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.     

Evidence for a dynamic-dominance hypothesis of handedness

Handedness is a prominent behavioral phenomenon that emerges from asymmetrical neural organization of human motor systems. However, the aspects of motor performance that correspond to handedness remain largely undetermined. A recent study examining interlimb differences in coordination of reaching demonstrated dominant arm advantages in controlling limb segment inertial dynamics (Sainburg and Kalakanis 2000). Based on these findings, I now propose the dynamic-dominance hypothesis, which states that the essential factor that distinguishes dominant from nondominant arm performance is the facility governing the control of limb dynamics. The purpose of this study is to test two predictions of this hypothesis: 1) adaptation to novel intersegmental dynamics, requiring the development of new dynamic transforms, should be more effective for the dominant arm; 2) there should be no difference in adapting to visuomotor rotations performed with the dominant as compared with the nondominant arm. The latter prediction is based on the idea that visual information about target position is translated into an internal reference frame prior to transformation of the movement plan into dynamic properties, which reflect the forces required to produce movement. To test these predictions, dominant arm adaptation is compared to nondominant arm adaptation during exposure to novel inertial loads and to novel visuomotor rotations. The results indicate substantial interlimb differences in adaptation to novel inertial dynamics, but equivalent adaptation to novel visuomotor rotations. Inverse dynamic analysis revealed better coordination of dominant arm muscle torques across both shoulder and elbow joints, as compared with nondominant arm muscle torques. As a result, dominant arm movements were produced with a fraction of the mean squared muscle torque computed for nondominant arm movements made at similar speeds. These results support the dynamic-dominance hypothesis, indicating that interlimb asymmetries in control arise downstream to visuomotor transformations, when dynamic variables that correspond to the forces required for motion are specified.     

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     

Influence of predominant patterns of coordination on the exploitation of interaction torques in a two-joint rhythmic arm movement

In this study we investigate the coordination between rhythmic flexion–extension (FE) and supination–pronation (SP) movements at the elbow joint-complex, while manipulating the intersegmental dynamics by means of a 2-degrees of freedom (df) robot arm. We hypothesized that constraints imposed by the structure of the neuromuscular-skeletal system would (1) result in predominant pattern(s) of coordination in the absence of interaction torques and (2) influence the capabilities of participants to exploit artificially induced interaction torques. Two experiments were conducted in which different conditions of interaction torques were applied on the SP-axis as a function of FE movements. These conditions promoted different patterns of coordination between the 2-df. Control trials conducted in the absence of interaction torques revealed that both the in-phase (supination synchronized with flexion) and the anti-phase (pronation synchronized with flexion) patterns were spontaneously established by participants. The predominance of these patterns of coordination is explained in terms of the mechanical action of bi-articular muscles acting at the elbow joint-complex, and in terms of the reflexes that link the activity of the muscles involved. Results obtained in the different conditions of interaction torques revealed that those neuromuscular-skeletal constraints either impede or favor the exploitation of intersegmental dynamics depending on the context. Interaction torques were indeed found to be exploited to a greater extent in conditions in which the profiles of interaction torques favored one of the two predominant patterns of coordination (i.e., in-phase or anti-phase) as opposed to other patterns of coordination (e.g., 90° or 270°). Those results are discussed in relation to recent studies reporting exploitation of interaction torques in the context of rhythmic movements.     

The development of goal-directed reaching in infants: hand trajectory formation and joint torque control

Nine young infants were followed longitudinally from 4 to 15 months of age. We recorded early spontaneous movements and reaching movements to a stationary target. Time-position data of the hand (endpoint), shoulder, and elbow were collected using an optoelectronic measurement system (ELITE). We analyzed the endpoint kinematics and the intersegmental dynamics of the shoulder and elbow joint to investigate how changes in proximal torque control determined the development of hand trajectory formation. Two developmental phases of hand trajectory formation were identified: a first phase of rapid improvements between 16 and 24 weeks of age, the time of reaching onset for all infants. During that time period the number of movement units per reach and movement time decreased dramatically. In a second phase (28–64 weeks), a period of fine-tuning of the sensorimotor system, we saw slower, more gradual changes in the endpoint kinematics. The analysis of the underlying intersegmental joint torques revealed the following results: first, the range of muscular and motiondependent torques (relative to body weight) did not change significantly with age. That is, early reaching was not confined by limitations in producing task-adequate levels of muscular torque. Second, improvements in the endpoint kinematics were not accomplished by minimizing amplitude of muscle and reactive torques. Third, the relative timing of muscular and motion-dependent torque peaks showed a systematic development toward an adult timing profile with increasing age. In conclusion, the development toward invariant characteristics of the hand trajectory is mirrored by concurrent changes in the control of joint forces. The acquisition of stable patterns of intersegmental coordination is not achieved by simply regulating force amplitude, but more so by modulating the correct timing of joint force production and by the system's use of reactive forces. Our findings support the view that development of reaching is a process of unsupervised learning with no external or innate teacher prescribing the desired kinematics or kinetics of the movement.     

Interlimb transfer of novel inertial dynamics is asymmetrical

Mechanisms underlying interlimb transfer of adaptation to visuomotor rotations have recently been explored in depth. However, little data are available regarding interlimb transfer of adaptation to novel inertial dynamics. The present study thus investigated interlimb transfer of dynamics by examining the effect of initial training with one arm on subsequent performance with the other in adaptation to a 1.5-kg mass attached eccentrically to the forearm. Using inverse dynamic analysis, we examined the changes in torque strategies associated with adaptation to the extra mass, and with interlimb transfer of that adaptation. Following initial training with the dominant arm, nondominant arm performance improved substantially in terms of linearity and initial direction control as compared with na?ve performance. However, initial training with the nondominant arm had no effect on subsequent performance with the dominant arm. Inverse dynamic analysis revealed that improvements in kinematics were implemented by increasing flexor muscle torques at the elbow to counter load-induced increases in extensor interaction torques as well as increasing flexor muscle torques at the shoulder to counter the extensor actions of elbow muscle torque. Following opposite arm adaptation, the nondominant arm adopted this dynamic strategy early in adaptation. These findings suggest that dominant arm adaptation to novel inertial dynamics leads to information that can be accessed and utilized by the opposite arm controller, but not vice versa. When compared with our previous findings on interlimb transfer of visuomotor rotations, our current findings suggest that adaptations to visuomotor and dynamic transformations are mediated by distinct neural mechanisms.     

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.     

Hand and joint paths during reaching movements with and without vision

 This study examines whether the kinematics of pointing movements are altered by the sensory systems used to select spatial targets and to guide movement. Hand and joint paths of visually guided reaching movements of human subjects were compared with two non-visual conditions where only proprioception was available: (1) movements of the same subjects with blindfolds, and (2) movements by congenitally blind subjects. While hand-path curvatures were overall quite small, sighted subjects wearing a blindfold showed a statistical increase in hand-path curvature compared with their visually guided movements. Blindfolded subjects also showed greater hand-path curvature than blind subjects. These increases in hand-path curvature for blindfolded subjects did not always lead to a decrease in joint-path curvature. While there were differences between blind subjects and sighted subjects using vision for some movement directions, there was no systematic difference between these two groups. The magnitude of joint-path curvature showed much greater variation than hand-path curvature across the movement directions. We found variation in joint-path curvature to be correlated to two factors, one spatial and one geometrical. For all subject groups, joint-path curvature tended to be smaller for sagittal-plane movements than for transverse or diagonal movements. As well, we found that the magnitude of joint-path curvature was also related to the relative motion at each joint. Joint-path curvature tended to increase when movements predominantly involved changes in shoulder angle and was minimal when movements predominantly involved elbow motion. The consistently small curvatures of hand trajectory across blind and sighted subjects emphasize the powerful tendency of the motor system to generate goal-directed reaching movements with relatively straight hand trajectories, even when deprived of visual feedback from very early in life. Received: 16 July 1997 / Accepted: 20 May 1998     

The organization of intralimb and interlimb synergies in response to different joint dynamics

We sought to understand differences in joint coordination between the dominant and nondominant arms when performing repetitive tasks. The uncontrolled manifold approach was used to decompose the variability of joint motions into components that reflect the use of motor redundancy or movement error. First, we hypothesized that coordination of the dominant arm would demonstrate greater use of motor redundancy to compensate for interaction forces than would coordination of the nondominant arm. Secondly, we hypothesized that when interjoint dynamics were more complex, control of the interlimb relationship would remain stable despite differences in control of individual hand paths. Healthy adults performed bimanual tracing of two orientations of ellipses that resulted in different magnitudes of elbow interaction forces. For the dominant arm, joint variance leading to hand path error was the same for both ellipsis orientations, whereas joint variance reflecting the use of motor redundancy increased when interaction moment was highest. For the nondominant arm, more joint error variance was found when interaction moment was highest, whereas motor redundancy did not differ across orientations. There was no apparent difference in interjoint dynamics between the two arms. Thus, greater skill exhibited by the dominant arm may be related to its ability to utilize motor redundancy to compensate for the effect of interaction forces. However, despite the greater error associated with control of the nondominant hand, control of the interlimb relationship remained stable when the interaction moment increased. This suggests separate levels of control for inter- versus intra-limb coordination in this bimanual task.     

Deficits in the coordination of multijoint arm movements in patients with hemiparesis: evidence for disturbed control of limb dynamics

This study provides a detailed analysis of disturbances in the kinematics and dynamics of the acceleration phase of multijoint arm movements in six patients with chronic hemiparesis. Movements of the dominant and nondominant limbs were also examined in three control subjects. Subjects performed rapid movements from a central starting point to 16 targets located equidistantly around the circumference of a circle. Support of the upper limb was provided by an air-bearing apparatus, which allowed very low friction movements in the horizontal plane. We found that patients retained the capacity to modulate, in response to target direction, the initial direction of movements performed with the paretic limb. However, in comparison to the nonparetic limb or control subjects, movements of the paretic limb were misdirected systematically. An inverse dynamics analysis revealed an abnormal spatial tuning of the muscle torque at the elbow used to initiate movements of the paretic limb. Based on electromyographic recordings, similar spatial abnormalities were also apparent in the initial activations of elbow muscles. We argue that these spatial abnormalities result from a systematic disturbance in the control signal to limb muscles that cannot be attributed to previously identified mechanisms such as weakness, spasticity mediated restraint, or stereotypic muscle activation patterns (muscle synergies). Instead, our analysis of movement dynamics and simulation studies demonstrate that the spatial abnormalities are consistent with an impaired feedforward control of the passive interaction torques which arise during multijoint movements. This impaired control is hypothesized to reflect a degradation of the internal representation of limb dynamics that occurs either as a primary consequence of brain injury or secondary to disuse.     

Influence of joint interactional effects on the coordination of planar two-joint arm movements

We have examined EMG-movement relations in two-joint planar arm movements to determine the influence of interactional torques on movement coordination. Explicitly defined combinations of elbow movements (ranging from 20 to 70°) and wrist movements (ranging from 20 to 40°) were performed during a visual, step-tracking task in which subjects were specifically required to attend to the initial and final angles at each joint. In all conditions the wrist and elbow rotated in the same direction, that is, flexion-flexion or extension-extension. Elbow movement kinematics were only slightly influenced by motion about the wrist. In contrast, the trajectory of the wrist movement was significantly influenced by uncompensated reaction torques resulting from movement about the elbow joint. At any given wrist amplitude, wrist movement duration increased and peak velocity decreased as elbow amplitude increased. In addition, as elbow amplitude increased, wrist movement on-set was progressively delayed relative to this elbow movement. Surprisingly, the changes between joint movement onsets were not accompanied by corresponding changes between agonist EMG onsets at the elbow and wrist joints. The mean difference in onset times between elbow and wrist agonists (22–30 ms) remained unchanged across conditions. In addition, a basic pattern of muscle activation that scaled with movement amplitude was observed at each joint. Phasic agonist activity at the wrist and elbow joints remained remarkably similar across conditions and thus the changes in joint movement onset could not be attributed to changes in the motor commands. Rather, the calculated torques from the averaged data showed that the difference in timing of joint movement onsets was influenced by joint interactional torques. These findings suggest that during simple two-joint planar movements of the elbow and the wrist joint, the central nervous system does not alter the basic motor commands at each joint and as a result the actual trajectory of each joint is determined by interactional torques.     

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.     

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     

Variant and invariant features characterizing natural and reverse whole-body pointing movements

Our previous studies of interlimb asymmetries during reaching movements have given rise to the dynamic-dominance hypothesis of motor lateralization. This hypothesis proposes that dominant arm control has become optimized for efficient intersegmental coordination, which is often associated with straight and smooth hand-paths, while non-dominant arm control has become optimized for controlling steady-state posture, which has been associated with greater final position accuracy when movements are mechanically perturbed, and often during movements made in the absence of visual feedback. The basis for this model of motor lateralization was derived from studies conducted in right-handed subjects. We now ask whether left-handers show similar proficiencies in coordinating reaching movements. We recruited right- and left-handers (20 per group) to perform reaching movements to three targets, in which intersegmental coordination requirements varied systematically. Our results showed that the dominant arm of both left- and right-handers were well coordinated, as reflected by fairly straight hand-paths and low errors in initial direction. Consistent with our previous studies, the non-dominant arm of right-handers showed substantially greater curvature and large errors in initial direction, most notably to targets that elicited higher intersegmental interactions. While the right, non-dominant, hand-paths of left-handers were slightly more curved than those of the dominant arm, they were also substantially more accurate and better coordinated than the non-dominant arm of right-handers. Our results indicate a similar pattern, but reduced lateralization for intersegmental coordination in left-handers. These findings suggest that left-handers develop more coordinated control of their non-dominant arms than right-handers, possibly due to environmental pressure for right-handed manipulations.     

Control of joint rotations in overarm throws of different speeds made by dominant and nondominant arms

We tested the hypothesis that dominant and nondominant overarm throws of different speeds are made by time-scaling of joint rotations, i.e., by joint rotations that have the same positions and amplitudes but that are scaled in time. Eight skilled subjects stood and made overarm throws with both their dominant and nondominant arms. Six joint rotations were computed from recordings of arm segments made with the search-coil technique. Throws made with nondominant arms were less accurate and had lower ball speeds. In contrast to the hypothesis, dominant arms showed large and consistent differences between fast and slow throws in six-dimensional angular position joint space. These same throws showed similar hand angular paths when these were time-scaled based on ball speed. Nondominant arms showed only small differences in angular position joint space in fast and slow throws. It is concluded that a joint space pattern resembling that predicted by time-scaling occurs in nondominant arm throwing when it is unskilled. However, time-scaling does not occur in dominant arm throwing, i.e., a skilled fast throw is not simply a skilled slow throw whose joint positions and amplitudes remain constant but whose joint velocities are sped-up. We hypothesize for future study that, when subjects first learn to throw at different speeds with their dominant arms, they use time-scaling of joint rotations that involves compensating for interaction torques; then as they become skilled at throwing fast, time-scaling is superseded by a more complex pattern of interjoint coordination that involves exploiting interaction torques.     

Coordinated turn-and-reach movements. II. Planning in an external frame of reference

The preceding study demonstrated that normal subjects compensate for the additional interaction torques generated when a reaching movement is made during voluntary trunk rotation. The present paper assesses the influence of trunk rotation on finger trajectories and on interjoint coordination and determines whether simultaneous turn-and-reach movements are most simply described relative to a trunk-based or an external reference frame. Subjects reached to targets requiring different extents of arm joint and trunk rotation at a natural pace and quickly in normal lighting and in total darkness. We first examined whether the larger interaction torques generated during rapid turn-and-reach movements perturb finger trajectories and interjoint coordination and whether visual feedback plays a role in compensating for these torques. These issues were addressed using generalized Procrustes analysis (GPA), which attempts to overlap a group of configurations (e.g., joint trajectories) through translations and rotations in multi-dimensional space. We first used GPA to identify the mean intrinsic patterns of finger and joint trajectories (i.e., their average shape irrespective of location and orientation variability in the external and joint workspaces) from turn-and-reach movements performed in each experimental condition and then calculated their curvatures. We then quantified the discrepancy between each finger or joint trajectory and the intrinsic pattern both after GPA was applied individually to trajectories from a pair of experimental conditions and after GPA was applied to the same trajectories pooled together. For several subjects, joint trajectories but not finger trajectories were more curved in fast than slow movements. The curvature of both joint and finger trajectories of turn-and-reach movements was relatively unaffected by the vision conditions. Pooling across speed conditions significantly increased the discrepancy between joint but not finger trajectories for most subjects, indicating that subjects used different patterns of interjoint coordination in slow and fast movements while nevertheless preserving the shape of their finger trajectory. Higher movement speeds did not disrupt the arm joint rotations despite the larger interaction torques generated. Rather, subjects used the redundant degrees of freedom of the arm/trunk system to achieve similar finger trajectories with differing joint configurations. We examined finger movement patterns and velocity profiles to determine the frame of reference in which turn-and-reach movements could be most simply described. Finger trajectories of turn-and-reach movements had much larger curvatures and their velocity profiles were less smooth and less bell-like in trunk-based coordinates than in external coordinates. Taken together, these results support the conclusion that turn-and-reach movements are controlled in an external frame of reference.     

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.     

Control of the dominant and nondominant hand: exploitation and taming of nonmuscular forces

Movements of the dominant and nondominant hand have been claimed to differ with respect to how they take intersegmental dynamics into account. Consistent with this claim, movements of the dominant hand are hypothesized to better exploit the intrinsic limb dynamics, whereas movements of the nondominant hand are controlled to make the intrinsic dynamics ineffective as far as this is possible. For rapid finger oscillations this hypothesis implies a higher level of co-contractions in the nondominant than in the dominant hand. Replicating previous findings on finger tapping, finger oscillations of the dominant hand were faster and less variable than those of the nondominant hand. More importantly, the variance of the relative difference between myoelectric signals of antagonistic muscles and thus the power of reciprocal myoelectric activity was smaller in the nondominant hand, indicating a relatively higher level of co-contractions than in the dominant hand. In addition, a spectral decomposition of the total power of the relative-difference signal revealed stronger relative power in the frequency band of the finger oscillations in the dominant than in the nondominant hand. These findings are consistent with the hypothesis that for the dominant hand more accurate feedforward control is possible based on a more accurate internal model of limb dynamics.     

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.     

Interlimb differences of directional biases for stroke production

Directional preferences during center-out horizontal shoulder–elbow movements were previously characterized for the dominant arm. These preferences were attributed to a tendency to actively accelerate one joint, while exploiting largely passive motion at the other joint. Since the non-dominant arm is known for inefficient coordination of inter-segmental dynamics, here we hypothesized that directional preferences would differ between the arms. A center-out free-stroke drawing task was used that allowed freedom in the selection of movement directions. The task was performed both with and without a secondary cognitive task that has been shown to increase directional biases of the dominant arm. Mirror-symmetrical directional preferences were observed in both arms, with similar bias strength and secondary task effects. The preferred directions were characterized by maximal exploitation of interaction torques for movement production, but only in the dominant arm. The non-dominant arm failed to benefit from interaction torques. The results point to a hierarchical architecture of control. At the higher level, a movement capable to perform the task while satisfying preferences in joint control is specified through forward dynamic transformations. This process is mediated for both arms from a common neural network adapted to the dominant arm and, specifically, to its ability to exploit interaction torques. Dynamic transformations that determine actual control commands are specified at the lower level of control. An alternative interpretation that strokes might be planned evenly across directions, and biases emerge during movement execution due to anisotropic resistance of intrinsic factors that do not depend on arm dominance is also discussed.