The Role of Extra-motor Networks in Upper Limb Motor Performance Post-stroke.

BACKGROUND: Motor improvement post-stroke may happen even if resting state functional connectivity between the ipsilesional and contralesional components of the sensorimotor network is not fully recovered. Therefore, we investigated which extra-motor networks might support upper limb motor gains in response to treatment post-stroke. METHODS: Both resting state functional connectivity and upper limb capacity were measured prior to and after an 8-week intervention of task-specific training in 29 human participants [59.24 ± (SD) 10.40 yrs., 12 females and 17 males] with chronic stroke. The sensorimotor and five extra-motor networks were defined: default mode, frontoparietal, cingulo-opercular, dorsal attention network, and salience networks. The Network Level Analysis toolbox was used to identify network pairs whose connectivities were enriched in connectome-behavior relationships. RESULTS: Mean upper limb capacity score increased 5.45 ± (SD) 5.55 following treatment. Baseline connectivity of some motor but mostly extra-motor network interactions of cingulo-opercular and default-mode networks were predictive of upper limb capacity following treatment. Also, changes in connectivity for extra-motor interactions of salience with default mode, cingulo-opercular, and dorsal attention networks were correlated with gains in upper limb capacity. CONCLUSIONS: These connectome-behavior patterns suggest larger involvement of cingulo-opercular networks in prediction of treatment response and of salience networks in maintenance of improved skilled behavior. These results support our hypothesis that cognitive networks may contribute to recovery of motor performance after stroke and provide additional insights into the neural correlates of intensive training.

Effects of Age and Attentional Focus on the Performance and Coordination of the Sit-to-Stand Task.

This study investigated whether age and attentional focus affect synergy organization of sit-to-stand (STS). Young and older adults performed STS while holding a cup under internal (IF) and external focus (EF) instructions. Uncontrolled manifold analysis was used to decompose trial-to-trial variability in joint kinematics into variability that preserves (VUCM) and interferes (VORT) with the horizontal and vertical positions of the center of mass (CoM) and cup. VUCM was significantly higher than VORT for all variables in both age groups and focus conditions. Older adults demonstrated higher VUCM for all variables and higher VORT for all variables except the vertical position of the cup. IF instructions benefited older adults, leading to decreased VORT of the vertical position of CoM and horizontal and vertical positions of the cup.

White matter integrity of contralesional and transcallosal tracts may predict response to upper limb task-specific training in chronic stroke.

OBJECTIVE: To investigate white matter (WM) plasticity induced by intensive upper limb (UL) task specific training (TST) in chronic stroke. METHODS: Diffusion tensor imaging data and UL function measured by the Action Research Arm Test (ARAT) were collected in 30 individuals with chronic stroke prior to and after intensive TST. ANOVAs tested the effects of training on the entire sample and on the Responders [ΔARAT ≥ 5.8, N = 13] and Non-Responders [ΔARAT < 5.8, N = 17] groups. Baseline fractional anisotropy (FA) values were correlated with ARATpost TST controlling for baseline ARAT and age to identify voxels predictive of response to TST. RESULTS: While ARAT scores increased following training (p < 0.0001), FA changes within major WM tracts were not significant at p < 0.05. In the Responder group, larger baseline FA of both contralesional (CL) and transcallosal tracts predicted larger ARAT scores post-TST. Subcortical lesions and more severe damage to transcallosal tracts were more pronounced in the Non-Responder than in the Responder group. CONCLUSIONS: The motor improvements post-TST in the Responder group may reflect the engagement of interhemispheric processes not available to the Non-Responder group. Future studies should clarify differences in the role of CL and transcallosal pathways as biomarkers of recovery in response to training for individuals with cortical and subcortical stroke. This knowledge may help to identify sources of heterogeneity in stroke recovery, which is necessary for the development of customized rehabilitation interventions.

Attention Focus Does Not Influence Performance of Sit-to-Stand in Young and Older Adults.

An external focus of attention can improve performance, but there is little research on effects for the elderly in every day, well-learned mobility tasks. 57 older and 59 young adults performed the sit-to-stand and stand-to-sit while holding a cup, at three difficulty levels (cup empty or full, at normal or fast speed). Half were instructed to focus internally (on their movements) and half externally (on the cup). The effects of focus, age, and difficulty level were tested for movement time, mean inclination of the cup, inclination variability, and smoothness with 2 × 2 × 3 ANOVAs. Significant effects of difficulty were consistent across variables (p < 0.05). An effect of focus was present only for the inclination variability of the stand-to-sit (p < 0.03), favoring an internal focus (less variability). The age × focus interaction was significant for mean cup inclination, but post hoc tests failed to reveal any significant differences. The results of this study, together with the literature, suggest that an external focus may not benefit the performance of young or older adults in general mobility activities of daily living. The prevalent assumption that an external focus is always beneficial for performance needs further empirical testing.

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' (VUCM) and 'bad' (VORT) 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 VUCM, VORT, 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 VUCM and VORT. 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 VUCM or decrease in VORT, 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.

Grasping with a new hand: Improved performance and normalized grasp-selective brain responses despite persistent functional changes in primary motor cortex and low-level sensory and motor impairments.

Hand loss can now be reversed through surgical transplantation years or decades after amputation. Remarkably, these patients come to use their new hand to skilfully grasp and manipulate objects. The brain mechanisms that make this possible are unknown. Here we test the hypothesis that the anterior intraparietal cortex (aIPC) - a multimodal region implicated in hand preshaping and error correction during grasping - plays a key role in this compensatory grasp control. Motion capture and fMRI are used to characterize hand kinematics and brain responses during visually guided grasping with a transplanted hand at 26 and 41 months post-transplant in patient DR, a former hand amputee of 13 years. Compared with matched controls, DR shows increasingly normal grasp kinematics paralleled by increasingly robust grasp-selective fMRI responses within the very same brain areas that show grasp-selectivity in controls, including the aIPC, premotor and cerebellar cortices. Paradoxically, over this same time DR exhibits significant limitations in basic sensory and motor functions, and persistent amputation-related functional reorganization of primary motor cortex. Movements of the non-transplanted hand positively activate the ipsilateral primary motor hand area - a functional marker of persistent interhemispheric amputation-related reorganization. Our data demonstrate for the first time that even after more than a decade of living as an amputee the normative functional brain organization governing the control of grasping can be restored. We propose that the aIPC and interconnected premotor and cerebellar cortices enable grasp normalization by compensating for the functional impact of reorganizational changes in primary sensorimotor cortex and targeting errors in regenerating peripheral nerves.

Systematic, Unintended Drifts in the Cyclic Force Produced with the Fingertips.

Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and in opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at various initial amplitudes and frequencies. We hypothesized that on feedback removal, the amplitude will be attracted toward a preferred value that is frequency dependent. We found that the amplitude always increased after feedback removal. The magnitude of the amplitude increase changed with initial frequency, but it was invariant over the explored range of initial amplitudes. Thus, the existence of a preferred amplitude of force oscillations was not supported. We interpret these results within the referent configuration and the referent configuration back-coupling hypotheses. These data will inform a mathematical model of finger-force drifts. However, currently, they raise more questions than they answer, and a coherent account of finger-force drifts remains a challenge.

Changes in Multidigit Synergies and Their Feed-Forward Adjustments in Multiple Sclerosis.

The authors explored the changes in multidigit synergies in patients with multiple sclerosis (MS) within the framework of the uncontrolled manifold hypothesis. The specific hypotheses were 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 MS patients and age-matched controls (n = 13 in both groups) performed one-finger and multifinger force production tasks involving both accurate steady-state force production and quick force pulses. The patients showed significantly lower maximal finger forces and a tendency toward slower force pulses. Enslaving was increased in MS, but only in the lateral fingers (index and little). Indices of multifinger synergies during steady-state force production were lower in MS, mainly due to the lower amount of intertrial variance that did not affect total force. Anticipatory synergy adjustments were significantly delayed in MS. The results show that MS leads to significant changes in multidigit synergies and feed-forward adjustments of the synergies prior to a quick action. The authors discuss possible contributions of subcortical structures to the impaired synergic control.

Unsteady steady-states: central causes of unintentional force drift.

We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely the fingertip referent coordinate (R FT) and its apparent stiffness (C FT). The system's state is defined by a point in the {R FT; C FT} space. We tested the hypothesis that, after visual feedback removal, this point (1) moves along directions leading to drop in the output fingertip force, and (2) has even greater motion along directions that leaves the force unchanged. Subjects produced a prescribed fingertip force using visual feedback and attempted to maintain this force for 15 s after the feedback was removed. We used the "inverse piano" apparatus to apply small and smooth positional perturbations to fingers at various times after visual feedback removal. The time courses of R FT and C FT showed that force drop was mostly due to a drift in R FT toward the actual fingertip position. Three analysis techniques, namely hyperbolic regression, surrogate data analysis, and computation of motor-equivalent and non-motor-equivalent motions, suggested strong covariation in R FT and C FT stabilizing the force magnitude. Finally, the changes in the two hypothetical control variables {R FT; C FT} relative to their average trends also displayed covariation. On the whole, the findings suggest that unintentional force drop is associated with (a) a slow drift of the referent coordinate that pulls the system toward a low-energy state and (b) a faster synergic motion of R FT and C FT that tends to stabilize the output fingertip force about the slowly drifting equilibrium point.

The nature of constant and cyclic force production: unintentional force-drift characteristics.

We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed constant or cyclic isometric F-tasks with the index finger under visual feedback. The cyclic task was paced by a metronome. Removing visual feedback resulted in a consistent force drop in constant F-tasks and a qualitatively similar drift in the mean force in the cyclic F-task. Both were slow with characteristic times of about 10-20 s. In contrast, force amplitude in the cyclic F-task increased quickly (within 1-2 s). When the subjects were asked to stop producing force for 5 s after the visual feedback disappeared and then resume force production, no downward force drift was seen in constant F-tasks, while in cyclic F-tasks, the drift of the mean force was present and an exaggerated increase in force amplitude was also observed. We conclude that while working memory limitations may influence cyclic F-tasks, their role in determining the force drift in constant F-tasks is limited. The results of both experiments are interpreted within the referent configuration hypothesis supplemented with an idea of unintentional drift of referent coordinates (RC-back-coupling) induced by differences between the referent and actual body configurations.

Motor equivalence during multi-finger accurate force production.

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

Motor equivalence (ME) during reaching: is ME observable at the muscle level?

The concept of motor equivalent combinations of arm muscles, or M-modes, was investigated during reaching to insert a pointer into a cylindrical target with and without an elbow perturbation. Five M-modes across 15 arm/scapula muscles were identified by principal component analysis with factor extraction. The relationship between small changes in the M-modes and changes in the position/orientation of the pointer were investigated by linear regression analyses. The results revealed a motor equivalent organization of the M-modes for perturbed compared with non-perturbed reaches, both with respect to hand position and orientation, especially in the first 100-ms postperturbation. Similar findings were obtained for motor equivalence computed based on changes in the joint configuration, although the kinematically defined motor equivalence was stronger for pointer orientation. The results support the hypothesis that the nervous system organizes muscles into M-modes and flexibly scales M-mode activation to preserve stable values of variables directly related to performance success.

Effect of fatigue on grip force control during object manipulation in carpal tunnel syndrome.

Eight subjects with carpal tunnel syndrome (CTS) (47.13 ± 7.83 years) and 8 matched controls (46.29 ± 7.27 years) manipulated a test object fitted with an accelerometer and force sensor, both before and after hand muscle fatigue. Grip force and object acceleration were recorded and used to calculate grip force control variables that included Grip Force Peak, Safety Margin, and Time to Grip Force Peak. Individuals with CTS exhibited a higher Safety Margin (p = .010) and longer Time to Peak of Grip Force (p = .012) than healthy controls during object manipulation. Once fatigued, both groups significantly decreased their grip force to perform the task (Grip Force Peak; p = .017 and Safety Margin; p < .001). Nevertheless, individuals with CTS maintained an unnecessarily high safety margin. Our results suggest that CTS can adversely affect how the central nervous system regulates grip force, which might aggravate the inflammatory process and exacerbate the symptoms of this disease.

Validity of a new stabilometric force platform for postural balance evaluation / Validade de uma nova plataforma de forças estabilométrica para avaliação do equilíbrio postural

The objective of this study was to validate a new stabilometric force platform (SFP). For this, three steps have been established: a) to determine the force threshold to reach an acceptable level of accuracy of the centre of pressure (CoP) measurement by the application of single point load; b) to determine the accuracy of the CoP measurement in the application of distributed load simulating the human feet; c) to verify the concurrent validity of the SFP by comparing it with a commercial force platform (FP). The tests performed in steps "a" and "b" were conducted by applying loads on the SFP using a universal testing machine. In the application of single point load, the mean force threshold presented by the SFP was 315.6 ± 140.5 N. The CoP measurement error in the points near the centre of the SFP was 1.04 ± 0.80 mm in medial-lateral (ML) and 1.31 ± 0.99 mm in anterior-posterior (AP) direction. In the points near the edges of the plate, the error was 2.03 ± 0.91 mm (ML) and 1.54 ± 0.96 mm (AP). In the test with distributed loads, errors of less than 1 mm were found. Additionally, no differences were found in the CoP parameters between SFP and the FP. The CoP measurement signal presented high correlation between both equipments in AP (r = 0.997 ± 0.001) and ML (r = 0.988 ± 0.003) directions. These findings suggest that the SFP can be used in scientific investigations of balance in quiet standing.

O objetivo deste estudo foi validar uma nova plataforma de forças estabilométrica (PFE). Para isso, três etapas foram estabelecidas: a) determinar o limiar de carga para chegar a um nível aceitável de exatidão da medida do centro de pressão (CP) pela aplicação de cargas pontuais; b) determinar a exatidão da medida do CP na aplicação de cargas distribuídas que simulam os pés humanos; c) verificar a validade concorrente da PFE comparando-a com uma plataforma de forças comercial (PF). Os testes das etapas "a" e "b" foram realizados pela aplicação de cargas sobre a PFE, utilizando uma máquina de ensaios universal. Na etapa de aplicação de carga pontual, a média do limiar de carga apresentado pela PFE foi de 315.6 ± 140.5 N. Os erros de medida do CP nos pontos próximos ao centro da PFE foram de 1.04 ± 0.80 mm na direção medio-lateral (ML) e 1.31 ± 0.99 mm na direção ântero-posterior (AP). Nos pontos próximos aos cantos da chapa, foram encontrados erros de 2.03 ± 0.91 mm (ML) e 1.54 ± 0.96 mm (AP). No teste com cargas distribuídas, os erros foram menores que 1 mm. Adicionalmente, não foram encontradas diferenças nos parâmetros do CP entre a PFE e a PF. O sinal do CP apresentou alta correlação entre os dois equipamentos, tanto na direção AP (r = 0.997 ± 0,001) quanto na direção ML (r = 0.988 ± 0,003). Os resultados sugerem que a PFE pode ser utilizada em estudos científicos do equilíbrio em postura ereta.

Validity of a new contact mat system for evaluating vertical jump / Validade de um novo tapete de contato para avaliação do salto vertical

The purpose of this study was to verify the validity of the flight and ground contact time measurement of a contact mat, SaltoBras (SB), comparing it to an oscilloscope (OS) and to a force plate (FP). For that, the SB was placed upon a FP. At first, four male and two female performed 15 jumps on the SB and the flight times were obtained. After, they performed six consecutive jumps and the ground contact times were obtained. SB software was valid in comparison with the OS, but underestimated the flight (~0.30 %) and ground contact (~1.01 %) time measurements when comparing it to the FP. Despite the differences, the error found between SB and FP was systematic, and two prediction equations were defined and added to the software for correction. The results suggest that SB is a valid instrument for the evaluation of vertical jump.

Este estudo teve como objetivo verificar a validade da medida de tempo de vôo e de contato de um tapete de contato, SaltoBras (SB), comparando-o com um osciloscópio (OS) e uma plataforma de forças (PF). Para isso o SB foi colocado sobre uma plataforma de forças. Primeiramente, quatro homens e duas mulheres realizaram quinze saltos sobre o SB e o tempo de vôo foi obtido. Depois, os mesmos realizaram seis saltos consecutivos e os tempos de contato foram obtidos. O SB apresentou-se valido em relação ao OS, mas subestimou o tempo de vôo (~0.30 %) e de contato (~1.01 %) quando comparado a PF. Apesar das diferenças, o erro encontrado entre SB e PF apresentou-se sistemático e duas equações de predição foram definidas e inseridas no software para a correção dos erros. Os resultados sugerem que SB é um instrumento válido para a avaliação do salto vertical.