Patterns of coordinated multi-joint movement

In multi-joint reaching movements, the motor system may choose any one of an infinite set of possible joint rotations to move the hand between given start and target positions. In order to find out whether reaching movements are represented in Cartesian hand coordinates or in joint coordinates, it is necessary to measure whether hand paths or joint paths have lower variability. We have measured hand paths and rotations of shoulder, elbow and wrist joints simultaneously in five subjects reaching in four orientations in the horizontal plane. As in earlier studies, we found a preference for nearly straight hand paths, despite different patterns of joint rotation for different orientations of movement. However, movements in three of four orientations showed a single principal joint, which rotated essentially without reversals. This may reflect optimisation in the motor system, preferring the simplest pattern of joint control for a desired hand path. We used generalised Procrustes analysis to quantify the variability in shape of repeated paths in hand space and joint space. Results showed that hand paths were less variable than the joint angles used to realise them, due to the kinematic redundancy of the limb, suggesting that hand paths, rather than joint angles, are directly represented by the motor system. Nevertheless, movements with straighter hand paths, on average, and those requiring coordinated activity at both shoulder and elbow joints also showed more variability in the shape of the hand path. Other orientations such as movement across the body use primarily a single joint and are less variable at the cost of a slightly curved path. These results suggest that coordinating multiple joints to produce a straight hand path has a definite computational cost. The motor system may perform a trade-off between the benefits of planning reaching movements as straight hand paths and the computational simplicity of executing them using patterns of joint rotation which simplify multi-joint coordination.     

Spatio-temporal and kinematic analysis of pointing movements performed by cerebellar patients with limb ataxia

Three patients with cerebellar limb ataxia and three age-matched controls performed arm-pointing movements towards a visual stimulus during an experimental procedure using a double-step paradigm in a three-dimensional space. Four types of trajectories were defined: P1, single-step pointing movement towards the visual stimulus in the initial position S1; P2, double-step pointing movement towards S1; P3, double-step straight pointing movement towards the second position S2; and P4, double-step pointing movement towards S2 with an initial direction towards S1. We found that the cerebellar patients, as well as the controls, were able to modify their motor programs, but with impaired timing, severe anomalies in the direction and amplitude of the changed movement trajectories and alteration of the precision of the pointing movements. Received: 26 February 1997 / Accepted: 13 October 1997     

Reprogramming of grip aperture in a double-step virtual grasping paradigm

 The present study investigated the control of manual prehension movements in humans. Subjects grasped luminous virtual discs with the thumb and index finger, and we recorded the instantaneous grip aperture, defined as the 3-D distance between the thumb and index finger. Target size could remain constant (single-step trials) or unexpectedly change shortly after target appearance (double-step trials). In single-step responses, grip aperture varied throughout the movement in a consistent fashion. Double-step responses exhibited distinct corrective modifications, which followed the target change with a latency similar to the normal reaction time. This suggests that visual size information has a fast and continuous access to the processes involved in grip formation. The grip-aperture profiles of single-step responses had a different shape when the target called for an increase than when it called for a decrease in the initial finger distance. The same asymmetry was observed for aperture corrections in double-step trials. These findings indicate that increases and decreases of grip aperture are controlled through separate processes, engaged equally by the appearance and by the size change of a target. Corrections of grip aperture in double-step trials had a higher peak velocity and reached their maximum as well as their final value earlier than the aperture profiles of single-step trials. Nevertheless, the total duration of double-step trials was prolonged. These response characteristics did not fit with either of the three corrective strategies previously proposed for double-step pointing movements, which could indicate that grasping and pointing movements are controlled by different mechanisms. However, more data are needed to substantiate this view. Received: 20 April 1998 / Accepted: 28 October 1998     

Differences in control of limb dynamics during dominant and nondominant arm reaching

This study compares the coordination patterns employed for the left and right arms during rapid targeted reaching movements. Six right-handed subjects reached to each of three targets, designed to elicit progressively greater amplitude interaction torques at the elbow joint. All targets required the same elbow excursion (20 degrees ), but different shoulder excursions (5, 10, and 15 degrees, respectively). Movements were restricted to the shoulder and elbow and supported on a horizontal plane by a frictionless air-jet system. Subjects received visual feedback only of the final hand position with respect to the start and target locations. For motivation, points were awarded based on final position accuracy for movements completed within an interval of 400-600 ms. For all subjects, the right and left hands showed a similar time course of improvement in final position accuracy over repeated trials. After task adaptation, final position accuracy was similar for both hands; however, the hand trajectories and joint coordination patterns during the movements were systematically different. Right hand paths showed medial to lateral curvatures that were consistent in magnitude for all target directions, whereas the left hand paths had lateral to medial curvatures that increased in magnitude across the three target directions. Inverse dynamic analysis revealed substantial differences in the coordination of muscle and intersegmental torques for the left and right arms. Although left elbow muscle torque contributed largely to elbow acceleration, right arm coordination was characterized by a proximal control strategy, in which movement of both joints was primarily driven by the effects of shoulder muscles. In addition, right hand path direction changes were independent of elbow interaction torque impulse, indicating skillful coordination of muscle actions with intersegmental dynamics. In contrast, left hand path direction changes varied directly with elbow interaction torque impulse. These findings strongly suggest that distinct neural control mechanisms are employed for dominant and non dominant arm movements. However, whether interlimb differences in neural strategies are a consequence of asymmetric use of the two arms, or vice versa, is not yet understood. The implications for neural organization of voluntary movement control are discussed.     

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.     

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.     

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     

Independent coactivation of shoulder and elbow muscles

 The aim of this study was to examine the possibility of independent muscle coactivation at the shoulder and elbow. Subjects performed rapid point-to-point movements in a horizontal plane from different initial limb configurations to a single target. EMG activity was measured from flexor and extensor muscles acting at the shoulder (pectoralis clavicular head and posterior deltoid) and elbow (biceps long head and triceps lateral head) and flexor and extensor muscles acting at both joints (biceps short head and triceps long head). Muscle coactivation was assessed by measuring tonic levels of electromyographic (EMG) activity after limb position stabilized following the end of the movements. It was observed that tonic EMG levels following movements to the same target varied as a function of the amplitude of shoulder and elbow motion. Moreover, for the movements tested here, the coactivation of shoulder and elbow muscles was found to be independent – tonic EMG activity of shoulder muscles increased in proportion to shoulder movement, but was unrelated to elbow motion, whereas elbow and double-joint muscle coactivation varied with the amplitude of elbow movement and were not correlated with shoulder motion. In addition, tonic EMG levels were higher for movements in which the shoulder and elbow rotated in the same direction than for those in which the joints rotated in opposite directions. In this respect, muscle coactivation may reflect a simple strategy to compensate for forces introduced by multijoint limb dynamics. Received: 7 July 1998 / Accepted: 28 July 1998     

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.     

The coordination between trunk and arm motion during pointing movements

The coordination between the trunk and arm of six subjects was examined during unrestrained pointing movements to five target locations. Two targets were within arm's length, three were beyond. The trunk participated in reaching primarily when the target could not be attained by arm and scapular motion. When the trunk did contribute to hand transport, its motion started simultaneously with arm movement and continued until target contact. Redundancy in the degrees of freedom used to execute the movement had no effect on the configuration of joints and segments used to attain a specified target; no difference in variability was noted regardless of whether redundancy existed. However, different configurations were used to achieve the same wrist coordinates along a common endpoint path, depending on the final position of the hand. The addition of trunk flexion, rotation and scapular motion did not alter the coupling between the elbow and shoulder joints and had no effect on the path of the hand or the smoothness of its velocity profile. Thus, trunk motion was integrated smoothly into the transport phase of the hand. As the trunk's contribution to hand transport increased, it played a progressively greater role in positioning the hand close to the target during the terminal stage of the reach. Of the movement components measured, trunk flexion was the last component to complete its motion when target reaches were made beyond arm's length. Hence, the trunk not only acts as a postural stabilizer during reaching, but becomes an integral component in positioning the hand close to the target.     

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.     

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.     

融合生物运动信息的上肢运动功能康复诊断方法研究

针对上肢运动功能的康复诊断问题,提出一种融合运动力学信息与生物电信息的综合性上肢运动功能评价方法,用于评价运动功能障碍患者肩肘腕关节的运动功能水平。在上肢进行动作时,对加速度信号和肌电信号进行信号采集、有效信号的选择、特征提取和特征筛选,并利用两类信号在动作分析中的不同优势,将信号的特征值进行优化组合。以简式Fugl-Meyer评分值为标准,构建多个线性回归模型,实现上肢运动功能的综合性评价。在对10位受试者肩肘腕的7个上肢动作（握拳、展拳、屈腕、伸腕、屈肘、伸肘、上肢平举）功能诊断实验中,提出的诊断方法不仅可以进行实时的信息提取和功能诊断,而且与Fugl-Meyer评价有很强的一致性,相关系数达99%以上。实验表明,该诊断方法能取代传统的上肢运动功能评价方法,并更细致地对上肢运动功能进行量化评分。     

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.     

Effect of motor planning on use of motor abundance

This study examined the hypothesis that the degree to which motor redundancy is used to coordinate joint motions for reaching is influenced by motor planning and enhanced when the task requires greater movement flexibility. Subjects reached at arm's length to the same centrally placed target under conditions where the target location was either certain or uncertain, using a double-step paradigm. The hypothesis was evaluated by partitioning the across-trials variance of the joint configuration at each percent of the reach into a component corresponding to the use of different joint angle combinations to achieve an equivalent hand position (GEV) and a component leading to a variable hand position (NGEV). Pointer-tip movement variability along the path and variable targeting error did not differ between conditions. Larger overall joint variance was found for the uncertain target condition. Most of this increase was GEV, which was significantly higher in the uncertain condition for control of both movement extent and movement direction. In contrast, NGEV differed between the two conditions only for the control of movement extent early in the reach, suggesting that target uncertainty led to inter-trial timing variability along the movement path. The results suggest that more flexible patterns of joint coordination are used when the nervous system must plan reaching movements to an uncertain target direction.     

Invariant geometric characteristics of spatial arm motion

This paper examines up to third-order geometric properties of wrist path and the first-order property of wrist trajectory (wrist speed) for spatial pointing movements. Previous studies report conflicting data regarding the time invariance of wrist-path shape, and most analyses are limited to the second-order geometric property (straightness, or strictly speaking, curvature). Subjects performed point-to-point reaching movements between targets whose locations ensured that the wrist paths spanned a range of lengths and 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. Analysis revealed that wrist paths tend to lie in planes and to curve more as movement speed decreases. The orientation of the wrist-path plane depends on the reaching task but does not vary significantly with movement speed. The planarity of wrist paths indicates that the paths have close to zero torsion—a third-order geometric property. Wrist-speed profiles showed multiple peaks for sufficiently slow and long lasting movements, indicating deviation from the well-known, bell-shaped profile. These kinematic findings are discussed in light of various motor control theories.     

Effects of altering initial position on movement direction and extent

The purpose of this study was to examine the relative influence of initial hand location on the direction and extent of planar reaching movements. Subjects performed a horizontal-plane reaching task with the dominant arm supported above a table top by a frictionless air-jet system. A start circle and a target were reflected from a horizontal projection screen onto a horizontally positioned mirror, which blocked the subject's view of the arm. A cursor, representing either actual or virtual finger location, was only displayed between each trial to allow subjects to position the cursor in the start circle. Prior to occasional "probe trials," we changed the start location of the finger relative to the cursor. Subjects reported being unaware of the discrepancy between cursor and finger. Our results indicate that regardless of initial hand location, subjects did not alter the direction of movement. However, movement distance was systematically adjusted in accord with the baseline target position. Thus when the hand start position was perpendicularly displaced relative to the target direction, neither the direction nor the extent of movement varied relative to that of baseline. However, when the hand was displaced along the target direction, either anterior or posterior, movements were made in the same direction as baseline trials but were shortened or lengthened, respectively. This effect was asymmetrical such that movements from anterior displaced positions showed greater distance adjustment than those from posterior displaced positions. Inverse dynamic analysis revealed substantial changes in elbow and shoulder muscle torque strategies for both right/left and anterior/posterior pairs of displacements. In the case of right/left displacements, such changes in muscle torque compensated changes in limb configuration such that movements were made in the same direction and to the same extent as baseline trials. Our results support the hypothesis that movement direction is specified relative to an origin at the current location of the hand. Movement extent, on the other hand, appears to be affected by the workspace learned during baseline movement experience.     

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     

Within-arm somatotopy in human motor areas determined by positron emission tomography imaging of cerebral blood flow

Within-arm somatotopy was identified in multiple motor areas of six normal human subjects who performed a visuomotor tracking task during positron emission tomography (PET) measurement of relative cerebral blood flow (relCBF). A randomly moving target, presented on a computer monitor, was continuously followed with the index finger (movement at the metacarpopha-langeal joint), thumb, fist (movement at the wrist), forearm (movement at the elbow), elbow (movement at the shoulder), and eyes alone (control task) during sequential imaging. Segmental limb movements were associated with relCBF responses in the contralateral motor, supplementary motor, cingulate, and parietal cortex, and in the ipsilateral cerebellum. Localization of responses after stereotaxic transformation into Talairach atlas space, as well as within-subject analysis without anatomic deformations, demonstrated an overlapping somatotopic distribution in the motor cortex, with thumb responses most ventrolateral and shoulder responses most dorsomedial. Proximal limb movements induced relCBF responses of greater magnitude than distal movements. Somatotopy was also identified in the supplementary motor area, with index finger responses dorsal and anterior to shoulder responses. An additional set of somatotopic responses were located in the cingulate cortex, also with finger responses anterior to shoulder responses. Somatotopy was not identified in the anterior cerebellum. The distribution of relCBF responses is concordant with electrophysiologic studies in nonhuman primates that demonstrate a fractured somatotopy on a fine scale and a general somatotopic scheme of the limb on a large scale in multiple discrete motor areas.     

The time course for kinetic versus kinematic planning of goal-directed human motor behavior

The present psychophysical study compares motor planning during goal-directed reaching movements and isometric spatial force generation. Our objective is to characterize the extent to which the motor system accounts for the biomechanical details of an impending reach. One issue that the nervous system must take into account when transforming a spatial sensory signal into an intrinsic pattern of joint torques is that of limb dynamics, including intersegmental dynamics and inertial anisotropy of the arm. These will act to displace the hand away from a straight path to an object. In theory, if the nervous system accounts for movement-related limb dynamics prior to its initial motor output, early force direction for a movement will differ from an isometric force to the same spatial target. Alternatively, biomechanical details of motor behavior may be implemented into the motor act following its initiation. Limb position and force output at the wrist were recorded while subjects displaced a cursor to targets viewed on a computer monitor. To generate isometric forces, a magnetic brake held a mechanical linkage supporting the arm in place. Subjects were cued to displace the cursor by using either isometric force or limb movement. On random trials, a movement was cued but an isometric force was unexpectedly required. Results show that there is not a significant directional difference in the initial force trajectory when planning a movement versus planning an isometric force. These findings suggest that the motor system may initially use a coarse approximation of movement-related limb dynamics, allowing for the refinement of the motor plan as the movement unfolds.