Changes in movement variables associated with transient overshoot of the final position

Transient overshoot (TO), which is assessed as the distance between the movement amplitude and the final position, was measured in a series of rapid, discrete elbow flexion movements performed under different distance and loading conditions by 7 participants. A positive relationship was found between kinematic variables (peak velocity, peak acceleration and deceleration, and the symmetry ratio) and the magnitude of TO, particularly in short movements performed against a light load. The relationships between TO and electromyographic (EMG) variables were low and mainly insignificant. Thus, TO contributes to the variability of rapid, discrete movements and therefore should be taken into account as an additional parameter in studies of the scaling of movement variables with movement mechanical conditions. TO could also represent a consequence of mechanical properties of the single-joint system rather than an independently programmed primary submovement.     

Asymmetries in the Regulation of Visually Guided Aiming

An experiment was conducted to examine the contribution of sensory information to asymmetries in manual aiming. Movements were performed in four vision conditions. In the full-vision condition (FV), subjects were afforded vision of both the hand and the target throughout the course of the movement. In the ambient-illumination-off condition (AO), the room lights were extinguished at movement initiation, preventing vision of the moving limb. In the target-off (TO) condition, the target was extinguished upon initiation of the movement. In a no-vision (NV) condition, ambient illumination was removed and the target was extinguished upon initiation of the response movement. Results indicated that accuracy was superior in the full-vision and target-off conditions and when movements were made by the right hand. Movements made by the right hand were also of shorter mean duration. The magnitudes of performance asymmetries were uninfluenced by vision condition. Analyses of movement kinematics revealed that movements made in conditions in which there was vision of the limb exhibited a greater number of discrete modifications of the movement trajectory. On an individual-trial basis, no relationship existed between accuracy and the occurrence of discrete modifications. These data suggest that although vision greatly enhances accuracy, discrete modifications subserved by vision reflect the imposition of nonfunctional zero-order control processes upon continuous higher-order control regimes.     

Asymmetries in the regulation of visually guided aiming

An experiment was conducted to examine the contribution of sensory information to asymmetries in manual aiming. Movements were performed in four vision conditions. In the full-vision condition (FV), subjects were afforded vision of both the hand and the target throughout the course of the movement. In the ambient-illumination-off condition (AO), the room lights were extinguished at movement initiation, preventing vision of the moving limb. In the target-off (TO) condition, the target was extinguished upon initiation of the movement. In a no-vision (NV) condition, ambient illumination was removed and the target was extinguished upon initiation of the response movement. Results indicated that accuracy was superior in the full-vision and target-off conditions and when movements were made by the right hand. Movements made by the right hand were also of shorter mean duration. The magnitudes of performance asymmetries were uninfluenced by vision condition. Analyses of movement kinematics revealed that movements made in conditions in which there was vision of the limb exhibited a greater number of discrete modifications of the movement trajectory. On an individual-trial basis, no relationship existed between accuracy and the occurrence of discrete modifications. These data suggest that although vision greatly enhances accuracy, discrete modifications subserved by vision reflect the imposition of nonfunctional zero-order control processes upon continuous higher-order control regimes.     

An Impulse-Timing Theory for Reciprocal Control of Muscular Activity in Rapid,Discrete Movements

Much remains to be learned about how agonist and antagonist muscles are controlled during the production of rapid, voluntary movements. In an effort to summarize a wide body of existing knowledge and stimulate future research on this subject, an impulse-timing theory is presented which attempts to predict the activity of reciprocal muscles based on certain characteristics of a movement. The basic tenet of the theory is that variables of movement time, movement distance, and inertial load have fairly predictable effects on the underlying muscular activity of the agonist and antagonist muscles during the production of rapid and discrete, voluntary movements. The theory is derived from the kinematic work of Schmidt, Zelaznik, Hawkins, Frank, and Quinn (1979) and supporting evidence from studies which have used electromyographic (EMG) recordings of agonist and antagonist muscles during rapid movements. Issues related to synergistic muscle control, central and peripheral control of reciprocal muscle activity, muscle control, and neurological disorder and the relationship between impulse-timing and mass-spring control are discussed in the final section.     

An impulse-timing theory for reciprocal control of muscular activity in rapid, discrete movements

Much remains to be learned about how agonist and antagonist muscles are controlled during the production of rapid, voluntary movements. In an effort to summarize a wide body of existing knowledge and stimulate future research on this subject, an impulse-timing theory is presented which attempts to predict the activity of reciprocal muscles based on certain characteristics of a movement. The basic tenet of the theory is that variables of movement time, movement distance and inertial load have fairly predictable effects on the underlying muscular activity of the agonist and antagonist muscles during the production of rapid and discrete, voluntary movements. The theory is derived from the kinematic work of Schmidt, Zelaznik, Hawkins, Frank and Quinn (1979) and supporting evidence from studies which have used electromyographic (EMG) recordings of agonist and antagonist muscles during rapid movements. Issues related to synergistic muscle control, central and peripheral control of reciprocal muscle activity, muscle control, and neurological disorder and the relationship between impulse-timing and mass-spring control are discussed in the final section.     

Parameter value switching in discrete and continuous aiming movements

The effect of practice variations on spatial and temporal accuracy was investigated in both discrete and continuous aiming movements in the preferred hand of college-aged participants (N=25). In a completely within-subject design, participants made rapid reversal movements with a lightweight lever in the sagittal plane, practicing 20 degrees and 60 degrees movements in repeated (same distance) and alternating (switching between 20 degrees and 60 degrees) conditions. Movements were also made one at a time (discretely) or in sequences of 20 movements (continuously). Spatial constant error, spatial variable error, spatial overall error, the coefficient of variation, movement time, and the relative timing were calculated for each set of 20 movements and analyzed by within-subject analyses of variance. Movements in the repeated conditions for both discrete and continuous movements were more accurate and consistent compared to the alternating condition where the short movements were overshot and the long movements were undershot. Discrete movements were more spatially and temporally variable than continuous movements. The discrete and continuous movements showed different relative timing patterns, suggesting that the temporal structure of the motor program is affected by task characteristics.     

Changes in Movement Symmetry Associated With Strengthening and Fatigue of Agonist and Antagonist Muscles

The hypothesis that strengthening or fatiguing procedures applied on active muscles can affect the symmetry of rapid, discrete movements was tested. Subjects (N = 12) performed rapid, consecutive elbow flexions and extensions between 2 targets before and after (a) applying a strength training program, (b) fatiguing elbow flexors, and (c) fatiguing elbow extensors. The results demonstrated that an increase in strength of elbow extensors caused by applied strength training is associated with an increase in the symmetry ratio (i.e., acceleration time divided by deceleration time) of elbow flexion movements. The symmetry ratio also increased and decreased in movements when agonists and antagonists were fatigued, respectively. Because the strength training and fatiguing procedures are both known to affect muscle force, the data are interpreted as changes in muscles' ability to exert the force while acting as agonists or antagonists. Namely, muscles need equal impulses of force (torque multiplied by time) to accelerate and, thereafter, to decelerate the limb while performing a rapid, discrete movement. The symmetry ratio may therefore be changed so that more time will be provided for muscles that become relatively weaker (compared with their antagonists) because a strengthening or fatiguing procedure has been applied, whereas a shorter time period should be sufficient for action of their stronger antagonists. Although, in the literature, the studied phenomenon has been discussed as a predominantly motor control phenomenon, the present data suggest that the movement symmetry could also be related to agonists' and antagonists' ability to exert force, particularly while performing rapid, discrete movements.     

Changes in movement symmetry associated with strengthening and fatigue of agonist and antagonist muscles

The hypothesis that strengthening or fatiguing procedures applied on active muscles can affect the symmetry of rapid, discrete movements was tested. Subjects (N = 12) performed rapid, consecutive elbow flexions and extensions between 2 targets before and after (a) applying a strength training program, (b) fatiguing elbow flexors, and (c) fatiguing elbow extensors. The results demonstrated that an increase in strength of elbow extensors caused by applied strength training is associated with an increase in the symmetry ratio (i.e., acceleration time divided by deceleration time) of elbow flexion movements. The symmetry ratio also increased and decreased in movements when agonists and antagonists were fatigued, respectively. Because the strength training and fatiguing procedures are both known to affect muscle force, the data are interpreted as changes in muscles' ability to exert the force while acting as agonists or antagonists. Namely, muscles need equal impulses of force (torque multiplied by time) to accelerate and, thereafter, to decelerate the limb while performing a rapid, discrete movement. The symmetry ratio may therefore be changed so that more time will be provided for muscles that become relatively weaker (compared with their antagonists) because a strengthening or fatiguing procedure has been applied, whereas a shorter time period should be sufficient for action of their stronger antagonists. Although, in the literature, the studied phenomenon has been discussed as a predominantly motor control phenomenon, the present data suggest that the movement symmetry could also be related to agonists' and antagonists' ability to exert force, particularly while performing rapid, discrete movements.     

Cancelling discrete and stopping ongoing rhythmic movements: Do they involve the same process of motor inhibition?

Motor inhibition is considered to be an important process of executive control and to be implicated in numerous activities in order to cancel prepared actions and, supposedly, to suppress ongoing ones. Usually, it is evaluated using a “stop-signal task” in which participants have to inhibit prepared discrete movements. However, it is unknown whether other movement types involve the same inhibition process. We therefore investigated whether the inhibition process for discrete movements is involved in stopping ongoing rhythmic movements as well.Twenty healthy adults performed two counterbalanced tasks. The first task was used to estimate the stop-signal reaction time (SSRTd) needed to inhibit prepared discrete key-pressing movements. In the second task, participants drew graphic patterns on a tablet and had to stop the movement when a stop-signal occurred. We calculated the rhythmic stop signal-reaction time as the time needed to initiate stopping such ongoing rhythmic movement (SSRTr) and the same latency relative to the period of the rhythmic movement (relSSRTr). We measured these delays under different movement frequencies and motor coordination conditions and further investigated whether they varied as a function of several parameters of the rhythmic movements (speed, mean and variance of the relative phase, and movement phase at several time events).We found no correlation between inhibition measures in the two tasks. In contrast, generalized linear models showed a moderate yet significant influence of the motion parameters on the inhibition of ongoing rhythmic movements. We therefore conclude that the motor inhibition processes involved in cancelling prepared discrete movements and stopping ongoing rhythmic movements are dissimilar.     

The postural control can be optimized by the first movement initiation condition encountered when submitted to muscle fatigue

We investigated whether and how the movement initiation condition (IC) encountered during the early movements performed following focal muscle fatigue affects the postural control of discrete ballistic movements. For this purpose, subjects performed shoulder flexions in a standing posture at maximal velocity under two movement IC, i.e., in self-paced conditions and submitted to a Stroop-like task in which participants had to trigger fast shoulder flexions at the presentation of incongruent colors. Shoulder flexion kinematics, surface muscle activity of focal and postural muscles as well as center-of-pressure kinematics were recorded. The initial IC and the order in which subjects were submitted to these two conditions were varied within two separate experimental sessions. IC schedule was repeated before and after fatigue protocols involving shoulder flexors. The aim of this fatigue procedure was to affect acceleration-generating capacities of focal muscles. In such conditions, the postural muscle activity preceding and accompanying movement execution is expected to decrease. Following fatigue, when subjects initially moved in self-paced conditions, postural muscle activity decreased and scaled to the lower focal peak acceleration. This postural strategy then transferred to the Stroop-like task. In contrast, when subjects initially moved submitted to the Stroop-like task, postural muscle activity did not decrease and this transferred to self-paced movements. Regarding the center-of-pressure peak velocity, which is indicative of the efficiency of the postural actions generated in stabilizing posture, no difference appeared between the two sessions post-fatigue. This highlights an optimization of the postural actions when subjects first moved in self-paced conditions, smaller postural muscle activation levels resulting in similar postural consequences. In conclusion, the level of neuromuscular activity associated with the postural control is affected and can be optimized by the initial movement IC experienced post-fatigue. Beyond the fundamental contributions arising from these results, we point out potential applications for trainers and sports instructors.     

Effect of orienting the finger opposition space in the control of reach-to-grasp movements

Participants (N = 13) made reach-to-grasp movements to an elongated object with or without a forearm pronation movement. Grasp and transport components of movements performed without forearm pronation differed from those performed when participants preplanned forearm pronation. The transport distance traveled after peak aperture (aperture closure distance) was unchanged, however, suggesting that participants initiated aperture closure on the basis of the distance of the hand from the target. When they suddenly pronated the forearm in response to a perturbation, aperture kinematics were altered from a monophasic to a biphasic profile and aperture closure distance was shortened. Conversely, a sudden reorientation to a nonpronated position minimized those changes. Thus, the relationship between transport and aperture components is differentially altered depending on online reorientation of the forearm.     

Accuracy constraints upon rapid elbow movements

Kinematic and myoelectric variables associated with rapid elbow-flexion movements of various distances to targets of various widths were studied. The movement time in these experiments conformed to Fitts' law: movement time increased with target distance and decreased with target width. Peak movement velocity, electromyograph (EMG) duration, and EMG quantity were poorly described by Fitts' law, for increases in target width were accompanied by increases in these variables. We show with regression equations, using separate weighting coefficients, that kinematic and myoelectric variables can be related to distance and target width. The use of distance and target width as independent variables allows us to suggest that the literature does not agree on the relation between EMG and distance moved partly because of the influences of the target on this relationship. We propose that human voluntary movement involves a subject "strategy," or set of internal constraints, that affect movement outcome. Significant elements of this strategy, such as how accurately to perform the task, may not be recognized or controlled in many movement paradigms, in spite of uniform instruction to subjects and similar apparatus.     

Kinematic markers of distance-specific control in linear hand movements

In this article, the authors analyze kinematic characteristics of reaching movements to memorized visual target locations. An increase in target distance was associated with a decrease in correlation between peak acceleration and movement distance and with a simultaneous increase in correlation between peak acceleration and movement time. According to the previous work on motor control in isometric force responses and in reaching movements these results seem to indicate a continuous transition from a rather preplanned to a more corrective mode of movement control, which may be associated with an adaptive mechanism serving to counteract an increase in signal-dependent noise of the motor system.     

Impulse Characteristics in Rapid Movement

Three untested assumptions of the impulse-variability model were examined in two experiments utilizing rapid, uni-planar limb movements. Experiment 1 varied movement distance (A) and movement time (MT) in a rapid-timing paradigm where the subject moved a lever through a certain distance in a certain time. Experiment 2 varied A in a reversal response where the S made a rapid elbow flexion and extension in a given MT. Displacement recordings were made on every trial. KR (knowledge of results) about MT was given after every trial. The results can be summarized as follows: (a) As predicted by the model, variations in impulse size and velocity were directly related to the impulses size; (b) There was no correlation between the accelerative and decelerative impulse durations recorded during the reversal response supporting the notion that the impulses might be independent; (c) Negative correlations (–.20 to –.50) were demonstrated between peak acceleration and impulse duration for both experiments, counter to the predictions of the model; and (d) Counter to the predictions of the model, timing error (VEt) increased as A decreased for rapid-timing responses. When the correlational results are taken into account, the model has the capacity to account for curvilinear relationships between relative timing error and movement speed. Overall, the results suggest that the impulse variability model requires some restructuring before it can be considered a viable model for the control of rapid limb movements.     

Conditions for a linear speed-accuracy trade-off in aimed movements

A linear speed-accuracy trade-off has been found for rapid, precisely timed movements from a home position toward a target point. In this trade-off, W_e = K₁ + K₂(D/T), where D is the distance between the home position and the target, T is a pre-specified movement time, and W_e is the standard deviation of the distances actually moved. This result differs from Fitts' law, the commonly observed logarithmic trade-off in aimed movements. A new experiment with wrist rotations was performed to determine what conditions induce the linear trade-off rather than Fitts' law. Three types of condition are considered: movement brevity, feedback deprivation, and temporal precision. The experiment yielded a linear trade-off for precisely timed movements even when their durations significantly exceeded an amount of time (200 ms) sufficient to process visual feedback. This result suggests that the linearity does not depend on movement brevity and/or feedback deprivation per se. Instead it supports a temporal-precision hypothesis that the linear trade-off occurs when aimed movements must have precisely specified durations.

A linear speed-accuracy trade-off has been found for rapid, precisely timed movements from a home position toward a target point. In this trade-off, W_e = K₁ + K₂(D/T), where D is the distance between the home position and the target, T is a pre-specified movement time, and W_e is the standard deviation of the distances actually moved. This result differs from Fitts' law, the commonly observed logarithmic trade-off in aimed movements. A new experiment with wrist rotations was performed to determine what conditions induce the linear trade-off rather than Fitts' law. Three types of condition are considered: movement brevity, feedback deprivation, and temporal precision. The experiment yielded a linear trade-off for precisely timed movements even when their durations significantly exceeded an amount of time (200 ms) sufficient to process visual feedback. This result suggests that the linearity does not depend on movement brevity and/or feedback deprivation per se. Instead it supports a temporal-precision hypothesis that the linear trade-off occurs when aimed movements must have precisely specified durations.     

Impulse characteristics in rapid movement: implications for impulse-variability models

Three untested assumptions of the impulse-variability model were examined in two experiments utilizing rapid, uni-planar limb movements. Experiment 1 varied movement distance (A) and movement time (MT) in a rapid-timing paradigm where the subject moved a lever through a certain distance in a certain time. Experiment 2 varied A in a reversal response where the S made a rapid elbow flexion and extension in a given MT. Displacement recordings were made on every trial. KR (knowledge of results) about MT was given after every trial. The results can be summarized as follows: (a) As predicted by the model, variations in impulse size and velocity were directly related to the impulses size; (b) There was no correlation between the accelerative and decelerative impulse durations recorded during the reversal response supporting the notion that the impulses might be independent; (c) Negative correlations (-.20 to -.50) were demonstrated between peak acceleration and impulse duration for both experiments, counter to the predictions of the model; and (d) Counter to the predictions of the model, timing error (VEt) increased as A decreased for rapid-timing responses. When the correlational results are taken into account, the model has the capacity to account for curvilinear relationships between relative timing error and movement speed. Overall, the results suggest that the impulse variability model requires some restructuring before it can be considered a viable model for the control of rapid limb movements.     

Matching of Movements Made Independently by the Two Arms in Normal Humans

Comparisons were made of voluntary movements of the right and left arms in normal human subjects. A series of movements of different amplitudes, made at the subject’s own speed, was performed with one limb. After a rest period, the same series was repeated with the contralateral limb. The relation between movement peak velocity and movement amplitude was linear and was the same for both arms. With repeated testing over periods up to two months, the slope of the peak velocity—amplitude relation decreased during the first week, thereafter remaining unchanged. In a second series of experiments, six normal subjects continuously wore a 1 lb (0.45 kg) weight strapped to their left (non-dominant) forearm for up to 1 week. This resulted in an increase in the slope of the peak-velocity/amplitude relation in this arm. A parallel change occurred in movements made independently by the right (non-loaded) arm. A similar matching of movement performance of the two limbs was seen following removal of the weight. The data is interpreted as providing support for the hypothesis that there is a single movement “command” which is applied to both limbs. The interaction of this command with the limbs which have similar second-order mechanical properties yields similar movements even when they are made independently.     

Matching of movements made independently by the two arms in normal humans

Comparisons were made of voluntary movements of the right and left arms in normal human subjects. A series of movements of different amplitudes, made at the subject' own speed, was performed with one limb. After a rest period, the same series was repeated with the contralateral limb. The relation between movement peak velocity and movement amplitude was linear and was the same for both arms. With repeated testing over periods up to two months, the slope of the peak velocity-amplitude relation decreased during the first week, thereafter remaining unchanged. In a second series of experiments, six normal subjects continuously wore a 1 lb (0.45 kg) weight strapped to their left (non-dominant) forearm for up to 1 week. This resulted in an increase in the slope of the peak-velocity/amplitude relation in this arm. A parallel change occurred in movements made independently by the right (non-loaded) arm. A similar matching of movement performance of the two limbs was seen following removal of the weight. The data is interpreted as proving support for the hypothesis that there is a single movement "command" which is applied to both limbs. The interaction of this command with the limbs which have similar second-order mechanical properties yields similar movements even when they are made independently.     

Distance and Movement Time Effects on the Timing of Agonist and Antagonist Muscles

The experiment examined the effects of movement time (MT) and distance on the timing of electromyographic (EMG) activity from an agonist and antagonist muscle during rapid, discrete elbow movements in the horizontal plane. According to impulse-timing theory (Wallace, 1981) MT, not distance moved, should have a pronounced effect on the timing of EMG activity (duration of initial agonist and antagonist burst and time to onset of initial antagonist burst). The levels of MT were 100 and 160 msec and the levels of distance were 27° and 45° of elbow flexion. In general support of impulse-timing theory, the results of the three EMG timing measures showed that MT had a more pronounced effect on these measures than distance. In addition, the timing of EMG activity in relation to total MT remained fairly consistent across the four MT-distance conditions.     

Distance and movement time effects on the timing of agonist and antagonist muscles: a test of the impulse-timing theory

The experiment examined the effects of movement time (MT) and distance on the timing at electromyographic (EMG) activity from an agonist and antagonist muscle during rapid, discrete elbow movements in the horizontal plane. According to impulse-timing theory (Wallace, 1981) MT, not distance moved, should have a pronounced effect on the timing of EMG activity (duration of initial agonist and antagonist burst and time to onset of initial antagonist burst). The levels of MT were 100 and 160 msec and the levels of distance were 27 degrees and 45 degrees of elbow flexion. In general support of impulse-timing theory, the results of the three EMG timing measures showed that MT had a more pronounced effect on these measures than distance. In addition, the timing of EMG activity in relation to total MT remained fairly consistent across the four MT-distance conditions.