An Examination of Rapid Positioning Movements with Spatiotemporal Constraints

Unidirectional positioning movements with spatiotemporal-constraints were examined as a test of impulse-timing theory (Schmidt, 1976; 1980; Wallace, 1981). Movements were examined at the kinematic, kinetic, and neuromuscular levels in three experiments. In the first experiment, displacement was held constant while five different movement times were examined. Both amplitudes and durations of the EMG and the kinetic variables were related to movement time. The results generally support the impulse-timing model. In the second experiment, movements were performed to a target at each of four distances in a constant movement time. EMG and force amplitudes and, unexpectedly, accelerative-force duration were modulated to achieve changes in displacement when movement time was constant. In the third experiment, movement time and displacement were simultaneously varied resulting in four conditions with equal average velocities. The results of this experiment were not as clear and exhibited individual differences. EMG duration did not always vary with changes in movement time. The results of all three experiments could not be adequately accounted for by the impulse-timing model.     

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.     

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.     

Effects of initial limb position on the accuracy, reaction time and electromyographic patterns of rapid movements

An experiment was performed to determine the effects of initial limb condition on final accuracy of rapid, elbow flexion movements in the horizontal plane. Electromyographic (EMG) recordings were also taken from an agonist (biceps) and antagonist (triceps) muscle by means of bipolar surface electrodes. In the experiment the subject's forearm was passively oscillated by means of an electric motor, and when an auditory buzzer sounded, the subject was required to react as quickly as possible and rapidly move to the previously learned target angle. Thus, movements could be initiated from either static or moving starting positions. The results indicated that general accuracy was not greatly affected by these manipulations, however, constant error and pre-motor reaction time suggested that subjects may have been utilizing initial limb condition information contrary to a mass-spring view. EMG data showed that the timing characteristics of the agonist and antagonist muscles were modulated, depending on the type of movement produced, supporting an impulse-timing model (Wallace 1981).     

Kinetic Attraction During Bimanual Coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.     

Kinetic attraction during bimanual coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.     

Temporal modulations of agonist and antagonist muscle activities accompanying improved performance of ballistic movements

Although many studies have examined performance improvements of ballistic movement through practice, it is still unclear how performance advances while maintaining maximum velocity, and how the accompanying triphasic electromyographic (EMG) activity is modified. The present study focused on the changes in triphasic EMG activity, i.e., the first agonist burst (AG1), the second agonist burst (AG2), and the antagonist burst (ANT), that accompanied decreases in movement time and error. Twelve healthy volunteers performed 100 ballistic wrist flexion movements in ten 10-trial sessions under the instruction to "maintain maximum velocity throughout the experiment and to stop the limb at the target as fast and accurately as possible". Kinematic parameters (position and velocity) and triphasic EMG activities from the agonist (flexor carpi radialis) and antagonist (extensor carpi radialis) muscles were recorded. Comparison of the results obtained from the first and the last 10 trials, revealed that movement time, movement error, and variability of amplitudes reduced with practice, and that maximum velocity and time to maximum velocity remained constant. EMG activities showed that AG1 and AG2 durations were reduced, whereas ANT duration did not change. Additionally, ANT and AG2 latencies were reduced. Integrated EMG of AG1 was significantly reduced as well. Analysis of the alpha angle (an index of the rate of recruitment of the motoneurons) showed that there was no change in either AG1 or AG2. Correlation analysis of alpha angles between these two bursts further revealed that the close relationship of AG1 and AG2 was kept constant through practice. These findings led to the conclusion that performance improvement in ballistic movement is mainly due to the temporal modulations of agonist and antagonist muscle activities when maximum velocity is kept constant. Presumably, a specific strategy is consistently applied during practice.     

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.     

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.     

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'. 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.     

EMG and motor performance changes with practice of a forearm movement by children

The purpose of this research was to investigate changes in the control of movement, using EMG and kinematic variables, over practice by children. Children in three age groups, 7, 9, and 11 yr., performed 60 trials of an elbow-flexion movement. Correct movements consisted of a 60 degrees angular movement of the forearm in 800 msec. The analysis of biceps brachii and triceps brachii muscle EMG activity, movement displacement and timing error, and movement velocity patterns indicated changes in motor performance with practice. All age groups improved performance with practice and also exhibited a decrease in biceps EMG activity with practice. Only movement-time error and time to peak triceps muscle activity differed between the age groups. The 11-yr.-old group significantly altered the timing of the antagonistic response to stop the movement over the practice session. This change is suggested to be related to the greater information-processing ability of these children and the development of appropriate movement strategies to perform the movement task successfully. Other changes observed in the EMG data appear similar to changes observed in studies of adults.     

Influence of Strategy on Muscle Activity During Impact Movements

Participants (N = 10) made flexions or extensions about the elbow. Movements either were pointing (i.e., self-terminated) or terminated by impact on a barrier. The author examined how the trajectory and the electromyographic (EMG) patterns varied according to the distance moved, the instruction provided concerning speed, or the type of termination. Variations in kinematics induced by changes in the target distance or the instruction regarding speed were the same for impact and pointing movements. In comparison with a pointing movement of similar distance and speed instruction, an impact movement (a) accelerated longer and reached a higher velocity, (b) had a longer agonist EMG burst, and (c) had a low level of contraction that started slightly after the agonist burst and continued throughout the movement but had little or no antagonist burst. Because the different types of movements required different forces from the muscles, there were systematic, task-specific differences in EMG patterns that reflected task-specific differences in central control. The results of this experiment demonstrate that impact movements share some of the rules used in the control of other tasks, such as pointing and reversing movements. The sharing is not imposed by mechanical or physiological constraints but, rather, represents the imposition of internal constraints.     

Influence of strategy on muscle activity during impact movements

Participants (N = 10) made flexions or extensions about the elbow. Movements either were pointing (i.e., self-terminated) or terminated by impact on a barrier. The author examined how the trajectory and the electromyographic (EMG) patterns varied according to the distance moved, the instruction provided concerning speed, or the type of termination. Variations in kinematics induced by changes in the target distance or the instruction regarding speed were the same for impact and pointing movements. In comparison with a pointing movement of similar distance and speed instruction, an impact movement (a) accelerated longer and reached a higher velocity, (b) had a longer agonist EMG burst, and (c) had a low level of contraction that started slightly after the agonist burst and continued throughout the movement but had little or no antagonist burst. Because the different types of movements required different forces from the muscles, there were systematic, task-specific differences in EMG patterns that reflected task-specific differences in central control. The results of this experiment demonstrate that impact movements share some of the rules used in the control of other tasks, such as pointing and reversing movements. The sharing is not imposed by mechanical or physiological constraints but, rather, represents the imposition of internal constraints.     

The spatial spread of attentional modulation of the motion aftereffect

The spatial spread of attentional modulation of selective adaptation was investigated in four experiments in which the duration of the movement aftereffect (MAE) was measured with and without processing of intermittently changing digits at the fixation point. In the first experiment, the effects of diverting attention on MAE duration were found to reduce as the distance between the fixation digits and the inner edge of the surrounding adapt/test grating was increased. A second experiment suggested that eye movements were unlikely to underlie the attentional effects. In experiment 3, the attentional effect stayed constant as the outer diameter of the adapt/test gratings was increased. In experiment 4 (as in experiment 1) the modulatory effects of attention were larger the closer the adapt/test gratings were to the locus of attention, when the area of the grating was held constant but its eccentricity varied. In experiments 1 and 4, an intermittently changing fixation digit was found to reduce MAE durations more than an unchanging digit, even when subjects were not required to process it, suggesting that exogenous as well as endogenous attentional processes modulate early motion processing.     

Binary choice reaction time as a function of the relationship between durations and forms of responses

In a choice between responding with the left or right hand, some kinds of differences between the movements increase RT (Reaction Time) while others do not. Of the first kind are differences in form, while differences in the finger used are of the latter kind. In previous experiments differences in form were confounded with differences in duration. Since there is some indication that a difference in duration is sufficient to lengthen RT, both characteristics were varied separately. It turned out that a difference in form (duration being constant) has essentially the same effects as a difference in duration (form being constant): Mean RT is longer, variability of RT and MT (movement time) is larger, and frequency of choice errors is smaller than in choice between identical movements. These effects, which seem to be associated with choice between movements of different temporal patterns, are interpreted in terms of advance specification of movement parameters. Additional results on the relationship between response duration and RT suggest that RT does not depend on duration (or velocity) per se, but on how much the duration deviates from quickest performance.     

Modification in movement accuracy in the triphasic pattern during a rapid forearm-flexion task

The present study was designed to investigate modifications in the triphasic EMG pattern during a forearm-flexion task at maximum speed which required three levels of movement accuracy. 36 subjects participated in 4 training sessions, performing a total of 200 repetitions of each movement. The fastest movement time was associated with the least accurate movement task. Likewise, the slowest movement time was found for the movement requiring the greatest accuracy. Differences in the duration and amplitude of agonist 1 activity, the start of agonist 2 activity, and the start and amplitude of antagonist activity were observed for the three movements. The results indicate that agonist 1 provides a propulsive force to initiate limb movement. The antagonist EMG activity was thought responsible for braking and correcting limb movement. Modifications in agonist 2 activity suggest this burst is related to movement velocity.     

Binary Choice Reaction Time as a Function of the Relationship between Durations and Forms of Responses

In a choice between responding with the left or right hand, some kinds of differences between the movements increase RT (Reaction Time) while others do not. Of the first kind are differences in form, while differences in the finger used are of the latter kind. In previous experiments differences in form were confounded with differences in duration. Since there is some indication that a difference in duration is sufficient to lengthen RT, both characteristics were varied separately. It turned out that a difference in form (duration being constant) has essentially the same effects as a difference in duration (form being constant): Mean RT is longer, variability of RT and MT (movement time) is larger, and frequency of choice errors is smaller than in choice between identical movements. These effects, which seem to be associated with choice between movements of different temporal patterns, are interpreted in terms of advance specification of movement parameters. Additional results on the relationship between response duration and RT suggest that RT does not depend on duration (or velocity) per se, but on how much the duration deviates from quickest performance.     

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.     

Adaptation to visual displacement with active and passive limb movements: effect of movement frequency and predictability of movement

Three experiments were performed to evaluate the influence of active and passive limb movements on adaptation to visual displacement. Over a wide frequency range (0·5-1·25 Hz) with constant amplitude, 30°, significant adaptation was achieved with active and passive movements. When arm movement frequency was constant at 1·0 Hz but amplitude of movement was varied, less adaptation was achieved for both active and passive movements than when amplitude was held constant. Even at a frequency above that of most naturally occurring limb movements, 1·67 Hz, and with variable amplitude motion, significant adaptation was achieved with active and passive limb movements. These findings emphasize the importance of visual-proprioceptive discordances for adaptation to visual displacement when only sight of the hand is permitted. Significant differences did not appear between the active and passive movement conditions in any of the experiments.