young and elderly persons

Two aspects of feed-forward control of action stability: effects of action speed and unexpected events

We explored two types of anticipatory synergy adjustments (ASA) during accurate four-finger total force production task. The first type is a change in the index of force-stabilizing synergy during a steady state when a person is expecting a signal to produce a quick force change, which is seen even when the signal does not come (steady-state ASA). The other type is the drop in in the synergy index prior to a planned force change starting at a known time (transient ASA). The subjects performed a task of steady force production at 10% of maximal voluntary contraction (MVC) followed by a ramp to 20% MVC over 1 s, 3 s, and as a step function (0 s). In another task, in 50% of the trials during the steady-state phase, an unexpected signal could come requiring a quick force pulse to 20% MVC (0-surprise). Inter-trial variance in the finger force space was used to quantify the index of force-stabilizing synergy within the uncontrolled manifold hypothesis. We observed significantly lower synergy index values during the steady state in the 0-ramp trials compared to the 1-ramp and 3-ramp trials. There was also larger transient ASA during the 0-ramp trials. In the 0-surprise condition, the synergy index was significantly higher compared to the 0-ramp condition whereas the transient ASA was significantly larger. The finding of transient ASA scaling is of importance for clinical studies, which commonly involve populations with slower actions, which can by itself be associated with smaller ASAs. The participants varied the sharing pattern of total force across the fingers more in the task with "surprises". This was coupled to more attention to precision of performance, i.e., inter-trial deviations from the target as reflected in smaller variance affecting total force, possibly reflecting higher concentration on the task, which the participants perceived as more challenging compared to a similar task without surprise targets.

Effects of walking speeds on lower extremity kinematic synergy in toe vertical position control: An experimental study

BACKGROUND

This study aimed to investigate whether lower limb joints mutually compensate for each other, resulting in motor synergy that suppresses toe vertical position fluctuation, and whether walking speeds affect lower limb synergy.

METHODS

Seventeen male university students walked at slow (0.85 ± 0.04 m/s), medium (1.43 ± 0.05 m/s) and fast (1.99 ± 0.06 m/s) speeds on a 15-m walkway while lower limb kinematic data were collected. Uncontrolled manifold analysis was used to quantify the strength of synergy. Two-way (speed × phase) repeated-measures analysis of variance was used to analyze all dependent variables.

RESULTS

A significant speed-by-phase interaction was observed in the synergy index (SI) (P  < .001). At slow walking speeds, subjects had greater SI during mid-swing (P  < .001), while at fast walking speeds, they had greater SI during early-swing (P  < .001). During the entire swing phase, fast walking exhibited lower SI values than medium (P  = .005) and slow walking (P  = .027).

CONCLUSION

Kinematic synergy plays a crucial role in controlling toe vertical position during the swing phase, and fast walking exhibits less synergy than medium and slow walking. These findings contribute to a better understanding of the role of kinematic synergy in gait stability and have implications for the development of interventions aimed at improving gait stability and reducing the risk of falls.

Optimality, Stability, and Agility of Human Movement: New Optimality Criterion and Trade-Offs

This review of movement stability, optimality, and agility is based on the theory of motor control with changes in spatial referent coordinates for the effectors, the principle of abundance, and the uncontrolled manifold hypothesis. A new optimality principle is suggested based on the concept of optimal sharing corresponding to a vector in the space of elemental variables locally orthogonal to the uncontrolled manifold. Motion along this direction is associated with minimal components along the relatively unstable directions within the uncontrolled manifold leading to a minimal motor equivalent motion. For well-practiced actions, this task-specific criterion is followed in spaces of referent coordinates. Consequences of the suggested framework include trade-offs among stability, optimality, and agility, unintentional changes in performance, hand dominance, finger specialization, individual traits in performance, and movement disorders in neurological patients.

Recent Advances in the Neural Control of Movements: Lessons for Functional Recovery

We review the current views on the control and coordination of movements following the traditions set by Nikolai Bernstein. In particular, we focus on the theory of neural control of effectors - from motor units to individual muscles, to joints, limbs, and to the whole body - with spatial referent coordinates organized into a hierarchy with multiple few-to-many mappings. Further, we discuss synergies ensuring stability of natural human movements within the uncontrolled manifold hypothesis. Synergies are organized within the neural control hierarchy based on the principle of motor abundance. Movement disorders are discussed as consequences of an inability to use the whole range of changes in referent coordinates (as in spasticity) and an inability to ensure controlled stability of salient variables as reflected in indices of multi-element synergies and their adjustments in preparation to actions (as in brain disorders, including Parkinson's disease, multiple-system atrophy, and stroke). At the end of the review, we discuss possible implications of this theoretical approach to peripheral disorders and their rehabilitations using, as an example, osteoarthritis. In particular, "joint stiffening" is viewed as a maladaptive strategy, which can compromise stability of salient variables during walking.

Sex-specific effects of a repetitive fatiguing task on stability: Analysis with motor equivalence model

Although studies showed that several internal factors affect task-specific stability, the sex-specific effects of fatigue on whole-limb stability during a semi-cycle repetitive pointing task remain unidentified. Synergy and Motor Equivalence concepts in the UCM framework have been developed to explain task-specific stability. The motor equivalence model quantifies the amount of deviation in the space of elemental variables that occurs in two directions; one that preserves the performance variable (good variance), and the other that affects it (bad variance). Synergy index (the difference between good and bad variance divided by the total variance > 0) represent stability in performing a task. Healthy adults (n = 26, 13F; age: 35.3 ± 10.6 yrs.) performed an RPT by moving their dominant arm between a proximal target and a distal target in a standing position until near fatigue (Borg CR10 rating 8/10). Tridimensional kinematics of trunk, upper arm, forearm, and hand segments were captured by high-resolution cameras every minute, and joint angles were extracted according to the ZX'Y″ Euler sequence. Results showed the synergy > 0 for both women and men, reflecting synergies stabilizing the endpoint coordinate in both Non-Fatigue and Fatigue conditions. Statistics (ANOVA) showed a significant Condition * Sex effect (p = 0.01), with higher good (by 0.19 ± 0.1 rad) and bad variances (by 0.15 ± 0.09 rad) in women compared to men after fatigue. Higher good and bad variability, with no change in women's performance could represent a less stable strategy, leading to the development of risk factors for neck-shoulder disorders.

One more time about motor (and non-motor) synergies

We revisit the concept of synergy based on the recently translated classical book by Nikolai Bernstein (On the construction of movements, Medgiz, Moscow 1947; Latash, Bernstein's Construction of Movements, Routledge, Abingdon 2020b) and progress in understanding the physics and neurophysiology of biological action. Two aspects of synergies are described: organizing elements into stable groups (modes) and ensuring dynamical stability of salient performance variables. The ability of the central nervous system to attenuate synergies in preparation for a quick action-anticipatory synergy adjustments-is emphasized. Recent studies have demonstrated synergies at the level of hypothetical control variables associated with spatial referent coordinates for effectors. Overall, the concept of synergies fits naturally the hierarchical scheme of control with referent coordinates with an important role played by back-coupling loops within the central nervous system and from peripheral sensory endings. Further, we review studies showing non-trivial changes in synergies with development, aging, fatigue, practice, and a variety of neurological disorders. Two aspects of impaired synergic control-impaired stability and impaired agility-are introduced. The recent generalization of the concept of synergies for non-motor domains, including perception, is discussed. We end the review with a list of unresolved and troubling issues.

Perturbation-induced fast drifts in finger enslaving

We explored changes in finger forces and in an index of unintentional finger force production (enslaving) under a variety of visual feedback conditions and positional finger perturbations. In particular, we tested a hypothesis that enslaving would show a consistent increase with time at characteristic times of about 1-2 s. Young healthy subjects performed accurate force production tasks under visual feedback on the total force of the instructed fingers (index and ring) or enslaved fingers (middle and little). Finger feedback was covertly alternated between master and enslaved fingers in a random fashion. The feedback could be presented over the first 5 s of the trial only or over the whole trial duration (21 s). After 5 s, the fingers were lifted by 1 cm, and after 15 s, the fingers were lowered to the initial position. The force of the instructed fingers drifted toward lower magnitudes in all conditions except the one with continuous feedback on that force. The force of enslaved fingers showed variable behavior across conditions. In all conditions, the index of enslaving showed a consistent increase with the time constant varying between 1 and 3 s. We interpret the results as pointing at the spread of excitation to enslaved fingers (possibly, in the cortical M1 areas). The relatively fast changes in enslaving under positional finger perturbations suggest that quick changes of the input into M1 from pre-M1 areas can accelerate the hypothesized spread of cortical excitation.

On the origin of finger enslaving: control with referent coordinates and effects of visual feedback

When a person tries to press with a finger, other fingers of the hand produce force unintentionally. We explored this phenomenon of enslaving during unintentional force drifts in the course of continuous force production by pairs of fingers of a hand. Healthy subjects performed accurate force production tasks by finger pairs Index-Middle, Middle-Ring, and Ring-Little with continuous visual feedback on the combined force of the instructed (master) fingers or of the noninstructed (enslaved) fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Across all finger pairs, enslaved force showed a drift upward under feedback on the master finger force, and master force showed a drift downward under feedback on the enslaved finger force. The subjects were unaware of the force drifts, which could reach over 50% of the initial force magnitude over 15 s. Across all conditions, the index of enslaving increased by ∼50% over the trial duration. The initial moment of force magnitude in pronation-supination was not a consistent predictor of the force drift magnitude. These results falsify the hypothesis that the counter-directional force drifts reflected drifts in the moment of force. They suggest that during continuous force production, enslaving increases with time, possibly due to the spread of excitation over cortical finger representations or other mechanisms, such as increased synchronization of firing of α-motoneurons innervating different compartments of extrinsic flexors. These changes in enslaving, interpreted at the level of control with referent coordinates for the fingers, can contribute to a variety of phenomena, including unintentional force drifts.NEW & NOTEWORTHY We report a consistent slow increase in finger enslaving (force production by noninstructed fingers) when visual feedback was presented on the force produced by either two instructed fingers or two noninstructed fingers of the hand. In contrast, force drifts could be in opposite directions depending on the visual feedback. We interpret enslaving and its drifts at the level of control with referent coordinates for the involved muscles, possibly reflecting spread of cortical excitation.

Long-term practice of isolated finger movements reduces enslaved response of tonically contracting little finger abductor to tonic index finger abduction

The purpose of this study was to elucidate whether the long-term practice of isolated finger movements reduces the enslaved response of the little finger abductor to the index finger abduction. The right-handed participants tonically or phasically abducted the index finger, while they maintained at rest or tonic abduction of the little finger. The enslaved response of the tonically contracting little finger abductor to the tonic abduction of the index finger was greater than the response of the same muscle at rest in the nonpianists. This indicates that the tonic contraction of the little finger abductor enhances the enslaving drive from the tonically contracting index finger abductor to the little finger abductor. The enslaved response of the tonically contracting little finger abductor to the tonic abduction of the index finger in the pianists was significantly smaller than that in the nonpianists, but such a significant group difference was absent when the little finger abductor was at rest. This indicates that the inhibitory process on the enslaving drive from the tonically contracting index finger abductor to the tonically active little finger abductor is unmasked through the long-term practice.

Effect of dance on multi-muscle synergies in older adults: a cross-sectional study

BACKGROUND

The purpose of this study was to investigate the efficacy of dance in the experienced older dancers compared to the inexperienced older adults. We explored the effect of dance on the composition of muscle groups and multi-muscle synergies stabilizing the center of pressure (COP) displacement in preparation to take a step during support surface translation.

METHODS

Eight dance experienced elderly participants were asked to take a step in response to support surface perturbations. Uncontrolled manifold analysis was used to identify muscle modes (M-modes) as factors in the muscle activation space. Variance components in the M-mode space and indices of M-mode synergy stabilizing COP displacement were computed.

RESULTS

The reciprocal M-modes were observed more frequently in the dance group than in the control group prior to the step initiation. Dance led to higher indices of multi-muscle synergies and earlier anticipatory synergy adjustments during preparation for making a step in response to the support surface translations.

CONCLUSIONS

Dance appeared to be associated with adjustments in both the composition of M-modes and M-mode co-variation patterns resulting in stronger synergies stabilizing COP coordinate in older adults. The results reported here could have clinical relevance when offering a dance approach to balance training for impaired individuals.

Age-related changes to motor synergies in multi-joint and multi-finger manipulative skills: a meta-analysis

Purpose

The aim of the current meta-analysis was to examine the extent to which there are differences in upper extremity motor synergies across different age groups in manipulative tasks.

Methods

The studies that used the uncontrolled manifold method to examine the effect of age on motor synergies in multi-joint and multi-finger tasks were selected. Sixteen relevant studies from 1154 articles were selected for the meta-analysis—4 and 12 studies considered multi-joint kinematics and multi-finger kinetic tasks respectively.

Results

The results of the meta-analysis suggested reduced strength of synergies in multi-finger task in older adults, but this was not the case for synergies in multi-joint task. Part of this age-related difference in finger function is related to the increased variability in total force in grasping tasks. However, reductions in the strength of multi-finger synergies in hand functions following ageing appear to depend on the characteristics of the task.

Conclusions

These findings indicate that the cooperation among fingers to stabilise the total required force to apply for grasping and other fine motor skills is less efficient in older adults that might affect the quality of manipulative tasks.

Explaining Individual Differences in Fine Motor Performance and Learning in Older Adults: The Contribution of Muscle Strength and Cardiovascular Fitness

It remains controversial whether aging influences motor learning and whether physiological factors, such as local strength or fitness, are associated with fine motor performance and learning in older adults (OA). OA (n = 51) and young adults (YA, n = 31) performed a short-term motor learning session using a precision grip force modulation task. The rate of improvement of OA compared with YA was steeper with respect to performance variability and temporal precision. Both age groups showed positive transfer during an unpracticed variant of the force modulation task. Local muscle strength (pinch and grip strength) and high cardiovascular fitness positively predicted fine motor performance, whereas initial performance, muscle strength, and motor fitness (heterogeneous motor test battery) negatively predicted rate of improvement. Analyses indicated potentials, but also limits of plasticity for OA.

Coordination in adults with neurological impairment - A systematic review of uncontrolled manifold studies

BACKGROUND

Analysis of sensorimotor synergies has been greatly advanced by the Uncontrolled Manifold (UCM) approach. The UCM method is based on partitioning inter-trial variance displayed by elemental variables into 'good' (V_UCM) and 'bad' (V_ORT) variability that, respectively, indicate maintenance or loss of task stability. In clinical populations, these indices can be used to investigate the strength, flexibility, stereotypy and agility of synergistic control.

RESEARCH QUESTION

How are synergies affected by neurological impairment in adults? Specifically, this study aimed to determine i) the impact of pathology on V_UCM, V_ORT, and their ratio (synergy index); ii) the relationship between synergy indices and functional performance; iii) changes in anticipatory synergy adjustments (ASAs); and iv) the effects of interventions on synergies.

METHODS

Systematic review of UCM studies on adults with neurological impairment.

RESULTS

Most of the 17 studies had moderate to high quality scores in the adapted Critical Review Form and the UCM reporting quality checklist developed for this review. i) Most of the studies found reduced synergy indices for patients with Parkinson's disease (PD), olivo-ponto-cerebellar atrophy, multiple sclerosis and spinocerebellar degeneration, with variable levels of change in V_UCM and V_ORT. Reduction in synergy indices was not as consistent for stroke, in three out of six studies it was unchanged. ii) Five of seven studies found no significant correlations between scores on motor function scales and UCM indices. iii) Seven studies consistently reported ASAs that are smaller in magnitude, delayed, or both, for patients compared to healthy controls. iv) Two studies reported increased synergy indices, either via increase in V_UCM or decrease in V_ORT, after dopaminergic drugs for patients with PD. There were similar synergy indices but improved ASAs after deep brain stimulation for patients with PD.

SIGNIFICANCE

UCM can provide reliable and sensitive indicators of altered synergistic control in adults with neurological impairment.

Flexibility in joint coordination remains unaffected by force and balance demands in young and old adults during simple sit-to-stand tasks

PURPOSE

We examined the possibility that old adults use flexibility in joint coordination as a compensatory mechanism for the age-related decline in muscle strength when performing the sit-to-stand (STS) task repeatedly under high force and balance demands.

METHOD

Young (n = 14, 22.4 ± 2.1) and old (n = 12, 70 ± 3.2) healthy adults performed repeated STSs under high and low force and balance demands. The balance demand was manipulated by reducing the base of support and the force demand by increasing body weight with a weight vest. Uncontrolled manifold analysis was used to quantify age differences in motor flexibility.

RESULTS

While there were age-typical differences in kinematic STS strategies, flexibility in joint coordination was independent of age and task difficulty during repeated STSs.

DISCUSSION

That simple manipulations of force and balance demands did not affect flexibility in joint coordination in old and young adults suggests that motor flexibility acts as a compensatory mechanism only at the limits of available muscle strength and balance abilities during STS movements. Intervention studies should identify how changes in specific neuromuscular functions affect flexibility in joint coordination during activities of daily living such as STS.

Mediolateral footpath stabilization during walking in people following stroke

Community dwelling stroke survivors most often fall while walking. Understanding how post-stroke individuals control mediolateral footpath during walking may help elucidate the mechanisms that contribute to walking instability. By applying the Uncontrolled Manifold (UCM) approach, we investigated (1) how post-stroke individuals coordinate lower-extremity joint motions to stabilize mediolateral footpath of the swing leg, and (2) how the inter-joint coordination in footpath stabilization correlates to their walking stability. Nine stroke subjects and nine healthy controls walked on a treadmill at four different speeds. UCM analysis partitions the variance of kinematic configurations across gait cycles into "good variance" (i.e., the variance component leading to a consistent footpath) or "bad variance" (i.e., the variance component leading to an inconsistent footpath). We found that both groups had a significantly greater "good" than "bad" variance (p<0.05) for most of the swing phase, suggesting that mediolateral footpath is an important variable stabilized by the central nervous system during walking. Stroke subjects had significantly greater relative variance difference (ΔV) (i.e. normalized difference between "good" and "bad" variance) (p<0.05), indicating a stronger kinematic synergy in footpath stabilization, than the controls. In addition, the kinematic synergy in mediolateral footpath stabilization is strongest during mid-swing but weakest during late swing in healthy gait. However, this phase-dependent strategy is preserved for mid-swing but not for late swing in stroke gait. Moreover, stroke and healthy subjects demonstrated different relationships between UCM and walking stability measures. A stronger kinematic synergy in healthy gait is associated with better walking stability whereas having more "good variance" or stronger kinematic synergy in stroke gait is associated with less walking stability. The current findings suggest that walking with too much "good variance" in people following stroke, despite no effect on the footpath, may adversely affect their walking stability to some extent.

Quantitative analysis of multi-element synergy stabilizing performance: comparison of three methods with respect to their use in clinical studies

A number of analyses associated with the uncontrolled manifold (UCM) hypothesis have been used recently to investigate stability of actions across populations. We explored whether some of those methods have an advantage for clinical studies because they require fewer trials to achieve consistent findings. We compared the number of trials needed for the analysis of inter-trial variance, analysis of motor equivalence, and analysis in the space of referent coordinates. Young healthy adults performed four-finger accurate force production tasks under visual feedback with the right (dominant) and left hand over three days. Three methods [analytical (M1), experimental (M2), and cumulative mean (M3) methods] were used to define the minimal number of trials required to reach certain statistical criteria. Two of these methods, M1 and M2, showed qualitatively similar results. Fewer trials (M1: 5-13, M2: 4-10) were needed for analysis of motor equivalence compared to inter-trial variance analysis (M1: 14-24, M2: 10-14). The third method (M3) showed no major differences among the outcome variables. The index of synergy in the inter-trial variance analysis required a very small number of trials (M1, M2: 2-4). Variables related to referent coordinates required only a few trials (under 3), whereas the synergy index in this analysis required the largest number of trials (M1: 24-34, M2: 12-16). This is the first study to quantify the number of trials needed for UCM-based methods of assessing motor coordination broadly used in clinical studies. Clinical studies can take advantage of specific recommendations based on the current data regarding the number of trials needed for each analysis thus allowing minimizing the test session duration without compromising data reliability.

Stability of steady hand force production explored across spaces and methods of analysis

We used the framework of the uncontrolled manifold (UCM) hypothesis and explored the reliability of several outcome variables across different spaces of analysis during a very simple four-finger accurate force production task. Fourteen healthy, young adults performed the accurate force production task with each hand on 3 days. Small spatial finger perturbations were generated by the "inverse piano" device three times per trial (lifting the fingers 1 cm/0.5 s and lowering them). The data were analyzed using the following main methods: (1) computation of indices of the structure of inter-trial variance and motor equivalence in the space of finger forces and finger modes, and (2) analysis of referent coordinates and apparent stiffness values for the hand. Maximal voluntary force and the index of enslaving (unintentional finger force production) showed good to excellent reliability. Strong synergies stabilizing total force were reflected in both structure of variance and motor equivalence indices. Variance within the UCM and the index of motor equivalent motion dropped over the trial duration and showed good to excellent reliability. Variance orthogonal to the UCM and the index of non-motor equivalent motion dropped over the 3 days and showed poor to moderate reliability. Referent coordinate and apparent stiffness indices co-varied strongly and both showed good reliability. In contrast, the computed index of force stabilization showed poor reliability. The findings are interpreted within the scheme of neural control with referent coordinates involving the hierarchy of two basic commands, the r-command and c-command. The data suggest natural drifts in the finger force space, particularly within the UCM. We interpret these drifts as reflections of a trade-off between stability and optimization of action. The implications of these findings for the UCM framework and future clinical applications are explored in the discussion. Indices of the structure of variance and motor equivalence show good reliability and can be recommended for applied studies.

Development of finger force coordination in children

Coordination is often observed as body parts moving together. However, when producing force with multiple fingers, the optimal coordination is not to produce similar forces with each finger, but rather for each finger to correct mistakes of other fingers. In this study, we aim to determine whether and how this skill develops in children aged 4-12 years. We measured this sort of coordination using the uncontrolled manifold hypothesis (UCM). We recorded finger forces produced by 60 typically developing children aged between 4 and 12 years in a finger-pressing task. The children controlled the height of an object on a screen by the total amount of force they produced on force sensors. We found that the synergy index, a measure of the relationship between "good" and "bad" variance, increased linearly as a function of age. This improvement was achieved by a selective reduction in "bad" variance rather than an increase in "good" variance. We did not observe differences between males and females, and the synergy index was not able to predict outcomes of upper limb behavioral tests after controlling for age. As children develop between the ages of 4 and 12 years, their ability to produce negative covariation between their finger forces improves, likely related to their improved ability to perform dexterous tasks.

Uncontrolled manifold hypothesis: Organization of leg joint variance in humans while walking in a wide range of speeds

This study aimed at investigating the organization of joint angle variability during walking by using the uncontrolled manifold (UCM) theory. We tested two hypotheses: i. the coordinative mechanism underlying joint angle variance during the stance phase is compatible with a kinematic synergy that stabilizes the centre of mass (CoM) position; ii. the walking speed affects the variance components onto and orthogonal to the UCM. Eight healthy subjects (26.0±2.0years old) steadily walked on a treadmill at five normalised speeds (from 0.62±0.03m/s to 1.15±0.07m/s). Joint angles and foot orientation, and components of the CoM position were, respectively, used as elemental variables and task performance for the UCM implementation. The effect of speed, time events, and variance components on the distribution of data variance in the space of joint angles was analyzed by the ANOVA test. Results corroborated the hypothesis that the variance of elemental variables is structured in order to minimize the stride-to-stride variability of the CoM position, at all speeds. Noticeably, both variance components increase during the propulsive phase, albeit that parallel to the UCM was always grater than the orthogonal one. Accordingly, the observed kinematic synergy is supposed to contribute to accomplishing an efficient transition between two steps. Results also revealed that the walking speed does not affect the partitioning of elemental variables-related variance onto and orthogonal to the UCM. Accordingly, the organization of leg joint variance underlying the stabilization of CoM position remains almost unaltered across speeds.

Does practicing a wide range of joint angle configurations lead to higher flexibility in a manual obstacle-avoidance target-pointing task?

Flexibility in motor actions can be defined as variability in the use of degrees of freedom (e.g., joint angles in the arm) over repetitions while keeping performance (e.g., fingertip position) stabilized. We examined whether flexibility can be increased through enlarging the joint angle range during practice in a manual obstacle-avoidance target-pointing task. To establish differences in flexibility we partitioned the variability in joint angles over repetitions in variability within (GEV) and variability outside the solution space (NGEV). More GEV than NGEV reflects flexibility; when the ratio of the GEV and NGEV is higher, flexibility is higher. The pretest and posttest consisted of 30 repetitions of manual pointing to a target while moving over a 10 cm high obstacle. To enlarge the joint angle range during practice participants performed 600 target-pointing movements while moving over obstacles of different heights (5-9 cm, 11-15 cm). The results indicated that practicing movements over obstacles of different heights led participants to use enlarged range of joint angles compared to the range of joint angles used in movements over the 10 cm obstacle in the pretest. However, for each individual obstacle neither joint angle variance nor flexibility were higher during practice. We also did not find more flexibility after practice. In the posttest, joint angle variance was in fact smaller than before practice, primarily in GEV. The potential influences of learning effects and the task used that could underlie the results obtained are discussed. We conclude that with this specific type of practice in this specific task, enlarging the range of joint angles does not lead to more flexibility.

Assessing postural stability via the correlation patterns of vertical ground reaction force components

Background

Many methods have been proposed to assess the stability of human postural balance by using a force plate. While most of these approaches characterize postural stability by extracting features from the trajectory of the center of pressure (COP), this work develops stability measures derived from components of the ground reaction force (GRF).

Methods

In comparison with previous GRF-based approaches that extract stability features from the GRF resultant force, this study proposes three feature sets derived from the correlation patterns among the vertical GRF (VGRF) components. The first and second feature sets quantitatively assess the strength and changing speed of the correlation patterns, respectively. The third feature set is used to quantify the stabilizing effect of the GRF coordination patterns on the COP.

Results

In addition to experimentally demonstrating the reliability of the proposed features, the efficacy of the proposed features has also been tested by using them to classify two age groups (18–24 and 65–73 years) in quiet standing. The experimental results show that the proposed features are considerably more sensitive to aging than one of the most effective conventional COP features and two recently proposed COM features.

Conclusions

By extracting information from the correlation patterns of the VGRF components, this study proposes three sets of features to assess human postural stability during quiet standing. As demonstrated by the experimental results, the proposed features are not only robust to inter-trial variability but also more accurate than the tested COP and COM features in classifying the older and younger age groups. An additional advantage of the proposed approach is that it reduces the force sensing requirement from 3D to 1D, substantially reducing the cost of the force plate measurement system.

Motor rehabilitation should be based on knowledge of motor control

Neurorehabilitation is at a crossroads. Indeed, there is inconclusive, but promising evidence about clinical effectiveness of rehabilitation in the field of neurological impairments. Translating the new theories on motor control into clinical research may help to develop new treatment strategies and guide rehabilitation approaches. The concepts of synergy and the uncontrolled manifold hypothesis provide a strong theoretical framework to explain how the nervous system controls and coordinates movements, ensuring stability during daily actions. Moreover, this approach can increase the understanding of the neural control of action stability with implications for clinical practice and may help the development of new treatment strategies.

Interpersonal synergies: static prehension tasks performed by two actors

We investigated multidigit synergies stabilizing components of the resultant force vector during joint performance of a static prehension task by two persons as compared to similar tasks performed by a single person using both hands. Subjects transferred the instrumented handle from the right hand to the left hand (one-person condition) or passed that handle to another person (two-person condition) while keeping the handle's position and orientation stationary. Only three digits were involved per hand, the thumb, the index finger, and the middle finger; the forces and moments produced by the digits were measured by six-component sensors. We estimated the performance-stabilizing synergies within the uncontrolled manifold framework by quantifying the intertrial variance structure of digit forces and moments. The analysis was performed at three levels: between hands, between virtual finger and virtual thumb (imagined digits producing the same mechanical variables as the corresponding actual digits combined) produced by the two hands (in both interpersonal and intrapersonal conditions), and between the thumb and virtual finger for one hand only. Additionally, we performed correlation and phase synchronization analyses of resultant tangential forces and internal normal forces. Overall, the one-person conditions were characterized by higher amount of intertrial variance that did not affect resultant normal force components, higher internal components of normal forces, and stronger synchronization of the normal forces generated by the hands. Our observations suggest that in two-person tasks, when participants try to achieve a common mechanical outcome, the performance-stabilizing synergies depend on non-visual information exchange, possibly via the haptic and proprioceptive systems. Therefore, synergies quantified in tasks using visual feedback only may not be generalizable to more natural tasks.

Effects of unilateral stroke on multi-finger synergies and their feed-forward adjustments

We explored the changes in multi-finger synergies in patients after a single cortical stroke with mild motor impairments. We hypothesized that both synergy indices and anticipatory synergy adjustments prior to the initiation of a self-paced quick action would be diminished in the patients compared to age-matched controls. The patients with history of cortical stroke, and age-matched controls (n=12 in each group) performed one-finger and multi-finger accurate force production tasks involving both steady-state and quick force pulse production. Finger interdependence (enslaving) and multi-finger synergies stabilizing total force were quantified. The stroke patients showed lower maximal finger forces, in particular in the contralesional hand, which also showed increased enslaving indices. Multi-finger synergies during steady-state force production were, however, unchanged after stroke. In contrast, a drop in the synergy index prior to the force pulse generation was significantly delayed in the stroke patients. Our results show that mild cortical stroke leads to no significant changes in multifinger synergies, but there is impairment in feed-forward adjustments of the synergies prior to a quick action, a drop in the maximal force production, and an increase in enslaving. We conclude that studies of synergies reveal two aspects of synergic control differentially affected by cortical stroke.

Changes in motor synergies for tracking movement and responses to perturbations depend on task-irrelevant dimension constraints

We investigated the changes in the motor synergies of target-tracking movements of hands and the responses to perturbation when the dimensionalities of target positions were changed. We used uncontrolled manifold (UCM) analyses to quantify the motor synergies. The target was changed from one to two dimensions, and the direction orthogonal to the movement direction was switched from task-irrelevant directions to task-relevant directions. The movement direction was task-relevant in both task conditions. Hence, we evaluated the effects of constraints on the redundant dimensions on movement tracking. Moreover, we could compare the two types of responses to the same directional perturbations in one- and two-dimensional target tasks. In the one-dimensional target task, the perturbation along the movement direction and the orthogonal direction were task-relevant and -irrelevant perturbations, respectively. In the two-dimensional target task, the both perturbations were task-relevant perturbations. The results of the experiments showed that the variabilities of the hand positions in the two-dimensional target-tracking task decreased, but the variances of the joint angles did not significantly change. For the task-irrelevant perturbations, the variances of the joint angles within the UCM that did not affect hand position (UCM component) increased. For the task-relevant perturbations, the UCM component tended to increase when the available UCM was large. These results suggest that humans discriminate whether the perturbations were task-relevant or -irrelevant and then adjust the responses of the joints by utilizing the available UCM.

Normalized Index of Synergy for Evaluating the Coordination of Motor Commands

Humans perform various motor tasks by coordinating the redundant motor elements in their bodies. The coordination of motor outputs is produced by motor commands, as well properties of the musculoskeletal system. The aim of this study was to dissociate the coordination of motor commands from motor outputs. First, we conducted simulation experiments where the total elbow torque was generated by a model of a simple human right and left elbow with redundant muscles. The results demonstrated that muscle tension with signal-dependent noise formed a coordinated structure of trial-to-trial variability of muscle tension. Therefore, the removal of signal-dependent noise effects was required to evaluate the coordination of motor commands. We proposed a method to evaluate the coordination of motor commands, which removed signal-dependent noise from the measured variability of muscle tension. We used uncontrolled manifold analysis to calculate a normalized index of synergy. Simulation experiments confirmed that the proposed method could appropriately represent the coordinated structure of the variability of motor commands. We also conducted experiments in which subjects performed the same task as in the simulation experiments. The normalized index of synergy revealed that the subjects coordinated their motor commands to achieve the task. Finally, the normalized index of synergy was applied to a motor learning task to determine the utility of the proposed method. We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands. In a motor learning task, subjects tracked a target trajectory of the total torque. The change in the coordination of muscle tension through learning was dominated by that of motor commands, which supported the hypothesis. We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.

Neural control of movement stability: Lessons from studies of neurological patients

The concept of synergy provides a theoretical framework for movement stability resulting from the neural organization of multiple elements (digits, muscles, etc.) that all contribute to salient performance variables. Although stability of performance is obviously important for steady-state tasks leading to high synergy indices, a feed-forward drop in synergy indices is seen in preparation to a quick action (i.e., anticipatory synergy adjustments, ASAs). We review recent studies of multi-finger and multi-muscle synergies that show decreased indices of synergies and ASAs in patients with Parkinson's disease (PD) or multisystem atrophy. In PD, the impairments in synergies and ASAs are partially reversed by dopaminergic drugs, and changes in synergy indices are present even in PD patients at earliest diagnosis. Taken together, these results point at subcortical structures that are crucial for proper control of movement stability. It is timely to introduce the concept of impaired control of stability as an objective, quantifiable, and theory-based clinical descriptor of movement disorders that can increase our understanding of the neural control of movement with all of its implications for clinical practice.

Force control in chronic stroke

Force control deficits are common dysfunctions after a stroke. This review concentrates on various force control variables associated with motor impairments and suggests new approaches to quantifying force control production and modulation. Moreover, related neurophysiological mechanisms were addressed to determine variables that affect force control capabilities. Typically, post stroke force control impairments include: (a) decreased force magnitude and asymmetrical forces between hands, (b) higher task error, (c) greater force variability, (d) increased force regularity, and (e) greater time-lag between muscular forces. Recent advances in force control analyses post stroke indicated less bimanual motor synergies and impaired low-force frequency structure. Brain imaging studies demonstrate possible neurophysiological mechanisms underlying force control impairments: (a) decreased activation in motor areas of the ipsilesional hemisphere, (b) increased activation in secondary motor areas between hemispheres, (c) cerebellum involvement, and (d) relatively greater interhemispheric inhibition from the contralesional hemisphere. Consistent with identifying neurophysiological mechanisms, analyzing bimanual motor synergies as well as low-force frequency structure will advance our understanding of post stroke force control.

Task-specific stability of multifinger steady-state action

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.

Stability of multifinger action in different state spaces

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

The effects of practice on coordination

We review practice-induced changes in two variance components defined based on the uncontrolled manifold hypothesis. One of them affects task performance, whereas the other one does not. Practice leads to a drop in the former component (higher accuracy), whereas the latter can drop, stay unchanged, or even increase. The last scenario can be achieved with practice that challenges performance stability.

Enslaving in a serial chain: interactions between grip force and hand force in isometric tasks

This study was motivated by the double action of extrinsic hand muscles that produce grip force and also contribute to wrist torque. We explored interactions between grip force and wrist torque in isometric force production tasks. In particular, we tested a hypothesis that an intentional change in one of the two kinetic variables would produce an unintentional change in the other (enslaving). When young healthy subjects produced accurate changes in the grip force, only minor effects on the force produced by the hand (by wrist flexion/extension action) were observed. In contrast, a change in the hand force produced consistent changes in grip force in the same direction. The magnitude of such unintentional grip force change was stronger for intentional hand force decrease as compared to hand force increase. These effects increased with the magnitude of the initial grip force. When the subjects were asked to produce accurate total force computed as the sum of the hand and grip forces, strong negative covariation between the two forces was seen across trials interpreted as a synergy stabilizing the total force. An index of this synergy was higher in the space of "modes," hypothetical signals to the two effectors that could be changed by the controller one at a time. We interpret the complex enslaving effects (positive force covariation) as conditioned by typical everyday tasks. The presence of synergic effects (negative, task-specific force covariation) can be naturally interpreted within the referent configuration hypothesis.

Force control improvements in chronic stroke: bimanual coordination and motor synergy evidence after coupled bimanual movement training

Bimanual movement disorders are common dysfunctions post stroke. This stroke study investigated bimanual force control capabilities to determine the effect of coupled bimanual movement training on bimanual coordination and motor synergy. Stroke participants (N = 11) completed three bimanual force control tasks at 5, 25, and 50 % of maximum voluntary contraction before and after coupled bimanual movement training. Root mean square error (RMSE), approximate entropy, correlation, and bimanual motor synergy were analyzed in two-way completely within-subjects ANOVAs (Test Session × Force Level: 2 × 3). Multiple linear regression analysis determined the relationship between RMSE and other force control measures. The analyses revealed three important findings: (1) RMSE decreased from baseline to posttest (2) negative correlation (e.g., error compensation) and bimanual motor synergy increased at 25 and 50 % after rehabilitation, and (3) increased bimanual motor synergy was strongly associated with decreased RMSE after training. The findings indicate that coupled bimanual movement training improved force control performance, bimanual coordination, and motor synergies. Indeed, the present findings extend bimanual motor synergies as a meaningful indicator for estimating task performance improvements. Finally, bimanual force control is a valid outcome measure in quantifying progress toward motor recovery post stroke.

An apparent contradiction: increasing variability to achieve greater precision?

To understand the relationship between variability of foot placement in the frontal plane and stability of gait patterns, we explored how constraining mediolateral foot placement during walking affects the structure of kinematic variance in the lower-limb configuration space during the swing phase of gait. Ten young subjects walked under three conditions: (1) unconstrained (normal walking), (2) constrained (walking overground with visual guides for foot placement to achieve the measured unconstrained step width) and, (3) beam (walking on elevated beams spaced to achieve the measured unconstrained step width). The uncontrolled manifold analysis of the joint configuration variance was used to quantify two variance components, one that did not affect the mediolateral trajectory of the foot in the frontal plane ("good variance") and one that affected this trajectory ("bad variance"). Based on recent studies, we hypothesized that across conditions (1) the index of the synergy stabilizing the mediolateral trajectory of the foot (the normalized difference between the "good variance" and "bad variance") would systematically increase and (2) the changes in the synergy index would be associated with a disproportionate increase in the "good variance." Both hypotheses were confirmed. We conclude that an increase in the "good variance" component of the joint configuration variance may be an effective method of ensuring high stability of gait patterns during conditions requiring increased control of foot placement, particularly if a postural threat is present. Ultimately, designing interventions that encourage a larger amount of "good variance" may be a promising method of improving stability of gait patterns in populations such as older adults and neurological patients.