Workspace location influences joint coordination during reaching in post-stroke hemiparesis

Integrating Motor Variability Evaluation Into Movement System Assessment

Common assessment tools for determining therapeutic success in rehabilitation typically focus on task-based outcomes. Task-based outcomes provide some understanding of the individual's functional ability and motor recovery; however, these clinical outcomes may have limited translation to a patient's functional ability in the real world. Limitations arise because (1) the focus on task-based outcome assessment often disregards the complexity of motor behavior, including motor variability, and (2) mobility in highly variable real-world environments requires movement adaptability that is made possible by motor variability. This Perspective argues that incorporating motor variability measures that reflect movement adaptability into routine clinical assessment would enable therapists to better evaluate progress toward optimal and safe real-world mobility. The challenges and opportunities associated with incorporating variability-based assessment of pathological movements are also discussed. This Perspective also indicates that the field of rehabilitation needs to leverage technology to advance the understanding of motor variability and its impact on an individual's ability to optimize movement.

IMPACT

This Perspective contends that traditional therapeutic assessments do not adequately evaluate the ability of individuals to adapt their movements to the challenges faced when negotiating the dynamic environments encountered during daily life. Assessment of motor variability derived during movement execution can address this issue and provide better insight into a patient's movement stability and maneuverability in the real world. Creating such a shift in motor system assessment would advance understanding of rehabilitative approaches to motor system recovery and intervention.

A Dynamical Approach to the Uncontrolled Manifold: Predicting Performance Error During Steady-State Isometric Force Production

The uncontrolled manifold (UCM) approach quantifies the presence of compensatory variability between musculoskeletal elements involved in a motor task. This approach has proved useful for identifying synergistic control strategies for a variety of everyday motor tasks and for investigating how control strategies are affected by motor pathology. However, the UCM approach is limited in its ability to relate compensatory motor variance directly to task performance because variability along the UCM is mathematically agnostic to performance. We present a new approach to UCM analysis that quantifies patterns of irregularity in the compensatory variability between motor elements over time. In a bimanual isometric force stabilization task, irregular patterns of compensation between index fingers predicted greater performance error associated with difficult task conditions, in particular for individuals who exploited a larger set of compensatory strategies (i.e., a larger subspace of the UCM). This relationship between the amount and structure of compensatory motor variance might be an expression of underlying processes supporting performance resilience.

Postural control during turn on the light task assisted by functional electrical stimulation in post stroke subjects

Postural control mechanisms have a determinant role in reaching tasks and are typically impaired in post-stroke patients. Functional electrical stimulation (FES) has been demonstrated to be a promising therapy for improving upper limb (UL) function. However, according to our knowledge, no study has evaluated FES influence on postural control. This study aims to evaluate the influence of FES UL assistance, during turning on the light task, in the related postural control mechanisms. An observational study involving ten post-stroke subjects with UL dysfunction was performed. Early and anticipatory postural adjustments (EPAs and APAs, respectively), the weight shift, the center of pressure and the center of mass (CoM) displacement were analyzed during the turning on the light task with and without the FES assistance. FES parameters were adjusted to improve UL function according to a consensus between physiotherapists' and patients' perspectives. The ANOVA repeated measures, Paired sample t and McNemar tests were used to compare postural control between the assisted and non-assisted conditions. When the task was assisted by FES, the number of participants that presented APAs increased (p = 0.031). UL FES assistance during turning on the light task can improve postural control in neurological patients with UL impairments.

The effects of an object's height and weight on force calibration and kinematics when post-stroke and healthy individuals reach and grasp

Impairment in force regulation and motor control impedes the independence of individuals with stroke by limiting their ability to perform daily activities. There is, at present, incomplete information about how individuals with stroke regulate the application of force and control their movement when reaching, grasping, and lifting objects of different weights, located at different heights. In this study, we assess force regulation and kinematics when reaching, grasping, and lifting a cup of two different weights (empty and full), located at three different heights, in a total of 46 participants: 30 sub-acute stroke participants, and 16 healthy individuals. We found that the height of the reached target affects both force calibration and kinematics, while its weight affects only the force calibration when post-stroke and healthy individuals perform a reach-to-grasp task. There was no difference between the two groups in the mean and peak force values. The individuals with stroke had slower, jerkier, less efficient, and more variable movements compared to the control group. This difference was more pronounced with increasing stroke severity. With increasing stroke severity, post-stroke individuals demonstrated altered anticipation and preparation for lifting, which was evident for either cortical lesion side.

Impairment and Compensation in Dexterous Upper-Limb Function After Stroke. From the Direct Consequences of Pyramidal Tract Lesions to Behavioral Involvement of Both Upper-Limbs in Daily Activities

Impairments in dexterous upper limb function are a significant cause of disability following stroke. While the physiological basis of movement deficits consequent to a lesion in the pyramidal tract is well demonstrated, specific mechanisms contributing to optimal recovery are less apparent. Various upper limb interventions (motor learning methods, neurostimulation techniques, robotics, virtual reality, and serious games) are associated with improvements in motor performance, but many patients continue to experience significant limitations with object handling in everyday activities. Exactly how we go about consolidating adaptive motor behaviors through the rehabilitation process thus remains a considerable challenge. An important part of this problem is the ability to successfully distinguish the extent to which a given gesture is determined by the neuromotor impairment and that which is determined by a compensatory mechanism. This question is particularly complicated in tasks involving manual dexterity where prehensile movements are contingent upon the task (individual digit movement, grasping, and manipulation…) and its objective (placing, two step actions…), as well as personal factors (motivation, acquired skills, and life habits…) and contextual cues related to the environment (presence of tools or assistive devices…). Presently, there remains a lack of integrative studies which differentiate processes related to structural changes associated with the neurological lesion and those related to behavioral change in response to situational constraints. In this text, we shall question the link between impairments, motor strategies and individual performance in object handling tasks. This scoping review will be based on clinical studies, and discussed in relation to more general findings about hand and upper limb function (manipulation of objects, tool use in daily life activity). We shall discuss how further quantitative studies on human manipulation in ecological contexts may provide greater insight into compensatory motor behavior in patients with a neurological impairment of dexterous upper-limb function.

Optimal multi-field functional electrical stimulation parameters for the "drinking task - reaching phase" and related upper limb kinematics repeatability in post stroke subjects

BACKGROUND

No specific guidelines for the management of functional electrical stimulation (FES) parameters in post stroke patients have been defined yet, despite its frequent use. The purpose of this study is to characterize the optimal FES parameters that assist the reaching phase of drinking task ("drinking task - reaching phase") on post stroke subjects and to analyze the related upper limb (UL) movement quality indicators repeatability.

METHODS

An observational study with a test and re-test design involving ten post stroke subjects with UL dysfunction was performed. End-point and joint kinematics of contralesional UL were assessed during the "drinking task - reaching phase" with FES through a test and retest design. FES parameters were adjusted to improve UL function according to a consensus between physiotherapists and patients' perspective.

FINDINGS

It was possible to establish reliable FES parameters that assisted the "drinking task - reaching phase". All FES parameters presented high to very high repeatability and led to moderate to very high repeatability in almost UL movement quality indicators during the "drinking task - reaching phase".

INTERPRETATION

These findings show that the main characteristics of FES parameters that improves patient perception of change are quite stable, which facilitate its implementation in clinical practice by allowing consistence between intervention sessions.

Number of Trials Necessary to Apply Analysis Within the Framework of the Uncontrolled Manifold Hypothesis at Different Levels of Hierarchical Synergy Control

The uncontrolled manifold hypothesis is a method used to quantify motor synergies, defined as a specific central nervous system organization that maintains the task-specific stability of motor actions. The UCM allows for inter-trial variance analysis between consecutive trials. However, despite the large body of literature within this framework, there is no report on the number of movement repetitions required for reliable results. Based on the hypothetical hierarchical control of motor synergies, this study aims to determine the minimum number of trials necessary to achieve a good to excellent level of reliability. Thirteen young, healthy participants performed fifteen bilateral isometric contractions of elbow flexion when visual feedback was provided. The force and electromyography data were recorded to investigate synergies at different levels of hierarchical control. The intraclass correlation coefficient was used to determine the reliability of the variance indices. Based on the obtained results, at least twelve trials are required to analyze the inter-trial variance in both force and muscle synergies within the UCM framework.

Optimal Multifield Functional Electrical Stimulation Parameters for the "Turn on the Light" Task and Related Upper Limb Kinematics Repeatability in Poststroke Subjects

OBJECTIVE

To characterize the optimal functional electrical stimulation (FES) parameters that assist the turn on the light task (TOTL) on poststroke participants and to analyze the related upper limb (UL) kinematics repeatability.

DESIGN

Cross-sectional study.

SETTING

Human movement research center.

PARTICIPANTS

Poststroke individuals (N=11) with history of a single unilateral stroke that resulted in a motor control dysfunction of the contralesional UL.

INTERVENTIONS

FES based on surface multifield technology applied to the contralesional wrist and finger extensors during the TOTL.

MAIN OUTCOME MEASURES

FES outcome metrics (virtual electrodes, stimulation duration, intensity) and kinematic metrics (end-point kinematics [absolute and relative duration, mean and peak velocities, relative instant of peak velocity, index of curvature, number of movement units] and joint kinematics [shoulder, elbow, wrist end position and range of movement]). Outcome measures were assessed 2 times with a 72-hour maximum time interval.

CONCLUSION

It was possible to establish reliable FES parameters that assisted the TOTL on poststroke participants. These stimulation parameters led to high to very high repeatability in terms of UL kinematics for most of the cases.

Movement Quality: A Novel Biomarker Based on Principles of Neuroscience

A major problem in neurorehabilitation is the lack of objective outcomes to measure movement quality. Movement quality features, such as coordination and stability, are essential for everyday motor actions. These features allow reacting to continuously changing environment or to resist external perturbations. Neurological disorders affect movement quality, leading to functionally impaired movements. Recent findings suggest that the central nervous system organizes motor elements (eg, muscles, joints, fingers) into task-specific ensembles to stabilize motor tasks performance. A method to quantify this feature has been previously developed based on the uncontrolled manifold (UCM) hypothesis. UCM quantifies movement quality in a spatial-temporal domain using intertrial analysis of covariation between motor elements. In this point-of-view article, we first describe major obstacles (eg, the need for group analysis) that interfere with UCM application in clinical settings. Then, we propose a process of quantifying movement quality for a single individual with a novel use of bootstrapping simulations and UCM analysis. Finally, we reanalyze previously published data from individuals with neurological disorders performing a wide range of motor tasks, that is, multi-digit pressing and postural balance tasks. Our method allows one to assess motor quality impairments in a single individual and to detect clinically important motor behavior changes. Our solution may be incorporated into a clinical setting to assess sensorimotor impairments, evaluate the effects of specific neurological treatments, or track movement quality recovery over time. We also recommended the proposed solution to be used jointly with a typical statistical analysis of UCM parameters in cohort studies.

Stability of reaching during standing in stroke

Reaching from standing requires simultaneous adjustments of focal and postural task elements. We investigated the ability of people with stroke to stabilize the endpoint trajectory while maintaining balance during standing reaches. Nineteen stroke and 11 age-equivalent healthy subjects reached toward a target (n = 30 trials) located beyond arm length from standing. Endpoint and center-of-mass (COM) trajectories were analyzed using the uncontrolled manifold (UCM) approach, with segment angles as elemental variables. A synergy index (SI) represented the normalized difference between segment angle combinations, leading to endpoint or COM trajectory stabilization (V_UCM) and lack of stabilization (in an orthogonal space; V_ORT). A higher SI reflects greater stability. In both groups, the endpoint SI (SI_END) decreased in parallel with endpoint velocity and returned close to baseline at the end of the movement. The range of SI_END was significantly greater in stroke (median: 0.87; QR:0.54) compared with healthy subjects (median: 0.58; QR: 0.33; P = 0.009). In both groups, the lowest SI_END occurred at the endpoint peak velocity, whereas the minimal SI_END of the stroke group (median: 0.51; QR:0.41) was lower than the healthy group (median: 0.25; QR: 0.50; P = 0.033). The COM SI (SI_COM) remained stable in both groups (~0.8). The maintenance of a high SI_COM despite a large reduction of SI_END in stroke subjects suggests that kinematic redundancy was effectively used to stabilize the COM position, but less so for endpoint position stabilization. Both focal and postural task elements should be considered when analyzing whole body reaching deficits in patients with stroke.NEW & NOTEWORTHY Reaching from standing requires simultaneous adjustments of endpoint and center-of-mass (COM) positions. We used uncontrolled manifold analysis to investigate the impact of stroke on the ability to use kinematic redundancy in this task. Our results showed that COM position was stabilized, whereas endpoint trajectory was more variable in stroke than healthy subjects. Enhancing the capacity to meet multiple task goals may be beneficial for motor recovery after stroke.

Differentiating Successful and Unsuccessful Single-Leg Drop Landing Performance Using Uncontrolled Manifold Analysis

Biomechanical analysis can effectively identify factors associated with task performance and injury risk, but often does not account for the interaction among the components that underlie task execution. Uncontrolled manifold (UCM) analyses were applied to data from 38 female, adolescent athletes performing single-leg drop landings and were used to differentiate successful and unsuccessful task performance by examining the frontal plane joint variance within the UCM (VUCM) that stabilized the horizontal center of mass position (VUCM) and within the orthogonal subspace (VORT). The UCM revealed stronger coordination, indicated by the VUCM/VORT ratio, in the successful condition. This may inform future research examining reduced motor coordination in failed movement tasks and its relation to injury risk and allow for targeted interventions that consider coordination processes rather than joint-specific outcomes.

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.

Kinematic Components of the Reach-to-Target Movement After Stroke for Focused Rehabilitation Interventions: Systematic Review and Meta-Analysis

Background: Better upper limb recovery after stroke could be achieved through tailoring rehabilitation interventions directly at movement deficits. Aim: To identify potential; targets for therapy by synthesizing findings of differences in kinematics and muscle activity between stroke survivors and healthy adults performing reach-to-target tasks. Methods: A systematic review with identification of studies, data extraction, and potential risk of bias was completed independently by two reviewers. Online databases were searched from their inception to November 2017 to find studies of reach-to-target in people-with-stroke and healthy adults. Potential risk-of-bias was assessed using the Down's and Black Tool. Synthesis was undertaken via: (a) meta-analysis of kinematic characteristics utilizing the standardized mean difference (SMD) [95% confidence intervals]; and (b), narrative synthesis of muscle activation. Results: Forty-six studies met the review criteria but 14 had insufficient data for extraction. Consequently, 32 studies were included in the meta-analysis. Potential risk-of-bias was low for one study, unclear for 30, and high for one. Reach-to-target was investigated with 618 people-with-stroke and 429 healthy adults. The meta-analysis found, in all areas of workspace, that people-with-stroke had: greater movement times (seconds) e.g., SMD 2.57 [0.89, 4.25]; lower peak velocity (millimeters/second) e.g., SMD -1.76 [-2.29, -1.24]; greater trunk displacement (millimeters) e.g. SMD 1.42 [0.90, 1.93]; a more curved reach-path-ratio e.g., SMD 0.77 [0.32, 1.22] and reduced movement smoothness e.g., SMD 0.92 [0.32, 1.52]. In the ipsilateral and contralateral workspace, people-with-stroke exhibited: larger errors in target accuracy e.g., SMD 0.70 [0.39, 1.01]. In contralateral workspace, stroke survivors had: reduced elbow extension and shoulder flexion (degrees) e.g., elbow extension SMD -1.10 [-1.62, -0.58] and reduced shoulder flexion SMD -1.91 [-1.96, -0.42]. Narrative synthesis of muscle activation found that people-with-stroke, compared with healthy adults, exhibited: delayed muscle activation; reduced coherence between muscle pairs; and use of a greater percentage of muscle power. Conclusions: This first-ever meta-analysis of the kinematic differences between people with stroke and healthy adults performing reach-to-target found statistically significant differences for 21 of the 26 comparisons. The differences identified and values provided are potential foci for tailored rehabilitation interventions to improve upper limb recovery after stroke.

[The dynamics of shoulder joint function in patients with hemiparesis in the acute period of carotid stroke]

AIM

To evaluate the recovery of shoulder joint function in patients with hemiparesis in the acute period of hemispheric stroke on the basis of the analysis of electromyography (fEMG) of the muscles of this region before and after rehabilitation measures, including targeted training with biofeedback (BFB).

MATERIAL AND METHODS

Three groups of 25 people each were studied. Patients of the physical therapy (PT) group received standard treatment and physical therapy; patients of the PT+BFB group received BFB training in addition to PT; the control group consisted of people without neurological and orthopedic symptoms. A clinical study and EMG of the muscles of the shoulder girdle during testing movements was performed.

RESULTS

On the side of paresis, the function of muscles was characterized by a decrease in the bioelectric activity (movement amplitude was decreased as well) and later phase of the maximum EMG activity compared to the norm. The time of maximum EMG activity had a trend towards the shift to the normal values during the treatment process, but the difference reached a statistically significant level not for all muscles and all movements. There were variants of the functions of the paretic muscles accompanied by the increased EMG activity. In the early stages (up to 21 days) of stroke, no significant changes in the EMG activity of shoulder girdle muscles were observed. The PT+BFB group showed better results than the PT group not only in the onset of activity, but also in the reduction of the amplitude when performing the same movement that indirectly suggested the more optimal inclusion of muscles in the motor act. In the period of acute hemispheric stroke, there were changes not only in the EMG activity of muscles of the affected side, but also of the contralateral side.

CONCLUSION

A fEMG of the muscles is a more sensitive and informative method of the diagnosis of disorders of motor function and assessment of recovery process of the muscles of the shoulder joint in patients with hemiparesis.

Upper Limb Coordination in Individuals With Stroke: Poorly Defined and Poorly Quantified

Background. The identification of deficits in interjoint coordination is important in order to better focus upper limb rehabilitative treatment after stroke. The majority of standardized clinical measures characterize endpoint performance, such as accuracy, speed, and smoothness, based on the assumption that endpoint performance reflects interjoint coordination, without measuring the underlying temporal and spatial sequences of joint recruitment directly. However, this assumption is questioned since improvements of endpoint performance can be achieved through different degrees of restitution or compensation of upper limb motor impairments based on the available kinematic redundancy of the system. Confusion about adequate measurement may stem from a lack a definition of interjoint coordination during reaching. Methods and Results. We suggest an operational definition of interjoint coordination during reaching as a goal-oriented process in which joint degrees of freedom are organized in both spatial and temporal domains such that the endpoint reaches a desired location in a context-dependent manner. Conclusions. In this point-of-view article, we consider how current approaches to laboratory and clinical measures of coordination comply with our definition. We propose future study directions and specific research strategies to develop clinical measures of interjoint coordination with better construct and content validity than those currently in use.

Examining impairment of adaptive compensation for stabilizing motor repetitions in stroke survivors

The hand, one of the most versatile but mechanically redundant parts of the human body, suffers more and longer than other body parts after stroke. One of the rehabilitation paradigms, task-oriented rehabilitation, encourages motor repeatability, the ability to produce similar motor performance over repetitions through compensatory strategies while taking advantage of the motor system's redundancy. The previous studies showed that stroke survivors inconsistently performed a given motor task with limited motor solutions. We hypothesized that stroke survivors would exhibit deficits in motor repeatability and adaptive compensation compared to healthy controls in during repetitive force-pulse (RFP) production tasks using multiple fingers. Seventeen hemiparetic stroke survivors and seven healthy controls were asked to repeatedly press force sensors as fast as possible using the four fingers of each hand. The hierarchical variability decomposition model was employed to compute motor repeatability and adaptive compensation across finger-force impulses, respectively. Stroke survivors showed decreased repeatability and adaptive compensation of force impulses between individual fingers as compared to the control (p < 0.05). The stroke survivors also showed decreased pulse frequency and greater peak-to-peak time variance than the control (p < 0.05). Force-related variables, such as mean peak force and peak force interval variability, demonstrated no significant difference between groups. Our findings indicate that stroke-induced brain injury negatively affects their ability to exploit their redundant or abundant motor system in an RFP task.

Synergy as a new and sensitive marker of basal ganglia dysfunction: A study of asymptomatic welders

BACKGROUND

Multi-digit synergies, a recently developed, theory-based method to quantify stability of motor action, are shown to reflect basal ganglia dysfunction associated with parkinsonian syndromes. In this study, we tested the hypothesis that multi-digit synergies may capture early and subclinical basal ganglia dysfunction. We chose asymptomatic welders to test the hypothesis because the basal ganglia are known to be most susceptible to neurotoxicity caused by welding-related metal accumulation (such as manganese and iron).

METHODS

Twenty right-handed welders and 13 matched controls were invited to perform single- and multi-finger pressing tasks using the fingers of the right or left hand. Unified Parkinson's Disease Rating Scale and Grooved Pegboard scores were used to gauge gross and fine motor dysfunction, respectively. High-resolution (3T) T1-weighted, T2-weighted, T1 mapping, susceptibility, and diffusion tensor MRIs were obtained to reflect manganese, iron accumulation, and microstructural changes in basal ganglia. The synergy index stabilizing total force and anticipatory synergy adjustments were computed, compared between groups, and correlated with estimates of basal ganglia manganese [the pallidal index, R1 (1/T1)], iron [R2* (1/T2*)], and microstructural changes [fractional anisotropy and mean diffusivity].

RESULTS

There were no significant differences in Unified Parkinson's Disease Rating Scale (total or motor subscale) or Grooved Pegboard test scores between welders and controls. The synergy index during steady-state accurate force production was decreased significantly in the left hand of welders compared to controls (p=0.004) but did not reach statistical significance in the right hand (p=0.16). Anticipatory synergy adjustments, however, were not significantly different between groups. Among welders, higher synergy indices in the left hand were associated significantly with higher fractional anisotropy values in the left globus pallidus (R=0.731, p<0.001) but not with the pallidal index, R1, or R2* values in the basal ganglia.

CONCLUSIONS

These data suggest that multi-digit synergy metrics may serve as preclinical markers for basal ganglia dysfunction in welders and other populations at risk for neurodegenerative diseases involving parkinsonian symptoms. This finding may have important clinical, scientific, and public/occupational health implications.

Upper Limb Obstacle Avoidance Behavior in Individuals With Stroke

BACKGROUND

Upper limb (UL) poststroke hemiparesis commonly leads to chronic disability. Despite moderate-to-good clinical recovery, many patients with UL hemiparesis still do not fully use their arm in daily tasks. Decreased arm use may be related to deficits in performance of more complex movement than what is usually assessed clinically.

OBJECTIVE

To identify differences between poststroke and nondisabled control subjects in making complex corrective movements to avoid an obstacle in the reaching path.

METHODS

Subjects rapidly reached for a juice bottle on a refrigerator shelf with their hemiparetic or dominant (controls) arm viewed in a large-screen projected 3D virtual environment. In random trials, a sliding door partially obstructed the reaching path. A successful trial was one in which subjects touched the bottle without their arm or hand hitting the door.

RESULTS

Fewer participants with stroke (12%) were successful at a 65% success rate in avoiding the door compared to controls (42%). Subjects with stroke also initiated corrections later (further) in the reaching path (100.7 ± 77.6 mm) compared to controls (51.6 ± 31.0 mm) resulting in a reduced margin of error. While both groups used similar endpoint movement strategies for obstructed reaching, subjects with stroke used less elbow and more trunk movement. Participants who reported being more confident using their hemiparetic arm had higher success rates.

CONCLUSION

Arm movement deficits can be identified when complex tasks are evaluated. Deficits in higher-order motor function such as obstacle avoidance behavior may decrease actual arm use in individuals with mild-to-moderate hemiparesis and should be evaluated in routine clinical practice.

Deficits in motor abilities for multi-finger force control in hemiparetic stroke survivors

The ability to control redundant motor effectors is one of hallmarks in human motor control, and the topic has been studied extensively over several decades since the initial inquiries proposed by Nicholi Bernstein. However, our understanding of the influence of stroke on the control of redundant motor systems is very limited. This study aimed to investigate the effect of stroke-related constraints on multi-finger force control abilities in a visuomotor task. Impaired (IH) and less-impaired hands (LH) of 19 hemiparetic stroke survivors and 19 age-matched control subjects were examined. Each hand repeatedly produced isometric forces to match a target force of 5 N shown on a computer screen using all four fingers. The hierarchical variability decomposition (HVD) model was used to separate force-matching errors (motor performance) into task-relevant measures (accuracy, steadiness, and reproducibility). Task-irrelevant sources of variability in individual finger force profiles within and between trials (flexibility and multiformity) were also quantified. The IH in the stroke survivors showed deficits in motor performance attributed mainly to lower accuracy and reproducibility as compared to control hands (p < 0.05). The LH in stroke survivors showed lower reproducibility and both hands in stroke also had higher multiformity than the control hands (p < 0.05). The findings from our HVD model suggest that accuracy, reproducibility, and multiformity were mainly impaired during force-matching task in the stroke survivors. The specific motor deficits identified through the HVD model with the new conceptual framework may be considered as critical factors for scientific investigation on stroke and evidence-based rehabilitation of this population.

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.

Relationship of diminished interjoint coordination after stroke to hand path consistency

Differences between 12 left-brain (LCVA, 65.4 ± 11.7 years old) and 10 right-brain (RCVA, 61 ± 12.1 years old) chronic stroke survivors and 10 age-matched control adults in coordinating specific joint motions of the arm to stabilize hand path when reaching to a central target were investigated in this study. The importance of coordinating joints to stabilize hand path was tested by comparing results from uncontrolled manifold (UCM) analysis performed on experimental data versus simulated data where the covariation (coordination) between particular joint motions was removed from the original data set. UCM analysis allowed estimation of the joint configuration variance magnitude that led to hand path variability (V ORT), where the extent of increase in V ORT after removing a joint's covariation indicated how well coordinated its motion actually was with those of the other joints. The more strongly coordinated a joint was with other joints, the greater effect removal of its covariance should have on indices of hand path stability. For the paretic arm of stroke survivors, simulated removal of a joint's covariation, mainly that of shoulder with elbow and wrist, led to less change in the magnitude of V ORT compared to the same arm of control subjects. These findings confirm a reduced ability of the motion of proximal joint from paretic arm to combine flexibly with motions of the distal joints to stabilize hand path.

Novel Kinetic Strategies Adopted in Asymmetric Split-Belt Treadmill Walking

The hip and ankle strategies that affect learning of a novel gait have not been fully determined, and could be of importance in design of clinical gait interventions. The authors' purpose was to determine the effects of asymmetric split-belt treadmill walking on ankle and hip work during propulsion. Participants were randomized into either a gradual training group or a sudden training group and later returned for a retention test. The gradual training group performed significantly more work at the hip joint of the slow limb during acquisition, and decreased the hip joint work performed during retention. These findings reveal the hip joint on the slow limb during initial swing as a possible site of adaptation to a novel locomotor pattern.

Carry-over coarticulation in joint angles

Coarticulation indicates a dependence of a movement segment on a preceding segment (carry-over coarticulation) or on the segment that follows (anticipatory coarticulation). Here we study coarticulation in multidegrees of freedom human arm movements. We asked participants to transport a cylinder from a starting position to a center target and on to a final target. In this naturalistic setting, the human arm has ten degrees of freedom and is thus comfortably redundant for the task. We studied coarticulation by comparing movements between the same spatial locations that were either preceded by different end-effector paths (carry-over coarticulation) or followed by different end-effector paths (anticipatory coarticulation). We found no evidence for coarticulation at the level of the end-effector. We found very clear evidence, however, for carry-over, not for anticipatory coarticulation at the joint level. We used the concept of the uncontrolled manifold to systematically establish coarticulation as a form of motor equivalence, in which most of the difference between different movement contexts lies within the uncontrolled manifold that leaves the end-effector invariant. The findings are consistent with movement planning occurring at the level of the end-effector, and those movement plans being transformed to the joint level by a form of inverse kinematics. The observation of massive self-motion excludes an account that is solely based on a kinematic pseudo-inverse.

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.

Robotic exoskeletons: a perspective for the rehabilitation of arm coordination in stroke patients

Upper-limb impairment after stroke is caused by weakness, loss of individual joint control, spasticity, and abnormal synergies. Upper-limb movement frequently involves abnormal, stereotyped, and fixed synergies, likely related to the increased use of sub-cortical networks following the stroke. The flexible coordination of the shoulder and elbow joints is also disrupted. New methods for motor learning, based on the stimulation of activity-dependent neural plasticity have been developed. These include robots that can adaptively assist active movements and generate many movement repetitions. However, most of these robots only control the movement of the hand in space. The aim of the present text is to analyze the potential of robotic exoskeletons to specifically rehabilitate joint motion and particularly inter-joint coordination. First, a review of studies on upper-limb coordination in stroke patients is presented and the potential for recovery of coordination is examined. Second, issues relating to the mechanical design of exoskeletons and the transmission of constraints between the robotic and human limbs are discussed. The third section considers the development of different methods to control exoskeletons: existing rehabilitation devices and approaches to the control and rehabilitation of joint coordinations are then reviewed, along with preliminary clinical results available. Finally, perspectives and future strategies for the design of control mechanisms for rehabilitation exoskeletons are discussed.

Co-activation of upper limb muscles during reaching in post-stroke subjects: an analysis of the contralesional and ipsilesional limbs

The purpose of this study was to analyze the change in antagonist co-activation ratio of upper-limb muscle pairs, during the reaching movement, of both ipsilesional and contralesional limbs of post-stroke subjects. Nine healthy and nine post-stroke subjects were instructed to reach and grasp a target, placed in the sagittal and scapular planes of movement. Surface EMG was recorded from postural control and movement related muscles. Reaching movement was divided in two sub-phases, according to proximal postural control versus movement control demands, during which antagonist co-activation ratios were calculated for the muscle pairs LD/PM, PD/AD, TRIlat/BB and TRIlat/BR. Post-stroke's ipsilesional limb presented lower co-activation in muscles with an important role in postural control (LD/PM), comparing to the healthy subjects during the first sub-phase, when the movement was performed in the sagittal plane (p<0.05). Conversely, the post-stroke's contralesional limb showed in general an increased co-activation ratio in muscles related to movement control, comparing to the healthy subjects. Our findings demonstrate that, in post-stroke subjects, the reaching movement performed with the ipsilesional upper limb seems to show co-activation impairments in muscle pairs associated to postural control, whereas the contralesional upper limb seems to have signs of impairment of muscle pairs related to movement.

Diminished joint coordination with aging leads to more variable hand paths

Differences in joint coordination between arms and due to aging were studied in healthy young and older adults reaching to either a fixed, central target or to the same target when it could unexpectedly change location after reach initiation. Joint coordination was investigated by artificially removing the covariation of each joint's motions with other joints' motions. Uncontrolled manifold analysis was used to partition joint configuration variance into variance reflecting motor abundance (VUCM) and variance causing hand path variability (VORT). The extent to which VORT, related to the consistency of the hand path, increased after removing a joint's covariation indicated the strength of its coordination with other joints. Young adults exhibited stronger indices of joint coordination, evidenced by a larger increase in VORT after removing joint covariation than for older adults. This effect was more striking for the dominant right compared to the left arm for young adults, but not for older adults, especially with target uncertainty. The results indicate that interjoint coordination in young adults leads to less hand path variability compared to older adults.

Temporal structure of variability decreases in upper extremity movements post stroke

BACKGROUND

The objective of this study was to determine movement variability in the more-affected upper-extremity in chronic stroke survivors. We investigated two hypotheses: (1) individuals with stroke will have increased amount of variability and altered structure of variability in upper-extremity joint movement patterns as compared to age-matched controls; and (2) the degree of motor impairment and joint kinematics will be correlated with the temporal structure of variability.

METHODS

Sixteen participants with chronic stroke and nine age-matched controls performed three trials of functional reach-to-grasp. The amount of variability was quantified by computing the standard deviation of shoulder, elbow, wrist and index finger flexion/extension joint angles. The temporal structure of variability was determined by calculating approximate entropy in shoulder, elbow, wrist and index finger flexion/extension joint angles.

FINDINGS

Individuals with stroke demonstrated greater standard deviations and significantly reduced approximate entropy values as compared to controls. Furthermore, motor impairments and kinematics demonstrated moderate to strong correlations with temporal structure of variability.

INTERPRETATION

Changes in the temporal structure of variability in upper-extremity joint angles suggest that movement patterns used by stroke survivors are less adaptable. This knowledge may yield additional insights into the impaired motor system and suggest better interventions that can enhance upper-extremity movement adaptability.

Developing functional workspace for the movement of trunk circumduction in healthy young subjects: a reliability study

Background

The lumbar range of motion has traditionally been used to assess disability in patients with low back disorders. Controversy exists about how movement ranges in static positions or in a single straight plane is related to the functional status of the patients. The trunk circumduction, as the result of neuromuscular coordination, is the integrated movements from three dimensions. The functional workspace stands for the volume of movement configuration from the trunk circumduction and represents all possible positions in three dimensions. By using single quantitative value, the functional workspace substitutes the complicated joint linear or angular motions. The aim of this study is to develop the functional workspace of the trunk circumduction (FWTC) considering possible functional positions in three dimensional planes. The reliability of the trunk circumduction is examined.

Methods

Test-retest reliability was performed with 18 healthy young subjects. A three-dimensional (3-D) Motion Analysis System was used to record the trunk circumduction. The FWTC was defined and calculated based on the volume of the cone that was formed as the resultant scanned area of markers, multiplied by the length of the body segment. The statistical analysis of correlation was performed to describe the relation of maximal displacements of trunk circumduction and straight planes: sagittal and coronal.

Results

The results of this study indicate that the movement of trunk circumduction measured by motion analysis instruments is a reliable tool. The ICC value is 0.90-0.96, and the means and standard deviations of the normalized workspace are: C7 0.425 (0.1162); L1 0.843 (0.2965); and knee 0.014 (0.0106). Little correlations between the maximal displacement of trunk circumduction and that of straight planes are shown and therefore suggest different movement patterns exist.

Conclusions

This study demonstrates high statistical reliability for the FWTC, which is important for the potential development as the functional assessment technique. The FWTC provides a single integrated value to represent angular and linear measurements of different joints and planes. Future study is expected to carry out the FWTC to evaluate the amount of workspace for the functional status of patients with low back injuries or patients with spinal surgery.

Constraining upper limb synergies of hemiparetic patients using a robotic exoskeleton in the perspective of neuro-rehabilitation

The aim of this paper was to explore how an upper limb exoskeleton can be programmed to impose specific joint coordination patterns during rehabilitation. Based on rationale which emphasizes the importance of the quality of movement coordination in the motor relearning process, a robot controller was developed with the aim of reproducing the individual corrections imposed by a physical therapist on a hemiparetic patient during pointing movements. The approach exploits a description of the joint synergies using principal component analysis (PCA) on joint velocities. This mathematical tool is used both to characterize the patient's movements, with or without the assistance of a physical therapist, and to program the exoskeleton during active-assisted exercises. An original feature of this controller is that the hand trajectory is not imposed on the patient: only the coordination law is modified. Experiments with hemiparetic patients using this new active-assisted mode were conducted. Obtained results demonstrate that the desired inter-joint coordination was successfully enforced, without significantly modifying the trajectory of the end point.

Timing variability of reach trajectories in left versus right hemisphere stroke

This study investigated trajectory timing variability in right and left stroke survivors and healthy controls when reaching to a centrally located target under a fixed target condition or when the target could suddenly change position after reach onset. Trajectory timing variability was investigated with a novel method based on dynamic programming that identifies the steps required to time warp one trial's acceleration time series to match that of a reference trial. Greater trajectory timing variability of both hand and joint motions was found for the paretic arm of stroke survivors compared to their non-paretic arm or either arm of controls. Overall, the non-paretic left arm of the LCVA group and the left arm of controls had higher timing variability than the non-paretic right arm of the RCVA group and right arm of controls. The shoulder and elbow joint warping costs were consistent predictors of the hand's warping cost for both left and right arms only in the LCVA group, whereas the relationship between joint and hand warping costs was relatively weak in control subjects and less consistent across arms in the RCVA group. These results suggest that the left hemisphere may be more involved in trajectory timing, although the results may be confounded by skill differences between the arms in these right hand dominant participants. On the other hand, arm differences did not appear to be related to differences in targeting error. The paretic left arm of the RCVA exhibited greater trajectory timing variability than the paretic right arm of the LCVA group. This difference was highly correlated with the level of impairment of the arms. Generally, the effect of target uncertainty resulted in slightly greater trajectory timing variability for all participants. The results are discussed in light of previous studies of hemispheric differences in the control of reaching, in particular, left hemisphere specialization for temporal control of reaching movements.

Unpredictable elbow joint perturbation during reaching results in multijoint motor equivalence

Motor equivalence expresses the idea that movement components reorganize in the face of perturbations to preserve the value of important performance variables, such as the hand's position in reaching. A formal method is introduced to evaluate this concept quantitatively: changes in joint configuration due to unpredictable elbow perturbation lead to a smaller change in performance variables than expected given the magnitude of joint configuration change. This study investigated whether motor equivalence was present during the entire movement trajectory and how magnitude of motor equivalence was affected by constraints imposed by two different target types. Subjects pointed to spherical and cylindrical targets both with and without an elbow joint perturbation produced by a low- or high-stiffness elastic band. Subjects' view of their arm was blocked in the initial position, and the perturbation condition was randomized to avoid prediction of the perturbation or its magnitude. A modification of the uncontrolled manifold method variance analysis was used to investigate how changes in joint configuration on perturbed vs. nonperturbed trials (joint deviation vector) affected the hand's position or orientation. Evidence for motor equivalence induced by the perturbation was present from the reach onset and increased with the strength of the perturbation after 40% of the reach, becoming more prominent as the reach progressed. Hand orientation was stabilized more strongly by motor equivalent changes in joint configuration than was three-dimensional position regardless of the target condition. Results are consistent with a recent model of neural control that allows for flexible patterns of joint coordination while resisting joint configuration deviations in directions that affect salient performance variables. The observations also fit a general scheme of synergic control with referent configurations defined across different levels of the motor hierarchy.

Weakening of synergist muscle coupling during reaching movement in stroke patients

BACKGROUND

After hemiparetic stroke, coordination of the shoulder flexor and elbow extensor muscles during a reaching movement is impaired and contributes to poor performance.

OBJECTIVE

The aim was to determine whether functional coupling between electromyographic signals of synergist muscles during reaching was weakened in stroke patients who had poor motor coordination.

METHODS

Surface electromyography (EMG) from the anterior deltoid, triceps brachii, biceps brachii, pectoralis major, supraspinatus, and latissimus dorsi of the affected upper limb in 11 stroke patients (mean Fugl-Meyer upper extremity score 27 ± 8) and in the dominant arm of 8 healthy controls were measured.

RESULTS

Coherence between the EMG of the anterior deltoid and triceps brachii, 2 synergists for reaching, was lower in patients compared with controls, in the 0- to 11-Hz range. Detailed segmented frequency-range analysis indicated significant differences in the coherence between groups in 0- to 3.9-Hz and 4- to 7.9-Hz ranges.

CONCLUSIONS

This weakened functional coupling may contribute to poor reaching performance and could be a consequence of a loss of common drive at the frequency bands as a result of interruption of information flow in the corticospinal pathway.

The complexity of childhood development: variability in perspective

The complexity of childhood development is exemplified in the variability of development that is seen across tasks and individuals. Furthermore, variability in performance is omnipresent within individuals across repetitions of a task and across individuals performing the same task. Previously, this variability was thought to reflect error of measurement or error of execution. On this account, variability reflects noise that should be filtered or averaged out of the data in order to reveal the "true" underlying characteristics of the performance. Although errors of measurement and execution indeed contribute to variability in movements, research in the last 2 decades has revealed characteristics of variability that are far more interesting than just noise. These characteristics can be deeply informative about underlying control processes and point to directions for clinical practice. This perspective article reviews different ways of characterizing variability, illustrates changes in variability as a result of development and learning, and discusses different theoretical perspectives on the role of variability that give clues about how to understand changes in variability and how to deal with variability in clinical settings.

Coordinate dependence of variability analysis

Analysis of motor performance variability in tasks with redundancy affords insight about synergies underlying central nervous system (CNS) control. Preferential distribution of variability in ways that minimally affect task performance suggests sophisticated neural control. Unfortunately, in the analysis of variability the choice of coordinates used to represent multi-dimensional data may profoundly affect analysis, introducing an arbitrariness which compromises its conclusions. This paper assesses the influence of coordinates. Methods based on analyzing a covariance matrix are fundamentally dependent on an investigator's choices. Two reasons are identified: using anisotropy of a covariance matrix as evidence of preferential distribution of variability; and using orthogonality to quantify relevance of variability to task performance. Both are exquisitely sensitive to coordinates. Unless coordinates are known a priori, these methods do not support unambiguous inferences about CNS control. An alternative method uses a two-level approach where variability in task execution (expressed in one coordinate frame) is mapped by a function to its result (expressed in another coordinate frame). An analysis of variability in execution using this function to quantify performance at the level of results offers substantially less sensitivity to coordinates than analysis of a covariance matrix of execution variables. This is an initial step towards developing coordinate-invariant analysis methods for movement neuroscience.

Motor control theories and their applications

We describe several influential hypotheses in the field of motor control including the equilibrium-point (referent configuration) hypothesis, the uncontrolled manifold hypothesis, and the idea of synergies based on the principle of motor abundance. The equilibrium-point hypothesis is based on the idea of control with thresholds for activation of neuronal pools; it provides a framework for analysis of both voluntary and involuntary movements. In particular, control of a single muscle can be adequately described with changes in the threshold of motor unit recruitment during slow muscle stretch (threshold of the tonic stretch reflex). Unlike the ideas of internal models, the equilibrium-point hypothesis does not assume neural computations of mechanical variables. The uncontrolled manifold hypothesis is based on the dynamic system approach to movements; it offers a toolbox to analyze synergic changes within redundant sets of elements related to stabilization of potentially important performance variables. The referent configuration hypothesis and the principle of abundance can be naturally combined into a single coherent scheme of control of multi-element systems. A body of experimental data on healthy persons and patients with movement disorders are reviewed in support of the mentioned hypotheses. In particular, movement disorders associated with spasticity are considered as consequences of an impaired ability to shift threshold of the tonic stretch reflex within the whole normal range. Technical details and applications of the mentioned hypo-theses to studies of motor learning are described. We view the mentioned hypotheses as the most promising ones in the field of motor control, based on a solid physical and neurophysiological foundation.

Coordination changes demonstrated by subjects with hemiparesis performing hand-arm training using the NJIT-RAVR robotically assisted virtual rehabilitation system

Various authors have described pre and post testing improvements in upper limb coordination as a result of intensive upper limb interventions. While the ability to alter coordination patterns as a result of repetitive hand-arm movement is established, patterns of change in the relationship between proximal and distal effectors of the UE over the course of multiple sessions of training have not been described in the rehabilitation literature. In this study eight subjects (5 male, 3 female) with a mean age of 56.4 years (SD=14.2) and a mean time since CVA of 54.7 months post-stroke (SD=51.7) were trained for eight, 2-3 hour sessions on four robotically facilitated virtual rehabilitation activities. This paper will present 1) Functional performance and pre and post testing kinematic analysis for the eight subjects 2) More extensive analysis of the change in hand and arm coordination over the course of the eight session intervention demonstrated by one of the subjects from this sample 3) Kinematic analysis of another subject from this sample performing an un-trained reaching and grasping activity, before and after training.

Synergies during reach-to-grasp: a comparative study between healthy and C6-C7 quadriplegic subjects

Analysis of functional capacities modifications after spinal cord injuries may have an interest in clinical rehabilitation programs. In this framework, this study is focused on the analysis of motor synergies i.e. how the upper-limb degrees of freedom (DOFS) are organized when considering C6-C7 quadriplegic subjects performing a reaching task. A previous work has shown that sharing patterns between DOFS are preserved. Using the uncontrolled manifold hypothesis (UCM), we investigate the second aspect of motor synergies i.e. how performance is allowed to show both stability against perturbations and flexibility to solve concurrent tasks. This procedure allows to evaluate the stabilization of a specific and selective performance variable through the variability of joint angles. In the present study, the wrist 3D path was considered and the variability of its trajectory is decomposed into two parts: one which does not affect the value of this performance variable (goal equivalent variance or GEV) and one which does (non goal equivalent variance or NGEV). Results showed that quadriplegic subjects present the same variance decomposition than healthy subjects suggesting that, despite the motor deficiency, flexibility is still allowed during movement execution.