Timing and the Control of Rhythmic Upper-Limb Movements

Accurate timing of limb displacement is crucial for effective motor control. The authors examined the effects of movement velocity, duration, direction, added mass, and auditory cueing on timing, spatial, and trajectory variability of single- and multijoint rhythmic movements. During single-joint movements, increased velocity decreased timing and spatial variability, whereas increased movement duration increased timing variability but decreased spatial variability. For multijoint movements, regardless of condition, increasing velocity decreased joint timing, spatial, and trajectory variability, but all hand variabilities were unaffected by velocity, duration, load, or direction. Timing, spatial, and trajectory variability was greater at the shoulder compared with the elbow and minimal at the hand, supporting the notion that reaching movements are planned in hand space as opposed to joint space.     

Response Amendment Latencies During Discrete Arm Movements

Examinations of the ability to rapidly modify ongoing movements have found that amendment latencies are shorter when a visual stimulus specifies an increase in movement velocity as compared to when a movement reversal is required. Our present study examined the hypothesis that amendment latencies for both continue and reverse instructions are dependent on the muscle activation state when the amendment is produced. Visual stimuli representing continue and reverse instructions were presented at four different phases of a discrete arm movement, with amendment latencies measured to modified electromyographic (EMG) patterns. Amendment latencies for the continue instruction were shortest when the stimulus was presented early in the movement and increased when the stimulus was presented later in the response. The opposite trend was true for the reversal condition with long latencies occurring when the stimulus was presented early in the movement. Our findings support the notion that amendment latencies are directly related to the active state of the motor system when the modification is required, and the nature of the amendment to be generated.     

Advantages of Rhythmic Movements at Resonance: Minimal Active Degrees of Freedom,Minimal Noise,and Maximal Predictability

Using time delay embedding, the authors applied phase space reconstruction to the time series of rhythmic movements of a hand-held pendulum. Subjects (N = 6) produced the manual oscillations about the wrist at the pendulum's resonant frequency and at a higher and a lower frequency. The number of active degrees of freedom required to capture the dynamics of the rhythmic behavior was 3 for the resonant frequency and 4 for each of the nonresonant frequencies. The residual high-dimensional noise was similarly lowest for the resonant frequency. Whereas 33% and 20%, respectively, of the vectors in the phase spaces of the dynamics higher and lower than resonance were unpredictable, only 12% were unpredictable at resonance. Finally, the predictability of the evolving dynamics extended farther into the future for oscillations at the resonant frequency. At resonance, the prediction horizon was 5 times farther than the prediction horizon for the higher than resonance behavior and 2.5 times farther than that for the lower than resonance behavior. The results suggest that, in pendular oscillations of a limb or limb segment, attunement of the central nervous system to the resonant frequency minimizes the variables to be controlled and maximizes the predictability of the rhythmic movement's chaotic dynamics.     

Investigating a Nonconservative Invariant of Motion in Coordinated Rhythmic Movements

The dimensions of an animal's limbs are fixed, but in locomotion and other rhythmic activities, they oscillated at a number of different frequencies. How might the physical conditions for this frequency variation be characterized? Kugler and Turvey (1987) hypothesized that the conditions might be adiabatic. A rhythmic system undergoes an adiabatic transformation of frequency when the stiffness is changed without a transfer of energy by heating. By standard definitions, adiabatic transformability is achievable only in conservative systems and only at infinitely slow rates of transformation. Kugler and Turvey's (1987) hypothesis extends adiabatic to systems that are dissipative and transformed rapidly by internal sources of energy, such as biological movement systems. Two predictions follow from the hypothesis. The first prediction is that a relation should be obtained in frequency-energy coordinates that has constant slope (Ehrenfest's adiabatic relation, a semipermanent invariant of motion) and an energy intercept less than zero (constant energy dissipation regardless of frequency). The second prediction is that the positive linear relation in frequency-energy coordinates can be satisfied by different relations in period-amplitude coordinates; amplitude increasing, amplitude increasing then decreasing, amplitude decreasing. The predictions were evaluated in four experiments with the same three participants. In each experiment, the rhythmic movement unit was defined by a pendulum of fixed dimensions held in the right hand that was made to oscillate at frequencies in the range 0.6 Hz to 1.8 Hz by the requirement of 1:1 frequency locking with a pendulum of different dimensions held in the other hand. Changes in the period of a pendular rhythmic movement were accompanied by statistically significant changes in the amplitude. Amplitude's systematic dependence on period differed, however, among the three participants in the experiments.     

Rapid Error Correction During Human Arm Movements

Studies were made of rapid error correction movements in eight subjects performing a visually guided tracking task involving flexion-extension movements about the elbow. Subjects were required to minimize reaction times in this two-choice task. Errors in initial movement direction occurred in about 3% of the trials. Error correction times (time from initiation to reversal of movement in incorrect direction) ranged from 30-150 ms. The first sign of correction of the error movement was a suppression of the electromyographic (EMG) activity in the muscle producing the error movement. This suppression started as early as 20-40 ms after the initiation of the error-related EMG activity and as much as 50 ms before any overt sign of limb movement. The correction of the error movement was also accompanied by an increase in the drive to the muscle which moved the arm in the correct direction. This increased activity always occurred after the initiation of the error movement. It is concluded that the first step in the error correction, suppression of drive to the muscle producing the error movement, cannot be based on information from the moving limb. It is thus suggested that this earliest response to the error movement is based on central monitoring of the commands for movement.     

Task-Dependent Compensatory Responses to Perturbations Applied During Rhythmic Movements in Humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans—running, cycling, and hopping—was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi-and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movement in humans.     

Coordination Between Arm and Leg Movements During Locomotion

To evaluate the contrasting dynamical and biomechanical interpretations of the 2:1 frequency coordination between arm and leg movements that occurs at low walking velocities and the 1:1 frequency coordination that occurs at higher walking velocities, the authors conducted an experiment in which they quantified the effect of walking velocity on the stability of the frequency and phase coordination between the individual limb movements. Spectral analyses revealed the presence of 2:1 frequency coordination as a consistent feature of the data in only 3 out of 8 participants at walking velocities ranging from 1.0 to 2.0 km/h, in spite of the fact that the eigenfrequencies of the arms were rather similar across participants. The degree of interlimb coupling, as indexed by weighted coherence and variability of relative phase, was lower for the arm movements and for ipsilateral and diagonal combinations of arm and leg movements than for the leg movements. Furthermore, the coupling between all pairs of limb movements was found to increase with walking velocity, whereas no clear signs were observed that the switches from 2:1 to 1:1 frequency coordination and vice versa were preceded by loss of stability. Therefore, neither a purely biomechanical nor a purely dynamical model is optimally suited to explain these results. Instead, an integrative model involving elements of both approaches seems to be required.     

Developmental Features of Rapid Aiming Arm Movements Across the Lifespan

Using a lifespan approach, the authors investigated developmental features of the control of ballistic aiming arm movements by manipulating movement complexity, response uncertainty, and the use of precues. Four different age groups of participants (6- and 9-year-old boys and girls and 24- and 73-year- old men and women, 20 participants in each age group) performed 7 types of rapid aiming arm movements on the surface of a digitizer. Their movement characteristics such as movement velocity, normalized jerk, relative timing, movement linearity, and intersegment intervals were profiled. Analyses of variance with repeated measures were conducted on age and task effects in varying movement complexity (Study 1), response uncertainty (Study 2), and precue use (Study 3) conditions. Young children and senior adults had slower, more variant, less smooth, and less linear arm movements than older children and young adults. Increasing the number of movement segments resulted in slower and more variant responses. Movement accuracy demands or response uncertainty interacted with age so that the 6- and 74-year-old participants had poorer performances but responded similarly to the varying treatments. Even though older children and young adults had better performances than young children and senior adults, their arm movement performance declined when response uncertainty increased. The analyses suggested that young children's and senior adults' performances are poorer because less of their movement is under central control, and they therefore use on-line adjustments. In addition, older children and young adults use a valid precue more effectively to prepare for subsequent movements than do young children and senior adults, suggesting that older children and young adults are more capable of organizing motor responses than arc young children and senior adults.     

Position of the Egocentric Reference and Directional Arm Movements in Right-Brain-Damaged Patients

We asked 12 right-brain-damaged patients (6 with left neglect signs and 6 without left neglect signs) to perform a straight ahead pointing task and a visual detection task with lateralized motor response, in order to investigate the relationship between the position of the egocentric reference and response time and accuracy in producing lateralized arm movements. Results showed that there was no correlation between the position of the egocentric reference and neglect signs, nor between the position of the egocentric reference and the latencies to direct a motor response toward either side of space. These findings were interpreted within the context of egocentric hypotheses of neglect. In particular, it was suggested that attentional or intentional neglect signs cannot be considered as a direct consequence of an ipsilesional deviation of the egocentric reference.     

Amplitude Scaling Compensates for Serial Delays in Correcting Eye and Arm Movements

Spatial and metrical parameters of the eye and arm movements made by human subjects (N = 7) in response to visual targets that were stepped unexpectedly either once (single step) or twice (double step) were studied. For the double-step, the displacement of a visual target was decreased or increased in amplitude at intervals before and during a movement. Provided the second target step occurred more than 100 ms before the onset of movement, the amplitude of the subjects' first response was altered in the direction of the new target location. But this amplitude scaling was not always sufficient to reach the new target location, and a second corrective response was required. The latency in producing this second response was greatly increased above reaction time latencies of movements to single-step targets, especially when the target change occurred 100 ms or more before movement onset. These findings suggest that even though serial processing limitations delay the production of a second corrective response, continuous parallel processing of visual information enables the amplitude of the first response to be altered with minimal delay. This enables some degree of real-time continuous control by the visuomotor control system.     

Kinematics of Goal-Directed Arm Movements in Neglect: Control of Hand Velocity

Do patients with unilateral neglect exhibit direction-specific deficits in the control of movement velocity when performing goal-directed arm movements? Five patients with left-sided neglect performed unrestrained three-dimensional pointing movements to visual targets presented at body midline, the left and right hemispace. A group of healthy adults and a group of patients with right-hemispheric brain damage but no neglect served as controls. Pointing was performed under normal room light or in darkness. Time-position data of the hand were recorded with an opto-electronic camera system. We found that compared to healthy controls, movement times were longer in both patient groups due to prolonged acceleration and deceleration phases. Tangential peak hand velocity was lower in both patient groups, but not significantly different from controls. Single peak, bell-shaped velocity profiles of the hand were preserved in all right hemispheric patients and in three out of five neglect patients. Most important, the velocity profiles of neglect patients to leftward targets did not differ significantly from those to targets in the right hemispace. In summary, we found evidence for general bradykinesia in neglect patients, but not for a direction-specific deficit in the control of hand velocity. We conclude that visual neglect induces characteristic changes in exploratory behavior, but not in the kinematics of goal-directed movements to objects in peripersonal space.     

Amplitude Scaling Compensates for Serial Delays in Correcting Eye and Arm Movements

Spatial and metrical parameters of the eye and arm movements made by human subjects (N = 7) in response to visual targets that were stepped unexpectedly either once (single step) or twice (double step) were studied. For the double-step, the displacement of a visual target was decreased or increased in amplitude at intervals before and during a movement. Provided the second target step occurred more than 100 ms before the onset of movement, the amplitude of the subjects' first response was altered in the direction of the new target location. But this amplitude scaling was not always sufficient to reach the new target location, and a second corrective response was required. The latency in producing this second response was greatly increased above reaction time latencies of movements to single-step targets, especially when the target change occurred 100 ms or more before movement onset. These findings suggest that even though serial processing limitations delay the production of a second corrective response, continuous parallel processing of visual information enables the amplitude of the first response to be altered with minimal delay. This enables some degree of real-time continuous control by the visuomotor control system.     

Extracting Global and Local Dynamics From the Stochastics of Rhythmic Forearm Movements

The authors examined the dynamics governing rhythmic forearm movements that 9 participants performed under a variety of task constraints by using a generic, unbiased analysis technique for extracting the drift coefficients of Fokker-Planck equations from stochastic data. From those coefficients, they reconstructed and analyzed vector fields and phase portraits to identify characteristic, task-dependent kinematic and dynamical features. They first directly estimated the parameters of weakly nonlinear self-sustaining oscillators from the extracted drift coefficients. The estimated parameters that the authors had selected instinctively and then particularized by using averaging methods largely confirmed previously derived limit-cycle models. Next, they ventured beyond limit-cycle models to examine global and local dynamical features that those models cannot adequately address, particularly task-dependent changes in flow strength and curvature and distinct dynamical features associated with flexion and extension. The authors argue that those features should be focal points of researchers' future modeling efforts to formulate a more adequate and encompassing account of the dynamics of rhythmic movement.     

Moving the Arm at Different Rates: Slow Movements are Avoided

Qualitative and quantitative changes characterize locomotion and rhythmic interlimb coordination at different speeds. Legs and hands do not move more or less quickly; they also adopt different relative coordination patterns. In the present article, the authors asked whether similar transitions occur for unimanual hand movements when speed is slowed below the preferred speed. Participants moved a handheld dowel back and forth between 2 large circular targets in time with a metronome at periods between 370 ms and 1667 ms. The authors analyzed the kinematics of participants’ movements at each period and found that proportional dwell time and number of peaks in the velocity profile increased as driving periods increased. Path lengths and peak velocities remained relatively constant for driving periods exceeding 800 ms. Participants made only gradual changes to their movement parameters, so that they went from a continuous mode to a more discrete mode of behavior for longer driving periods. Thus, unlike for rhythmic bimanual movements or locomotory patterns, there are quantitative but no clear qualitative changes for unimanual movements. The results suggest that participants tried to move close to their preferred tempo at different rates, and that they avoided moving slowly.     

Relation Between EMG Activation Patterns and Kinematic Properties of Aimed Arm Movements

Aimed flexion movements of the arm of different amplitude and duration were studied. Velocity and acceleration traces of movements with equal duration but different amplitude were equal, apart from a scaling factor (ratio between movement amplitudes). After appropriate scaling, EMG activity of the first agonist burst for these movements superimposed. This was not true for EMG activity in the antagonist muscle.

For movements with equal amplitude, but different duration, the time to peak acceleration was constant for all MT’s. Except for this fact, traces of acceleration, velocity, and agonist activity following the time of peak acceleration were about equal after appropriate scaling in time and amplitude. The integral of EMG activity in the first agonist burst increased linearly with peak velocity. For the antagonist burst, the integrated EMG activity increased more than proportionally.

During movements made as fast as possible, subjects used a different strategy by varying the duration of the accelerating phase for movements of different amplitude. Movement amplitude was achieved by adjusting the duration of the agonist burst and the onset time for the antagonist muscle. Amplitude of the antagonist burst was constant within a narrow range for movements of different amplitude.

These results did not change when the inertial mass was doubled by loading the arm with an additional mass.     

Control of Rapid Arm Movements When Target Position Is Altered During Saccadic Suppression

This experiment examined whether rapid arm movements can be corrected in response to a change in target position that occurs just prior to movement onset, during saccadic suppression of displacement. Because the threshold of retinal input reaches its highest magnitude at that time, displacement of the visual target of a saccade is not perceived. Subjects (N = 6) were instructed to perform very rapid arm movements toward visual targets located 16, 20, and 24 degrees from midline (on average, movement time was 208 ms). On some trials the 20 degrees target was displaced 4 degrees either to the right or to the left during saccadic suppression. For double-step trials, arm movements did not deviate from their original trajectory. Movement endpoints and movement structure (i.e., velocity-and acceleration-time profiles) were similar whether or not target displacements occurred, showing the failure of proprioceptive signals or internal feedback loops to correct the arm trajectory. Following this movement, terminal spatially oriented movements corrected the direction of the initial movement (as compared with the single-step control trials) when the target eccentricity decreased by 4 degrees. Subjects were unaware of these spatial corrections. Therefore, spatial corrections of hand position were driven by the goal level of the task, which was updated by oculomotor corrective responses when a target shift occurred.     

The Effect of Load on Torques in Point-to-Point Arm Movements: A 3D Model

A dynamic, 3-dimensional model was developed to simulate slightly restricted (pronation-supination was not allowed) point-to-point movements of the upper limb under different external loads, which were modeled using 3 objects of distinct masses held in the hand. The model considered structural and biomechanical properties of the arm and measured coordinates of joint positions. The model predicted muscle torques generated by muscles and needed to produce the measured rotations in the shoulder and elbow joints. The effect of different object masses on torque profiles, magnitudes, and directions were studied. Correlation analysis has shown that torque profiles in the shoulder and elbow joints are load invariant. The shape of the torque magnitude-time curve is load invariant but it is scaled with the mass of the load. Objects with larger masses are associated with a lower deflection of the elbow torque with respect to the sagittal plane. Torque direction–time curve is load invariant scaled with the mass of the load. The authors propose that the load invariance of the torque magnitude–time curve and torque direction–time curve holds for object transporting arm movements not restricted to a plane.     

Ipsilesional Arm Aiming Movements After Stroke: Influence of the Degree of Contralesional Impairment

The authors examined the effects of the degree of impairment of the contralesional upper limb and the side of the hemispheric damage on ipsilesional upper limb performance in chronic stroke individuals. Right- and left-side stroke resulting in mild-to-severe impairment and healthy participants took part in simple and choice reaction time tasks involving aiming movements. The stroke individuals performed the aiming movements with the ipsilesional upper limb using a digitizing tablet to ipsi- or contralateral targets presented in a monitor. The global performance of the group with severe right hemispheric damage was worse than that of the other groups, indicating that the side of hemispheric damage and degree of motor impairment can adversely affect aiming movement performance.     

Diffusive,Synaptic, and Synergetic Coupling: An Evaluation Through In-Phase and Antiphase Rhythmic Movements

The in-phase and antiphase patterns of interlimb l:1 frequency locking were contrasted with respect to models of coordination dynamics in biological movement systems that are based on diffusive coupling, synaptic coupling, and synergetic principles. Predictions were made from each model concerning the stable relative phase phi between the rhythmic units, its standard deviation SDphi and the self-chosen coupled frequency omegasubc;. The experimental task involved human subjects oscillating two handheld pendulums either in-phase or antiphase. The eigenfrequencies of the two hand-pendulum systems were manipulated by varying the length and mass of each pendulum individually. Relative to an eigenfrequency difference of Delta equal to zero, |Deltaomega| > 0 displaced phi from phi = 0 and phi = pi, and amplified SDphi. omegasubc; decreased with |Deltaomega|. Both the displacement of phi and SDphi were greater in the antiphase mode. Additionally, the displacement of phi increased more sharply with |Delta| for antiphase than for in-phase coordination. In contrast, omegasubc; was identical for the two coordination modes. Of the models of interlimb coordination dynamics, the synergetic model was the most successful in addressing the pattern of dependencies of phi and SDphi. The specific forms of the functions relating omegasubc; and phi to Deltaomega pose challenges for all three models, however