Validation of a full body 3-D dynamic linked segment model

In studies related to human movement, linked segment models (LSM's) are often used to quantify forces and torques, generated in body joints. Some LSM's represent only a few body segments. Others, for instance used in studies on the control of whole body movements, include all body segments. As a consequence of the complexity of 3-dimensional (3-D) analyses, most LSM's are restricted to one plane of motion. However, in asymmetric movements this may result in a loss of relevant information. The aim of the current study was to develop and validate a 3-D LSM including all body segments. Braces with markers, attached to all body segments, were used to record the body movements. The validation of the model was accomplished by comparing the measured with the estimated ground reaction force and by comparing the torques at the lumbo-sacral joint that resulted from a bottom-up and a top-down mechanical analysis. For both comparisons, reasonable to good agreement was found. Sources of error that could not be analysed this way, were subjected to an additional sensitivity analysis. It was concluded that the internal validity of the current model is quite satisfactory.     

Attention as a mediating variable in the dynamics of bimanual coordination

The influence of focal attention on the coordination dynamics in a bimanual circle drawing task was investigated. Six right-handed and seven left-handed subjects performed bimanual circling movements, in two modes of coordination, symmetrical or asymmetrical. The frequency of movement was scaled by an auditory metronome from 1.50 Hz to 3.00 Hz in 7 steps. On each trial, subjects were required to attend either to the dominant hand, to a neutral position, or to the nondominant hand.The uniformity of the relative tangential angle was lower in asymmetrical than in symmetrical conditions, but was not influenced by the direction of attention. In the asymmetrical mode, shifts in RTA relations, suggestive of loss of stability, were evident as the movement frequency was increased. Typically, these shifts were mediated by distortions of the trajectory of the nondominant limb. When the nondominant hand was the focus of attention, movements of this hand were more circular, and temporal variability was reduced, at the cost of a greater deviation from the target frequency. Movements of the dominant hand were not affected by the direction of attention. The findings show that although directed attention acts to modify the coordination dynamics, it does so primarily at the level of the individual hands, rather then in terms of the relation between them.     

Impact of instruction on the acquisition of sequence knowledge in a sensorimotor task

We examined whether and to what extent a sequence of finger movements can be learned and transferred to the untrained hand according to the muscle homology depending on the relative salience of response locations and effectors. Participants performed a discrete sequence production task, in which they were asked to learn a sequence of either key locations or of finger movements. Each training block was followed by a transfer block in which responding with the opposite hand was required. Before the last transfer block participants received an unexpected instruction. They had to reproduce the sequence of key locations instead of the sequence of finger movements and conversely, the sequence of finger movements instead of the sequence of key locations. The results do not support the existence of a sequence representation for the order of finger movements irrespective of the hand used.     

Coordination modes in sensorimotor synchronization of whole-body movement: a study of street dancers and non-dancers

This study investigated whole-body sensorimotor synchronization (SMS) in street dancers and non-dancers. Two kinds of knee bending movement in a standing position to a metronome beat were explored in terms of stability under different movement frequencies: down-movement condition (knee flexion on the beat) and up-movement condition (knee extension on the beat). Analyses of phase relation between movement and beat revealed several distinct differences between the down- and up-movement conditions, and between dancers and non-dancers. In both groups under the up-movement condition, deviation from intended phase relation at higher beat rates, and enhanced fluctuations were observed. The deviation from intended phase relation under up-movement condition, and movement fluctuations were greater in non-dancers than in dancers. Moreover, subjective difficulty rating revealed that both groups felt that the up-movement condition was more difficult at higher beat rates. These findings suggest that down and up movements are two distinguishable coordination modes in whole-body coordination, and that street dancers have superior whole-body SMS ability.     

Interlimb coordination: Learning and transfer under different feedback conditions

The role of intrinsic and extrinsic information feedback in learning a new bimanual coordination pattern was investigated. The pattern required continuous flexion-extension movements of the upper limbs with a 90 ° phase offset. Separate groups practiced the task under one of the following visual feedback conditions: (a) blindfolded (reduced FB group), (b) with normal vision (normal FB group), or (c) with concurrent relative motion information (enhanced FB group). All groups were subjected to three different transfer test conditions at regular intervals during practice. These tests included reduced, normal vision, and enhanced vision conditions. Experiment 1 showed that the group receiving augmented information feedback about its relative motions in real-time produced the required coordination pattern more successfully than the remaining two groups, irrespective of the transfer conditions under which performance was evaluated. Experiment 2 replicated and extended the superiority of the enhanced feedback group during acquisition and retention. Experiment 3 demonstrated that successful transfer to various transfer test conditions was not a result of test-trial effects. Overall, the data suggest that the conditions that optimized performance of the coordination pattern during acquisition also optimized transfer performance.     

Understanding social motor coordination

Recently there has been much interest in social coordination of motor movements, or as it is referred to by some researchers, joint action. This paper reviews the cognitive perspective’s common coding/mirror neuron theory of joint action, describes some of its limitations and then presents the behavioral dynamics perspective as an alternative way of understanding social motor coordination. In particular, behavioral dynamics’ ability to explain the temporal coordination of interacting individuals is detailed. Two experiments are then described that demonstrate how dynamical processes of synchronization are apparent in the coordination underlying everyday joint actions such as martial art exercises, hand-clapping games, and conversations. The import of this evidence is that emergent dynamic patterns such as synchronization are the behavioral order that any neural substrate supporting joint action (e.g., mirror systems) would have to sustain.     

Human movement variability, nonlinear dynamics, and pathology: is there a connection?

Fields studying movement generation, including robotics, psychology, cognitive science, and neuroscience utilize concepts and tools related to the pervasiveness of variability in biological systems. The concept of variability and the measures for nonlinear dynamics used to evaluate this concept open new vistas for research in movement dysfunction of many types. This review describes innovations in the exploration of variability and their potential importance in understanding human movement. Far from being a source of error, evidence supports the presence of an optimal state of variability for healthy and functional movement. This variability has a particular organization and is characterized by a chaotic structure. Deviations from this state can lead to biological systems that are either overly rigid and robotic or noisy and unstable. Both situations result in systems that are less adaptable to perturbations, such as those associated with unhealthy pathological states or absence of skillfulness.     

Generalization of action knowledge following observational learning

Both observational and physical practices support the acquisition of motor skill knowledge in the form of spatiotemporal coordination patterns. The current experiment examined the extent that observation and physical practice can support the transfer of spatiotemporal knowledge and amplitude knowledge associated with motor skills. Evidence from a multijoint limb task revealed that knowledge about spatiotemporal patterns (relative phase) acquired by observers and models can be generalized exceptionally well within the trained arm (right) and across to the untrained arm (left). Transfer of relative phase occurred even when untrained combinations of joint amplitudes were required. This indicates that observation and physical practice both lead to the development of an effector-independent representation of the spatiotemporal knowledge in this task. Both observers and models showed some transfer of the relative amplitude knowledge, with observers demonstrating superior transfer for both a trained and untrained-arm transfer test, while the models were limited to positive transfer on an untrained-arm transfer test. The representation of movement amplitude knowledge is effector-independent in this task, but the use of that knowledge is constrained by the specific practice context and the linkage between the elbow and wrist.     

Contextual interference and augmented feedback: is there an additive effect for motor learning?

Learning to perform a skilled behavior is affected by the context of the practice session and the frequency of augmented feedback. We studied the combined effect of these variables in the acquisition of a ballistic, bi-directional lever movement pattern involving four different target locations as measured by performance in practice, retention, and transfer tests. Augmented feedback was presented in either an every-trial or a faded schedule during random and blocked practice. Consistent with the contextual interference effect, the blocked practice group produced lower errors in acquisition, but the random practice group outperformed the blocked practice group in both retention and transfer. In contrast, faded feedback did not have a beneficial effect on learning and degraded learning when provided during blocked practice. While the results were consistent with previous findings of random and blocked practice, they were not consistent with previous findings of reduced feedback frequencies.     

The learning of 90° continuous relative phase with and without Lissajous feedback: external and internally generated bimanual coordination

Results from recent experiments (e.g., Kovacs, Buchanan, & Shea, 2009a–b, 2010a,b) suggest that when salient visual information is presented using Lissajous plots bimanual coordination patterns typically thought to be very difficult to perform without extensive practice can be performed with remarkably low relative phase error and variability with 5 min or less of practice. However, when this feedback is removed, performance deteriorates. The purpose of the present experiment was to determine if reducing the frequency of feedback presentation will decrease the participant's reliance on the feedback and will facilitate the development of an internal representation capable of sustaining performance when the Lissajous feedback is withdrawn. The results demonstrated that reduced frequency Lissajous feedback results in very effective bimanual coordination performance on tests with Lissajous feedback available and when feedback is withdrawn. Taken together the present experiments add to the growing literature that supports the notion that salient perceptual information can override some aspects of the system's intrinsic dynamics typically linked to motor output control. Additionally, the present results suggest that the learning of both externally and internally driven bimanual coordination is facilitated by providing reduced frequency Lissajous feedback.     

Coordination of complex bimanual multijoint movements under increasing cycling frequencies: The prevalence of mirror-image and translational symmetry

The present study examined the principles underlying inter and intralimb coordination constraints during performance of bimanual elbow–wrist movements at different cycling frequencies (from 0.75 Hz to 2.50 Hz). Participants performed eight coordination tasks that consisted of a combination of in-phase (IN) and/or anti-phase (AN) coordination modes between both elbows and wrists (interlimb), with isodirectional (Iso) or non-isodirectional (NonI) coordination modes within each limb (intralimb). As expected, the principle of muscle homology (in-phase coordination), giving rise to mirror symmetrical movements with respect to the mid-sagittal plane, had a powerful influence on the quality of global coordinative behavior both between and within limbs. When this principle was violated (i.e., when the anti-phase mode was introduced in one or both joint pairs), the non-isodirectional intralimb mode exhibited a (de)stabilizing role in coordination, which became more pronounced at higher cycling frequencies. However, pattern loss with increasing cycling frequency resulted not only in convergence toward the more stable in-phase patterns with the elbows and wrists but also to the anti-phase patterns (which were associated with directional compatibility of within-limb motions). Moreover, participants generally preserved their initial mode of coordination (either in-phase or anti-phase) in the proximal joints (i.e., elbows) while shifting from anti-phase to in-phase (or vice versa) with their distal joint pair (i.e., wrists). Taken together, these findings reflect the impact of two immanent types of symmetry in bimanual coordination: mirror-image and translational symmetry.     

Implicit and explicit adjustments to extrinsic visuo-motor transformations and their age-related changes

Humans have unique abilities in using tools. The skilled and goal-directed use of a tool implies that processes of motor control can be adjusted to the transformation of the movement of a part of the body into the movement of the effective part of the tool. A common example is the transformation of a hand movement in the motion of a cursor on a computer monitor. In part the adjustments to such transformations are implicit, that is, without conscious awareness of the novel transformation and the appropriate change of one’s own movements. However, the adjustments can also be explicit and intentional. We review a series of experiments which show that implicit and explicit adjustments to a novel visuo-motor gain are additive. This finding suggests that the processes which generate different types of adjustment are functionally independent. In a second series of experiments it turned out that at older adult age explicit adjustments to novel visuo-motor transformations are impaired, whereas implicit adjustments remain unaffected across working age.     

Common functional principal components analysis: a new approach to analyzing human movement data

In many human movement studies angle-time series data on several groups of individuals are measured. Current methods to compare groups include comparisons of the mean value in each group or use multivariate techniques such as principal components analysis and perform tests on the principal component scores. Such methods have been useful, though discard a large amount of information. Functional data analysis (FDA) is an emerging statistical analysis technique in human movement research which treats the angle-time series data as a function rather than a series of discrete measurements. This approach retains all of the information in the data. Functional principal components analysis (FPCA) is an extension of multivariate principal components analysis which examines the variability of a sample of curves and has been used to examine differences in movement patterns of several groups of individuals. Currently the functional principal components (FPCs) for each group are either determined separately (yielding components that are group-specific), or by combining the data for all groups and determining the FPCs of the combined data (yielding components that summarize the entire data set). The group-specific FPCs contain both within and between group variation and issues arise when comparing FPCs across groups when the order of the FPCs alter in each group. The FPCs of the combined data may not adequately describe all groups of individuals and comparisons between groups typically use t-tests of the mean FPC scores in each group. When these differences are statistically non-significant it can be difficult to determine how a particular intervention is affecting movement patterns or how injured subjects differ from controls. In this paper we aim to perform FPCA in a manner allowing sensible comparisons between groups of curves. A statistical technique called common functional principal components analysis (CFPCA) is implemented. CFPCA identifies the common sources of variation evident across groups but allows the order of each component to change for a particular group. This allows for the direct comparison of components across groups. We use our method to analyze a biomechanical data set examining the mechanisms of chronic Achilles tendon injury and the functional effects of orthoses.     

Model for a flexible motor memory based on a self-active recurrent neural network

Using recent recurrent network architecture based on the reservoir computing approach, we propose and numerically simulate a model that is focused on the aspects of a flexible motor memory for the storage of elementary movement patterns into the synaptic weights of a neural network, so that the patterns can be retrieved at any time by simple static commands. The resulting motor memory is flexible in that it is capable to continuously modulate the stored patterns. The modulation consists in an approximately linear inter- and extrapolation, generating a large space of possible movements that have not been learned before. A recurrent network of thousand neurons is trained in a manner that corresponds to a realistic exercising scenario, with experimentally measured muscular activations and with kinetic data representing proprioceptive feedback. The network is “self-active” in that it maintains recurrent flow of activation even in the absence of input, a feature that resembles the “resting-state activity” found in the human and animal brain. The model involves the concept of “neural outsourcing” which amounts to the permanent shifting of computational load from higher to lower-level neural structures, which might help to explain why humans are able to execute learned skills in a fluent and flexible manner without the need for attention to the details of the movement.     

Contemporary theories of 1/f noise in motor control

1/f noise has been discovered in a number of time series collected in psychological and behavioral experiments. This ubiquitous phenomenon has been ignored for a long time and classical models were not designed for accounting for these long-range correlations. The aim of this paper is to present and discuss contrasted theoretical perspectives on 1/f noise, in order to provide a comprehensive overview of current debates in this domain. In a first part, we propose a formal definition of the phenomenon of 1/f noise, and we present some commonly used methods for measuring long-range correlations in time series. In a second part, we develop a theoretical position that considers 1/f noise as the hallmark of system complexity. From this point of view, 1/f noise emerges from the coordination of the many elements that compose the system. In a third part, we present a theoretical counterpoint suggesting that 1/f noise could emerge from localized sources within the system. In conclusion, we try to draw some lines of reasoning for going beyond the opposition between these two approaches.     

Artificial neural networks for analyzing inter-limb coordination: the golf chip shot

Motor control research relies on theories, such as coordination dynamics, adapted from physical sciences to explain the emergence of coordinated movement in biological systems. Historically, many studies of coordination have involved inter-limb coordination of relatively few degrees of freedom. This study looked at the high-dimensional inter-limb coordination used to perform the golf chip shot toward six different target distances. This study also introduces a visualization of high-dimensional coordination relevant within the coordination dynamics theoretical framework. A specific type of Artificial Neural Network (ANN), the Self-Organizing Map (SOM), was used for the analysis. In this study, the trajectory of consecutive best-matching nodes on the output map was used as a collective variable and subsequently fed into a second SOM which was used to create visualization of coordination stability. The SOM trajectories showed changes in coordination between movement patterns used for short chip shots and movement patterns used for long chip shots. The attractor diagrams showed non-linear phase transitions for three out of four players. The methods used in this study may offer a solution for researchers from a coordination dynamics perspective who intend to use data obtained from discrete high-dimensional movements.     

Muscle coordination and force variability during static and dynamic tracking tasks

This study examined muscular activity patterns of extensor and flexor muscles and variability of forces during static and dynamic tracking tasks using compensatory and pursuit display. Fourteen volunteers performed isometric actions in two conditions: (i) a static tracking task consisting of flexion/pronation, ulnar deviation, extension/supination and radial deviation of the wrist at 20% maximum voluntary contraction (MVC), and (ii) a dynamic tracking task aiming at following a moving target at 20% MVC in the four directions of contraction. Surface electromyography (SEMG) from extensor carpi ulnaris, extensor carpi radialis, flexor carpi ulnaris and flexor digitorum superficialis muscles and exerted forces in the transverse and sagittal plane were recorded. Normalized root mean square and mutual information (index of functional connectivity within muscles) of SEMGs and the standard deviation and sample entropy of force signals were extracted. Larger SEMG amplitudes were found for the dynamic task (p < .05), while normalized mutual information between muscle pairs was larger for the static task (p < .05). Larger size of variability (standard deviation of force) concomitant with smaller sample entropy was observed for the dynamic task compared with the static task (p < .01 for both). These findings underline a rescaling of the muscles’ respective contribution influencing force variability relying on feedback and feed-forward control strategies in relation to display modes during static and dynamic tracking tasks.     

Retrieval practice in motor learning

In this study we sought to determine whether testing promotes the generalization of motor skills during the process of encoding and/or consolidation. We used a dynamic arm movement task that required participants to reproduce a spatial-temporal pattern of elbow extensions and flexions with their dominant right arm. Generalization of motor learning was tested by the ability to transfer the original pattern (extrinsic transformation) or the mirrored pattern (intrinsic transformation) to the unpractised left arm. To investigate the testing effects during both encoding and consolidation processing, participants were administered an initial testing session during early practice before being evaluated on a post-practice testing session administered either 10 min (Testing-Encoding group) or 24 hr apart (Testing-Consolidation group), respectively. Control groups were required to perform a post-practice testing session administered after either a 10-min (Control-Encoding group) or 24-hr delay (Control-Consolidation group). The findings revealed that testing produced rapid, within-practice skill improvements, yielding better effector transfer at the 10-min testing for the Testing-Encoding group on both extrinsic and intrinsic transformation tests when compared with the Control-Encoding group. Furthermore, we found better performance for the Testing-Consolidation group at the 24-hr testing for extrinsic and intrinsic transformations of the movement pattern when compared with the Control-Consolidation group. However, our results did not indicate any significant testing advantage on the latent, between-session development of the motor skill representation (i.e., from the 10-min to the 24-hr testing). The testing benefits expressed at the 10-min testing were stabilised but did not extend during the period of consolidation. This indicates that testing contributes to the generalisation of motor skills during encoding but not consolidation.     

Non-differential return of fear in humans after a reinstatement procedure

The present experiment investigated reinstatement of fear in humans using a differential fear conditioning preparation. In this experiment, one neutral stimulus (conditioned stimulus; CS+) was paired with an aversive stimulus (unconditioned stimulus; US) during the acquisition phase, while another neutral stimulus was not (CS−). This procedure led to a difference in responding between the CS+ and the CS− (i.e., differential conditioning). After this acquisition phase, an extinction phase followed, during which both CSs were presented without the US, resulting in a decrease in differential conditioned responding. Reinstatement refers to the return of extinguished conditioned responses due to the experience of US-only trials after the extinction phase. This phenomenon was investigated by presenting half of the participants (reinstatement group) with unpredictable USs after the extinction phase. The control group did not receive these USs after the extinction procedure. The results show that return of fear was clearly apparent after the reinstating USs. This return of fear was, however, not limited to the CS+. An increase in ‘conditioned’ responding was also observed for the control stimulus. This interesting observation will be discussed against the background of a number of recent theoretical conceptualizations of reinstatement.