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

Elastic Resistance and Shoulder Movement Patterns: An Analysis of Reaching Tasks Based on Proprioception

This study departs from the conventional research on horizontal plane reach movements by examining human motor control strategies in vertical plane elastic load reach movements conducted without visual feedback. Here, participants performed shoulder presses with elastic resistances at low, moderate, and high intensities without access to visual information about their hand position, relying exclusively on proprioceptive feedback and synchronizing their movements with a metronome set at a 3 s interval. The results revealed consistent performance symmetry across different intensities in terms of the reach speed (p = 0.254-0.736), return speed (p = 0.205-0.882), and movement distance (p = 0.480-0.919). This discovery underscores the human capacity to uphold bilateral symmetry in movement execution when relying solely on proprioception. Furthermore, this study observed an asymmetric velocity profile where the reach duration remained consistent irrespective of the load (1.15 s), whereas the return duration increased with higher loads (1.39 s-1.45 s). These findings suggest that, in the absence of visual feedback, the asymmetric velocity profile does not result from the execution of the action but rather represents a deliberate deceleration post-reach aimed at achieving the target position as generated by the brain's internal model. These findings hold significant implications for interpreting rehabilitation approaches under settings devoid of visual feedback.

Does Object Height Affect the Dart Throwing Motion Angle during Seated Activities of Daily Living?

Complex wrist motions are needed to complete various daily activities. Analyzing the multidimensional motion of the wrist is crucial for understanding our functional movement. Several studies have shown that numerous activities of daily livings (ADLs) are performed using an oblique plane of wrist motion from radial-extension to ulnar-flexion, named the Dart Throwing Motion (DTM) plane. To the best of our knowledge, the DTM plane angle performed during ADLs has not been compared between different heights (e.g. table, shoulder and head height), as is common when performing day-to-day tasks. In this study, we compared DTM plane angles when performing different ADLs at three different heights and examined the relationship between DTM plane angles and limb position. We found that height had a significant effect on the DTM plane angles - the mean DTM plane angle was greater at the lower level compared to the mid and higher levels. A significant effect of shoulder orientation on mean DTM plane angles was shown in the sagittal and coronal planes. Our findings support the importance of training daily tasks at different heights during rehabilitation following wrist injuries, in order to explore a large range of DTM angles, to accommodate needs of common ADLs.

Shoulder reflexes integrate elbow information at "long-latency" delay throughout a corrective action

Previous studies have demonstrated a progression of function when healthy subjects counter a sudden mechanical load. Short-latency reflexes are linked to local stretch of the particular muscle and its antagonist. Long-latency reflexes integrate stretch information from both local sources and muscles crossing remote joints appropriate for a limb's mechanical interactions. Unresolved is how sensory information is processed throughout the corrective response, since capabilities at some time can be produced by circuits acting at that delay and at briefer delays. One possibility is that local abilities are always expressed at a short-latency delay and integrative abilities are always expressed at a long-latency delay. Alternatively, the neural circuits may be altered over time, leading to a temporal shift in expressing certain abilities; a refractory period could retard integrative responses to a second perturbation, whereas priming could enable integrative responses at short latency. We tested between these three hypotheses in a shoulder muscle by intermixing trials of step torque with either torque pulses ( experiment 1) or double steps of torque ( experiment 2). The second perturbation occurred at 35, 60, and 110 ms after the first perturbation to probe processing throughout the corrective action. The second perturbation reliably evoked short-latency responses in the shoulder muscle linked to only shoulder motion and long-latency responses linked to both shoulder and elbow motion. This pattern is best accounted by the continuous action of controllers with fixed functions. NEW & NOTEWORTHY Sudden displacement of the limb evokes a short-latency reflex, 20-50 ms, based on local muscle stretch and a long-latency reflex based on integrating muscle stretch at different joints. A novel double-perturbation paradigm tested if these abilities are temporally conserved throughout the corrective response or are shifted (retarded or delayed) due to functional changes in the responsible circuits. Multi-joint integration was reliably expressed at a long-latency delay consistent with the continuous operation of circuits with fixed abilities.

Hand Switching Costs are not Uniform Across Response Components

We investigated the extent to which a complex finger sequence impacts on hand switching costs in a sequential action. Response component latencies (premotor, motor, and movement) were compared in no-switch (same finger performed the action of pressing and reaching) and switch conditions (pressing with one finger and completing the reaching action with the homologous finger from the other hand). Results showed that the switch condition presented longer latency for premotor and movement components. For the motor component, however, switch condition was faster. This expands the previous literature investigating switching costs using simple finger movements in more complex tasks. A mechanical explanation of the interplay between response subcomponents is provided to explain the inversion of response pattern for the motor component.

High-Frequency Intermuscular Coherence between Arm Muscles during Robot-Mediated Motor Adaptation

Adaptation of arm reaching in a novel force field involves co-contraction of upper limb muscles, but it is not known how the co-ordination of multiple muscle activation is orchestrated. We have used intermuscular coherence (IMC) to test whether a coherent intermuscular coupling between muscle pairs is responsible for novel patterns of activation during adaptation of reaching in a force field. Subjects (N = 16) performed reaching trials during a null force field, then during a velocity-dependent force field and then again during a null force field. Reaching trajectory error increased during early adaptation to the force-field and subsequently decreased during later adaptation. Co-contraction in the majority of all possible muscle pairs also increased during early adaptation and decreased during later adaptation. In contrast, IMC increased during later adaptation and only in a subset of muscle pairs. IMC consistently occurred in frequencies between ~40–100 Hz and during the period of arm movement, suggesting that a coherent intermuscular coupling between those muscles contributing to adaptation enable a reduction in wasteful co-contraction and energetic cost during reaching.

Optimizing effort: increased efficiency of motor memory with time away from practice

In motor tasks, efficiency can be measured via the commands that are produced to accomplish a goal. To maximize efficiency, the nervous system should produce task-relevant motor commands while avoiding behaviors that are task-irrelevant. The current view is that this is achieved through training, i.e., the optimum motor commands are learned by trial and error. However, in contrast to this view, there are numerous examples in which during an experiment, task-irrelevant commands are continuously produced. To address this, we trained human volunteers to reach in a force field. With practice, they learned to produce forces that compensated for the field, generating task-relevant commands that were necessary to achieve success. As expected, training also resulted in generalization, the transfer of learning to other movements. We designed the task so that any forces produced as a result of generalization were unnecessary and therefore task-irrelevant. Importantly, there were no explicit cues to indicate that production of these forces was task-irrelevant. Rather, the only indicator was effort itself. Could this inefficiency of the motor commands be reduced? We found that even with extensive practice, the production of task-irrelevant forces persisted. However, if subjects were given sufficient time away from practice (6 or 24 h but not 3 or 30 min), they spontaneously reduced production of the task-irrelevant forces. Therefore, practice alone was insufficient to allow for increased efficiency of motor output. Time away from practice was a required element for optimization of effort.

Influence of prior use of the same or different effectors in a reaching action

Use of different effectors in two consecutive actions could generate an attentional shift between the effectors with shorter latencies in the second action of reaching. 18 participants (10 men; M age = 21.3 yr.) participated in an experiment with two main variables: (1) effector switching with two levels (Switching and No Switching), where the participants use or do not use a different motor effector for each action; (2) lifting muscles, i.e., the muscles involved in the first phase of the reaching, with two levels (finger-lifting muscle and palm-lifting muscle). Premotor time, Motor time, Reaction time, Movement time, and Total time were measured. For Premotor, Movement, and Total times, faster responses were observed when there was no switching of the effector. This delay could be due to an attentional shift between motor effectors and its relations with motor processes. Possible applications include the ergonomic design of device controls, considering that the use of the same effector is beneficial when fast reaction times are sought.

Immediate compensation for variations in self-generated Coriolis torques related to body dynamics and carried objects

We have previously shown that the Coriolis torques that result when an arm movement is performed during torso rotation do not affect movement trajectory. Our purpose in the present study was to examine whether torso motion-induced Coriolis and other interaction torques are counteracted during a turn and reach (T&R) movement when the effective mass of the hand is augmented, and whether the dominant arm has an advantage in coordinating intersegmental dynamics as predicted by the dynamic dominance hypothesis (Sainburg RL. Exp Brain Res 142: 241-258, 2002). Subjects made slow and fast T&R movements in the dark to just extinguished targets with either arm, while holding or not holding a 454-g object. Movement endpoints were equally accurate at both speeds, with either hand, and in both weight conditions, but subjects tended to angularly undershoot and produce more variable endpoints for targets requiring greater torso rotation. There were no changes in endpoint accuracy or trajectory deviation over repeated movements. The dominant right arm was more stable in its control of trajectory direction across targets, whereas the nondominant left arm had an improved ability to stop accurately on the target for higher levels of interaction torques. The trajectories to more eccentric targets were straighter when performed at higher speeds but slightly more deviated when subjects held the weight. Subjects did not slow their torso velocity or change the timing of the arm and torso velocities when holding the weight, although there was a slight decrease in their hand velocity relative to the torso. The delay between the onsets of torso and finger movements was almost twice as large for the right arm than the left, suggesting the right arm was better able to account for torso rotation in the arm movement. Holding the weight increased the peak Coriolis torque by 40% at the shoulder and 45% at the elbow and, for the most eccentric target, increased the peak net torque by 12% at the shoulder and 34% at the elbow. In accordance with Sainburg's dynamic dominance hypothesis, the right arm exhibited an advantage for coordinating intersegmental dynamics, showing a more stable finger velocity in relation to the torso across targets, decreasing error variability with movement speed, and more synchronized peaks of finger relative and torso angular velocities in conditions with greater joint torque requirements. The arm used had little effect on the movement path and the magnitude of the joint torques in any of the conditions. These results indicate that compensations for forthcoming Coriolis torque variations take into account the dynamic properties of the body and of external objects, as well as the planned velocities of the torso and arm.

On-line coordination in complex goal-directed movements: a matter of interactions between several loops

Motor flexibility is the ability to rapidly modify behavior when unexpected perturbations occur. In goal directed movements, this process may be involved during the motor execution itself, by using on-line motor corrections, or off-line, on a trial-by-trial basis. A consensus has emerged to describe and unify these two dependant processes within the framework of the internal models theory in which the cerebellum is involved in error processing. However, this general framework may be incomplete to describe on-line motor corrections when complex motor coordination is involved in the task. In particular, interaction torques existing between different effectors limit the independence between different controllers that could be considered to control various body parts. In addition, recent findings suggest that different (sub)-cortical loops may be involved during orienting responses to visual stimuli but also during on-line motor corrections following visual perturbations. The way these different loops with different dynamics interact but achieve the same motor goal is an important problem in motor control. The simplest organization may be sequential, as in the well-known stretch reflex. This implies that during on-line corrections, the nervous system may be involved in a distributed fashion and that motor plans and synergies depend both on anatomical and temporal constraints. More particularly, motor plans and synergies may be stored and may differ according to the (sub)-cortical loops involved during the whole on-line correction process. Finally, questions concerning the independence (or not) of these loops remain unanswered. The case of strict independence would mean that between the various corrective loops, (i) error processing and (ii) motor plans/synergies would be different. By contrast, in a situation of dependency, it would probably mean that interactions would link lower (and faster) to upper (and longer) loops by informing these latter of the motor corrections sent by the former, similarly to an efference copy.

Limitations on coupling of bimanual movements caused by arm dominance: when the muscle homology principle fails

Studies of bimanual movements typically report interference between motions of the two arms and preference to perform mirror-symmetrical patterns. However, recent studies have demonstrated that the two arms differ in the ability to control interaction torque (INT). This predicts limitations in the capability to perform mirror-symmetrical movements. Here, two experiments were performed to test this prediction. The first experiment included bimanual symmetrical and asymmetrical circle drawing at two frequency levels. Unimanual circle drawing was also recorded. The increases in cycling frequency caused differences between the two arms in movement trajectories in both bimanual modes, although the differences were more pronounced in the asymmetrical compared with the symmetrical mode. Based on torque analysis, the differences were attributed to the nondominant arm's decreased capability to control INT. The intraarm differences during the symmetrical pattern of bimanual movements were similar (although more pronounced) to those during unimanual movements. This finding was verified in the second experiment for symmetrical bimanual oval drawing. Four oval orientations were used to provide variations in INT. Similar to the first experiment, increases in cycling frequency caused spontaneous deviations from perfect bimanual symmetry associated with inefficient INT control in the nondominant arm. This finding supports the limitations in performing mirror-symmetrical bimanual movements due to differences in joint control between the arms. Based on our results and previous research, we argue that bimanual interference occurs during specification of characteristics of required motion, whereas lower-level generation of muscle forces is independent between the arms. A hierarchical model of bimanual control is proposed.

Performance Evaluation of a Planar 3DOF Robotic Exoskeleton for Motor Assessment

A new planar robotic exoskeleton for upper-limb motor assessment has been developed. It provides independent control of a user’s shoulder, elbow, and wrist joints in the horizontal plane. The lightweight backdriveable robot is based on a novel cable-driven curved track and carriage system that enables the entire mechanism to be located underneath the user’s arm. It has been designed to extend the assessment capabilities of an existing planar robotic exoskeleton. This paper presents the design and performance of the new robot.

Limb stiffness is modulated with spatial accuracy requirements during movement in the absence of destabilizing forces

The motor system can use a number of mechanisms to increase movement accuracy and compensate for perturbing external forces, interaction torques, and neuromuscular noise. Empirical studies have shown that stiffness modulation is one adaptive mechanism used to control arm movements in the presence of destabilizing external force loads. Other work has shown that arm muscle activity is increased at movement end for reaching movements to small visual targets and that changes in stiffness at movement end are oriented to match changes in visual accuracy requirements such as target shape. In this study, we assess whether limb stiffness is modulated to match spatial accuracy requirements during movement, conveyed using visual stimuli, in the absence of external force loads. Limb stiffness was estimated in the middle of reaching movements to visual targets located at the end of a narrow (8 mm) or wide (8 cm) visual track. When greater movement accuracy was required, we observed modest but reliable increases in limb stiffness in a direction perpendicular to the track. These findings support the notion that the motor system uses stiffness control to augment movement accuracy during movement and does so in the absence of external unstable force loads, in response to changing accuracy requirements conveyed using visual cues.

Interaction torque contributes to planar reaching at slow speed

Background

How the central nervous system (CNS) organizes the joint dynamics for multi-joint movement is a complex problem, because of the passive interaction among segmental movements. Previous studies have demonstrated that the CNS predictively compensates for interaction torque (INT) which is arising from the movement of the adjacent joints. However, most of these studies have mainly examined quick movements, presumably because the current belief is that the effects of INT are not significant at slow speeds. The functional contribution of INT for multijoint movements performed in various speeds is still unclear. The purpose of this study was to examine the contribution of INT to a planer reaching in a wide range of motion speeds for healthy subjects.

Methods

Subjects performed reaching movements toward five targets under three different speed conditions. Joint position data were recorded using a 3-D motion analysis device (50 Hz). Torque components, muscle torque (MUS), interaction torque (INT), gravity torque (G), and net torque (NET) were calculated by solving the dynamic equations for the shoulder and elbow. NET at a joint which produces the joint kinematics will be an algebraic sum of torque components;

NET = MUS - G - INT.

Dynamic muscle torque (DMUS = MUS-G) was also calculated. Contributions of INT impulse and DMUS impulse to NET impulse were examined.

Results

The relative contribution of INT to NET was not dependent on speed for both joints at every target. INT was additive (same direction) to DMUS at the shoulder joint, while in the elbow DMUS counteracted (opposed to) INT. The trajectory of reach was linear and two-joint movements were coordinated with a specific combination at each target, regardless of motion speed. However, DMUS at the elbow was opposed to the direction of elbow movement, and its magnitude varied from trial to trial in order to compensate for the variability of INT.

Conclusion

Interaction torque was important at slow speeds. Muscle torques at the two joints were not directly related to each other to produce coordinated joint movement during a reach. These results support Bernstein's idea that coordinated movement is not completely determined by motor command in multi-joint motion. Based on the data presented in this study and the work of others, a model for the connection between joint torques (muscle and passive torques including interaction torque) and joint coordination is proposed.

Posture Control and Complex Arm Coordination: Analysis of Multijoint Coordinative Movements and Stability of Stance

The authors addressed the interactions between control of bimanual multijoint coordination tasks and posture. Participants (N = 6) performed 8 coordination patterns that differed in degree of complexity by using their bilateral elbows and wrists under 3 scaled motion speeds while standing on 2 force plates. Results indicated that producing complex bimanual multijoint coordinative tasks affected postural sway, thus resulting in an increase of sway activity. Behavioral as well as mechanical factors accounted for the increased disturbance in postural sway. Those findings suggest that performing complex coordination tasks disrupts postural control in normal young adults.

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.

Toy-oriented changes in early arm movements II--joint kinematics

Our recent work suggests that infants begin to change their hand and joint kinematics in the presence of a toy months before the onset of purposeful reaching. Moreover, these 'toy-oriented' changes in hand kinematics cluster into Early, Mid and Late phases. The purpose of the present study was to test hypotheses regarding toy-oriented changes in joint kinematics in the same infants.

METHODS

Thirteen infants were observed every other week from 8 weeks up to the first week of reaching around 20 weeks. At each session, a high-speed motion analysis system recorded infants' arm movements with and without a toy present.

RESULTS

During the Early phase, infants scaled down their movements. In contrast, during the Mid phase infants scaled up their movements and did not change the relationship between the shoulder and elbow for speed and smoothness-related variables. In addition, infants showed toy-oriented changes such as increase in shoulder flexion and adduction. In the Late phase, infants continued to produce toy-oriented changes in shoulder orientation, and increased shoulder excursion and speed relative to the elbow. Thus, the toy-oriented changes in hand kinematics in the Mid and Late phases [Bhat, A. N., & Galloway, J. C. (2006). Toy-oriented changes in early arm movements of young infants: Hand kinematics. Infant Behavior and Development, 29(3), 358-372] more closely followed changes in shoulder kinematics. Lastly, results are discussed in terms of shoulder-elbow dissociations, speed-amplitude relationships, and the key role of spontaneous movements in the development of reaching.

Comparison of kinematics in skilled and unskilled arms of the same recreational baseball players

We examined mechanisms of coordination that enable skilled recreational baseball players to make fast overarm throws with their skilled arm and which are absent or rudimentary in their unskilled arm. Arm segment angular kinematics in three dimensions at 1000 Hz were recorded with the search-coil technique from the arms of eight individuals who on one occasion threw with their skilled right arm and on another with their unskilled left arm. Compared with their unskilled arm, the skilled arm had: a larger angular deceleration of the upper arm in space in the forward horizontal direction; a larger shoulder internal rotation velocity at ball release (unskilled arms had a negative velocity); a period of elbow extension deceleration before ball release; and an increase in wrist velocity with an increase in ball speed. It is suggested that some of these differences in arm kinematics occur because of differences between the skilled and unskilled arms in their ability to control interaction torques (the passive torque at one joint due to motion at adjacent joints). It is proposed that one reason unskilled individuals cannot throw fast is that, unlike their skilled counterparts, they have not developed the coordination mechanisms to effectively exploit interaction torques.

Efficient control of arm movements in advanced age

The present study addresses the influence of aging on the ability to regulate mechanical effects arising during arm movements due to the multi-joint structure of the arm. Two mechanical factors were considered, interaction torque (IT) and inertial resistance (IR). Regulation of these two factors can be demanding in terms of the timing and magnitude of the required muscle torque (MT), specifically during fast movements. We hypothesized that aging exacerbates the challenge regarding the regulation of these effects with muscular control due to declines in the motor system. This hypothesis was tested by comparing performance of a cyclic line-drawing task in two age groups, young and older adults. Only two joints, the shoulder and elbow, participated in motion. Four orientations of the lines were used to provide variations in the requirements for regulation of IT and IR. Cyclic frequency was manipulated to emphasize the dependence of the mechanical factors on movement speed. Various characteristics of fingertip motion showed that there were no age-related deteriorations in accuracy of line drawing. However, older adults were systematically slower, particularly in the directions of high IR. A detailed analysis of the magnitude of MT and the contribution of this torque to production of net torque at each joint demonstrated that older adults modified joint control and decreased the demands for MT by skillful exploitation of IT in a way specific for each particular line orientation. The results point to a tendency in older adults to decrease the production of muscle force. Nevertheless, older adults also demonstrated an ability to partially compensate for declines in the force production by developing sophisticated strategies of joint control that exploit the multi-joint mechanical structure of the arm. This ability suggests that the internal representation of inter-segmental dynamics and the capability to use it for movement control does not decay with age. The study emphasizes the importance of analysis of joint motion and control characteristics for the investigation of arm movements and for comparison of these movements between different subject populations.

Are there distinct neural representations of object and limb dynamics?

In recent studies of human motor learning, subjects learned to move the arm while grasping a robotic device that applied novel patterns of forces to the hand. Here, we examined the generality of force field learning. We tested the idea that contextual cues associated with grasping a novel object promote the acquisition and use of a distinct internal model, associated with that object. Subjects learned to produce point-to-point arm movements to targets in a horizontal plane while grasping a robotic linkage that applied either a velocity-dependent counter-clockwise or clockwise force field to the hand. Following adaptation, subjects let go of the robot and were asked to generate the same movements in free space. Small but reliable after-effects were observed during the first eight movements in free space, however, these after-effects were significantly smaller than those observed for control subjects who moved the robot in a null field. No reduction in retention was observed when subjects subsequently returned to the force field after moving in free space. In contrast, controls who reached with the robot in a NF showed much poorer retention when returning to a force field. These findings are consistent with the idea that contextual cues associated with grasping a novel object may promote the acquisition of a distinct internal model of the dynamics of the object, separate from internal models used to control limb dynamics alone.

The effects of observational feedback and verbal cues on the motor learning of an aimed reach-and-point task

PURPOSE

Current technology allows the recording of movement for both motion analysis and providing observational feedback. The most effective type of observational feedback is under debate. We compared a child's reach-and-point performance after viewing a videotaped playback of a model's performance and after viewing a split-screen comparison of the model's and child's performances while simultaneously receiving verbal cues.

METHODS

A PTVision system provided observational feedback and recorded spatial trajectory, target accuracy, movement time, and joint angles while a 13 year-old boy with cerebral palsy reached for three targets.

RESULTS

The split-screen comparison had the largest effect on reach performance, including slower-yet-more-accurate movements and a more extended wrist, curved spatial trajectories, and an ulnar-deviated wrist.

CONCLUSIONS

Feedback using split-screen comparison between a model's and the child's performance with verbal cues appears to promote motor learning. When using technology to augment therapy, the intervention should be designed considering current motor learning principles.

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.

The internal model and the leading joint hypothesis: implications for control of multi-joint movements

This article presents a theoretical generalization of recent experimental findings accumulated in support of two concepts of inter-segmental dynamics regulation during multi-joint movements. The concepts are the internal model of inter-segmental dynamics and the leading joint hypothesis (LJH). The internal model of limb dynamics is a well-established interpretation of feed-forward control. Recent experiments have generated new information about the organization of the internal model and its role in regulation of inter-segmental dynamics. The LJH, which proposes a simplified principle of the regulation of inter-segmental dynamics, is at the beginning stage of development. This paper outlines major results obtained in these two research directions and demonstrates that the two groups of findings complement and augment each other, suggesting a simple and robust hierarchical strategy of multi-joint movement control that exploits specific mechanical properties of human limbs.

Motor cortex neural correlates of output kinematics and kinetics during isometric-force and arm-reaching tasks

We recorded the activity of 132 proximal-arm-related neurons in caudal primary motor cortex (M1) of two monkeys while they generated either isometric forces against a rigid handle or arm movements with a heavy movable handle, in the same eight directions in a horizontal plane. The isometric forces increased in monotonic fashion in the direction of the force target. The forces exerted against the handle in the movement task were more complex, including an initial accelerating force in the direction of movement followed by a transient decelerating force opposite to the direction of movement as the hand approached the target. EMG activity of proximal-arm muscles reflected the difference in task dynamics, showing directional ramplike activity changes in the isometric task and reciprocally tuned "triphasic" patterns in the movement task. The apparent instantaneous directionality of muscle activity, when expressed in hand-centered spatial coordinates, remained relatively stable during the isometric ramps but often showed a large transient shift during deceleration of the arm movements. Single-neuron and population-level activity in M1 showed similar task-dependent changes in temporal pattern and instantaneous directionality. The momentary dissociation of the directionality of neuronal discharge and movement kinematics during deceleration indicated that the activity of many arm-related M1 neurons is not coupled only to the direction and speed of hand motion. These results also demonstrate that population-level signals reflecting the dynamics of motor tasks and of interactions with objects in the environment are available in caudal M1. This task-dynamics signal could greatly enhance the performance capabilities of neuroprosthetic controllers.

Braking of elbow extension in fast overarm throws made by skilled and unskilled subjects

A previous computer simulation study of overarm throws in 2D showed that reversal of elbow torque by antagonist muscle action late in the throw led to increased wrist flexion velocity and to increased ball speeds. We tested the hypothesis that the skill of making fast overarm throws in 3D involves deceleration (braking) of elbow extension before ball release, and that this is an active mechanism. Skilled and unskilled throwers were instructed to throw baseballs at a fast speed. Arm segment angular positions in 3D at 1,000 Hz were recorded with the search-coil technique (which records angular motions). In skilled throws, but not in unskilled throws, there was a period (mean 17 ms) of rapid elbow extension deceleration before ball release. However, there was relatively little biceps EMG activity associated with the very large magnitude of elbow deceleration. This finding and other work suggests that elbow extension deceleration results in part from interaction torques associated with late-occurring shoulder rotations, and only in part from elbow flexor contraction. During the period when elbow extension was decelerating, the forearm in space was undergoing angular acceleration (because of internal rotation at the shoulder) which would be expected to produce a torque at the wrist in the extensor (not flexor) direction. The results show that elbow extension deceleration occurs before ball release in fast (skilled) 3D throws, and that it does not produce forearm angular deceleration. Whether it produces forearm translational deceleration, which could increase wrist flexion velocity, remains to be determined.

Novel muscle patterns for reaching after cervical spinal cord injury: a case for motor redundancy

A fundamental issue in the neuromotor control of arm movements is whether the nervous system can use distinctly different muscle activity patterns to obtain similar kinematic outcomes. Although computer simulations have demonstrated several possible mechanical and torque solutions, there is little empirical evidence that the nervous system actually employs fundamentally different muscle patterns for the same movement, such as activating a muscle one time and not the next, or switching from a flexor to an extensor. Under typical conditions, subjects choose the same muscles for any given movement, which suggests that in order to see the capacity of the nervous system to make a different choice of muscles, the nervous system must be pushed beyond the normal circumstances. The purpose of this study, then, was to examine an atypical condition, reaching of cervical spinal cord injured (SCI) subjects who have a reduced repertoire of available distal arm muscles but otherwise a normal nervous system above the level of lesion. Electromyography and kinematics of the shoulder and elbow were examined in the SCI subjects performing a center-out task and then compared to neurologically normal control subjects. The findings showed that the SCI-injured subjects produced reaches with typical global kinematic features, such as straight finger paths, bell-shaped velocities, and joint excursions similar to control subjects. The SCI subjects, however, activated only the shoulder agonist muscle for all directions, unlike the control pattern that involved a reciprocal pattern at each joint (shoulder, elbow, and wrist). Nonetheless, the SCI subjects could activate their shoulder antagonist muscles, elbow flexors, and wrist extensor (extensor carpi radialis) for isometric tasks, but did not activate them during the reaching movements. These results demonstrate that for reaching movements, the SCI subjects used a strikingly different pattern of intact muscle activities than control subjects. Hence, the findings imply that the nervous system is capable of choosing either the control pattern or the SCI pattern. We would speculate that control subjects do not select the SCI pattern because the different choice of muscles results in kinematic features (reduced fingertip speed, multiple shoulder accelerations) other than the global features that are somehow less advantageous or efficient.

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

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

Age-related differences in the control of multijoint movements

The present study investigates whether regulation of interactive torque during multijoint movements decays with advanced age as a result of declines in the motor system. Young and elderly adults repeatedly drew a circle and ovals oriented in different directions using shoulder and elbow joint movements. Each template was traced at three levels of cycling frequency with and without vision. Although vision did not affect performance, increases in cycling frequency caused distortions of movement trajectories in both groups. The pattern of distortions differed, however, between the groups. These differences were accounted for by differences in elbow control. Young adults provided regulation of elbow amplitude and timing by matching muscle torque magnitude with increased interactive torque. In contrast, elderly adults did not increase muscle torque magnitude and modulated torque timing for elbow motion regulation. This strategy is discussed as adaptation to decrements in the aging motor system.

Inter-joint coupling strategy during adaptation to novel viscous loads in human arm movement

When arm movements are perturbed by a load, how does the nervous system adjust control signals to reduce error? While it has been shown that the nervous system is capable of compensating for the effects of limb dynamics and external forces, the strategies used to adapt to novel loads are not well understood. We used a robotic exoskeleton [kinesiological instrument for normal and altered reaching movements (KINARM)] to apply novel loads to the arm during single-joint elbow flexions in the horizontal plane (shoulder rotation was allowed). Loads varied in magnitude with the instantaneous velocity of elbow flexion, and were applied to the shoulder in experiment 1 (interaction loads) and the elbow in experiment 2 (direct loads). Initial exposure to both interaction and direct loads resulted in perturbations at both joints, even though the load was applied to only a single joint. Subjects tended to correct for the kinematics of the elbow joint while perturbations at the shoulder persisted. Electromyograms (EMGs) and computed muscle torque showed that subjects modified muscle activity at the elbow to reduce elbow positional deviations. Shoulder muscle activity was also modified; however, these changes were always in the same direction as those at the elbow. Current models of motor control based on inverse-dynamics calculations and force-control, as well as models based on positional control, predict an uncoupling of shoulder and elbow muscle torques for adaptation to these loads. In contrast, subjects in this study adopted a simple strategy of modulating the natural coupling that exists between elbow and shoulder muscle torque during single-joint elbow movements.

Multijoint movement control: the importance of interactive torques

The underlying mechanisms of the neural control of movement have long been explored, with a focus primarily on central control aspects and often overlooking the intrinsic mechanical properties of the motor system. To fully understand the control and regulation of movements, the biomechanical properties of the moving subject, specifically interactive torques, must be considered in the design, evaluation, and interpretation of empirical data. We first discuss the difficulty of extrapolating information from a wide variety of tasks due to their varying inherent task constraints. Examples are subsequently given where a biomechanical perspective provides a more informative interpretation of existing data. Finally, we focus on research examining the role of interactive torques with a discussion of how discoordinated movements may be explained by an inability to modulate interactive torques. Inclusion of biomechanical considerations in motor control research is a step toward incorporating multilevel methodologies and interpretations into the field, and providing a more comprehensive understanding of the neural control and regulation of movement.

Shoulder muscle activity in Parkinson's disease during multijoint arm movements across a range of speeds

Bradykinesia is one of the primary symptoms of Parkinson disease and leads to significant functional limitations for patients. Single joint movement studies, that have investigated the mechanism of bradykinesia, suggest that several features of muscle activity are disrupted, including modulation of burst amplitude and duration, and the number of bursts. It has been proposed that it is the blending of these different burst deficits that collectively defines bradykinesia. This study adds two new approaches to the study of bradykinesia. First, we examined the features of shoulder muscle activities during multijoint arm movement in bradykinetic and control subjects, such that previously reported single joint hypotheses could be tested for generalized arm movement. Second, we directly manipulated speed while keeping distance constant for a large range of speeds. In this manner, we could compare individual trials of muscle activity between controls and subjects with Parkinson's disease (PD) for movements matched for both speed and movement duration. Our results showed that while a multiple burst pattern of shoulder muscles was a common strategy for all subjects (young, elderly controls and PD), subjects with PD showed several burst abnormalities, including deficits in initial agonist burst amplitude and duration at both fast and slow speeds. Subjects with PD also (1) failed to produce a one-burst pattern at fast speeds and, instead, produced a predominance of multiple burst patterns and (2) showed a relationship between the number of burst deficits and the severity of disease. These results extend the findings of single joint studies to multi-joint and similarly indicate that a combination of burst modulation abnormalities correlate with bradykinesia and disease severity.

Counteractive relationship between the interaction torque and muscle torque at the wrist is predestined in ball-throwing

Many investigators have demonstrated that in swing motions such as ball-throwing, the motion of the proximal joint (shoulder) produced assistive interaction torque for the distal joint (elbow). In line with these studies, the shoulder and elbow motions would be expected to produce the assistive interaction torque for the wrist joint as well. However, we recently showed that the interaction torque at the wrist was always counteractive to the wrist muscle torque during ball-throwing. The purpose of this study is to clarify, by means of computer simulation, whether the counteractive relationship at the wrist during ball-throwing is caused by the neural contribution or the musculoskeletal mechanical properties of the human arm. First, we simulated the throwing motions of the normal forearm-hand model by systematically changing the proximal-to-distal delay of muscle activities and could line up two candidates for the determinant of the counteractive relationship: the rest angle (neutral angle) of the wrist and the length and mass of the hand. Second, we simulated the throwing motions of the virtual forearm-hand models, showing that only nonrealistic elongation of these two parameters produced the assistive relationship between the interaction torque and muscle torque. These results suggested that the mechanical properties of the human wrist are the main determinant of the counteractive relationship, which is advantageous for keeping the state of the wrist joint stable in multi-joint upper-limb movements and would lead to avoidance of excessive wrist extension or flexion and simplification of extrinsic finger control.

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.

Role of cocontraction in arm movement accuracy

Cocontraction (the simultaneous activation of antagonist muscles around a joint) provides the nervous system with a way to adapt the mechanical properties of the limb to changing task requirements-both in statics and during movement. However, relatively little is known about the conditions under which the motor system modulates limb impedance through cocontraction. The goal of this study was to test for a possible relationship between cocontraction and movement accuracy in multi-joint limb movements. The electromyographic activity of seven single- and double-joint shoulder and elbow muscles was recorded using surface electrodes while subjects performed a pointing task in a horizontal plane to targets that varied randomly in size. Movement speed was controlled by providing subjects with feedback on a trial-to-trial basis. Measures of cocontraction were estimated both during movement and during a 200-ms window immediately following movement end. We observed an inverse relationship between target size and cocontraction: as target size was reduced, cocontraction activity increased. In addition, trajectory variability decreased and endpoint accuracy improved. This suggests that, although energetically expensive, cocontraction may be a strategy used by the motor system to facilitate multi-joint arm movement accuracy. We also observed a general trend for cocontraction levels to decrease over time, supporting the idea that cocontraction and associated limb stiffness are reduced over the course of practice.

Utilization and compensation of interaction torques during ball-throwing movements

The manner in which the CNS deals with interaction torques at each joint in ball throwing was investigated by instructing subjects to throw a ball at three different speeds, using two (elbow and wrist) or three joints (shoulder, elbow, and wrist). The results indicated that the role of the muscle torque at the most proximal joint was to accelerate the most proximal joint and to produce the effect of interjoint interaction on the distal joints. In the three-joint throwing, shoulder muscle torque produced the assistive interaction torque for the elbow, which was effectively utilized to generate large elbow angular velocity when throwing fast. However, at the wrist, the muscle torque always counteracted the interaction torque. By this kinetic mechanism, the wrist angular velocity at the ball-release time was kept relatively constant irrespective of ball speed, which would lead to an accurate ball release. Thus it was concluded that humans can adjust the speed and accuracy of ball-throwing by utilizing interaction torque or compensating for it.

Coordinated turn-and-reach movements. I. Anticipatory compensation for self-generated coriolis and interaction torques

When reaching movements involve simultaneous trunk rotation, additional interaction torques are generated on the arm that are absent when the trunk is stable. To explore whether the CNS compensates for such self-generated interaction torques, we recorded hand trajectories in reaching tasks involving various amplitudes and velocities of arm extension and trunk rotation. Subjects pointed to three targets on a surface slightly above waist level. Two of the target locations were chosen so that a similar arm configuration relative to the trunk would be required for reaching to them, one of these targets requiring substantial trunk rotation, the other very little. Significant trunk rotation was necessary to reach the third target, but the arm's radial distance to the body remained virtually unchanged. Subjects reached at two speeds-a natural pace (slow) and rapidly (fast)-under normal lighting and in total darkness. Trunk angular velocity and finger velocity relative to the trunk were higher in the fast conditions but were not affected by the presence or absence of vision. Peak trunk velocity increased with increasing trunk rotation up to a maximum of 200 degrees /s. In slow movements, peak finger velocity relative to the trunk was smaller when trunk rotation was necessary to reach the targets. In fast movements, peak finger velocity was approximately 1.7 m/s for all targets. Finger trajectories were more curved when reaching movements involved substantial trunk rotation; however, the terminal errors and the maximal deviation of the trajectory from a straight line were comparable in slow and fast movements. This pattern indicates that the larger Coriolis, centripetal, and inertial interaction torques generated during rapid reaches were compensated by additional joint torques. Trajectory characteristics did not vary with the presence or absence of vision, indicating that visual feedback was unnecessary for anticipatory compensations. In all reaches involving trunk rotation, the finger movement generally occurred entirely during the trunk movement, indicating that the CNS did not minimize Coriolis forces incumbent on trunk rotation by sequencing the arm and trunk motions into a turn followed by a reach. A simplified model of the arm/trunk system revealed that additional interaction torques generated on the arm during voluntary turning and reaching were equivalent to < or =1.8 g (1 g = 9.81 m/s(2)) of external force at the elbow but did not degrade performance. In slow-rotation room studies involving reaching movements during passive rotation, Coriolis forces as small as 0.2 g greatly deflect movement trajectories and endpoints. We conclude that compensatory motor innervations are engaged in a predictive fashion to counteract impending self-generated interaction torques during voluntary reaching movements.

Influence of biomechanical constraints on horizontal arm movements

Influence of mechanical interactions between the shoulder and elbow on production of different coordination patterns during horizontal arm movements is investigated in the present study. Subjects performed cyclical movements along a circle and along lines of 4 different orientations. Cycling frequency was manipulated to highlight control features responsible for interactive torque regulation. When the shoulder was involved in motion, torque analysis revealed that this joint was controlled similarly during all movement types. At the elbow, however, each movement type required a specific pattern of regulation of interactive torque with muscle torque. When interactive torque acted in the direction of the required elbow rotation, the demands for active control were lower than when the interactive torque resisted elbow motion and had to be actively suppressed. Kinematic analysis demonstrated that increases in cycling frequency systematically deformed the fingertip path. The amount of these deformations differed across movement types, being more pronounced for movements where the interactive torque resisted joint motion. It appears that interactive torque can assist or resist movement at the joints, making control of some movement types more difficult than others.

Hemispatial differences in visually guided aiming are neither hemispatial nor visual

Many studies have found differences in movements made to either side of the body midline. A popular interpretation of these differences has been that movements made by the arm, which is on same side of space in which the visual target appeared, are faster and better organised because they are processed within-hemisphere. Carey et al. (Experimental Brain Research 112 (1996) 496) showed that hemispatial movement differences cannot be accounted for by such a model. Their data suggested that biomechanical factors such as those proposed by Gordon et al. (Experimental Brain Research 99 (1994) 112) could better account for differences in movement duration and several characteristics of velocity and acceleration. The present study examines these arguments by requiring subjects to make rapid pointing movements in two experiments. In the first, results demonstrated that hemispatial effects occurred in pointing movements made without any visual target or vision of the limb. These findings suggest that intra- and inter-hemispheric models are untenable. Gordon et al. argued that hand path direction relative to the long axis of the upper arm accounts for hemispatial effects on kinematics. In the second experiment hand path direction and hemispace were dissociated. Contralateral movements were performed more efficiently than ipsilateral movements, when target and starting positions required an adductive movement to acquire the contralateral target and an abductive movement to acquire the ipsilateral target. These results provide strong support for the Gordon et al. model, although the possible contributions of other dynamic factors and/or differential control of proximal and distal muscles by the central nervous system cannot be ruled out.

Compensation for the effects of head acceleration on jaw movement in speech

Recent studies have demonstrated the ability of subjects to adjust the control of limb movements to counteract the effects of self-generated loads. The degree to which subjects change control signals to compensate for these loads is a reflection of the extent to which forces affecting movement are represented in motion planning. Here, we have used empirical and modeling studies to examine whether the nervous system compensates for loads acting on the jaw during speech production. As subjects walk, loads to the jaw vary with the direction and magnitude of head acceleration. We investigated the patterns of jaw motion resulting from these loads both in locomotion alone and when locomotion was combined with speech production. In locomotion alone, jaw movements were shown to vary systematically in direction and magnitude in relation to the acceleration of the head. In contrast, when locomotion was combined with speech, variation in jaw position during both consonant and vowel production was substantially reduced. Overall, we have demonstrated that the magnitude of load associated with head acceleration during locomotion is sufficient to produce a systematic change in the position of the jaw. The absence of variation in jaw position during locomotion with speech is thus consistent with the idea that in speech, the control of jaw motion is adjusted in a predictive manner to offset the effects of head acceleration.