Interlimb transfer and generalisation of learning in the context of persistent failure to accomplish a visuomotor task.

Transfer, in which capability acquired in one situation influences performance in another is considered, along with retention, as demonstrative of effectual learning. In this regard, interlimb transfer of functional capacity has commanded particular attention as a means of gauging the generalisation of acquired capability. Both theoretical treatments and prior empirical studies suggest that the successful accomplishment of a physical training regime is required to bring about generalised changes that extend to the untrained limb. In the present study, we pose the following question: Does interlimb transfer occur if and only if the training movements are executed? We report findings from JG-an individual recruited to a larger scale trial, who presented with (unilateral) deficits of motor control. We examined whether changes in the performance of the untrained right limb arose following practice undertaken by the impaired left limb, wherein the majority of JG's attempts to execute the training task were unsuccessful. Comparison was made with a group of "control" participants drawn from the main trial, who did not practice the task. For JG, substantial gains in the performance of the untrained limb (registered 3 days, 10 days and 1 year following training) indicated that effective learning had occurred. Learning was, however, expressed principally when the unimpaired (i.e. untrained) limb was utilised to perform the task. When the impaired limb was used, marked deficiencies in movement execution remained prominent throughout.

Sensorimotor Learning: Neurocognitive Mechanisms and Individual Differences.

Here we provide an overview of findings and viewpoints on the mechanisms of sensorimotor learning presented at the 2016 Biomechanics and Neural Control of Movement (BANCOM) conference in Deer Creek, OH. This field has shown substantial growth in the past couple of decades. For example it is now well accepted that neural systems outside of primary motor pathways play a role in learning. Frontoparietal and anterior cingulate networks contribute to sensorimotor adaptation, reflecting strategic aspects of exploration and learning. Longer term training results in functional and morphological changes in primary motor and somatosensory cortices. Interestingly, re-engagement of strategic processes once a skill has become well learned may disrupt performance. Efforts to predict individual differences in learning rate have enhanced our understanding of the neural, behavioral, and genetic factors underlying skilled human performance. Access to genomic analyses has dramatically increased over the past several years. This has enhanced our understanding of cellular processes underlying the expression of human behavior, including involvement of various neurotransmitters, receptors, and enzymes. Surprisingly our field has been slow to adopt such approaches in studying neural control, although this work does require much larger sample sizes than are typically used to investigate skill learning. We advocate that individual differences approaches can lead to new insights into human sensorimotor performance. Moreover, a greater understanding of the factors underlying the wide range of performance capabilities seen across individuals can promote personalized medicine and refinement of rehabilitation strategies, which stand to be more effective than "one size fits all" treatments.

Interhemispheric inhibition of corticospinal projections to forearm muscles.

OBJECTIVE: Interhemispheric inhibition (IHI) is typically examined via responses elicited in intrinsic hand muscles. As the cortical representations of proximal and distal muscles in the upper limb are distinguished in terms of their inter-hemispheric projections, we sought to determine whether the IHI parameters established for the hand apply more generally. METHODS: We investigated IHI at 5 different conditioning stimulus (CS) intensities and a range of short-latency inter-stimulus intervals (ISIs) in healthy participants. Conditioning and test stimuli were delivered over the M1 representation of the right and left flexor carpi radialis respectively. RESULTS: IHI increased as a function of CS intensity, and was present for ISIs between 7 and 15ms. Inhibition was most pronounced for the 10ms ISI at all CS intensities. CONCLUSIONS: The range of parameters for which IHI is elicited in projections to the forearm is similar to that reported for the hand. The specific utility lies in delineation of stimulus parameters that permit both potentiation and attenuation of IHI to be assessed. SIGNIFICANCE: In light of evidence that there is a greater density of callosal projections between cortical areas that represent proximal muscles than between those corresponding to distal limb muscles, and in view of the assumption that variations in functional connectivity to which such differences give rise may have important implications for motor behavior, it is critical to determine whether processes mediating the expression of IHI depend on the effector that is studied. This issue is of further broad significance given the practical utility of movements generated by muscles proximal to the wrist in the context of upper limb rehabilitation.

Superimposed vibration confers no additional benefit compared with resistance training alone.

Eighteen participants (22-43 years) were randomly allocated to one of two groups: resistance training combined with vibration (VIB; five males, four females) or resistance training alone (CON; five males, four females). Each participant trained three sessions per week (three sets of 10 seated calf raises against a load, which was increased progressively from 75% of one repetition maximum (1RM) to 90% 1RM for 4 weeks. For the VIB group, a vibratory stimulus (30 Hz, 2.5 mm amplitude) was applied to the soles of the feet by a vibration platform. The two groups did not differ significantly with respect to the total amount of work performed during training. Both groups showed a significant increase in maximum voluntary contraction and 1RM (P<0.01) with training. There were no significant changes in measures that assessed the rate at which force was developed. Countermovement jump height increased for the CON (P<0.01) but not for the VIB group. Comparisons between the groups revealed that they did not differ significantly from one another with respect to any measure of performance, before or following training. It appears that vibration superimposed upon resistance training does not alter or augment the increase in strength induced by resistance training alone.

The role of the primary motor cortex during skill acquisition on a two-degrees-of-freedom movement task.

One can partially eliminate motor skills acquired through practice in the hours immediately following practice by applying repetitive transcranial stimulation (rTMS) over the primary motor cortex. The disruption of acquired levels of performance has been demonstrated on tasks that are ballistic in nature. The authors investigated whether motor recall on a discrete aiming task is degraded following a disruption of the primary motor cortex induced via rTMS. Participants (N = 16) maintained acquired performance levels and patterns of muscle activity following the application of rTMS, despite a reduction in corticospinal excitability. Disruption of the primary motor cortex during a consolidation period did not influence the retention of acquired skill in this type of discrete visuomotor task.

The preparation of reach-to-grasp movements in adults, children, and children with movement problems.

This study explored the use of advance information in the control of reach-to-grasp movements. The paradigm required participants to reach and grasp illuminated blocks with their right hand. Four target blocks were positioned on a table surface, two each side of the mid-saggital plane. In the complete precue condition, advance information precisely specified target location. In the partial precue condition, advance information indicated target location relative to the midsaggital plane (left or right). In the null condition, the advance information was entirely ambiguous. Participants produced fastest responses in the complete precue condition, intermediate response times in the partial condition, and the slowest responses in the null condition. This result was observed in adults and four groups of children including a group aged 4-6 years. In contrast, children with Developmental Coordination Disorder (DCD, n=11, aged 7--13 years) showed no advantage of partial precueing. Movement duration was determined by target location but was unaffected by precue condition. Movement duration was a clear function of age apart from children in the DCD group who showed equivalent movement times to those of the youngest children. These findings provide important insights into the control of reach-to-grasp movements and highlight that partial cues are exploited by children as young as 4 years but are not used in situations of abnormal development.

Neural pathways mediating bilateral interactions between the upper limbs.

The ease with which we perform tasks such as opening the lid of a jar, in which the two hands execute quite different actions, belies the fact that there is a strong tendency for the movements of the upper limbs to be drawn systematically towards one another. Mirror movements, involuntary contractions during intended unilateral engagement of the opposite limb, are considered pathological, as they occur in association with specific disorders of the CNS. Yet they are also observed frequently in normally developing children, and motor irradiation, an increase in the excitability of the (opposite) homologous motor pathways when unimanual movements are performed, is a robust feature of the mature motor system. The systematic nature of the interactions that occur between the upper limbs has also given rise to the expectation that functional improvements in the control of a paretic limb may occur when movements are performed in a bimanual context. In spite of the ubiquitous nature of these phenomena, there is remarkably little consensus concerning the neural basis of their mediation. In the present review, consideration is given to the putative roles of uncrossed corticofugal fibers, branched bilateral corticomotoroneuronal projections, and segmental networks. The potential for bilateral interactions to occur in various brain regions including the primary motor cortex, the supplementary motor area, non-primary motor areas, the basal ganglia, and the cerebellum is also explored. This information may provide principled bases upon which to evaluate and develop task and deficit-specific programs of movement rehabilitation and therapy.

Developmental changes in the response to obstacles during prehension.

Adults are proficient at reaching to grasp objects of interest in a cluttered workspace. The issue of concern, obstacle avoidance, was studied in 3 groups of young children aged 11-12, 9-10, and 7-8 years (n=6 in each) and in 6 adults aged 18-24 years. Adults slowed their movements and decreased their maximum grip aperture when an obstacle was positioned close to a target object (the effect declined as the distance between target and obstacle increased). The children showed the same pattern, but the magnitude of the effect was quite different. In contrast to the adults, the obstacle continued to have a large effect when it was some distance from the target (and provided no physical obstruction to movement).

Visual feedback alters the variations in corticospinal excitability that arise from rhythmic movements of the opposite limb.

Augmented visual feedback can have a profound bearing on the stability of bimanual coordination. Indeed, this has been used to render tractable the study of patterns of coordination that cannot otherwise be produced in a stable fashion. In previous investigations (Carson et al. 1999), we have shown that rhythmic movements, brought about by the contraction of muscles on one side of the body, lead to phase-locked changes in the excitability of homologous motor pathways of the opposite limb. The present study was conducted to assess whether these changes are influenced by the presence of visual feedback of the moving limb. Eight participants performed rhythmic flexion-extension movements of the left wrist to the beat of a metronome (1.5 Hz). In 50% of trials, visual feedback of wrist displacement was provided in relation to a target amplitude, defined by the mean movement amplitude generated during the immediately preceding no feedback trial. Motor potentials (MEPs) were evoked in the quiescent muscles of the right limb by magnetic stimulation of the left motor cortex. Consistent with our previous observations, MEP amplitudes were modulated during the movement cycle of the opposite limb. The extent of this modulation was, however, smaller in the presence of visual feedback of the moving limb (FCR omega2=0.41; ECR omega2=0.29) than in trials in which there was no visual feedback (FCR omega2=0.51; ECR omega2=0.48). In addition, the relationship between the level of FCR activation and the excitability of the homologous corticospinal pathway of the opposite limb was sensitive to the vision condition; the degree of correlation between the two variables was larger when there was no visual feedback of the moving limb. The results of the present study support the view that increases in the stability of bimanual coordination brought about by augmented feedback may be mediated by changes in the crossed modulation of excitability in homologous motor pathways.

Early alterations in serum creatine kinase and total cholesterol following high intensity eccentric muscle actions.

AIM: The purpose of this experiment was to assess the levels of muscle soreness, serum total cholesterol (TC) and creatine kinase (CK) in the first 48 hours following fatiguing eccentric exercise performed with the triceps brachii. METHODS: Eleven untrained male college students performed a total of 50 eccentric elbow extensions in 8 sets (6 x 7 and 2 x 4) with a load equal to 85% of their maximal concentric elbow extension strength. Isometric elbow extension strength, muscle soreness and circumference, and serum CK and TC concentrations were measured before, immediately after, and 2, 24 and 48 hours after the exercise. RESULTS: Statistically reliable changes in isometric strength, serum CK and TC, muscle soreness and upper arm circumference occurred within the first 48 hours following eccentric exercise. Serum TC concentrations exhibited a very rapid (within 2 hours) reduction from pre-exercise values after eccentric exercise to a relatively stable concentration of approximately 85% of baseline. CONCLUSION: These results suggest that serum TC concentration may follow the time-course of reductions in force generating capacity more closely than other biochemical markers of muscle damage.

Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb.

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.

A simple and unified approach to human voluntary movements.

In his provocative article, F. Mechsner (2004) advances the thesis that human voluntary movements are subject to "psychological" or "perceptual-cognitive" control and are thus organized "without regard to efferent patterns" (p. 355). Rather than considering in detail the experiments that he proffered by way of support, the present author discusses the degree to which that supposition has appeal on the grounds of simplicity and is defined in terms that are compatible with a unified science.

Governing coordination: behavioural principles and neural correlates.

The coordination of movement is governed by a coalition of constraints. The expression of these constraints ranges from the concrete--the restricted range of motion offered by the mechanical configuration of our muscles and joints; to the abstract--the difficulty that we experience in combining simple movements into complex rhythms. We seek to illustrate that the various constraints on coordination are complementary and inclusive, and the means by which their expression and interaction are mediated systematically by the integrative action of the central nervous system (CNS). Beyond identifying the general principles at the behavioural level that govern the mutual interplay of constraints, we attempt to demonstrate that these principles have as their foundation specific functional properties of the cortical motor systems. We propose that regions of the brain upstream of the motor cortex may play a significant role in mediating interactions between the functional representations of muscles engaged in sensorimotor coordination tasks. We also argue that activity in these "supramotor" regions may mediate the stabilising role of augmented sensory feedback.

Interaction of directional, neuromuscular and egocentric constraints on the stability of preferred bimanual coordination patterns.

We investigated how the relative direction of limb movements in external space (iso- and non-isodirectionality), muscular constraints (the relative timing of homologous muscle activation) and the egocentric frame of reference (moving simultaneously toward/away the longitudinal axis of the body) contribute to the stability of coordinated movements. In the first experiment, we attempted to determine the respective stability of isodirectional and non-isodirectional movements in between-persons coordination. In a second experiment, we determined the effect of the relative direction in external space, and of muscular constraints, on pattern stability during a within-person bimanual coordination task. In the third experiment we dissociated the effects on pattern stability of the muscular constraints, relative direction and egocentric frame of reference. The results showed that (1) simultaneous activation of homologous muscles resulted in more stable performance than simultaneous activation of non-homologous muscles during within-subject coordination, and that (2) isodirectional movements were more stable than non-isodirectional movements during between-persons coordination, confirming the role of the relative direction of the moving limbs in the stability of bimanual coordination. Moreover, the egocentric constraint was to some extent found distinguishable from the effect of the relative direction of the moving limbs in external space, and from the effect of the relative timing of muscle activation. In summary, the present study showed that relative direction of the moving limbs in external space and muscular constraints may interact either to stabilize or destabilize coordination patterns.

Bimanual aiming and overt attention: one law for two hands.

Reaching to interact with an object requires a compromise between the speed of the limb movement and the required end-point accuracy. The time it takes one hand to move to a target in a simple aiming task can be predicted reliably from Fitts' law, which states that movement time is a function of a combined measure of amplitude and accuracy constraints (the index of difficulty, ID). It has been assumed previously that Fitts' law is violated in bimanual aiming movements to targets of unequal ID. We present data from two experiments to show that this assumption is incorrect: if the attention demands of a bimanual aiming task are constant then the movements are well described by a Fitts' law relationship. Movement time therefore depends not only on ID but on other task conditions, which is a basic feature of Fitts' law. In a third experiment we show that eye movements are an important determinant of the attention demands in a bimanual aiming task. The results from the third experiment extend the findings of the first two experiments and show that bimanual aiming often relies on the strategic co-ordination of separate actions into a seamless behaviour. A number of the task specific strategies employed by the adult human nervous system were elucidated in the third experiment. The general strategic pattern observed in the hand trajectories was reflected by the pattern of eye movements recorded during the experiment. The results from all three experiments demonstrate that eye movements must be considered as an important constraint in bimanual aiming tasks.

The control and learning of patterns of interlimb coordination: past and present issues in normal and disordered control.

The present paper provides a historical note on the evolution of the behavioral study of interlimb coordination and the reasons for its success as a field of investigation in the past decades. Whereas the original foundations for this field of science were laid down back in the seventies, it has steadily grown in the past decades and has attracted the attention of various scientific disciplines. A diversity of topics is currently being addressed and this is also expressed in the present contributions to the special issue. The main theme is centered on the brain basis of interlimb coordination. On the one hand, this pertains to the study of the control and learning of patterns of interlimb coordination in clinical groups. On the other hand, basic neural approaches are being merged together with behavioral approaches to reveal the neural basis of interlimb coordination.

Coordination and movement pathology: models of structure and function.

Here we consider the role of abstract models in advancing our understanding of movement pathology. Models of movement coordination and control provide the frameworks necessary for the design and interpretation of studies of acquired and developmental disorders. These models do not however provide the resolution necessary to reveal the nature of the functional impairments that characterise specific movement pathologies. In addition, they do not provide a mapping between the structural bases of various pathologies and the associated disorders of movement. Current and prospective approaches to the study and treatment of movement disorders are discussed. It is argued that the appreciation of structure-function relationships, to which these approaches give rise, represents a challenge to current models of interlimb coordination, and a stimulus for their continued development.

Role of peripheral afference during acquisition of a complex coordination task.

It has long been supposed that the interference observed in certain patterns of coordination is mediated, at least in part, by peripheral afference from the moving limbs. We manipulated the level of afferent input, arising from movement of the opposite limb, during the acquisition of a complex coordination task. Participants learned to generate flexion and extension movements of the right wrist, of 75 degrees amplitude, that were a quarter cycle out of phase with a 1-Hz sinusoidal visual reference signal. On separate trials, the left wrist either was at rest, or was moved passively by a torque motor through 50 degrees, 75 degrees or 100 degrees, in synchrony with the reference signal. Five acquisition sessions were conducted on successive days. A retention session was conducted 1 week later. Performance was initially superior when the opposite limb was moved passively than when it was static. The amplitude and frequency of active movement were lower in the static condition than in the driven conditions and the variation in the relative phase relation across trials was greater than in the driven conditions. In addition, the variability of amplitude, frequency and the relative phase relation during each trial was greater when the opposite limb was static than when driven. Similar effects were expressed in electromyograms. The most marked and consistent differences in the accuracy and consistency of performance (defined in terms of relative phase) were between the static condition and the condition in which the left wrist was moved through 50 degrees. These outcomes were exhibited most prominently during initial exposure to the task. Increases in task performance during the acquisition period, as assessed by a number of kinematic variables, were generally well described by power functions. In addition, the recruitment of extensor carpi radialis (ECR), and the degree of co-contraction of flexor carpi radialis and ECR, decreased during acquisition. Our results indicate that, in an appropriate task context, afferent feedback from the opposite limb, even when out of phase with the focal movement, may have a positive influence upon the stability of coordination.

Factores neuro-músculo-esqueléticos en la coordinación de movimientos de pronosupinacion / Neuromuscular-skeletal constraints in the coordination of prono-supination

Objetivo: valorar la influencia de los cambios de posición del eje de rotación del antebrazo sobre la estabilidad de la coordinación de movimientos de pronosupinación y sobre los patrones de activación de algunos de los músculos implicados. Participantes: 15 sujetos voluntarios realizaron ciclos completos de pronosupinación del antebrazo a distintas frecuencias de movimiento controladas por un metrónomo (desde 1.75Hz hasta 3.5Hz).Métodos: se registró el desplazamiento angular (grados de movimiento) durante la realización de cielos completos de pronación-supinación del antebrazo con el eje de rotación: por encima, en línea o por debajo del eje longitudinal del antebrazo; y bajo 2 modos de coordinación: p) haciendo coincidir la señal auditiva con la posición de pronación máxima y s) haciendo coincidir la señal auditiva con la posición de supinación máxima. Los tiempos en los que se produjeron las transiciones a un modo de coordinación distinto al solicitado fueron determinados para valorar la estabilidad de cada modo de coordinación. La actividad electromiográfica de los músculos pronador redondo, bíceps braquial, palmar mayor y primer radial fue registrada en 4 sujetos. Resultados: la estabilidad del modo de coordinación pronación en la señal fue mayor cuando el eje de rotación se situó por debajo del eje longitudinal del antebrazo. Por el contrario, la estabilidad del modo de coordinación supinación en la señal fue mayor cuando el eje de rotación se situó por encima. La dominancia relativa de las fases de pronación y supinación durante la rotación del antebrazo dependió mayormente del grado de participación de los músculos palmar mayor y primer radial. Conclusión: la estabilidad de los modos de coordinación estuvo supeditada al contexto mecánico. Los cambios en el contexto mecánico alteraron los patrones de activación de los músculos que contribuyen a la pronación y supinación del antebrazo, particularmente el palmar mayor (AU)