Effects of unexpected perturbations on trajectories and EMG patterns of rapid wrist flexion movements in humans

To investigate how motor programs can be modified by sensory inputs we recorded kinematic and EMG patterns from normal human subjects performing well-practised wrist flexion movements in response to an auditory tone. On random trials unexpected wrist perturbations were introduced at varying times after the signal to move had been given. Extension perturbations delivered before agonist EMG onset resulted in an increased maximum velocity (MV) during the wrist flexion movement and in an increased target overshoot even though the wrist was further from the target than expected by the subject at the onset of the movement. The first agonist EMG burst and the antagonist burst were both increased in magnitude in these perturbed trials. Flexion perturbations delivered before the agonist EMG onset moved the hand nearer to the target just prior to movement onset. These resulted in a reduced MV, but the expected increased target overshoot did not occur. The first agonist burst was reduced in magnitude, and the antagonist burst was increased in magnitude. Perturbations delivered after agonist EMG onset produced less change in the first agonist and antagonist EMG burst, and less compensation for the perturbation was evident in wrist position and velocity recordings. These results indicate that, at least in some situations, motor programs for rapid voluntary movements can be modified by afferent inputs. This interaction between central motor commands and sensory feedback might occur at the cortical or spinal level, depending on when perturbations occur relative to onset of EMG and movement. The timing of the EMG changes suggest that both reflex mechanisms and longer latency 'voluntary' adjustments contribute to the compensatory changes in movement trajectory.     

Relationship between EMG patterns and kinematic properties for flexion movements at the human wrist

Summary EMG patterns associated with voluntary wrist flexion movements were studied in normal human subjects. Initially, subjects were trained to produce movements within five specified velocity ranges while the amplitude of the movement and the opposing load remained constant. In a second set of experiments, subjects were required to produce movements at four different amplitudes, moving as rapidly as possible against a constant load. Finally, with movement velocity and amplitude kept constant, the external load was varied so that different forces were required to generate the movements. The slowest movements were associated with a prolonged burst of EMG activity from the agonist muscle with little or no antagonist activity. With increasing movement velocity, there was a gradual evolution to the characteristic triphasic pattern associated with rapid voluntary movements. As velocity of movement increased further, the amplitude and area of the EMG bursts increased while burst duration and interburst intervals decreased. Increases in movement amplitude were accomplished mainly by changing the timing of the EMG bursts; with larger amplitude movements the antagonist burst occurred later. With movements against larger loads there was an increase in the size of the agonist burst and a decrease in the antagonist burst, but no change in the relative timing of the EMG bursts. These systematic changes in EMG patterns associated with different types of movement provide an indirect method of obtaining information concerning the motor programs which generate the movements.     

Vibration-induced changes in movement-related EMG activity in humans

The effect of muscle tendon vibration during voluntary arm movement was studied in normal humans. Subjects made alternating step flexion and extension movements about the elbow. A small vibrator was mounted over either the biceps or the triceps muscle and vibration was applied during flexion or extension movements. The vibrator was turned off between movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the muscle antagonist to the movement being performed produced an undershoot of the required end-movement position. The undershoot was 20-30% of the total movement amplitude. In contrast, vibration of the muscle agonist to the movement resulted in no change in movement end position. The vibration-induced undershoot was associated with an increase in the EMG activity of the vibrated (antagonist) muscle and a resultant increase in the ratio of the antagonist to agonist EMG activity. The increase in antagonist EMG produced by the vibration occurred with a latency of approximately 60 ms from vibration onset. The observed results are consistent with vibration-induced activation of muscle spindle receptors in the lengthening muscle during movement. It is suggested that, during movement, the sensitivity of the spindle receptors in the shortening muscle is decreased and the information concerning limb position during movement comes primarily from the lengthening muscle.     

Principles for learning single-joint movements

This study investigated changes in myoelectric and mechanical variables for movements made as fast as possible as a function of practice in the context of the dual-strategy hypothesis of motor control (Gottlieb et al. 1989b). Five male subjects made 1400 rapid elbow flexion movements in ten blocks of 20 trials over seven experimental sessions. Improved performance was defined as increased peak movement velocity, decreased peak velocity variability, increased acceleration and deceleration, a proportionately greater increase in peak deceleration than peak acceleration, and greater consistency in terminal location. The changes observed over experimental sessions were very similar to (but larger and more consistent than) those seen for the first experimental session, with the partial exception of the timing of the antagonist electromyogram (EMG). In general, the increases in the values of the measured mechanical variables covary with myoelectric measures in the same way as when subjects are asked to intentionally change speed in accordance with the rules of the speed-sensitive strategy (Corcos et al. 1989). However, there are differences between subjects in the extent to which speed changes can be attributable to the agonist muscle, the antagonist muscle, or in the timing between the muscles. In one of the five subjects, the latency of the antagonist EMG decreased over blocks on the 1st day but increased over experimental sessions and was consequently activated proportionately later in the movement. This suggests that extended practice can give at least some subjects flexibility in modifying the motor programs that underlie movement.     

Ballistic reactions under different motor sets

In preparation for performing task specific ballistic movements, subjects may choose among different possibilities for setting up their motor apparatus, ranging from quiet resting to different types of muscle activation. In the study presented here, we investigated whether differences in the motor set modify either the reaction time or the kinematic characteristics of the movement. Subjects wearing surface EMG recording electrodes in the wrist extensor (WE) and wrist flexor (WF) muscles were requested to react to the presentation of a visual stimulus by performing a ballistic wrist extension movement of an amplitude of about 50° in the following experimental conditions: resting quietly, which was considered as the control condition (CC); isometric contraction (IC), in which subjects were required to activate WE and WF muscles isometrically; rapid oscillations (RO), in which subjects were requested to make a fast oscillatory wrist movement; and slow oscillations (SO), in which subjects were maintaining a slow oscillatory motion of the wrist. To constrain the movement to the wrist joint and limit the action of postural muscles, the subjects forearm and hand were attached to joined non-resistive metallic platforms, allowing for free non-frictional displacement. In the EMG recordings, we measured the size of the EMG bursts in agonist and antagonist muscles, and the inter-burst intervals. In movement recordings, we measured movement onset latency and the velocity profile. Movement onset was delayed in SO with respect to all other conditions. Conversely, peak velocity was larger in all test conditions in comparison to CC. There were no differences in the size of the first EMG burst of the agonist muscle, but significant changes occurred in the subsequent bursts recorded in the agonist and antagonist muscles. Our study indicates that the motor program used to execute a ballistic voluntary movement is influenced by the conditions of the motor system. The configuration of the motor set should be specifically considered in the search for improving the speed of the reaction and the kinematics of ballistic movements.JMC and MTS were funded by the Spanish Ministry of Education, Culture and Sport, with grant numbers PR 2003-0212 and AP2000-0913     

Effects of gravitational forces on single joint arm movements in humans

We have examined the kinematics and muscle activation patterns of single joint elbow movements made in the vertical plane. Movements of different amplitudes were performed during a visual, step-tracking task. By adjusting shoulder position, both elbow flexion and extension movements were made under three conditions: (a) in the horizontal plane, (b) in the vertical plane against gravity, and (c) in the vertical plane with gravity. Regardless of the gravitational load, all movements were characterized by time symmetric velocity profiles. In addition, no differences were found in the relationships between movement duration, peak velocity, and movement amplitude in movements with or against gravity. The pattern of muscle activation was influenced however, by the gravitational load. Both flexion and extension movements made with gravity were characterized by a reciprocally organized pattern of muscle activity in which phasic agonist activity was followed by phasic antagonist activity. Flexion and extension movements made against gravity were characterized by early phasic antagonist activity occurring at about the same time as the initial agonist burst. These findings suggest that EMG patterns are modified in order to preserve a common temporal structure in the face of different gravitational loads.     

Evidence that a disordered servo-like mechanism contributes to tremor in movements during cerebellar dysfunction

The characteristics of discontinuities and tremor that occurred in elbow flexions during cooling of the lateral cerebellar nuclei were investigated in five Cebus monkeys. Discontinuities in movements appeared as rhythmical oscillations (kinetic tremor) when movements were slow or when movements were made with a constant force that loaded the antagonist. These oscillations had similar properties to cerebellar terminal tremor following movements; e.g., their amplitude and frequency were decreased by addition of mass to the handle and they occurred in the absence of visual feedback. The abnormal initial decrease in velocity that initiated oscillations in flexion movements was associated with abnormally early or large antagonist (triceps) electromyogram (EMG) activity. This abnormal EMG activity did not follow the normal inverse relation between initial velocity and antagonist latency from onset of movement. The initial deflection from the expected trajectory was opposed by a second burst of EMG activity in the agonist (biceps). This second burst was not the continuation of a step of EMG activity because its amplitude was often larger than the amplitude of the first agonist burst. The second agonist burst had the properties of a servo-like response: it occurred when biceps shortening was slowed (but biceps was not stretched), its magnitude was proportional to the magnitude or the deflection in velocity, its latency was 50-80 ms from onset of the abnormal decrease in velocity, and it occurred in the absence of visual feedback. However, this servo-like response was disordered because it did not return the limb accurately to the expected trajectory. The servo-like mechanism was studied further by applying torque pulse perturbations during elbow flexions. When the cerebellar nuclei were cooled, agonist responses to the perturbation were proportional to the size of the velocity deflection, but they were prolonged and onset of antagonist activity was delayed. It is suggested that discontinuities and tremor in movements during cerebellar dysfunction result from the same mechanism: alternation between disordered stretch reflexes and disordered servo-assistance mechanisms, both partly involving transcortical pathways.     

Movement-related phasic muscle activation. II. Generation and functional role of the triphasic pattern

1. Electromyographic (EMG) activity of arm movements made at constant velocity was studied in humans. In these movements, acceleration was temporally separated from deceleration by a period of constant velocity (zero acceleration) lasting up to 600 ms. 2. Agonist (AG1) and antagonist (ANT1) bursts were associated with acceleration. AG1 began before acceleration onset. ANT1 started after the onset of AG1 and was often partially coextensive with AG1. The initial phasic activity was followed by tonic EMG activity during the constant-velocity phase of the movements. Movement deceleration was associated with an antagonist burst (ANT2) and an agonist (AG2) burst. 3. Subjects could alter the magnitudes of the acceleration- and deceleration-related activities independently, with resulting independent changes in the movement acceleration and deceleration. 4. When the duration of the constant-velocity phase was decreased, the agonist/antagonist burst pairs occurred progressively closer in time. When movement duration was decreased to the point at which the velocity profile resembled that of step-tracking movements, the four periods of phasic EMG activity formed the classic triphasic pattern. 5. Triphasic EMG patterns were occasionally seen at the beginning or end of long-duration, constant-velocity movements. When they occurred, these triphasic patterns were associated with an acceleration/deceleration pattern similar to that seen in step-tracking movements. 6. The data indicate that paired agonist/antagonist activation is the basic unit of movement control. The AG1/ANT1 burst pair determines the increase and decrease of acceleration, respectively, and the ANT2/AG2 burst pair the increase and decrease of deceleration. These muscle activation pairs can be combined as needed to produce movements having different temporal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electromyographic responses to constant position errors imposed during voluntary elbow joint movement in human

The role of reflexes in the control of stiffness during human elbow joint movement was investigated for a wide range of movement speeds (1.5–6 rad/s). The electromyographic (EMG) responses of the elbow joint muscles to step position errors (step amplitude 0.15 rad; rise time 100 ms) imposed at the onset of targeted flexion movements (1.0 rad amplitude) were recorded. For all speeds of movement, the step position disturbance produced large modulations of the usual triphasic EMG activity, both excitatory and inhibitory, with an onset latency of 25 ms. In the muscles stretched by the perturbation, the early EMG response (25–60 ms latency) magnitude was greater than 50% of the activity during the unperturbed movements (background activity). In all muscles the EMG responses integrated over the entire movement were greater than 25% of the background activity. The responses were relatively greater for slower movements. Perturbations assisting the movement caused a short-latency (25–60 ms) reflex response (in the antagonist muscle) that increased with movement speed and was constant as a percentage of the background EMG activity. In contrast, perturbations resisting the movement caused a reflex response (in the agonist muscle) that was of the same absolute magnitude at all movement speeds, and thus decreased with movement speed as a percentage of the background EMG activity. There was a directional asymmetry in the reflex response, which produced an asymmetry in the mechanical response during slow movements. When the step perturbation occurred in a direction assisting the flexion movement, the antagonist muscle activity increased, but the main component of this response was delayed until the normal time of onset of the antagonist burst. When the step perturbation resisted the movement the agonist muscles responded briskly at short latency (25 ms). A reflex reversal occurred in two of six subjects. A fixed reflex response occurred in the antagonist muscle, regardless of the perturbation direction. For the extension direction perturbations (resisting movement), this response represented a reflex reversal (50 ms onset latency) and it caused the torque resisting the imposed step (stiffness) to drop markedly (below zero for one subject). Reflex responses were larger when the subject was prevented from reaching the target. That is, when the perturbation remained on until after the normal time of reaching the target, the EMG activity increased, with a parallel increase in stiffness. Similarly, when the perturbations prevented the subject from reaching the target during a 1-rad voluntary cyclic movement, the EMG and stiffness increased markedly. Coactivation of the antagonist muscle with the agonist muscle was not prominent (<30% of antagonist activity) during unperturbed movements. The perturbations were resisted with reciprocal activity, and thus reflex action did not increase the coactivation. However, as a result of the low-pass properties of muscle, substantial cocontraction of the agonist and antagonists muscle forces may have occurred during rapid movements, thus leading to increased stiffness. As the relative changes in normal EMG activity produced by the perturbation were often comparable with the changes in mean muscle torque (stiffness) reported in the first paper of this series, we conclude that the action of reflexes produced a significant portion of the resistance to perturbations. This reflexive portion was greater for slower movements, it was greater when the subject neared the target, and it was variable according to the perturbation direction and the particular subject. Given that the perturbations were of similar frequency content to the movement itself (though of smaller amplitude) and that the reflexes contributed substantially to the resistance to these perturbations, we suggest that in normal unperturbed movements the observed EMG is likewise substantially determined by the reflex activity.     

Asymmetrical trajectory formation in cyclic forearm movements in man

Summary Predictions of the minimum-jerk model for a human cyclic motion were given in terms of asymmetry in movement trajectories. A detailed kinematic analysis of cyclic forearm motion, i.e., extension/flexion movements around the elbow joint in a horizontal plane ranging in frequency from 2–5.5 Hz, was made to examine the validity of the predictions. The kinematics of the trajectories were described in terms of deviation from symmetry in velocity and acceleration profiles, and jerk cost. The asymmetry could be accounted for by the solution of the minimum-jerk model using the boundary condition differences between extension and flexion during a movement cycle. The trajectory was asymmetrical at relatively low frequencies, and symmetrical at higher frequencies; the frequency boundary from asymmetrical to symmetrical trajectories differed among subjects with a range of 3–4.3 Hz. It was suggested for the asymmetrical trajectory formation that consecutive extension and flexion in a cycle could be processed as a unit in which speed and acceleration in each direction were differentiated. The shift from asymmetrical to symmetrical trajectories with increasing frequency was accompanied by a reduction in jerk cost and mechanical energy. The oscillators underpinning the high frequency movements were mainly non-linear. The results suggested a shift of control from the rhythmic sequencing of extension and flexion which resulted in trajectory asymmetry, to nonlinear oscillation with no directional difference.     

Movement-related phasic muscle activation. I. Relations with temporal profile of movement

1. The role of phasic muscle activation in determining the temporal properties of human arm movements was studied. The experiments show that subjects can modulate the triphasic electromyographic (EMG) pattern to produce movements of varied temporal structures. 2. Subjects performed horizontal forearm movements in which they varied movement accelerations and decelerations. All movements were of the same amplitude, duration, and peak velocity. A phase-plane (velocity vs. position) template of the desired movement was presented to the subject, who had to reproduce the template by appropriate movement of the forearm. 3. The ratio of the durations of acceleration to deceleration (termed the symmetry ratio, SR) was used as a measure of the temporal structure of the movements. Movements with SRs ranging from 0.4 (short acceleration-long deceleration) to 2.0 (long acceleration-short deceleration) were studied. 4. Subjects modulated the components of the triphasic EMG pattern to produce movements with different temporal profiles. As the SR was increased (increasing acceleration duration-decreasing deceleration duration), the following changes occurred: 1) the duration of the initial agonist burst (AG1) increased while its magnitude decreased; 2) the antagonist burst (ANT1) was progressively delayed relative to movement onset. ANT1 magnitude increased while its duration remained constant; and 3) the magnitude of the second agonist burst (AG2) increased and its duration decreased. 5. The triphasic EMG pattern can be modified to produce movements whose velocity profiles are not the same under simple scaling of duration or magnitude. It is concluded that previously described relations between components of the triphasic EMG pattern and movement parameters, such as amplitude, speed, and duration, are secondary to associated changes in their acceleration and deceleration characteristics.     

Role of agonist and antagonist muscle strength in performance of rapid movements

Six subjects performed rapid self-terminated elbow movements under different mechanical conditions prior to, and 5 weeks after an elbow extensor strengthening programme. Despite the large difference in the strengths of elbow flexors and extensors, the pretest did not demonstrate significant differences between the movement time of flexion and extension movements performed under the same mechanical conditions. The results obtained in the posttest demonstrated a decrease in movement time (i.e. an increase in movement speed) in both elbow flexion and extension movements under some mechanical conditions. In addition, flexion movements demonstrated a relative increase in the acceleration time (acceleration time as a proportion of the movement time). It was concluded that the strength of both the agonist and antagonist muscles was important for the performance of rapid movements. Stronger agonists could increase the acceleration of the limb being moved, while stronger antagonists could facilitate the arrest of the limb movement in a shorter time, providing a longer time for acceleration.     

Organizing principles for single joint movements: V. Agonist-antagonist interactions.

1. Normal human subjects made discrete elbow flexions in the horizontal plane under different task conditions of initial or final position, inertial loading, or instruction about speed. We measured joint angle, acceleration, and electromyographic signals (EMGs) from two agonist and two antagonist muscles. 2. For many of the experimental tasks, the latency of the antagonist EMG burst was strongly correlated with parameters of the first agonist EMG burst defined by a single equation, expressed in terms of the agonist's hypothetical excitation pulse. Latency is proportional to the ratio of pulse duration to pulse intensity, making it proportional to movement distance and inertial load and inversely proportional to planned movement speed. However, these rules are not sufficient to define the timing of every possible single joint movement. 3. For movements described by the speed-insensitive strategy, the quantity of both antagonist and agonist muscle activity can be uniformly associated with selected kinetic measures that incorporate muscle force-velocity relations. 4. For movements collectively described by the speed-sensitive strategy, (i.e., that have direct or indirect constraints on speed), no single rule can describe all the combinations of agonist-antagonist coordination that are used to perform these diverse tasks. 5. Estimates of joint viscosity were made by calculating the amount of velocity-dependent torque used to terminate movements on target. These estimates are similar to those that have previously been made of limb viscosity during postural maintenance. They imply that a significant component of muscle activity must be used to overcome these forces. 6. These and previous results are all consistent with a dual-strategy hypothesis for those single-joint movements that are sufficiently fast to require pulse-like muscle activation patterns. The major features of such patterns (pulse intensities, durations, and latencies) are determined by central commands programmed in advance of movement initiation. The selection between speed-insensitive or speed-sensitive rules of motoneuron pool excitation is implicitly specified by the nature of speed constraints of the movement task.     

The trajectory of human wrist movements

1. To determine the form of human movement trajectories and the factors that determine this form, normal subjects performed wrist flexion movements against various elastic, viscous, and inertial loads. The subjects were instructed with visual and auditory feedback to make a movement of prescribed amplitude in a present period of time, but were free to choose any trajectory that fulfilled these constraints. 2. The trajectories were examined critically to determine if they corresponded to those which would minimize the root mean square (RMS) value of some kinematic variable or of energy consumption. The data agreed better with the trajectory that minimized the RMS value of jerk (the third derivative of length) than that of acceleration. However, systematic deviations from the minimum jerk predictions were consistently observed whenever movements were made against elastic and viscous loads. 3. Improved agreement could generally be obtained by assuming that the velocity profile varied according to a normal (Gaussian) curve. We conclude that minimization of jerk is not a general principle used by the nervous system in organizing voluntary movements, although movements may approach the predicted form, particularly under inertial loading conditions. 4. The EMG of the agonist muscles consisted of relatively simple waveforms containing ramplike increases and approximately exponential decays. The form of the movements could often be predicted quite well by using the EMG as an input to a linear second-order model of the muscle plus load. Rather than rigorously minimizing a kinematic variable or energy consumption, the nervous system may generate simple waveforms and adjust the parameters of these waveforms by trial and error until a trajectory is achieved that meets the requirements for a given load.     

Trajectory control in targeted force impulses

The functional role of opposing muscles in the production of isometric force trajectories was studied in six adult subjects producing impulses and steps of elbow flexor force, with different rise times and amplitudes. Rapidly rising forces were invariably associated with an alternating pattern of EMG activity in agonist and antagonist muscles: an agonist burst (AG1) initiated the development of force in the desired direction while a reciprocal burst in the antagonist (ANT-R) led to the deceleration of the force trajectory prior to the peak force. The temporal pattern of agonist and antagonist activation was dependent on force rise time. Force trajectories with long rise times (greater than 200 ms) were entirely controlled by the agonist, and EMG activity closely followed the contours of the rising force trajectory. For rise times of about 120 to 200 ms, agonist activation formed a discrete EMG burst, and force continued to rise during the subsequent silent period. For brief force rise times (less than 120 ms), reciprocal activation of the antagonist muscle occurred at about the time of the peak dF/dt. The integrated magnitude of AG1 was dependent on peak force but was independent of force rise time. AG1 duration varied directly with both peak force and force rise time. The integrated value of ANT-R varied as an inverse function of force rise time and was minimally influenced by peak force. ANT-R was present with the same magnitude and timing in both force impulses and steps when rise times were equal; therefore it did not serve to return force to baseline. Rather it served to truncate the rising force when very brief rise times were required, thus compensating for the low-pass filter properties of the agonist muscle. Subjects were able to voluntarily suppress ANT-R in rapidly accelerated force trajectories, indicating that the linkage between the commands controlling agonist and antagonist is not obligatory; however AG1 was then prolonged. Our findings emphasize that neuronal commands to opposing muscles acting at a joint must be adapted to constraints imposed by the properties of the neuromuscular plant.     

Interjoint coordination during handwriting-like movements

The present study investigates intrinsic preferences and tendencies in coordination of the wrist and finger movements during handwriting-like tasks. Movement of the inkless pen tip in nine right-handed subjects was registered with a digitizer. One circle-drawing task and four line-drawing tasks were included in the experiment. The line-drawing task included: (1) drawing with the wrist only, (2) drawing with the fingers only, (3) an equivalent pattern consisting of the simultaneous flexion/extension of the wrist and fingers, and (4) a nonequivalent pattern in which wrist flexion was accompanied by finger extension and wrist extension was accompanied by finger flexion. Both the line and circle drawing were performed repetitively at four speed levels, ranging from slow to "as fast as possible" movements. The analysis of the line drawing revealed differential variability and temporal characteristics across the four movement patterns. While the equivalent pattern had characteristics of performance similar to those observed in the wrist-only and fingers-only pattern, the nonequivalent pattern was more variable and was executed slower when as fast as possible movement was required, compared to the other three patterns. The circle-drawing task also revealed intrinsic tendencies in coordination of the wrist and fingers. These tendencies were manifested by a spontaneous transition of the circular path of the pen tip to a tilted oval with increases in movement speed. The transition to the oval shape was accompanied by decreases in relative phase between the wrist and finger movements, whereas amplitudes of these movements were not affected by movement speed manipulations. The results suggest that subjects did not display a tendency to decrease the number of joints involved when executing the patterns that required simultaneous wrist and finger movements. Instead, there were preferences during these patterns to integrate wrist and finger movements with low relative phase. The findings are interpreted in terms of biomechanical constraints imposed on the wrist-finger linkage. This interpretation was further examined by testing two left-handed subjects. The data obtained showed symmetrical preferences in joint coordination. Collectively, the findings support a supposition that the shape of cursive letters may have been adjusted to the biomechanical structure of the hand to facilitate the motor act of handwriting.     

Role of proprioceptive information in the temporal coordination between joints

Ten subjects made rapid, simultaneous movements of jaw (elevation or lowering) and right foot (ankle flexion or extension) in two experimental situations: (1) in response to an external signal (reaction-time situation), and (2) in a self-paced situation. We calculated the mean time intervals between the onsets of electromyographic (EMG) activity of agonist muscles (tibialis anterior or gastrocnemius lateralis compared with masseter or digastricus pars anterior) and those between the onsets of movement acceleration at each joint. Despite the fact that subjects reported simultaneous jaw-foot movements, there was always a short time interval between the two movements as between the agonist EMG activities. When the subjects were asked to perform a jaw elevation movement simultaneously with an ankle movement (flexion or extension), the sign of the time interval was dependent on the situation of movement initiation. In the reaction-time situation, the jaw motor activity preceded that of the ankle, whereas the reverse temporal order was observed in the self-paced situation. This is consistent with a previous hypothesis suggesting that the simultaneity of two motor actions is centrally established through two separate central processes: reactive or predictive. When subjects tried to perform simultaneous jaw lowering and foot flexion or extension movements, the strict temporal order observed when considering jaw elevation and ankle movements disappeared. The jaw motor activity generally preceded that of ankle in the reactive situation, but, depending on the subjects, it preceded or followed the ankle motor activity during self-paced movements. It is likely that the specific spindle supply of jaw muscles accounts for these results. Indeed, the jaw depressor muscles, in contrast to the elevators, lack muscle spindles. Our results suggest that the kinesthetic inputs used by the upper central nervous system to synchronize two rapid voluntary movements are mainly those from spindles located in the muscles that accelerate the movement, suggesting a strong α-γ linkage. Received: 31 October 1996 / Accepted: 1 September 1997     

Rapid goal-directed elbow flexion movements: limitations of the speed control system due to neural constraints

Summary In rapid goal-directed elbow flexion movements the influence of both movement amplitude and inertial load on the three-burst pattern and the consequences on movement time were studied. Subjects performed visually guided, self-paced movements as rapidly and as accurately as possible. An increase of both the movement amplitude and the inertial load were found to be interacting factors for the modulation of the three-burst-pattern and movement time. The first biceps burst progressively increased in duration and amplitude for larger movements, resulting in prolonged movement times. Surplus inertial loads further prolonged the agonist burst for large, but not for small movement amplitudes. The activity of the antagonist burst, in contrast, was largest in small movements and successively decreased at increasing movement amplitudes. Its duration, however, remained fairly constant. As was similarly observed for the agonist burst, surplus inertial loads lead to a prolongation of antagonist burst duration and an increase of the activity integral for large, but not for small movement amplitudes. It is suggested that in elbow flexion movements the programming of fastest goal-directed movements must take into account neural constraints and biomechanical characteristics of the agonist muscle and the antagonist muscle. Due to neural constraints of the biceps muscle, in contrast to finger movements, the concept of movement time invariance does not hold for elbow movements. Furthermore, neural constraints of the antagonist muscle lead to a limited force production of the agonist muscle at small movement amplitudes in order to avoid an overload of the braking process. The complexity of the relationship between neural and mechanical factors indicate that the size and timing of the three-burst-pattern has to be subtly adjusted to the precise nature of the task and its biomechanical characteristics.Supported by the Deutsche Forschungsgemeinschaft (SFB 33)     

Coordination of multiple muscles in two degree of freedom elbow movements

The present study quantifies electromyographic (EMG) magnitude, timing, and duration in one and two degree of freedom elbow movements involving combinations of flexion-extension and pronation-supination. The aim is to understand the organization of commands subserving motion in individual and multiple degrees of freedom. The muscles tested in this study fell into two categories with respect to agonist burst magnitude: those whose burst magnitude varied with motion in a second degree of freedom at the elbow, and those whose burst magnitude depended on motion in one degree of freedom only. In multiarticular muscles contributing to motion in two degrees of freedom at the elbow, we found that the magnitude of the agonist burst was greatest for movements in which a muscle acted as agonist in both degrees of freedom. The burst magnitudes for one degree of freedom movements were, in turn, greater than for movements in which the muscle was agonist in one degree of freedom and antagonist in the other. It was also found that, for movements in which a muscle acted as agonist in two degrees of freedom, the burst magnitude was, in the majority of cases, not different from the sum of the burst magnitudes in the component movements. When differences occurred, the burst magnitude for the combined movement was greater than the sum of the components. Other measures of EMG activity such as burst onset time and duration were not found to vary in a systematic manner with motion in these two degrees of freedom. It was also seen that several muscles which produced motion in one degree of freedom at the elbow, including triceps brachii (long head), triceps brachii (lateral head), and pronator quadratus displayed first agonist bursts whose magnitude did not vary with motion in a second degree of freedom. However, for the monoarticular elbow flexors brachialis and brachioradialis, agonist burst magnitude was affected by pronation or supination. Lastly, it was observed that during elbow movements in which muscles acted as agonist in one degree of freedom and antagonist in the other, the muscle activity often displayed both agonist and antagonist components in the same movement. It was found that, for pronator teres and biceps brachii, the timing of the bursts was such that there was activity in these muscles concurrent with activity in both pure agonists and pure antagonists. The empirical summation of EMG burst magnitudes and the presence in a single muscle of both agonist and antagonist bursts within a movement suggest that central commands associated with motion in individual degrees of freedom at the elbow may be superimposed to produce elbow movements in two degrees of freedom.     

Does dystonia always include co-contraction? A study of unconstrained reaching in children with primary and secondary dystonia

Dystonia is a movement disorder in which involuntary or intermittent muscle contractions cause twisting and repetitive movements, abnormal postures, or both. Excessive co-contraction and abnormalities in the time course of reciprocal inhibition between antagonist groups of muscles are considered to be cardinal features of some types of dystonia and reduced speed of movement is often attributed to involuntary activation of antagonist muscles about a joint. In the present study we describe muscle activity during unconstrained multi-joint reaching movements. Children diagnosed with arm dystonia due to cerebral palsy (CP) or primary dystonia (n = 7, 4–16 years, 4 with CP, 3 primary) and similar age healthy subjects pointed alternately to two targets as fast as possible. The children with dystonia showed decreased speed, greater variability, and pauses at targets compared with controls. Decreased speed was mostly due to difficulty in reversing reaching direction, and increased variability was associated with large fluctuations in the duration of the pauses at targets, rather than with variations in the flexion/extension velocity profiles. Surface electromyographic (EMG) activities were examined to assess if the abnormalities observed in the children with dystonia could be explained in terms of increased levels of co-contraction. Unexpectedly, we found that the children with dystonia showed lower levels of co-contraction than the controls during movement, and the pauses at targets were associated with reduced levels of activation rather than with excessive activity in antagonist groups of muscles. Therefore reduced speed of movement during unconstrained reaching may not be due to involuntary activation of the antagonist muscle, and co-contraction of opposing muscles about a joint is not an obligatory feature of multi-joint movement in children with dystonia.