Julius P. A. Dewald

Abnormal joint torque patterns in the paretic upper limb of subjects with hemiparesis

This study presents the results of a novel paradigm for characterizing abnormal coordination in subjects with hemiparesis. Subjects generated maximum voluntary torques (MVTs) isometrically in four randomly ordered blocks consisting of elbow flexion/extension, shoulder flexion/extension, shoulder abduction/adduction, and shoulder external/internal rotation. A 6–degree‐of‐freedom (DOF) load cell was used to measure torques in secondary DOFs at the elbow and shoulder, as well as in the torque direction the subject was attempting to maximize. This allowed characterization of the multijoint torque patterns associated with the generation of MVTs in the eight directions examined. Significant differences were found between the torque patterns exhibited by the paretic limb of the hemiparetic group (n = 8) and those observed for the nonparetic limb and control group (n = 4). Potential neural and biomechanical mechanisms underlying these abnormal torque patterns are discussed along with implications for the functional use of the paretic limb.

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

Abstract. 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.

Redirection of cutaneous sensation from the hand to the chest skin of human amputees with targeted reinnervation

Amputees cannot feel what they touch with their artificial hands, which severely limits usefulness of those hands. We have developed a technique that transfers remaining arm nerves to residual chest muscles after an amputation. This technique allows some sensory nerves from the amputated limb to reinnervate overlying chest skin. When this reinnervated skin is touched, the amputees perceive that they are being touched on their missing limb. We found that touch thresholds of the reinnervated chest skin fall within near-normal ranges, indicating the regeneration of large-fiber afferents. The perceptual identity of the limb and chest was maintained separately even though they shared a common skin surface. A cutaneous expression of proprioception also occurred in one reinnervated individual. Experiments with peltier temperature probes and surface electrical stimulation of the reinnervated skin indicate the regeneration of small diameter temperature and pain afferents. The perception of an amputated limb arising from stimulation of reinnervated chest skin may allow useful sensory feedback from prosthetic devices and provides insight into the mechanisms of neural plasticity and peripheral regeneration in humans.

Shoulder abduction-induced reductions in reaching work area following hemiparetic stroke : neuroscientific implications

A stroke-related loss of corticospinal and corticobulbar pathways is postulated to result in an increased use of remaining neural substrates such as bulbospinal pathways as individuals with stroke are required to generate greater volitional shoulder abduction torques. The effect of shoulder abduction on upper extremity reaching range of motion (work area) was measured in 18 individuals with stroke using the Arm Coordination Training 3-D (ACT3D) device. This robotic system is capable of quantifying movement kinematics when a subject attempts to reach while simultaneously generating various levels of active shoulder abduction torque. We have provided data demonstrating an incremental increase of abnormal coupling of elbow flexion for greater levels of shoulder abduction in the paretic limb that results in a reduction in available work area as a function of active limb support. The progressive increase in the expression of abnormal shoulder/elbow coupling can be explained by a progressive reliance on the indirect cortico-bulbospinal connections that remain in individuals following a stroke-induced brain injury.

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 hand’s 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.

Joint dependent passive stiffness in paretic and contralateral limbs of spastic patients with hemiparetic stroke.

Torque-angle relations at the elbow and ankle joints of relaxed normal controls and patients with hemiparetic stroke were compared. Low velocity flexion/hold/extension angular perturbations were applied to the joint under examination. The resulting torque-angle profiles described a hysteresis loop with similar slopes during the extension and flexion stages but separated by a vertical torque offset. Torque-angle responses obtained in the absence of significant muscle activation, as recorded by surface electromyographic activity, were designated as passive. Elbow passive stiffness estimates were calculated from the slope of the torque-angle response during the flexion stage of the perturbation. The elbow torque-angle plots exhibited linear passive stiffness with magnitude significantly lower than the passive stiffness of the ankle in both normal subjects and spastic patients. Changing ramp velocity had no significant effect on the passive torque-angle hysteresis loop at the elbow. A comparison of the torque-angle relations between hemiparetic spastic and normal control arms showed no significant differences in passive stiffness. Furthermore, no significant differences were found between paretic and contralateral upper limbs of a given hemiparetic subject. By contrast, significant differences in the torque-angle hysteresis loop were present between the paretic and contralateral ankles in all hemiparetic patients tested. These differences were more significant during dorsiflexion, and therefore seem to be related to preferential changes in mechanical properties of plantar flexor muscles. It is hypothesised that the differences in the torque-angle hysteresis loop between elbow and angle joints are related primarily to the larger amount of connective tissue in the calf muscles, as well as to a larger total physiological cross sectional area of calf muscles compared with elbow muscles. It is further hypothesized that the preferential increases in passive stiffness at the ankle in spastic legs result from immobilisation induced changes in muscle connective tissue, which are most prominent in muscles with predominantly slow-twitch fibres (such as soleus). Connective tissue surrounding such slow twitch muscle fibres have been shown to be more sensitive to immobilisation than those in fast twitch muscle. The functional, pathophysiological, and clinical implications of our findings are reviewed.

Task-dependent weakness at the elbow in patients with hemiparesis

OBJECTIVE To investigate the task dependence of elbow weakness in patients with hemiparesis. DESIGN Descriptive study based on interlimb comparisons of maximum voluntary torques (MVTs) generated isometrically in elbow flexion and extension under four task conditions: without explicit control of the torques at adjacent joints and in combination with each of three submaximal shoulder abduction/adduction torque levels. SETTING Rehabilitation center research laboratory. PATIENTS Volunteer samples of six patients with chronic hemiparesis and four controls. MAIN OUTCOME MEASURE Residual strength (RS), defined as the ratio of MVTs for the paretic and nonparetic limbs of patients and nondominant and dominant limbs of controls. RESULTS For the patient group a significant effect of task condition on RS was found (analysis of variance, p = .0003 and p = .002 for elbow flexion and extension, respectively). With increasing shoulder abduction torque level, elbow flexion RS increased and elbow extension RS decreased. In contrast, for the control group, the effect of task condition on RS was not significant. CONCLUSION In hemiparetic patients, weakness of the paretic elbow musculature shows a strong task dependence. This task dependence likely reflects the existence of abnormal synergies between elbow and shoulder muscles of the paretic limb and has important implications for the rehabilitation of motor function following hemiparesis.

Upper-Limb Discoordination in Hemiparetic Stroke: Implications for Neurorehabilitation

Abstract Clinically, upper-limb discoordination after stroke is evident in the form of stereotypic movement patterns that reflect a loss of independent joint control. These movement abnormalities, in conjunction with our recent quantitative findings under isometric conditions, provide evidence for an impaired capacity to generate certain muscle coactivation patterns in the impaired limb. In this article, we examine the parallels that exist between coordination disturbances observed under isometric and movement conditions. Our results suggest that discoordination in stroke may largely represent a manifestation of additional neural constraints on motor outflow. The neurotherapeutic implications of our findings are discussed.

Estimation of muscle forces about the wrist joint during isometric tasks using an EMG coefficient method

A technique for estimating isometric muscle forces based on EMGs and anatomical parameters is presented. In the present study, we record EMGs from five muscles acting at the wrist, during a series of isometric contractions in flexion, extension, ulnar deviation and radial deviation. The method then uses these EMG signals and the necessary anatomical data to estimate individual muscle forces. For one subject, complete anatomical parameters were estimated by MRI reconstruction of muscle moment arms and lines of muscle action. In all subjects, the errors associated with variability in the EMG signals were reduced through the use of signal processing techniques and intensive subject training. These EMG-based force estimates were then validated by evaluations at torque directions in which no mechanical redundancy existed. The stability of the solution space was examined using Monte Carlo simulations. The results of our study show that individual muscle forces at the wrist can be estimated with considerable accuracy, without assuming any control strategy (as is done with optimization theories). However, due to the limited mechanical redundancy of the wrist, it is uncertain whether the method can be used to estimate muscle forces in more highly redundant systems.

Evaluation of different cortical source localization methods using simulated and experimental EEG data

Different cortical source localization methods have been developed to directly link the scalp potentials with the cortical activities. Up to now, these methods are the only possible solution to noninvasively investigate cortical activities with both high spatial and time resolutions. However, the application of these methods is hindered by the fact that they have not been rigorously evaluated nor compared. In this paper, the performances of several source localization methods (moving dipoles, minimum Lp norm, and low resolution tomography (LRT) with Lp norm, p equal to 1, 1.5, and 2) were evaluated by using simulated scalp EEG data, scalp somatosensory evoked potentials (SEPs), and upper limb motor-related potentials (MRPs) obtained on human subjects (all with 163 scalp electrodes). By using simulated EEG data, we first evaluated the source localization ability of the above methods quantitatively. Subsequently, the performance of the various methods was evaluated qualitatively by using experimental SEPs and MRPs. Our results show that the overall LRT Lp norm method with p equal to 1 has a better source localization ability than any of the other investigated methods and provides physiologically meaningful reconstruction results. Our evaluation results provide useful information for choosing cortical source localization approaches for future EEG/MEG studies.