Muscle Recruitment Strategies in a Redundant Task: Age Differences Through Network Analyses.

There are numerous studies comparing young and old adults in terms of muscle coordination in standard tasks (e.g., walking, reaching) and small variations of them. These tasks might hide differences: individuals would converge to similar behavior as they practice these throughout life. Also, we are unaware of studies that considered the muscle recruitment nested dynamics. For this reason, our study evaluated how young and old women coordinate and control the movement system while performing an unusual redundant motor control task through the network physiology approach. We acquired electromyographic signals from nine leg muscles of the dominant and non-dominant limbs during maximum voluntary isometric contractions (knee extension and flexion) and co-contraction bouts. Our results showed that young participants presented higher peak torque output, with similar EMG variability, compared to older participants. Considering firing rate frequencies, old and young women demonstrated different traits for network clustering and efficiency for the task. Age seems to affect muscle coordination at higher frequencies, even with a similar number of muscle synergies, indicating that younger women might have more integrated synergies than older women. The findings also point to differential muscle coordination adaptability.

Proficiency Barrier in Track and Field: Adaptation and Generalization Processes.

The literature on motor development and training assumes a hierarchy for learning skills-learning the "fundamentals"-that has yet to be empirically demonstrated. The present study addressed this issue by verifying (1) whether this strong hierarchy (i.e., the proficiency barrier) holds between three fundamental skills and three sport skills and (2) considering different transfer processes (generalization/adaptation) that would occur as a result of the existence of this strong hierarchy. Twenty-seven children/adolescents participated in performing the countermovement jump, standing long jump, leap, high jump, long jump, and hurdle transposition. We identified the proficiency barrier in two pairs of tasks (between the countermovement jump and high jump and between the standing long jump and long jump). Nonetheless, the transfer processes were not related to the proficiency barrier. We conclude that the proposed learning hierarchy holds for some tasks. The underlying reason for this is still unknown.

Consistent Individual Tendencies in Motor Speed-Accuracy Trade-Off.

The literature on speed-accuracy trade-off (SAT) in motor control has evidenced individuality in how individuals trade moments (e.g., mean and variance) of spatial and temporal errors. These individual tendencies could grasp tendencies of the system given previous experiences and constraints of the organism, a signature of the system control. Nonetheless, such tendency must be robust to small perturbations. Thirty participants performed nine conditions with different time and spatial criteria over 2 days (scanning). In between these scanning conditions, individuals performed a practice condition that required modifications of the individuals' preferred spatial and temporal tendency in the SAT. Our results demonstrated that there were no systematic effects of practice in SAT preferences. However, individual analyses demonstrated significant changes for 25 out of 30 individuals. The latter either attests against a consistent preference or to a more complex characterization of individual SAT tendencies.

Transitional Movement Skill Dependence on Fundamental Movement Skills: Testing Seefeldt's Proficiency Barrier.

Purpose: In 1979 Vern Seefeldt postulated that individuals that did not achieve a given level of proficiency in the fundamental movement skills (FMS) would be limited in performance on new and more complex skills during development. This hypothesis, the proficiency barrier, inspired research in motor development but, to the best of our knowledge, was never empirically tested. The present article tested three potential mathematical functions (linear, sigmoidal and piecewise) describing the proficiency barrier relating FMS with a transitional movement skill (TMS, a more complex movement skill). Methods: 87 children aged 7 to 10 years were tested on six skills of the TGMD-2 test battery (running, hopping, leaping, kicking, catching and stationary bouncing) and dribbling (a combination of running and stationary bouncing). Results: The results showed evidence for the proficiency barrier based on a specific sigmoidal relation. We also identified critical movement aspects from FMS that seem to induce this relation. Conclusion: There is some evidence supporting Seefeldt's Proficiency Barrier.

Are Sub-Movements Induced Visually in Discrete Aiming Tasks?

There is a long-held view that discrete movements aimed to a target are composed of a sequence of movement units (sub-movements) that have different roles in motor control (e.g., initial impulse, error correction and movement termination) depending on the task constraints (e.g., spatial-temporal requirements). Here we report findings from the manipulation of vision/no-vision on the prevalence and type of sub-movements in discrete movement tasks over a range of space-time task criteria. The presence of vison resulted in longer movement times compared to the no-vision counterpart in time-matching tasks. A similar vision effect was observed in the highest Index of Difficulty for time-minimization tasks. Conditions that resulted/required longer movement times demonstrated more pre-velocity-peak and post-velocity-peak types of sub-movements whereas short movement times increased the likelihood of overshooting sub-movements. The present study results are consistent with the idea that movement time is the variable associated with changes in sub-movement profiles.

Shaping Exploration: How Does the Constraint-Induced Movement Therapy Helps Patients Finding a New Movement Solution.

Despite the relative success of constraint-induced movement therapy in the recovery of injury-/trauma-related populations, the mechanisms by which it promotes its results are still unknown. From a dynamical systems approach, we investigated whether the induced exploratory patterns within and between trials during an exercise in Shaping (the therapy's practice) could shed light on this process. We analyzed data from four chronic spinal-cord injury patients during a task of placing and removing their feet from a step. We assessed the within and between trial dynamics through recurrent quantification analyses and task-space analyses, respectively. From our results, individuals found movement patterns directed to modulate foot height (to accomplish the task). Additionally, when the task was manipulated (increasing step height), individuals increased coupling and coupling variability in the ankle, hip, and knee over trials. This pattern of findings is in consonance with the idea of Shaping inducing exploration of different movements. Such exploration might be an important factor affording the positive changes observed in the literature.

Mind-Muscle Connection: Verbal Instructions Alter Electromyographic Activity for Elbow Flexors and Extensors During Co-Contraction Training.

Co-contraction training has demonstrated similar electromyographic (EMG) activity levels compared to conventional strength training. Since verbal instructions can increase EMG activity on target muscles during conventional exercises, the same should occur during co-contraction. In this study we analyzed whether different verbal instructions would alter the EMG activity of target muscles - biceps brachii (BB) and triceps brachii lateral head (TB) - during co-contraction training for the elbow joint. Seventeen males with experience in strength training performed a co-contraction set in two verbal instruction conditions to emphasize either elbow flexion or elbow extension. Surface electrodes were fixed over biceps brachii and triceps brachii lateral head muscles. We measured EMG mean amplitude and analyzed data with 2-way ANOVA. We found a significant interaction between muscle and verbal instruction (p = 0.002). Post hoc tests indicated that verbal instructions (p = 0.001) influenced the BB EMG activity (elbow flexion: M = 68.74, SD = 17.96%; elbow extension: M = 53.47, SD = 16.13%); and also showed difference (p = 0.006) in the EMG activity between BB and TB with verbal instruction emphasizing the elbow extension (BB: M = 53.47, SD = 16.13%; TB: M = 69.18, SD = 21.79%). There was a difference in the EMG ratio of BB/TB (p = 0.001) when focusing on elbow flexion (M = 1.09, SD = 0.30) versus elbow extension (M = 0.81, SD = 0.25). As verbal instruction modified the magnitude of muscle recruitment during co-contractions for elbow joint muscles, there is a clear mind-muscle connection of importance to this method of training. Also, of importance to trainers, verbal instructions seemed to affect individuals differentially.

Perceiving amputee gait from biological motion: kinematics cues and effect of experience level.

Physical therapists (PT) and clinicians must be skilled in identifying gait features through observation to assess motor deficits in patients and intervene appropriately. Inconsistent results in the literature have led researchers to question how clinical experience influences PT's gait perception and to seek the key kinematic features that should be trained to enhance PT's skill. Thus, this study investigated (1) what are the informative kinematic features that allow gait-deviation perception in amputee gait and (2) whether there are differences in observational gait skills between PT and individuals with less clinical experience (PT students [PTS] and Novices). We introduced a new method that combines biological motion and principal component analysis to gradually mesh amputee and typical walking patterns. Our analysis showed that on average the accuracy rate in identifying gait deviations between PT and PTS was similar and better than Novices. Also, we found that PT's experience was demonstrated by their better perception of gait asymmetry. The extracted principal components demonstrated that the major gait deviation of amputees was the medial-lateral body sway and spatial gait asymmetry.

Search Strategies in the Perceptual-Motor Workspace and the Acquisition of Coordination, Control, and Skill.

In this paper we re-visit and elaborate-on the theoretical framework of learning as searching within the perceptual-motor workspace for a solution to the task. The central focus is the nature of search strategies to locate and create stable equilibrium regions in the perceptual-motor workspace and how these strategies relate to the emergent movement forms in the acquisition of coordination, control, and skill. In the ecological theory of perception and action, the enhanced stability of performance occurs through the attunement of the perceptual systems to the task dynamics together with modifications of action as task and intrinsic dynamics cooperate and/or compete. Thus, through practice in this search process, individuals adapt to the pick-up of task relevant perceptual variables and change their movement form according to the stability of the performed action and its outcome in relation to the task demands. Contemporary experimental findings have revealed features of the search process given the interaction of individual intrinsic dynamics in the context of task requirements and principles that drive the change - e.g., exploitation of more tolerant task-space solutions and emergence of compensatory mechanisms. Finally, we outline how the search strategy framework relates to traditional learning-related phenomena: including the dynamical pathways of learning, learning curves, factors of learning, individuality, motor development, and sport and rehabilitation interventions.

The Development of Bimanual Coordination Across Toddlerhood.

As one of the hallmarks of human activity and cultural achievement, bimanual coordination has been the focus of research efforts in multiple fields of inquiry. Since the seminal work of Cohen (1971) and Kelso and colleagues (Haken, Kelso, & Bunz, 1985; Kelso, Southard, & Goodman, 1979), bimanual action has served as a model system used to investigate the role of cortical, perceptual, cognitive, and situational underpinnings of coordinated movement sequences (e.g., Bingham, 2004; Oliveira & Ivry, 2008). This work has been guided primarily by dynamical systems theory in general, and by the formal Haken-Kelso-Bunz (HKB; 1985) model of bimanual coordination, in particular. The HKB model describes the self-organizing relationship between a coordinated movement pattern and the underlying parameters that support that pattern, and can also be used to conceptualize and test predictions of how changes in coordination occur. Much of the work investigating bimanual control under the HKB model has been conducted with adults who are acting over time periods of a few seconds to a few days. However, there are also changes in bimanual control that occur over far longer time spans, including those that emerge across childhood and into adolescence (e.g., Wolff, Kotwica, & Obregon, 1998). Using the formal HKB model as a starting point, we analyzed the ontogenetic emergence of a particular pattern of bimanual coordination, specifically, the anti-phase (or inverse oscillatory motion) coordination pattern between the upper limbs in toddlers who are performing a drumming task (see Brakke, Fragaszy, Simpson, Hoy, & Cummins-Sebree, 2007). This study represents a first attempt to document the emergence of the anti-phase pattern by examining both microgenetic and ontogenetic patterns of change in bimanual activity. We report the results of a longitudinal study in which seven toddlers engaged monthly in a bimanual drumming task from 15 to 27 months of age. On some trials, an adult modeled in-phase or anti-phase action; on other trials, no action was modeled. We documented the motion dynamics accompanying the emergence of the anti-phase bimanual coordination pattern by assessing bout-to-bout and month-to-month changes in several movement parameters-oscillation frequency, amplitude ratio of the drumsticks, initial position of the limbs to begin bouts, and primary arm-joint involvement. These parameters provided a good starting point to understand how toddlers explore movement space in order to achieve greater stability in performing the anti-phase coordination pattern. Trained research assistants used Motus software to isolate each bout of drumming and to digitize the movement of the two drumstick heads relative to the stationary drum surface. Because we were primarily interested in the vertical movement of the drumsticks that were held in the child's hands, we relied on two-dimensional analyses and analyzed data that were tracked by a single camera. We used linear mixed effects analyses as well as qualitative analyses for each participant to help elucidate the emergence and stability of the child's use of anti-phase coordination. This approach facilitated descriptions of individual pathways of behavior that are possible only with longitudinal designs such as the one used here. Our analyses indicated that toddlers who were learning to produce anti-phase motion in this context employed a variety of strategies to adjust the topography of their action. Specifically, as we hypothesized, toddlers differentially exploited oscillation frequency and movement amplitude to support change to anti-phase action, which briefly appeared as early as 15 months of age but did not become relatively stable until approximately 20 months of age. We found evidence that many toddlers reduced oscillation frequency before transitioning from in-phase to anti-phase drumming. Toddlers also used different means of momentarily modulating the amplitude ratio between limbs to allow a change in coordination from in-phase to anti-phase. Nevertheless, these oscillation-frequency and amplitude-ratio strategies were interspersed by periods of nonsystematic exploration both within and between bouts of practice. We also observed that toddlers sometimes changed their initial limb positions to start a bout or altered which primary arm joints they used when drumming. When they enacted these changes, the toddlers increased performance of the anti-phase coordination pattern in their drumming. However, we found no evidence of systematic exploration with these changes in limb position and joint employment, suggesting that the toddlers did not intentionally employ these strategies to improve their performance on the task. Although bimanual drumming represents a highly specific behavior, our examination of the mechanisms underlying emergence of the anti-phase coordination pattern in this context is one of the missing pieces needed to understand the development of motor coordination more broadly. Our results document that the anti-phase coordination pattern emerges and stabilizes through modulation of the dynamics of the movement and change of the attractor landscape (i.e., the motor repertoire). Consistent with literatures in motor control, motor learning, and skill development, our results suggest that the acquisition of movements in ontogenetic development can be thought of as exploration of the emergent dynamics of perception and action. This conclusion is commensurate with a systemic approach to motor development in which functional dynamics, rather than specific structures, provide the basis for understanding developmental changes in skill. Based on our results as well as the relevant previous empirical literature, we present a conceptual model that incorporates developmental dynamics into the HKB model. This conceptual model calls for new investigations using a dynamical systems approach that allows direct control of movement parameters, and that builds on the methods and phenomena that we have described in the current work.

The precision demands of viewing distance modulate postural coordination and control.

There are contrasting views on the role of vision in modifying postural organization (information-driven and postural facilitation) and limited direct tests of the underlying postural mechanisms. Here, we examined whether the distinction between the two views is appropriate given that both are interrelated parts of task constraints modulating postural coordination and control. The study investigated whether changes in the organization of the postural system are a function of the visual precision demands of a task and, in addition, whether such organization could be described as reflecting an intermittent controller. Sixteen participants were instructed to maintain quiet postural stance while fixating a point at different viewing distances (25, 50, 135, 220, 305 cm) or standing with eyes closed. The 25-cm condition showed the lowest standard deviation of the center of pressure (COP) and the highest correlation dimension (CD) in the anterior posterior direction. Analyses revealed that, contrary to the intermittent controller hypothesis, adaptations in the continuous COP and center of mass (COM) coupling characterized the observed changes in CD. The findings show that the natural act of looking to the same feature in the environment as a function of visual viewing distance can lead to quantitative and qualitative changes in the dynamics of posture. This is consistent with the view that postural facilitation and information availability are integrated in the perceptual-motor dynamics.

Postural coordination and control to the precision demands of light finger touch.

We examine the proposition that information availability and postural facilitation-usually viewed as opposing views in postural control-are intertwined with the effects of one being related to the other. If that is the case, a single control parameter (precision demands) would capture the changes in postural control relating information and postural facilitation. Using the dynamical systems approach, we investigated whether, manipulating touch requirements as to increase precision demands, would induce quantitative and qualitative changes in postural dynamics. Additionally, we tested whether the COM-COP coupling reflects the qualitative dynamics of the system. Seventeen participants were instructed to maintain quiet standing while maintaining or not a light finger force with either precision or no precision. Standard deviation (SD) of the COP decreased with the precision demands and the correlation dimension (CD) of COP showed higher values for the touch conditions. Participants showed reduced synchronization of COP-COM coupling; following changes in CD. These results point out the integrated nature of information availability, task requirements, and the emergent postural organization reflected in COP-COM coupling.

Movement Speed and Accuracy in Space and Time: The Complementarity of Error Distributions.

Movement speed-accuracy trade-off is a function of the space-time constraints of the task. We investigated the space-time account of Hancock and Newell (1985) and the hypothesis of complementarity between the four moments of the error distribution in space and time. Twelve participants performed 15 conditions in a line drawing task composed of different spatial (10, 20, and 30 cm) and temporal (250 to 2,500 ms) criteria. The results showed that all moments of distributions changed systematically between conditions but there were some departures from the Hancock and Newell predictions. In contrast, individual analysis revealed the complementarity of the spatial and temporal error including a trade-off between the four moments of error. These findings support a complementary space-time account of movement speed and accuracy.

Learning a specific, individual and generalizable coordination function: evaluating the variability of practice hypothesis in motor learning.

Constant and variable practice conditions have been hypothesized to lead to different learning outcomes between them but similar within. However, experiments have found that within a constant practice condition, participants can show highly individual outcomes (i.e., coordination functions). Considering the contradictory evidence on the effects of variable practice, we tested the idea that measures of the individual learned outcome would be required to provide a full explanation for results in transfer tests rather than or in addition to the group task-related conditions on which individuals practiced. Twenty-four participants were divided into three groups with different practice conditions (constant, varied distance of the target, and varied angle of the target) and for 5 days performed a task of throwing for precision to a target. Pre-, post-, and transfer tests were used to evaluate our hypothesis. The results showed that although the group measures could predict certain aspects of the transfer tests, the coordination function characteristics were required to show higher levels of explanatory power. This finding supports the view that learning involves a specific, individual and generalizable solution although there are aspects of learning that are specific to the condition of practice.

Transfer of a learned coordination function: Specific, individual and generalizable.

It is generally held that transfer of practice is a function of the similarity between the originally learned task and the subsequent transfer task. In the present paper, we examine the proposition that individuals learn a coordination function that is specific to the task, individual and yet generalizable, depending on the characteristics of the learned coordination function. Seventeen individuals performed for 5 days the task of learning to throw for accuracy to a target and then performed two transfer tests that differed in terms of the axis of variation that individuals could vary in task outcome. The results showed individual differences in terms of the acquired movement pattern even when a similar performance was achieved. Additionally, the coordination function characterized by principal component analysis and its projection into the landing plane predicted performance in the transfer tests. These results support the proposition that individuals learn a coordination function that is specific, individual and generalizable.

Constraints specific influences of vision, touch and surface compliance in postural dynamics.

Studies that have manipulated vision and touch in posture usually emphasize the prescriptive closed-loop function of the information to reduce the amount of postural motion. In contrast, we examine here the hypothesis that the standard sensory manipulations to maintain quiet stance also change in specific ways the constraints on the task goal and the emergent movement organization. Twelve participants were instructed to maintain quiet postural stance under three sensory factors: surface compliance (foam/no foam), visual information (open/closed eyes) and tactile information (finger touch/no finger touch). The standard deviation of center of pressure (COP) motion decreased with the presence of vision, touch and rigid surface. The correlation dimension showed that the manipulation of touch and vision produced different attractor dynamics that also interacted with surface compliance. Vision decreased the correlation dimension in the foam surface while the touch manipulation increased dimension in the rigid surface. The sensory information manipulations changed the qualitative properties of the attractor dynamics as well as the quantitative properties of the amount of postural motion providing evidence for the specific nature of the postural organization across information conditions.

Search strategies in practice: Influence of information and task constraints.

The practice of a motor task has been conceptualized as a process of search through a perceptual-motor workspace. The present study investigated the influence of information and task constraints on the search strategy as reflected in the sequential relations of the outcome in a discrete movement virtual projectile task. The results showed that the relation between the changes of trial-to-trial movement outcome to performance level was dependent on the landscape of the task dynamics and the influence of inherent variability. Furthermore, the search was in a constrained parameter region of the perceptual-motor workspace that depended on the task constraints. These findings show that there is not a single function of trial-to-trial change over practice but rather that local search strategies (proportional, discontinuous, constant) adapt to the level of performance and the confluence of constraints to action.

A test of fixed and moving reference point control in posture.

This study investigated two contrasting assumptions of the regulation of posture: namely, fixed and moving reference point control. These assumptions were tested in terms of time-dependent structure and data distribution properties when stability is manipulated. Fifteen male participants performed a tightrope simulated balance task that is, maintaining a tandem stance while holding a pole. Pole length (and mass) and the standing support surface (fixed surface/balance board) were manipulated so as to mechanically change the balance stability. The mean and standard deviation (SD) of COP length were reduced with pole length increment but only in the balance board surface condition. Also, the SampEn was lower with greater pole length for the balance board but not the fixed surface. More than one peak was present in the distribution of COP in the majority of trials. Collectively, the findings provide evidence for a moving reference point in the maintenance of postural stability for quiet standing.

Bimanual coordination and the intermittency of visual information in isometric force tracking.

The effect of the intermittency of visual information in the bimanual coordination of an isometric force coordination task was investigated as a function of criterion force level. Eight levels of visual information intermittency (.2-25.6 Hz) were used in blocked fashion at each force level. Participants were required to produce a constant force output matching as accurately as possible the criterion force target. The results showed that performance improved as the intermittency of visual information was reduced-this effect being a function of force level. The distribution of the relative phase through the trial revealed a preference for the two hands to be coupled together (in-phase) at the slower rates of visual presentation (~.2 Hz). However, as the rate of visual feedback was increased (up to ~25.6 Hz), there was a transition to predominantly a negative correlation pattern (anti-phase). The pattern of bimanual coordination in this isometric tracking task is driven by the availability of information for error correction and the interactive influence of perceptual-motor constraints.

Adapting relative phase of bimanual isometric force coordination through scaling visual information intermittency.

Visual information plays an adaptive role in the relation between bimanual force coupling and error corrective processes of isometric force control. In the present study, the evolving distribution of the relative phase properties of bimanual isometric force coupling was examined by scaling within a trial the temporal feedback rate of visual intermittency (short to long presentation intervals and vice versa). The force error (RMSE) was reduced, and time-dependent irregularity (SampEn) of the force output was increased with greater amounts of visual information (shorter intermittency). Multi-stable coordination patterns of bimanual isometric force control were differentially shifted toward and away from the intrinsic dynamics by the changing the intermittency of visual information. The distribution of Hilbert transformed relative phase values showed progressively a predominantly anti-phase mode under less intermittent visual information to predominantly an in-phase mode with limited (almost no) visual information. Correlation between the hands showed a continuous reduction, rather than abrupt "transition," with increase in visual information, although no mean negative correlation was realized, despite the tendency towards an anti-phase distribution. Lastly, changes in both the performance outcome and bimanual isometric force coordination occurred at visual feedback rates faster than the minimal visual processing times established from single limb movement and isometric force protocols.