Reprogramming of muscle activation patterns at the wrist in compensation for elbow reaction torques during planar two-joint arm movements

The relationship between wrist kinematics, dynamics and the pattern of muscle activation were examined during a two-joint planar movement in which the two joints moved in opposite directions, i.e. elbow flexion/wrist extension and elbow extension/wrist flexion. Elbow movements (ranging from 10 to 70 deg) and wrist movements (ranging from 10 to 50 deg) were performed during a visual, step-tracking task in which subjects were required to attend to the initial and final angles at each joint. As the elbow amplitude increased, wrist movement duration increased and the wrist movement trajectories became quite variable. Analysis of the torques acting at the wrist joint showed that elbow movements produced reaction torques acting in the same direction as the intended wrist movement. Distinct patterns of muscle activation were observed at the wrist joint that were dependent on the relative magnitude of the elbow reaction torque in relation to the net wrist torque. When the magnitude of the elbow reaction torque was quite small, the wrist agonist was activated first. As the magnitude of the elbow reaction torque increased, activity in the wrist agonist decreased significantly. In conditions where the elbow reaction torque was much larger than the net wrist torque, the wrist muscle torque reversed direction to oppose the intended movement. This reversal of wrist muscle torque was directly associated with a change in the pattern of muscle activation where the wrist antagonist was activated prior to the wrist agonist. Our findings indicate that motion of the elbow joint is an important consideration in planning wrist movement. Specifically, the selection of muscle activation patterns at the wrist is dependent on the relative magnitude and direction of the elbow reaction torque in relation to the direction of wrist motion.     

Control of the wrist in three-joint arm movements to multiple directions in the horizontal plane

In a reaching movement, the wrist joint is subject to inertial effects from proximal joint motion. However, precise control of the wrist is important for reaching accuracy. Studies of three-joint arm movements report that the wrist joint moves little during point-to-point reaches, but muscle activities and kinetics have not yet been described across a range of movement directions. We hypothesized that to minimize wrist motion, muscle torques at the wrist must perfectly counteract inertial effects arising from proximal joint motion. Subjects were given no instructions regarding joint movement and were observed to keep the wrist nearly motionless during center-out reaches to directions throughout the horizontal plane. Consistent with this, wrist muscle torques exactly mirrored interaction torques, in contrast to muscle torques at proximal joints. These findings suggest that in this reaching task the nervous system chooses to minimize wrist motion by anticipating dynamic inertial effects. The wrist muscle torques were associated with a direction-dependent choice of muscles, also characterized by initial reciprocal activation rather than initial coactivation to stiffen the wrist joint. In a second experiment, the same pattern of muscle activities persisted even after many trials reaching with the wrist joint immobilized. These results, combined with similar features at the three joints, such as cosine-like tuning of muscle torques and of muscle onsets across direction, suggest that the nervous system uses similar rules for muscles at each joint, as part of one plan for the arm during a point-to-point reach.     

The leading joint hypothesis for spatial reaching arm motions

The leading joint hypothesis (LJH), developed for planar arm reaching, proposes that the interaction torques experienced by the proximal joint are low compared to the corresponding muscle torques. The human central nervous system could potentially ignore these interaction torques at the proximal (leading) joint with little effect on the wrist trajectory, simplifying joint-level control. This paper investigates the extension of the LJH to spatial reaching. In spatial motion, a number of terms in the governing equation (Euler’s angular momentum balance) that vanish for planar movements are non-trivial, so their contributions to the joint torque must be classified as net, interaction or muscle torque. This paper applies definitions from the literature to these torque components to establish a general classification for all terms in Euler’s equation. This classification is equally applicable to planar and spatial motion. Additionally, a rationale for excluding gravity torques from the torque analysis is provided. Subjects performed point-to-point reaching movements between targets whose locations ensured that the wrist paths lay in various portions of the arm’s spatial workspace. Movement kinematics were recorded using electromagnetic sensors located on the subject’s arm segments and thorax. The arm was modeled as a three-link kinematic chain with idealized spherical and revolute joints at the shoulder and elbow. Joint torque components were computed using inverse dynamics. Most movements were ‘shoulder-led’ in that the interaction torque impulse was significantly lower than the muscle torque impulse for the shoulder, but not the elbow. For the few elbow-led movements, the interaction impulse at the elbow was low, while that at the shoulder was high, and these typically involved large elbow and small shoulder displacements. These results support the LJH and extend it to spatial reaching motion.     

General coordination of shoulder,elbow and wrist dynamics during multijoint arm movements

Studies of multijoint arm movements have demonstrated that the nervous system anticipates and plans for the mechanical effects that arise from motion of the linked limb segments. The general rules by which the nervous system selects appropriate muscle activities and torques to best deal with these intersegmental effects are largely unknown. In order to reveal possible rules, this study examined the relationship of muscle and interaction torques to joint acceleration at the shoulder, elbow and wrist during point-to-point arm movements to a range of targets in the horizontal plane. Results showed that, in general, dynamics differed between the joints. For most movements, shoulder muscle torque primarily determined net torque and joint acceleration, while interaction torque was minimal. In contrast, elbow and wrist net torque were determined by a combination of muscle and interaction torque that varied systematically with target direction and joint excursion. This "shoulder-centered pattern" occurred whether subjects reached targets using straight or curved finger paths. The prevalence of a shoulder-centered pattern extends findings from a range of arm movement studies including movement of healthy adults, neurological patients, and simulations with altered interaction effects. The shoulder-centered pattern occurred for most but not all movements. The majority of the remaining movements displayed an "elbow-centered pattern," in which muscle torque determined initial acceleration at the elbow and not at the shoulder. This occurred for movements when shoulder excursion was <50% of elbow excursion. Thus, both shoulder- and elbow-centered movements displayed a difference between joints but with reversed dynamics. Overall, these findings suggest that a difference in dynamics between joints is a general feature of horizontal plane arm movements, and this difference is most commonly reflected in a shoulder-centered pattern. This feature fits well with other general shoulder-elbow differences suggested in the literature on arm movements, namely that: (a) agonist muscle activity appears more closely related to certain joint kinematics at the shoulder than at the elbow, (b) adults with neurological damage display less disruption of shoulder motion than elbow motion, and (c) infants display adult-like motion first in the shoulder and last at the wrist.     

Electromyographic responses to a mechanical perturbation applied during impending arm movements in different directions: one-joint and two-joint conditions

Directional tuning is a common finding for many physiological features of arm movements and related neuronal activity. We investigated whether the electromyographic response to a brief (30 ms) torque perturbation prior to voluntary movement depends on the direction of the impending movement. Pointing movements with the elbow joint alone and those involving both the shoulder and elbow joints were studied in separate experiments. Target direction was varied between flexion and extension for the one-joint experiments and among four spatial directions for the two-joint experiments. Movement trials in which a perturbation stretched the flexor muscles just prior to the pointing movement were randomly interspersed among unperturbed movement trials in each experiment. A small pre-load ensured some background activity of the flexor muscles. Results were remarkably similar for the one- and two-joint conditions. The short-latency reflex response of the stretched muscles (in a 30-60 ms window after perturbation onset) was not modulated with direction of target-reaching movement in a statistically significant manner, which confirms earlier findings for one-joint movements and extends these to the two-joint condition. Beyond the short-latency window, the perturbation provoked earlier onsets of target-reaching muscle activities for the agonist muscles, whether or not the muscle had been stretched by the perturbation. The onset of the braking activity of the antagonist muscles also occurred earlier in the presence of the brief perturbation prior to movement, irrespective of whether the muscle had been stretched or not. The magnitude of target-reaching muscle activity, in general, was greater for the perturbed trials, though not consistently for all muscles or all directions. These results suggest that, when movement is about to be initiated, in either single- or multi-joint conditions, the long-latency effects of the stretch strongly depend on the intended direction of movement. The dependence is such that the response serves to hasten and augment the intended movement, but not necessarily to oppose the perturbation.     

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     

Common principles underlying the control of rapid, single degree-of-freedom movements at different joints

Studies of rapid, single degree-of-freedom movements have shown different changes in electromyographic patterns for movement tasks that appear very similar (e.g., movements over different ranges of distance). However, it is not clear whether these differences are a result of joint-specific control schemes or whether they are instead due to the limited range of task parameters studied relative to the mechanical constraints of each joint (e.g., short compared with long movements relative to the range of motion of a particular joint). In this study, we measured and compared the kinematic trajectories and electromyograms recorded during various movement tasks at the wrist, elbow, and ankle. Subjects performed movements over a wide range of distances “as fast as possible,”“at a comfortable speed,” and against two inertial loads (at the elbow only), and they performed movements over a fixed distance at three different speeds at the wrist and ankle. For fast movements we show that, in spite of some joint-specific differences, the basic pattern of electromyographic (EMG) modulation is similar at all three joints; for example, the agonist EMG burst transitions from a fixed duration to an increasing duration with increasing movement distance at all three joints. Moreover, the distance at which this transition occurs in one joint relative to the distance at which this transition occurs in the other two joints is consistent across subjects. The transition occurs at the shortest distance at the ankle and the longest distance at the wrist. In general we suggest that the data are consistent with a single set of control rules applied at all three joints, with the biomechanical constraints at each joint accounting for the differences in the EMG and kinematic patterns observed across joints. Received: 3 September 1996 / Accepted: 10 June 1997     

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.     

Detection of simultaneous movement at two human arm joints

To detect joint movement, the brain relies on sensory signals from muscle spindles that sense the lengthening and shortening of the muscles. For single-joint muscles, the unique relationship between joint angle and muscle length makes this relatively straightforward. However, many muscles cross more than one joint, making their spindle signals potentially ambiguous, particularly when these joints move in opposite directions. We show here that simultaneous movement at adjacent joints sharing biarticular muscles affects the threshold for detecting movements at either joint whereas it has no effect for non-adjacent joints. The angular displacements required for 70% correct detection were determined in 12 subjects for movements imposed on the shoulder, elbow and wrist at angular velocities of 0.25–2 deg s⁻¹. When moved in isolation, detection thresholds at each joint were similar to those reported previously. When movements were imposed on the shoulder and wrist simultaneously, there were no changes in the thresholds for detecting movement at either joint. In contrast, when movements in opposite directions at velocities greater than 0.5 deg s⁻¹ were imposed on the elbow and wrist simultaneously, thresholds increased. At 2 deg s⁻¹, the displacement threshold was approximately doubled. Thresholds decreased when these adjacent joints moved in the same direction. When these joints moved in opposite directions, subjects more frequently perceived incorrect movements in the opposite direction to the actual. We conclude that the brain uses potentially ambiguous signals from biarticular muscles for kinaesthesia and that this limits acuity for detecting joint movement when adjacent joints are moved simultaneously.     

Hierarchical control of different elbow-wrist coordination patterns

The present paper focused on the role of mechanical factors arising from the multijoint structure of the musculoskeletal system and their use in the control of different patterns of cyclical elbow-wrist movements. Across five levels of cycling frequency (from 0.45 Hz up to 3.05 Hz), three movement patterns were analyzed: (1) unidirectional, including rotations at the elbow and wrist in the same direction; (2) bidirectional, with rotation at the joints in opposite directions, and (3) free-wrist pattern, which is characterized by alternating flexions and extensions at the elbow with the wrist relaxed. Angular position of both joints and electromyographic activity of biceps, triceps, the wrist flexor, and the wrist extensor were recorded. It was demonstrated that control at the elbow was principally different from control at the wrist. Elbow control in all three patterns was similar to that typically observed during single-joint movements: elbow accelerations-decelerations resulted from alternating activity of the elbow flexor and extensor and were largely independent of wrist motion at all frequency plateaus. The elbow muscles were responsible not only for the elbow movement, but also for the generation of interactive torques that played an important role in wrist control. There were two types of interactive torques exerted at the wrist: inertial torque arising from elbow motion and restraining torque arising from physical limits imposed on wrist rotation. These interactive torques were the primary source of wrist motion, whereas the main function of wrist-muscle activity was to intervene with the interactive effects and to adjust the wrist movement to comply with the required coordination pattern. The unidirectional pattern was more in agreement with interactive effects than the bidirectional pattern, thus causing their differential difficulty at moderate cycle frequencies. When cycling frequency was further increased, both the unidirectional and bidirectional movements lost their individual features and acquired features of the free-wrist pattern. The deterioration of the controlled patterns at high cycling frequencies suggests a crucial role for proprioceptive information in wrist control. These results are suppportive of a hierachical organization of control with respect to elbow-wrist coordination, during which the functions of control at the elbow and wrist are principally different: the elbow muscles generate movement of the whole linkage and the wrist muscles produce corrections of the movement necessary to fulfill the task. Received: 5 August 1997 / Accepted: 29 January 1998     

Cerebellar dysmetria at the elbow, wrist, and fingers

1. The objective was to investigate in cerebellar patients with dysmetria the kinematic and electromyographic (EMG) characteristics of large and small movements at the elbow, wrist, and finger and thereby to determine the nature of cerebellar dysmetria at distal as well as proximal joints. Flexions were made as fast as possible by moving relatively heavy manipulanda for each joint to the same end position through 5, 30, and 60 degrees. 2. In normal subjects flexions at all joints were accompanied by similar triphasic EMG activity. Movements of increasing amplitude were made with increasing movement durations and increasing durations and magnitudes of initial agonist EMG activity. Antagonist activity often appeared to have two components: one coactive with the initial agonist burst but starting later, the other reaching its peak at about peak velocity. 3. Cerebellar patients with dysmetria showed hypermetria followed by tremor at all three joints when movements were made with the manipulanda. Hypermetria was most marked for aimed movements of small amplitude (5 degrees) at all joints. 4. A characteristic of cerebellar disordered movements, which could be present at all amplitudes and all joints, was an asymmetry with decreased peak accelerations and increased peak decelerations compared to normal movements. Both the asymmetry and the hypermetria for small amplitude movements could be used clinically as sensitive indicators of cerebellar dysfunction. 5. The EMG abnormalities accompanying hypermetria and asymmetry were a more gradual buildup and a prolongation of agonist activity and delayed onset of antagonist activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expertise-dependent modulation of muscular and non-muscular torques in multi-joint arm movements during piano keystroke

The problem of skill-level-dependent modulation in the joint dynamics of multi-joint arm movements is addressed in this study using piano keystroke performed by expert and novice piano players. Using the measured kinematic and key-force data, the time varying net, gravitational, motion-dependent interaction (INT), key-reaction (REA), and muscular (MUS) torques at the shoulder, elbow, wrist, and metacarpophalangeal (MP) joints were computed using inverse dynamics techniques. INTs generated at the elbow and wrist joints, but not those at the MP joint, were greater for the experts as compared with the novices. REA at the MP joint, but not at the other joints, was less for the experts as compared with the novices. The MUSs at the MP, wrist, and elbow joints were smaller, and that at the shoulder joint was larger for the experts as compared with the novices. The experts also had a lesser inter-strike variability of key striking force and key descending velocity as compared with the novices. These findings indicated that the relationship among the INT, REA, and MUS occurring at the joints of the upper-extremity differed between the expert and novice piano players, suggesting that the organization of multi-joint arm movement is modulated by long-term motor training toward facilitating both physiological efficiency and movement accuracy.     

Effects of inactivation of the anterior interpositus nucleus on the kinematic and dynamic control of multijoint movement

We previously showed that inactivating the anterior interpositus nucleus in cats disrupts prehension; paw paths, normally straight and accurate, become curved, hypometric, and more variable. In the present study, we determined the joint kinematic and dynamic origins of this impairment. Animals were restrained in a hammock and trained to reach and grasp a cube of meat from a narrow food well at varied heights; movements were monitored using the MacReflex analysis system. The anterior interpositus nucleus was inactivated by microinjection of the GABA agonist muscimol (0.25-0.5 microgram in 0.5 microliter saline). For each joint, we computed the torque due to gravity, inertial resistance (termed self torque), interjoint interactions (termed interaction torque), and the combined effects of active muscle contraction and passive soft tissue stretch (termed generalized muscle torque). Inactivation produced significant reductions in the amplitude, velocity, and acceleration of elbow flexion. However, these movements continued to scale normally with target height. Shoulder extension was reduced by inactivation but wrist angular displacement and velocity were not. Inactivation also produced changes in the temporal coordination between elbow, shoulder, and wrist kinematics. Dynamic analysis showed that elbow flexion both before and during inactivation was produced by the combined action of muscle and interaction torque, but that the timing depended on muscle torque. Elbow interaction and muscle torques were scaled to target height both before and during inactivation. Inactivation produced significant reductions in elbow flexor interaction and muscle torques. The duration of elbow flexor muscle torque was prolonged to compensate for the reduction in flexor interaction torque. Shoulder extension was produced by extensor interaction and muscle torques both before and during inactivation. Inactivation produced a reduction in shoulder extension, primarily by reduced interaction torque, but without compensation. Wrist plantarflexion, which occurred during elbow flexion, was driven by plantarflexor interaction and gravitational torques both before and during inactivation. Muscle torque acted in the opposite direction with a phase lead to restrain the plantarflexor interaction torque. During inactivation, there was a reduction in plantarflexor interaction torque and a loss of the phase lead of the muscle torque. Our findings implicate the C1/C3 anterior interpositus zone of the cerebellum in the anticipatory control of intersegmental dynamics during reaching, which zone is required for coordinating the motions of the shoulder and wrist with those of the elbow. In contrast, this cerebellar zone does not play a role in scaling the movement to match a target.     

Muscle activation patterns and kinetics of human index finger movements

1. The present study was conducted to determine whether dynamic interaction torques are significant for control of digit movements and to investigate whether such torques are compensated by specific muscle activation patterns. 2. Angular positions of the metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints of the index finger in the flexion/extension plane were recorded with the use of planar electrogoniometers. Muscle activation patterns were monitored with the use of fine wire and surface electromyography of intrinsic and extrinsic finger muscles. 3. Dynamic interaction torques associated with index finger movements were large in relation to joint torques produced by muscles, especially in faster movements. The significance of dynamic interaction torques was demonstrated in model simulations of two-joint finger motion in response to joint torque inputs. Removal of interaction torques from the model inputs produced movements that differed greatly from digit motions produced by human subjects. 4. Electromyogram (EMG) and torque patterns associated with finger movements of different speeds indicated that muscle activity is necessary not only for producing motion at the joints but also to counteract segmental interaction torques. This was especially evident during movements that required voluntary maintenance of a constant MP joint angle during motion of the distal segment about the PIP joint. Under these conditions, muscle moments acting at the MP acted directly to counteract torques at the MP arising from motion at the PIP. 5. Neural mechanisms underlying control of index finger movement are discussed with reference to the implications of dynamic interaction torques. Potential control strategies include accurate programming of muscle activation patterns, appropriate use of motion-dependent peripheral afferent information, and control of the finger as a viscoelastic system through coactivation of flexor and extensor musculature. It is concluded that additional research incorporating study of motion in three dimensions and the use of mechanical models of the finger and related musculature is required to determine how interaction torques are compensated during finger motion.     

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.     

Kinetic and kinematic adaptation to anisotropic load

Different investigators have proposed that multi-joint arm movements are planned with respect to either the path of the hand or the forces and torques acting about the moving joints. In this experiment, we examined the kinematic and kinetic response of the motor system when a load was applied to the forearm, which reduced the natural anisotropy of the arm. We asked two questions: (1) when the movement path changes upon the introduction of the novel load, do muscle torques at the shoulder and elbow remain the same as they were before the load was applied? and (2) when the path is restored partially as the novel load is learned, do changes in muscle torque occur preferentially at one or the other joint? Participants performed rapid arm movements to a target with and without the novel load attached to their arm. Changes in hand path and muscle torque profiles were examined immediately after the application of the load and again following 30 practice trials. The introduction of the load increased the curvature of hand paths for each participant and resulted in changes in the magnitude and time course of muscle torque at both joints, although to a greater extent at the shoulder. After practice with the load, hand paths and elbow muscle torques resembled those produced with no load. Muscle torques produced at the shoulder, however, did not return to pre-load patterns. These observations provide support for the idea that movements are initiated by planned muscle torques and that as the movement proceeds muscle torques are regulated in order to produce hand paths that conform approximately to a kinematic plan.     

Effect of target size on spatial and temporal characteristics of a pointing movement in man

Summary The effects of constraints related to movement accuracy on the spatial and temporal characteristics of pointing movements of the arm to a target were investigated. It was found that movement time increased, even at slow speeds, when target size decreased. Spatial variability of the trajectory of the index finger was also reduced, but only in proximity to the target, when higher accuracy was demanded while variability of motion at the wrist showed little change. The effect of varying the angular orientation of the target on the trajectories of the wrist and finger was also investigated. The data support the hypothesis that motion at the shoulder and elbow joints, which is closely linked, is determined primarily by target position while motion at the wrist joint, which is only loosely coupled to the motion at the more proximal joints, is related principally to the angular orientation of the target in space. The data also suggest that wrist motion is controlled separately from motion at the more proximal joints.     

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.     

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.