Strength training as a dynamical model of motor learning.

This paper outlines a framework for strength training as a dynamical model of perceptual-motor learning. We show, with emphasis on fixed-point attractor dynamics, that strength training can be mapped to the general dynamical principles of motor learning that arise from the constraints on action, including the distribution of practice/training. The time scales of the respective dynamics of performance change (increment and decrement) in discrete strength training and motor learning tasks reveal superposition of exponential functions in fixed-point dynamics, but distinctive attractor and parameter dynamics in oscillatory limit cycle and more continuous tasks, together with unique timescales to process influences (including practice, learning, strength, fitness, fatigue, warm-up decrement). Increments and decrements of strength can be viewed within a dynamical model of change in motor performance that reflects the integration of practice and training processes at multiple levels of learning and skill development.

The evolving high bar longswing in elite gymnasts of three age groups.

Chronological age classifies elite male gymnasts into developmental performance classifications: senior (18+ years), junior (14-18 years) and development (8-14 years). Here, we examine the influence of age and experience on the biomechanics of the high bar longswing across classifications. Joint angular kinematics and kinetics were obtained from 30 gymnasts performing three sets each of eight consecutive longswings. Differences between groups and relations between age, experience and key biomechanical variables were correlated. Kinetic variables and range of motion of the hip and knee were highest for development gymnasts. In all age groups, a dominant shoulder kinetic contribution was found, although circle location of the peak joint kinetics occurred earliest for junior gymnasts. Hip work contributed more prominently in development gymnasts. Age and experience were positively correlated to an increase in peak shoulder moments and powers and negatively correlated to peak hip and knee moments. The findings reveal that age and experience combine to influence the functional phase, joint kinematics and relative joint kinetic contribution, particularly with the senior group demonstrating a shoulder dominant technique. Changes in musculoskeletal loading across the age groups suggest that factors such as relative strength and practice may have influenced this joint mode transition of the longswing.

Task Specific and General Patterns of Joint Motion Variability in Upright- and Hand-Standing Postures.

The preservation of static balance in both upright- and hand-stance is maintained by the projection of center of mass (CM) motion within the region of stability at the respective base of support. This study investigated, from a degrees of freedom (DF) perspective, whether the stability of the CM in both upright- and hand-stances was predicted by the respective dispersion and time-dependent regularity of joint (upright stance-ankle, knee, hip, shoulder, neck; hand stance-wrist, elbow, shoulder, neck) angle and position. Full body three-dimensional (3D) kinematic data were collected on 10 advanced level junior female gymnasts during 30 s floor upright- and hand-stands. For both stances the amount of the dispersion of joint angle and sway motion was higher than that of the CM and center of pressure (CP) with an inverse relation to time-dependent irregularity (SampEn). In upright-standing the variability of neck motion in the anterior-posterior direction was significantly greater than that of most joints consistent with the role of vision in the control of quiet upright posture. The findings support the proposition that there are both task specific and general properties to the global CM control strategy in the balance of upright- and hand-standing induced by the different active skeletal-muscular organization and the degeneracy revealed in the multiple distributional variability patterns of the joint angle and position in 3D.

Learning the high bar longswing:II. energetics and the emergence of the coordination pattern.

This paper is Part II of a study of the effects of practice on young adult novice gymnasts learning the movement coordination pattern of the longswing. The focus was the early stage of learning a critical component of the longswing, namely: through joint motion to inject mechanical energy into the upswing segment effectively to complete the longswing circle. Twenty-five novice male gymnasts received expert instruction while practicing two sessions a week for 3 weeks between a pre- and a post-practice assessment test trial. Seven novices completed a full circle by the end of Test 4. Completion of the longswing was positively related to the angular velocity generated in the gravity driven free fall of the initial segment and the greater rate of energy input in the upward segment. A successful performance in terms of the emergent movement pattern requires coordination of the onset, rate and level of energy input to counterbalance the negative influence of gravity on the second half upwards segment. The development of the complete longswing through the emergence of the collective variable dynamics (Paper 1) and biomechanical energetics of the gymnast (Paper 2) informs coaches, scientists and clinicians regarding task decomposition and learning the longswing.

Learning the High Bar Longswing:I. Task Dynamics and Emergence of the Coordination Pattern.

We studied novice gymnasts (N = 25) learning to form the longswing movement coordination pattern. The focus was the emerging behavioural organisation of centre of mass (CM) dynamics and the relative phase of the bar-CM angular motion. Seven novices learned a complete longswing by the end of the study, 8 novices showed no improvement in proportion of circle completed, and the remainder produced modest but persistent increments of final swing height without achieving a full circle. The radial angular velocity generated in the free fall phase and the circle location of bar-CM relative phase progressively and predominantly predicted circle completion. Bar-CM relative phase produced a consistent qualitative relation within- and between-subjects characteristic of a collective variable with the bar leading the CM on the initial downward antiphase segment and the CM leading on the upward antiphase segment. The ratio of these phases was related by the last practice session to degree of circle completion. The findings showed strong individual differences in the effect of practice on the early steps of learning the movement coordination where the progressive emergence of the longswing circle is driven by exploiting the positive- and minimising the negative-influence of gravity on the bar-CM coordination dynamics (candidate collective variable).

Cross-limb dynamics of postural tremor due to limb loading to fatigue: neural overflow but not coupling.

Many experiments have shown independence of the index finger dynamics under bilateral postural tremor protocols. Here we investigated in young adults the dynamics of bilateral multidirectional postural tremor and forearm muscle activity under the progressively fatiguing conditions supporting an external weight to the point of induced postural failure. When no loads were applied, tremor in the vertical (VT) and mediolateral (ML) directions was similar with prominent peaks within 2- to 4-Hz and 8- to 12-Hz bandwidths. Contrastingly tremor in the anterior-posterior (AP) direction was characterized by a single peak between 0 and 2 Hz. Although no tremor coupling occurred cross limbs, strong within-limb coupling was found between ML and VT directions when no loads were applied (coherence range: 0.77-0.85), implying that these oscillations are related and likely derived from mechanical sources. Applying an external load to the index finger(s) led to significant increases in the amplitude of VT tremor and EMG activity within that limb but also caused increases in tremor directions not aligned with the gravitational vector (AP and ML). Significant increases in VT and ML tremor and EMG activity in the contralateral (unloaded) limb were also found when a single index finger was loaded; however, this bilateral increase did not align with increases in interlimb coupling (coherence <0.21). The effects of fatigue caused by prolonged loading were widespread, affecting tremor and muscle activity in both limbs through a combination of neural and mechanical mechanisms. The single- and dual-limb loading to fatigue increased neural overflow but not tremor coupling between the index fingers.NEW & NOTEWORTHY This study investigated bilateral multidirectional tremor under unloaded and loaded conditions. We found that tremor in the mediolateral and vertical directions within a limb were strongly coupled, a result not reported previously. Furthermore, when holding a weight to failure, tremor in all directions increased. Tremor also increased in the contralateral (unloaded) limb despite no interlimb coupling. This contralateral increase in tremor following loading a limb until fatigue is hypothesized to stem from motor-overflow effects.

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.

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.

Temperature influences perception of the length of a wielded object via effortful touch.

Individuals can perceive the properties of an attached or grasped object by wielding it through muscular effort-an ability referred to as dynamic or effortful touch. Sensitivity to the forces required to move such objects and to the resulting global patterns of tissue deformation underlies such perception. Given that perception via dynamic touch is movement-based, we hypothesized that manipulations that affect the ability to produce and control muscular movements might affect perception via dynamic touch. Cooling muscles from 40 to 10 °C impedes the development and transmission of muscular force and diminishes muscle stretch-reflex sensitivity. Accordingly, we anticipated that changes in hand temperature would alter the ability to detect patterns of tissue deformation and thus perception of the properties of wielded objects. In two experiments, participants wielded dowels with different lengths and rotational inertias (Experiment 1) and objects with identical lengths and different rotational inertias (Experiment 2). They reported perceived lengths of these objects, in the absence of vision, in cool (~ 10 °C), neutral (~ 30 °C), and warm temperature conditions (~ 40 °C). Actual length predicted perceived length of the dowels (Experiment 1), and rotational inertia predicted perceived length of the objects (Experiment 2); perceived lengths were longer in the warm condition than in the cool condition. In consideration of known temperature-induced changes in tissue structure and function, our results support the hypothesis that comparable processes underlie the control of movement and perception via dynamic touch.

Location of a grasped object's effector influences perception of the length of that object via dynamic touch.

Perception of properties of a grasped object via dynamic touch (wielding) contributes to dexterity in tool use (e.g., using a hammer, screwdriver) and sports (e.g., hockey, tennis). These activities differ from simple object manipulation in that they involve making contact with an intended target. In the present study, we examined whether and how making (percussive) contact with a target influences perception of the length of a grasped object via dynamic touch. Making contact with a target by the tip resulted in a more accurate perception of the length than simple wielding. However, making contact with the target at a point along the length did not influence the accuracy of perception. These findings suggest that the location of a grasped object's effector influences perception of properties of that object via dynamic touch. We discuss these findings in terms of time-varying properties of vibrations generated by the percussive contact of the grasped object and target.

Skill level changes the coordination and variability of standing posture and movement in a pistol-aiming task.

The study investigated the coordination and variability of posture and pistol motion for skilled pistol shooters and novices in a pistol-aiming task. The participants stood on a force platform and held a pistol with the preferred arm to aim for accuracy to a target on 30 s trials. The results revealed that the amount of the centre of pressure (COP) and pistol motion was lower for the expert than novice group. The time-varying structure of COP as indexed by multiscale entropy (MSE) and detrended fluctuation analysis (DFA) was also lower for the expert than the novice group. The relative phase between the COP in the anterior-posterior (AP) and pistol in the AP and between the COP in the medial-lateral (ML) and pistol in AP was close to inphase for the both groups. However, for the novice group the coordination patterns of posture and pistol motion were more variable with the pistol motion leading the posture motion while it was lagging in the skilled group. The findings show different qualitative and quantitative dynamics in pistol-aiming as a function of skill level with postural control foundational to supporting the reduced dispersion and complexity of the skilled arm-pistol motion.

Differences in postural tremor dynamics with age and neurological disease.

The overlap of dominant tremor frequencies and similarly amplified tremor observed for Parkinson's disease (PD) and essential tremor (ET) means differentiating between these pathologies is often difficult. As tremor exhibits non-linear properties, employing both linear and non-linear analyses may help distinguish between the tremor dynamics of aging, PD and ET. This study was designed to examine postural tremor in healthy older adults, PD and ET using standard linear and non-linear metrics. Hand and finger postural tremor was recorded in 15 healthy older adults (64 ± 6 years), 15 older individuals with PD (63 ± 6 years), and 10 persons with ET (68 ± 7 years). Linear measures of amplitude, frequency, and between-limb coupling (coherence) were performed. Non-linear measures of regularity (ApEn) and coupling (Cross-ApEn) were also used. Additionally, receiver operating characteristic analyses were performed for those measures that were significantly different between all groups. The results revealed that the linear measures only showed significant differences between the healthy adults and ET/PD persons, but no differences between the two neurological groups. Coherence showed higher bilateral coupling for ET but no differences in inter-limb coupling between PD and healthy subjects. However, ApEn values for finger tremor revealed significant differences between all groups, with tremor for ET persons being more regular (lower ApEn) overall. Similarly, Cross-ApEn results also showed differences between all groups, with ET persons showing strongest inter-limb coupling followed by PD and elderly. Overall, our findings point to the diagnostic potential for non-linear measures of coupling and tremor structure as biomarkers for discriminating between ET, PD and healthy persons.

Real-time visual feedback of COM and COP motion properties differentially modifies postural control structures.

The experiment was setup to investigate the control of human quiet standing through the manipulation of augmented visual information feedback of selective properties of the motion of two primary variables in postural control: center of pressure (COP) and center of mass (COM). Five properties of feedback information were contrasted to a no feedback dual-task (watching a movie) control condition to determine the impact of visual real-time feedback on the coordination of the joint motions in postural control in both static and dynamic one-leg standing postures. The feedback information included 2D COP or COM position and macro variables derived from the COP and COM motions, namely virtual time-to-contact (VTC) and the COP-COM coupling. The findings in the static condition showed that the VTC and COP-COM coupling feedback conditions decreased postural motion more than the 2D COP or COM positional information. These variables also induced larger sway amplitudes in the dynamic condition showing a more progressive search strategy in exploring the stability limits. Canonical correlation analysis (CCA) found that COP-COM coupling contributed less than the other feedback variables to the redundancy of the system reflected in the common variance between joint motions and properties of sway motion. The COP-COM coupling had the lowest weighting of the motion properties to redundancy under the feedback conditions but overall the qualitative pattern of the joint motion structures was preserved within the respective static and dynamic balance conditions.

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.

Coordination as a function of skill level in the gymnastics longswing.

The purpose of this study was to investigate the nature of inter-joint coordination at different levels of skilled performance to: (1) distinguish learners who were successful versus unsuccessful in terms of their task performance; (2) investigate the pathways of change during the learning of a new coordination pattern and (3) examine how the learner's coordination patterns relate to those of experts in the longswing gymnastics skill. Continuous relative phase of hip and shoulder joint motions was examined for longswings performed by two groups of novices, successful (n = 4) and unsuccessful (n = 4) over five practice sessions, and two expert gymnasts. Principal component analysis showed that during longswing positions where least continuous relative phase variability occurred for expert gymnasts, high variability distinguished the successful from the unsuccessful novice group. Continuous relative phase profiles of successful novices became more out-of-phase over practice and less similar to the closely in-phase coupling of the expert gymnasts. Collectively, the findings support the proposition that at the level in inter-joint coordination a technique emerges that facilitates successful performance but is not more like an expert's movement coordination. This finding questions the appropriateness of inferring development towards a "gold champion" movement coordination.

Changes in joint kinetics during learning the longswing on high bar.

Biomechanics helps us understand the association between technique changes and performance improvement during learning. The aim of this research was to investigate joint kinetic characteristics of technique during learning of the longswing on the high bar. Twelve male, novice participants took part in the learning study. During swing attempts in 8 weekly testing sessions, kinematic data were collected. Inverse dynamics analysis was performed from known zero forces at the toes to quantify joint moments and power at the hips and shoulders. Key biomechanical constraints that limited performance outcome were identified based on changes in joint kinetics during learning. These constraints were the ability to perform a large shoulder power and to overcome passive kinetics acting during the downswing. Constraints to action at the level of joint kinetics differentially challenge learners and therefore could underpin more individual, specific learning interventions. Functional phases, defined by maximum hyperextension to flexion of the hips and maximum flexion to extension of the shoulders, did not describe the key joint kinetics of the hip and shoulder for novices. The functional phases may serve however to identify novices that were unable to overcome the passive kinetic constraint.

Dimension and Complexity in Human Movement and Posture.

There has been considerable effort over the last 25 years to understand the emergence of complexity in motor output and how this relates to properties of the individual (e.g., age, disease state, etc.), environment (e.g., information) and task (e.g., movement, posture, isometric force). This paper addresses the behavioral dimension of motor complexity in movement and posture from a degrees of freedom (DF) perspective together with the change of complexity through aging, disease and fatigue. The dimension of behavior for a given perceptual-motor output is shown to be relatively low, dependent on the interaction between the individual, environmental, and task constraints and varies within a limited adaptive range for a given motor task. The determination of dimension in movement and posture has taken us beyond the traditional motor performance scores of behavior but it is not a sufficient characterization of the adaptive and emergent processes of complexity.

Changing up the routine: intervention-induced variability in motor learning.

Variability is often introduced by an external agent (e.g., an instructor) during practice with the purpose of enhancing motor learning. Using a task analysis approach, we provide a framework to examine the effects of intervention-induced variability. We propose that variability may have markedly different consequences on learning depending on the task level at which it is introduced.

The learning of isometric force time scales is differentially influenced by constant and variable practice.

This experiment was set up to investigate the influence of constant and variable practice on performance accuracy and the time- and frequency-dependent structure of the force output dynamics in the learning of an irregular isometric force pattern. Traditional approaches to the variability of practice hypothesis have demonstrated benefits of task-induced variability at the outcome level of behavior, but there have been limited investigations of the effect of practice conditions on movement execution and particularly the multiple time scale processes of force output. During the practice phase, variability was induced along the force-time dimension of the target pattern for the variable practice condition (different wave forms), but the wave forms exhibited the same distributional properties of the frequency content (1/f noise: ß = -1.5) as the constant practice condition. The results showed that both practice conditions exhibited similar reductions in task error as a function of practice. However, constant practice produced greater changes in the time- and frequency-dependent properties of force output than variable practice, including a higher relative change in the contribution from faster (4-12 Hz) time scale mechanisms. Generalization tests to novel target patterns revealed that the task dynamics had a greater influence than the effect of practice conditions. Collectively, the findings support the adaptive nature of force output structure and the perspective that practice conditions can produce differential effects on the outcome and execution levels of motor behavior.

Postural coordination patterns as a function of rhythmical dynamics of the surface of support.

This study investigated the organization of postural coordination patterns as a function of the rhythmical dynamics of the surface of support. We examined how the number and nature of the dynamical degrees of freedom in the movement coordination patterns changed as a function of the amplitude and frequency of support surface motion. Young adult subjects stood on a moving platform that was translated sinusoidally in anterior-posterior (AP) direction with the task goal to maintain upright bipedal postural balance. A force platform measured the kinetics at the surface of support and a 3D motion analysis system recorded torso and joint kinematics. Principal components analysis (PCA) identified four components overall, but increasing the average velocity of the support surface reduced the modal number of components of the postural coordination pattern from three to two. The analysis of joint motion loadings on the components revealed that organizational properties of the postural pattern also changed as a function of platform dynamics. PC1 (61.6-73.2 %) was accounted for by ankle, knee, and hip motion at the lowest velocity conditions, but as the velocity increased, ankle and hip variance dominated. In PC2 (24.2-20.2 %), the contribution of knee motion significantly increased while that of ankle motion decreased. In PC3 (9.7-5.1 %) neck motion contributed significantly at the highest velocity condition. Collectively, the findings show that the amplitude and frequency of the motion of the surface of support maps redundantly though preferentially to a small set of postural coordination patterns. The higher platform average velocities led to a reduction in the number of dynamical degrees of freedom of the coordination mode and different weightings of joint motion contributions to each component.