Coordination of multi-joint arm movements in cerebellar ataxia: Analysis of hand and angular kinematics

Kinematic abnormalities of fast multijoint movements in cerebellar ataxia include abnormally increased curvature of hand trajectories and an increased hand path and are thought to originate from an impairment in generating appropriate levels of muscle torques to support normal coordination between shoulder and elbow joints. Such a mechanism predicts that kinematic abnormalities are pronounced when fast movements are performed and large muscular torques are required. Experimental evidence that systematically explores the effects of increasing movement velocities on movement kinematics in cerebellar multijoint movements is limited and to some extent contradictory. We, therefore, investigated angular and hand kinematics of natural multijoint pointing movements in patients with cerebellar degenerative disorders and healthy controls. Subjects performed self-paced vertical pointing movements with their right arms at three different target velocities. Limb movements were recorded in three-dimensional space using a two-camera infrared tracking system. Differences between patients and healthy subjects were most prominent when the subjects performed fast movements. Peak hand acceleration and deceleration were similar to normals during slow and moderate velocity movements but were smaller for fast movements. While altering movement velocities had little or no effect on the length of the hand path and angular motion of elbow and shoulder joints in normal subjects, the patients exhibited overshooting motions (hypermetria) of the hand and at both joints as movement velocity increased. Hypermetria at one joint always accompanied hypermetria at the neighboring joint. Peak elbow angular deceleration was markedly delayed in patients compared with normals. Other temporal movement variables such as the relative timing of shoulder and elbow joint motion onsets were normal in patients. Kinematic abnormalities of multijoint arm movements in cerebellar ataxia include hypermetria at both the elbow and the shoulder joint and, as a consequence, irregular and enlarged paths of the hand, and they are marked with fast but not with slow movements. Our findings suggest that kinematic movement abnormalities that characterize cerebellar limb ataxia are related to an impairment in scaling movement variables such as joint acceleration and deceleration normally with movement speed. Most likely, increased hand paths and decomposition of movement during slow movements, as described earlier, result from compensatory mechanisms the patients may employ if maximum movement accuracy is required.     

Multijoint arm movements in cerebellar ataxia: Abnormal control of movement dynamics

In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients’ deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients’ difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements. Received: 28 October 1996 / Accepted: 30 September 1997     

Cerebellar ataxia: torque deficiency or torque mismatch between joints?

Prior work has shown that cerebellar subjects have difficulty adjusting for interaction torques that occur during multi-jointed movements. The purpose of this study was to determine whether this deficit is due to a general inability to generate sufficient levels of phasic torque inability or due to an inability to generate muscle torques that predict and compensate for interaction torques. A second purpose was to determine whether reducing the number of moving joints by external mechanical fixation could improve cerebellar subjects' targeted limb movements. We studied control and cerebellar subjects making elbow flexion movements to touch a target under two conditions: 1) a shoulder free condition, which required only elbow flexion, although the shoulder joint was unconstrained and 2) a shoulder fixed condition, where the shoulder joint was mechanically stabilized so it could not move. We measured joint positions of the arm in the sagittal plane and electromyograms (EMGs) of shoulder and elbow muscles. Elbow and shoulder torques were estimated using inverse dynamics equations. In the shoulder free condition, cerebellar subjects made greater endpoint errors (primarily overshoots) than did controls. Cerebellar subjects' overshoot errors were largely due to unwanted flexion at the shoulder. The excessive shoulder flexion resulted from a torque mismatch, where larger shoulder muscle torques were produced at higher rates than would be appropriate for a given elbow movement. In the shoulder fixed condition, endpoint errors of cerebellar subjects and controls were comparable. The improved accuracy of cerebellar subjects was accompanied by reduced shoulder flexor muscle activity. Most of the correct cerebellar trials in the shoulder fixed condition were movements made using only muscles that flex the elbow. Our findings suggest that cerebellar subjects' poor shoulder control is due to an inability to generate muscle torques that predict and compensate for interaction torques, and not due to a general inability to generate sufficient levels of phasic torque. In addition, reducing the number of muscles to be controlled improved cerebellar ataxia.     

Multijoint movement control in Parkinson's disease

Impairments in the performance of complex actions in Parkinson's disease (PD) patients are well documented. The aim of the present study was to investigate potential mechanisms that may be contributing to impaired movement performance in PD patients. PD patients and age-matched control subjects performed rapid pointing movements to a series of four tabletop targets. The height of the table was adjusted until the targets could be achieved with arm movements in the horizontal plane. The targets were arranged such that target 1 required elbow extension only and targets 2–4 required increasing amounts of horizontal shoulder flexion in addition to the elbow extension. While the control subjects accelerated and decelerated the elbow and shoulder joints simultaneously regardless of the target location, the PD patients decomposed motion during the acceleration phase by accelerating first the shoulder and then the elbow joint. For PD patients this decomposition of arm segments was associated with greater coactivation of the muscles about the elbow when elbow extension and shoulder flexion were simultaneously required (targets 2–4), in contrast to the single joint action. The control subjects decreased elbow joint coactivation while the patients increased it across the four targets. The resulting peak interaction torques at both the elbow and shoulder joints occurred relatively later for the PD patients. The coactivation patterns observed in PD patients may reduce the ability to take advantage of interaction torques and may also contribute to joint motion decomposition. Electronic Publication     

Compensation for interaction torques during single- and multijoint limb movement

During multijoint limb movements such as reaching, rotational forces arise at one joint due to the motions of limb segments about other joints. We report the results of three experiments in which we assessed the extent to which control signals to muscles are adjusted to counteract these "interaction torques." Human subjects performed single- and multijoint pointing movements involving shoulder and elbow motion, and movement parameters related to the magnitude and direction of interaction torques were manipulated systematically. We examined electromyographic (EMG) activity of shoulder and elbow muscles and, specifically, the relationship between EMG activity and joint interaction torque. A first set of experiments examined single-joint movements. During both single-joint elbow (experiment 1) and shoulder (experiment 2) movements, phasic EMG activity was observed in muscles spanning the stationary joint (shoulder muscles in experiment 1 and elbow muscles in experiment 2). This muscle activity preceded movement and varied in amplitude with the magnitude of upcoming interaction torque (the load resulting from motion of the nonstationary limb segment). In a third experiment, subjects performed multijoint movements involving simultaneous motion at the shoulder and elbow. Movement amplitude and velocity at one joint were held constant, while the direction of movement about the other joint was varied. When the direction of elbow motion was varied (flexion vs. extension) and shoulder kinematics were held constant, EMG activity in shoulder muscles varied depending on the direction of elbow motion (and hence the sign of the interaction torque arising at the shoulder). Similarly, EMG activity in elbow muscles varied depending on the direction of shoulder motion for movements in which elbow kinematics were held constant. The results from all three experiments support the idea that central control signals to muscles are adjusted, in a predictive manner, to compensate for interaction torques-loads arising at one joint that depend on motion about other joints.     

Horizontal-plane arm movements with direction reversals performed by normal individuals and individuals with down syndrome

We examined the systematic variation in shoulder and elbow torque, as well as movement kinematics, for horizontal-plane arm movements with direction reversals performed by normal individuals and individuals with Down syndrome. Eight neurologically normal individuals and eight individuals with Down syndrome performed horizontal, planar reversal movements to four different target locations. The four locations of the targets were chosen such that there is a systematic increase in elbow interaction torque for each of the four different target locations. This systematic increase in interaction torque has previously been shown to lead to progressively larger movement reversal errors, and trajectories that do not show a sharp reversal of direction, for movements to and from the target in patients who have proprioceptive abnormalities. We computed joint torques at the elbow and shoulder and found a high correlation between elbow and shoulder torque for the neurologically normal subjects. The ratio of joint torques varied systematically with target location. These findings extend previously reported findings of a linear synergy between shoulder and elbow joints for a variety of point-to-point movements. There was also a correlation between elbow and shoulder torque in individuals with Down syndrome, but the magnitude of the correlation was less. The ratio of joint torques changed systematically with target direction in individuals with Down syndrome but was slightly different from the ratio observed for neurologically normal individuals. The difference in the ratio was caused by the generation of proportionately more elbow torque than shoulder torque. The fingertip path of individuals with Down syndrome showed a sharp reversal in moving toward and then away from the target. In this respect, they were similar to neurologically normal individuals but dissimilar to individuals with proprioceptive deficits. Finally, we observed that individuals with Down syndrome spend proportionately more time in the vicinity of the target than normal individuals. Collectively these results show that there is a systematic relationship between joint torques at the elbow and shoulder. This relationship is present for reversal movements and is also present in individuals with Down syndrome.     

Unraveling the interaction between pathological upper limb synergies and compensatory trunk movements during reach-to-grasp after stroke: a cross-sectional study

The aim of the present study was to identify how pathological limb synergies between shoulder and elbow movements interact with compensatory trunk movements during a functional movement with the paretic upper limb after stroke. 3D kinematic joint and trunk angles were measured during a reach-to-grasp movement in 46 patients with stroke and 12 healthy individuals. We used principal component analyses (PCA) to identify components representing linear relations between the degrees of freedom of the upper limb and trunk across patients with stroke and healthy participants. Using multivariate logistic regression analysis, we investigated whether component scores were related to the presence or absence of basic limb synergies as indicated by the arm section of the Fugl-Meyer motor assessment (FMA). Four and three principal components were extracted in patients with stroke and healthy individuals, respectively. Visual inspection revealed that the contribution of joint and trunk angles to each component differed substantially between groups. The presence of the flexion synergy (Shoulder Abduction and Elbow Flexion) was reflected by component 1, whereas the compensatory role of trunk movements for lack of shoulder and elbow movements was reflected by components 2 and 3 respectively. The presence or absence of basic limb synergies as determined by means of the FMA was significantly related to components 2 (p = 0.014) and 3 (p = 0.003) in patients with stroke. These significant relations indicate that PCA is a useful tool to identify clinically meaningful interactions between compensatory trunk movements and pathological synergies in the elbow and shoulder during reach-to-grasp after stroke.     

A kinematic comparison of single and multijoint pointing movements

Summary Rapid pointing movements (no accuracy or reaction time requirements) were performed under three conditions which limited motion to the shoulder, elbow or a combination of these two joints. Velocity profiles of the hand's trajectory differed during single and multijoint movements. For the same magnitude of displacement, the hand always had a higher peak velocity, shorter rise time (time to peak velocity) and shorter movement time during single joint movements. However, when the profiles were normalized with respect to amplitude and movement time, no significant differences were observed between these three movement conditions. The velocity profiles of the elbow and/or shoulder were also compared under single and multijoint movement conditions. Analysis of these profiles revealed that the relationships between peak velocity and displacement and between movement time and displacement remained the same at the shoulder joint during single and multijoint movements. In contrast, the elbow joint velocity profiles were significantly affected by movement conditions. These relationships (peak velocity/ displacement and movement time/displacement) changed during multijoint movements and became the same as those observed at the shoulder joint. The shape of the hand velocity profile and its invariance across movement conditions can best be explained by dynamic optimization theory and supports the notion that movement of the hand is of primary importance during rapid pointing. However, the consistency of the shoulder velocity profile and the highly significant relationships between the movement of the elbow and shoulder joints indicates that a subordinate joint planning strategy is also used. The purpose of this strategy is to functionally decrease the available degrees of freedom and to simplify coordination between the moving joints. Thus, the organization of arm movements is hierarchically structured with important, but different contributions being made on both the hand planning and joint planning levels.     

A novel shoulder–elbow mechanism for increasing speed in a multijoint arm movement

The speed of arm movements is normally increased by increasing agonist muscle activity, but in overarm throwing, an additional effect on speed may come from exploitation of interaction torques (a passive torque associated with motion at adjacent joints). We investigated how the central nervous system (CNS) controls interaction torques at the shoulder and elbow to increase speed in 2-D overarm throwing. Twelve experienced throwers made slow, medium, and fast 2-D throws in a parasagittal plane. Joint motions were computed from recordings made with search coils; joint torques were calculated using inverse dynamics. For slow and medium-speed throws, elbow extension was primarily produced by elbow muscle torque. For fast throws, there was an additional late-occurring elbow extensor interaction torque. Parceling out this elbow extension interaction torque revealed that it primarily arose from shoulder extension deceleration. Surprisingly, shoulder deceleration before ball release was not caused by shoulder flexor (antagonist) muscle torque. Rather, shoulder deceleration was produced by passive elbow-to-shoulder interaction torques that were primarily associated with elbow extension acceleration and velocity. It is concluded that when generating fast 2-D throws, the CNS utilized the arm’s biomechanical properties to increase ball speed. It did this by coordinating shoulder and elbow motions such that an instantaneous mechanical positive feedback occurred of interaction torques between shoulder and elbow before ball release. To what extent this mechanism is utilized in other fast multijoint arm movements remains to be determined.     

Commonalities and differences in control of various drawing movements

Characteristics of control at the shoulder and elbow during nine types of drawing movements were studied in the present work. The task was to repetitively track a template, depicted on a horizontal table, with the index finger at a cyclic frequency of 1.5 Hz. The templates were a circle, four ovals and four lines of different orientations. The wrist was immobilized and the movement consisted of rotations at the shoulder and elbow joints. The studied movements varied in a wide range with respect to the amplitude of elbow and shoulder movements and relative phase between them. Kinetic analysis included analysis of torque signs, impulses, and timing. It demonstrated that the role of muscle torque in movement production was different at the two joints. During eight out of the nine movement types, the muscle torque at the shoulder accelerated and decelerated this joint and almost completely coped with the influence of the interactive torque arising from elbow motion. Conversely, interactive torque generated by shoulder motion played a dominant role in elbow acceleration and deceleration, whereas muscle torque at the elbow adjusted passive elbow movement to the various template shapes. EMG data were in agreement with the conclusions made from the kinetic analysis. Collectively, these data support the hypothesis that the two joints have different functions in movement production. The shoulder creates a foundation for motion of the entire arm through the interactive torque, and the elbow serves as a fine-tuner of the end-point movement. Control of the shoulder was similar across the eight movement types and the differences in the end-point path were provided by variations in elbow control. The two joints exchanged roles during one movement type, namely, drawing the line tilted right. During this movement, the elbow musculature generated motion at this joint and the shoulder musculature counteracted mechanical influence of this motion on the shoulder position. The findings suggest that during drawing movements, the control strategy exploits intersegmental dynamics of the shoulder-elbow mechanical linkage.     

Interjoint coordination dynamics during reaching in stroke

A technique is described that characterizes the dynamics of the interjoint coordination of arm reaching movements in healthy subjects (n=10) and in patients who had sustained a left-sided cerebrovascular accident (n=18). All participants were right-handed. Data from the affected right arm of patients with stroke were compared with those from the right arm of healthy subjects. Seated subjects made 25 pointing movements in a single session. Movements were made from an initial target located ipsilaterally to the right arm beside the body, to a final target located in front of the subject in the contralateral arm workspace. Kinematic data from the finger, wrist, elbow, both shoulders and sternum were recorded in three dimensions at 200 Hz with an optical tracking system. Analysis of interjoint coordination was based on the patterns of temporal delay between rotations at two adjacent joints (shoulder and elbow). The data were reduced to a single graph (Temporal Coordination or TC index) integrating the essential temporal characteristics of joint movement (the angular displacements, velocities and timing). TC segments, duration and amplitude, were analysed. The analysis was sensitive to the differences in interjoint coordination between healthy subjects and patients with arm motor deficits. In patients, the temporal coordination between elbow and shoulder movements was disrupted from the middle to the end of the reach. More specifically, in mid-reach, all patients had difficulty coordinating elbow flexion with shoulder horizontal adduction. In addition, patients with severe arm hemiparesis had difficulty changing elbow movement direction from flexion to extension and in coordinating this change with shoulder movement. At the end of the reach, patients with severe hemiparesis had deficits in the execution of elbow extension while all patients had impaired coordination of elbow extension and shoulder horizontal adduction. In addition, active ranges of joint motions were significantly decreased in the stroke compared to the healthy subjects. Finally, TC analysis revealed significant relationships between specific aspects of disrupted interjoint coordination and the level of motor impairment, suggesting that it may be a useful tool in the identification of specific movement coordination deficits in neurological impaired populations that can be targeted in treatment for arm motor recovery.     

Influence of movement speed on accuracy and coordination of reaching movements to memorized targets in three-dimensional space in a deafferented subject

Multiarticular reaching movements at different speeds produce differential demands for the on-line control of ongoing movements and for the predictive control of intersegmental dynamics. The aim of this study was to assess the ability of a proprioceptively deafferented patient and aged-matched control subjects to make precise and coordinated three-dimensional reaching movements at different speeds without vision during the movement. A patient with a complete loss of proprioception below the neck (C.F.) and five control subjects made reaching movements to four remembered visual targets at slow, natural, and fast speeds. All movements were performed without vision of the arm during the movements. The spatial accuracy, the movement kinematics and the interjoint coordination of these movements were analyzed. Results showed that control subjects made larger spatial errors at both slow and fast speeds than at natural speed. However, they synchronized motions at the shoulder and elbow joints and kept most movement kinematic features invariant across speed conditions. In contrast, C.F. failed to produce smooth and simultaneous motions at the shoulder and elbow joints at all speeds. Surprisingly, however, he made much larger errors than control subjects at slow and natural speeds, but not at fast speed. Analysis of patterns of interjoint coordination revealed that, when instructed to move fast, C.F. initiated arm movements by fixing the elbow while moving the shoulder joint to damp interaction torques exerted on the elbow joint from motion of the upper arm. The results demonstrated that, although proprioceptive loss disrupted normal control of multijoint movements at all speeds, when performing relatively fast three-dimensional movements, C.F. could control intersegmental dynamics by reducing the number of active joints. More importantly, the results highlight the dual role of proprioception in controlling multijoint movements; that is, to provide important cues both for the predictive control of interaction torques and for the synchronization of adjacent joints even when interactive torques are very small. These findings support the idea that proprioceptive input is used by the CNS to update an internal model of limb dynamics that adapts the motor plan according to biomechanical contexts. Electronic Publication     

The effects of muscle fatigue and movement height on movement stability and variability

Performing repetitive manual tasks can lead to muscle fatigue, which may induce changes in motor coordination, movement stability, and kinematic variability. In particular, movements performed at or above shoulder height have been associated with increased shoulder injury risk. The purpose of this study was to determine the effects of repetitive motion-induced muscle fatigue on posture and on the variability and stability of upper extremity movements. Ten healthy subjects performed a repetitive task similar to sawing continuously until volitional exhaustion. This task was synchronized with a metronome to control movement timing. Subjects performed the sawing task at shoulder (“High”) and sternum height (“Low”) on two different days. Joint angles and muscle activity were recorded continuously. Local and orbital stability of joint angles, kinematic variability (within subject standard deviations), and peak joint angles were calculated for five bins of data spaced evenly across each trial. Subjects fatigued more quickly when movements were performed at the High height. They also altered their kinematic patterns significantly in response to muscle fatigue. These changes were more pronounced when the task was performed at the High height. Subjects also exhibited increased kinematic variability of their movements post-fatigue. Increases in variability and altered coordination did not lead to greater instability, however. Shoulder movements were more locally stable when the task was performed at the High height. Conversely, shoulder and elbow movements were more orbitally unstable for the High condition. Thus, people adapt their movement strategies in multi-joint redundant tasks and maintain stability in doing so.     

Shoulder and elbow joint power differ as a general feature of vertical arm movements

Consistent patterns of joint power underlie coordinated lower extremity behaviors such as running and walking. Recent work found that shoulder and elbow power consistently differed during reaching movements in the horizontal plane. Moreover, joint power during horizontal reaching appears correlated with motor cortical activity. It is not known if the feature of differential joint power extends to vertical plane reaches or to reaches of different movement conditions. The purpose of this study was to test for differential shoulder and elbow power during the acceleratory and deceleratory phases of fast and normal speed vertical reaches in sitting and supine positions. Our results suggest that shoulder and elbow power typically differed both within and across conditions. First, shoulder power values were positive or negative dependent largely on movement direction and movement phase. That is, for each direction and phase, the shoulder either generated or absorbed energy independent of speed or body position. Second, and unexpectedly, reaches of certain condition combinations had similar shoulder power magnitudes across directions. In contrast, elbow power values for each direction varied between positive and negative values depending on phase, speed and position, and no two condition combinations overlapped across directions. Third, as target direction, movement phase and body position varied, shoulder power at fast and normal speeds were linearly correlated, as was shoulder power in sitting and in supine. In contrast, elbow power was linearly correlated only between speeds. These results join other studies to suggest that the neuromotor control of the shoulder may be less complex as compared to the elbow as a general feature of reaching movements. This differential control has important implications for the study of reaching impairments in neurorehabilitation populations, and provides a potentially important variable in the study of cortical firing patterns.

Persistence of inter-joint coupling during single-joint elbow flexions after shoulder fixation

Single-joint elbow flexions are associated with muscle activity at the shoulder that opposes interaction torques arising from rotation of the elbow. We have previously shown that this activity is linearly related to elbow muscle torque and is robust in the presence of novel dynamic loads. Here we examined this relationship in the context of shoulder joint fixation. We tested the hypothesis that after mechanically fixing the shoulder the relationship between shoulder muscle activity and elbow muscle torque will be preserved. In contrast, proposals in which energetic variables are optimized predict that shoulder muscle activity should cease. Subjects performed single-joint elbow flexions in a horizontal plane while interacting with the KINARM robotic exoskeleton. After repeated movements with the shoulder joint fixed we observed a slight and gradual decrease in the activity of pectoralis major relative to movements in which the shoulder was free to rotate. However the strength of the coupling between the shoulder and elbow did not change after shoulder fixation. This is consistent with our previous findings and suggests that the nervous system maintains this inter-joint coupling relationship even when activity at the fixed joint is no longer needed for movement accuracy.     

Control of 3D limb dynamics in unconstrained overarm throws of different speeds performed by skilled baseball players

This study investigated how the human CNS organizes complex three-dimensional (3D) ball-throwing movements that require both speed and accuracy. Skilled baseball players threw a baseball to a target at three different speeds. Kinematic analysis revealed that the fingertip speed at ball release was mainly produced by trunk leftward rotation, shoulder internal rotation, elbow extension, and wrist flexion in all speed conditions. The study participants adjusted the angular velocities of these four motions to throw the balls at three different speeds. We also analyzed the dynamics of the 3D multijoint movements using a recently developed method called "nonorthogonal torque decomposition" that can clarify how angular acceleration about a joint coordinate axis (e.g., shoulder internal rotation) is generated by the muscle, gravity, and interaction torques. We found that the study participants utilized the interaction torque to generate larger angular velocities of the shoulder internal rotation, elbow extension, and wrist flexion. To increase the interaction torque acting at these joints, the ball throwers increased muscle torque at the shoulder and trunk but not at the elbow and wrist. These results indicates that skilled ball throwers adopted a hierarchical control in which the proximal muscle torques created a dynamic foundation for the entire limb motion and beneficial interaction torques for distal joint rotations.     

Effect of speed and precision demands on human shoulder muscle electromyography during a repetitive task

Effects of speed and precision on electromyography (EMG) in human shoulder muscles were studied during a hand movement task where five points were marked repeatedly with a pencil. Six female subjects performed with three precision demands and at four speeds. Three of the speeds were predefined, while the last speed was performed as fast as possible. The EMG were recorded from 13 shoulder muscles or parts of muscles. Elbow velocity, acceleration and rectified EMG were calculated for each task. The mean elbow velocity and acceleration increased with speed and precision demands. There was an increase in EMG as the speed demand increased for all three precision demands (P < 0.001), and as the precision demand increased for the two highest predefined speed demands (P < 0.05). The combination of a high speed and a high precision demand resulted in the highest EMG. Different EMG levels were attained for the 13 muscles and the supraspinatus muscle always showed the highest normalized EMG. However, analysis of variance showed the same relative increase for all muscles with speed and precision demands. The EMG changes in response to precision demand can only be explained in part by the differences in movement velocity and acceleration, and other factors such as increased co-contraction must also be taken into account.     

Causes of left-right ball inaccuracy in overarm throws made by cerebellar patients

Cerebellar patients throw inaccurately in the left-right direction but the cause of this multijoint ataxia is unclear. We tested whether it was due, as originally proposed, to variable left-right directions of the hand path, or, alternatively, to variable timing of ball release occurring on a right to left curved hand path. We also examined the cause of the variability in hand path direction per se. Six right-handed cerebellar patients and six control subjects were instructed to throw tennis balls at a slow, medium and fast speed from a seated position while angular positions in 3D of five arm segments were recorded at 1000 Hz with the search-coil technique. Compared to controls, cerebellar patients threw slower and less accurately, had more variable timing of ball release occurring on a right to left curved hand path and had more variable left-right directions of hand paths at a fixed point in front of the sternum. In all cerebellar patients, ball left-right inaccuracy was related both to timing of ball release and to hand path direction at the fixed point. The cause of the increased variability in hand path direction varied between patients and could not be explained by disorder in a single joint rotation. No evidence was found that it resulted from variable stabilization at the shoulder during elbow extension. Instead, the more variable left-right direction of the hand path was related to the initial pattern of joint rotations occurring early in the throw before the onset of elbow extension, and to the amplitudes of radioulnar pronation and wrist abduction occurring late in the throw. The results emphasize that in the presence of a cerebellar lesion, ball left-right inaccuracy in overarm throws cannot be explained by a single disorder. Rather ball inaccuracy was likely due to disorders in central commands to proximal joint rotations that produced the hand path and in central commands to distal joints that controlled the timing of finger opening.     

Target-dependent differences between free and constrained arm movements in chronic hemiparesis

This study compares the kinematic and kinetic characteristics of constrained and free upper limb movements in eight subjects with chronic hemiparesis. Movements of the dominant and nondominant limbs were also examined in five control subjects. Rapid movements were performed in the horizontal plane from a central starting point to five targets located to require various combinations of flexion/extension rotations at the elbow and shoulder. Support of the upper limb against gravity loading was provided either by a low-friction air-bearing apparatus (constrained condition) or by voluntary generation of abduction and external rotation torques at the shoulder (free condition). Data analysis focused on the peak joint torques generated during the acceleratory phase of movement, and on the net change in joint angles at the elbow and shoulder. We found that movement parameters were broadly invariant with support condition for either limb of control subjects, as well as for the nonparetic limb of hemiparetic subjects. In contrast, support condition had a target-dependent effect on movements of the paretic limb. Relative to the constrained condition, peak torques for free arm movements were significantly reduced for distal targets requiring elbow extension and/or shoulder flexion torques. However, peak elbow flexion and shoulder extension joint torques for proximal targets were relatively unaffected by support condition. Of perhaps more functional importance, free movements were characterized by a target-dependent restriction in the hands work area that reflected a reduced range of active elbow extension, relative to constrained movements. The target-dependent effects of support condition on movements of the paretic limb are consistent with the existence of abnormal constraints on muscle activation patterns in subjects with chronic hemiparesis, namely an abnormal linkage between activation of the elbow flexors and shoulder extensors, abductors, and external rotators.     

Shoulder and elbow muscle activity in goal-directed arm movements

 This research examined the electromyographic (EMG) activity of shoulder and elbow muscles during reaching movements of the upper limb. Subjects performed goal-directed arm movements in the horizontal plane. Movements which varied in amplitude, speed, and direction were performed in different sections of the workspace. EMG activity was recorded from the pectoralis major, posterior deltoid, biceps brachii short head, brachioradialis, triceps brachii long head, and triceps brachii lateral head; motion recordings were obtained with an optoelectric system. The analysis focused on the magnitude and timing of opposing muscle groups at the shoulder and elbow joints. For hand movements within any given direction of the workspace direction, kinematic manipulations changed agonist and antagonist EMG magnitude and intermuscle timing in a manner consistent with previous single-joint findings. To produce reaching movements in different directions and areas of the workspace, shoulder and elbow agonist EMG magnitude increased for those hand motions which required higher angular velocities, while the timing between opposing muscle groups at each joint was invariant. Received: 11 January 1996 / Accepted: 24 February 1997