Eric G. James

Short-term differential training decreases postural sway

Differential training has been shown to enhance motor learning in sports skills. In the present study differential training was applied to the minimization of postural sway. A differential training group performed 15 one minute practice trials, each with different postural movement instructions. A repetitive practice group performed 15 trials standing as still as possible for one minute. Pre- and post-tests were performed standing as still as possible in 1 and 2-leg stance. Accelerometry data were collected approximately at the level of the center of mass (COM) and at the head. The root mean square jerk (RMSJ) of movement at the COM and head was estimated for the anteroposterior and mediolateral axes of motion. A significant Group × Test interaction revealed that the differential training led to lower anteroposterior RMSJ on the post-test than on the Pre-test in both the 1 and 2-leg stance tasks. A significant Group × Effector × Test interaction revealed that the decrease in anteroposterior RMSJ with differential training occurred in the RMSJ of the head but not the COM. The repetitive practice did not lead to a significant change in anteroposterior RMSJ at either the COM or the head. Neither form of training led to a significant change in mediolateral RMSJ. The results indicated that differential training can enhance motor learning not only in complex sports skills but in relatively simple motor tasks such as maintaining quiet stance.

Hierarchical control in redundant and non-redundant postural tasks

Prior studies of postural coordination have shown inconsistencies between hip-ankle coordination in redundant and non-redundant coordination tasks as well as predictions of the HKB model. These inconsistencies were investigated by testing the hypothesis that there are different hierarchical control structures for redundant (multiple potential task solutions) and non-redundant (a single task solution) coordination tasks (Bernstein, 1996). The transfer between a non-redundant postural tracking task and a redundant scanning task consisting of 16 hip-ankle relative phase patterns from 0° to 337.5° was investigated. The results showed that the transfer between the tasks was transitory, negative and occurred only from the non-redundant to the redundant task. This finding supports the hypothesis that inconsistencies between redundant and non-redundant coordination dynamics may be due to a hierarchical relation between control structures for the performance of these types of tasks.

Body movement instructions facilitate synergy level motor learning, retention and transfer.

Prior work has suggested that the findings of research on attentional focus during human motor learning research generalize to the use of instructions regarding body movement. However, research on focus of attention has generally not included the use of instructions that prescribe body movement. The present study examined the effect of instructions regarding body movement or movement outcome in a motor task that principally relied upon the organization of an effective movement pattern, with little demand to adapt the movement to environmental task constraints. The use of instructions for efficient body movement produced an improvement in a seated turning range-of-motion task within the first 5 movement trials. This improvement was retained 24 h later and transferred across sitting positions. The instructions to optimize the movement outcome improved the turning range-of-motion significantly on the post-test but not on the retention or transfer tests. These findings indicate that instructions regarding movement form can be more advantageous than instructions regarding movement outcome in a task that relies upon the organization of an effective movement pattern with little demand to adapt this pattern to environmental constraints of the task. The results are interpreted with respect to task constraints and Bernsteins (1996) hierarchy of control.

Nonstationarity of Stable States in Rhythmic Bimanual Coordination

The HKB model and its variants characterize bimanual coordination with fixedpoint dynamics and predict stationarity of the mean and variance of relative phase in stable coordinative states. In the current study, participants performed in-phase and antiphase coordination modes in rhythmic bimanual finger and elbow flexionextension tasks. The results of runs tests revealed that discrete relative phase was nonstationary in 49.25%, 50.25%, and 54% of time-series in the 10, 20, and 30 box runs tests, respectively. In all individual Task conditions >38% of time-series were nonstationary. These findings contradicted model predictions that the mean and variance of relative phase are stationary in bimanual coordination and distinguish the concept of dynamical stability from statistical stationarity. The findings indicated that relative phase was not attracted to a stationary fixed-point and that fluctuations in relative phase are not Gaussian white noise as in existing models of bimanual coordination.

Effects of Practice Variability on Unimanual Arm Rotation

ABSTRACT High variability practice has been found to lead to a higher rate of motor learning than low variability practice in sports tasks. The authors compared the effects of low and high levels of practice variability on a simple unimanual arm rotation task. Participants performed rhythmic unimanual internal-external arm rotation as smoothly as possible before and after 2 weeks of low (LV) or high (HV) variability practice and after a 2-week retention interval. Compared to the pretest, the HV group significantly decreased hand, radioulnar, and shoulder rotation jerk on the retention test and shoulder jerk on the posttest. After training the LV group had lower radioulnar and shoulder jerk on the posttest but not the retention test. The results supported the hypothesis that high variability practice would lead to greater learning and reminiscence than low variability practice and the theoretical prediction of a bifurcation in the motor learning dynamics.

A model of postural coordination dynamics

Prior research has shown that voluntary postural movement is characterized by stable in-phase and antiphase hip-ankle coordination modes. Prior modeling of coordination dynamics does not capture the stable fixed-points, phase transitions and hysteresis found in hip-ankle coordination. In this article a model was created to capture the dynamics of hip-ankle postural coordination. The present model follows the synergetic approach and uses two nonlinear oscillators to capture the dynamics of hip-ankle coordination. Terms for symmetry breaking and additive stochastic noise are included in the model. The model captures phase transitions from in-phase to antiphase coordination as movement frequency is scaled up and from antiphase to in-phase coordination as movement frequency is scaled down. The model also exhibits hysteresis, with phase transitions occurring at different movement frequencies as the control parameter is scaled up and down.

Metastable postural coordination dynamics.

The present study examined the coordination dynamics of the head and center of mass (COM) using accelerometry in quiet 1 and 2 leg stance with and without vision. The root mean square jerk of effectors was greater in 1 leg stance and without vision, and was greater for the head in 2 leg stance and greater at the COM for 1 leg stance. The coordination of the COM and head was more variable in 1 leg stance with vision than in the other stance and vision combinations. Both grouped and individual participant data showed metastable coordination dynamics with the presence of ghost attractors on both axes of motion that varied with the task. The findings indicated that stance and visual information conditions acted as control parameters, with increments in task difficulty increasing relative phase variability until a bifurcation in the metastable dynamics occurred in 1 leg stance without vision.

Dimensionality in rhythmic bimanual coordination.

Newell and Vaillancourt (2001) hypothesized that the dimensionality of motor behavior is a function of the level of task performance and the task dynamic. The present study examined high (in-phase), moderate (antiphase) and low (45°, 90°, and 135° relative phase) levels of task performance in bimanual coordination. Estimates of dimensionality were calculated for the component (effector movements), coupling of components (coupling of effectors), and task output (the produced relative phase) levels of analysis. The in-phase coordination mode had lower Approximate Entropy within, and lower Cross-Approximate Entropy between, effector movements than all other modes. The in-phase mode had higher relative phase Approximate Entropy than all other modes. These findings indicate lower effector and coupling dimensionality, and higher relative phase dimensionality, in the in-phase mode. These results support the hypothesis that at the levels of analysis with limit-cycle dynamics high levels of task performance are characterized by lower dimensionality than lower levels of performance. The results also support the hypothesis that high task performance of the fixed-point task goal of maintaining a constant relative phase is characterized by higher dimensionality than low level performance. Together, these findings support and generalize the Newell and Vaillancourt hypothesis to the component, coupling, and task output levels of analysis.

Fixed-Point Drift and Hysteresis in Frequency-Scaled Unimanual Coordination

ABSTRACT Research on human rhythmic coordination has shown that the in-phase and antiphase coordination modes are typically stable and that the coordination of asymmetric effectors frequently exhibits fixed-point drift. The author extended research on symmetry breaking in coordination dynamics by examining a frequency-scaled unimanual pronation-supination task. The results showed symmetry breaking and fixed-point drift, with the radioulnar joint increasingly more phase advanced than the shoulder with increments in movement frequency. Hysteresis was also observed, as the relative phase patterns produced at 3 of the 4 movement frequencies were lower in the upward frequency scaling direction than in the downward direction. These results showed that the dynamic properties of symmetry breaking and fixed-point drift in unimanual pronation-supination movements were consistent with prior research and modeling. The hysteresis effect was explained as potentially being due to the control structures that organize this redundant coordination task.

Dynamical degrees of freedom and correlations in isometric finger force production

Prior research has concluded that the correlations of isometric finger forces represent the extent to which the fingers are controlled as a single unit. If this is the case, finger force correlations should be consistent with estimates of the controlled (dynamical) degrees of freedom in finger forces. The present study examined the finger force correlations and the dynamical degrees of freedom in four isometric force tasks. The tasks were to produce a preferred level of force with the (a) Index, (b) Ring, (c) Both fingers and also to (d) Rest the fingers on the load cells. Dynamical degrees of freedom in finger forces were lowest in the Both finger force task and progressively higher in the Ring, Index and Resting finger force tasks. The finger force correlations were highest in the Resting and lowest in the Index and Ring finger tasks. The results for the dynamical degrees of freedom in finger forces were consistent with a reduction in degrees of freedom in response to the degrees of freedom problem and the task constraints. The results for the finger force correlations were inconsistent with a reduction in the dynamical degrees of freedom. These findings indicate that finger force correlations do not necessarily reflect the coupling of finger forces. The findings also highlight the value of time-domain analyses to reveal the organization of control in isometric finger forces.