Directional effects of changes in muscle torques on initial path during simulated reaching movements

The effect of movement direction on joint torque covariation

It has been proposed that unconstrained upper limb movements are coordinated via a kinetic constraint that produces dynamic muscle torques at each moving joint that are a linear function of a single torque command. This constraint has been termed linear synergy (Gottlieb et al. J Neurophysiol 75:1760-1764, 1996). The current study tested two hypotheses: (1) that the extent of covariation between dynamic muscle torques at the shoulder and elbow varied with the direction of movement and (2) that the extent to which muscle torques deviated from linear synergy would be reproduced by a simulation of pointing movements in which the path of the hand was constrained to be straight. Dynamic muscle torques were calculated from sagittal plane pointing movements performed by 12 participants to targets in eight different directions. The results of principal component analyses performed on the muscle torque data demonstrated direction-dependent variation in the extent to which dynamic muscle torques covaried at the shoulder and elbow. Linear synergy was deviated from substantially in movement directions for which the magnitude of muscle torque was low at one joint. A simulation of movements with straight hand paths was able to accurately estimate the amount of covariation between muscle torques at the two joints in many directions. These results support the idea that a kinematic constraint is imposed by the central nervous system during unconstrained pointing movements. Linear synergy may also be applied as a coordinating constraint in circumstances where its application allows the path of the moving endpoint to remain close to a straight line.

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.

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.

The Role of Execution Noise in Movement Variability

The origin of variability in goal-directed movements is not well understood. Variability can originate from several neural processes such as target localization, movement planning, and movement execution. Here we examine variability resulting from noise in movement execution. In several experiments, subjects moved their unseen hand to visual targets, under conditions which were designed to minimize the variability expected from localization and planning processes. We tested short movements in 32 directions in a center-out reaching task. The variability in the movement endpoints and in the initial movement direction varied systematically with the movement direction, with some directions having up to twice the variability of others. In a second experiment we tested four movements in the same direction but with different extents. Here, the longer movements were systematically curved, and the endpoint ellipses were not aligned with the straight line between starting and end position, but they were roughly aligned with the last part of the trajectory. We show that the variability observed in these experiments cannot be explained by planning noise but is well explained by noise in movement execution. A combination of both signal-dependent and signal-independent noise in the amplitude of the motor commands and temporal noise in their duration can explain the observed variability. Our results suggest that, in general, execution noise accounts for at least a large proportion of movement variability.

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.

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.

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

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