Abnormal locomotor muscle recruitment activity is present in horses with shivering and Purkinje cell distal axonopathy.

BACKGROUND: Cerebellar Purkinje cell axonal degeneration has been identified in horses with shivering but its relationship with abnormal hindlimb movement has not been elucidated. OBJECTIVES: To characterise surface electromyographic (sEMG) hindlimb muscle activity in horses with shivering, correlate with clinical scores and examine horses for Purkinje axonal degeneration. STUDY DESIGN: Descriptive controlled clinical study. METHODS: The hindlimb of seven shivering and six control draught horses were clinically scored. Biceps femoris (BF), vastus lateralis (VL), tensor fasciae latae and extensor digitorum longus were recorded via sEMG during forward/backward walking and trotting. Integrated (iEMG) and peak EMG activity were compared between groups and correlated with clinical locomotor exam scores. Sections of the deep cerebellar nuclei (DCN) of six of the seven shivering horses were examined with calbindin immunohistochemistry. RESULTS: In control horses, backward walking resembled forward walking (right hindlimb peak EMG: backward: 47.5 ± 21.9%, forward: 36.9 ± 15.7%) but displayed significantly higher amplitudes during trotting (76.1 ± 3.4%). However, in shivering horses, backward walking was significantly different from forward (backward: 88.5 ± 21.5%, forward: 49.2 ± 8.9%), and resembled activity during trotting (81.4 ± 4.8%). Specific to backward walking, mean sEMG amplitude fell outside two standard deviations of mean control sEMG for ≥25% of the stride in the BF for all seven and the VL for six of the seven shivering horses. Locomotor exam scores were correlated with peak EMG (r = 0.87) and iEMG (r = 0.87). Calbindin-positive spheroids were present in Purkinje axons in DCN of all shivering horses examined. MAIN LIMITATIONS: The neuropathological examination focused specifically on the DCN and, therefore, we cannot fully exclude additional lesions that may have influenced abnormal sEMG findings in shivering horses. CONCLUSION: Shivering is characterised by abnormally elevated muscle recruitment particularly in BF and VL muscles during backward walking and associated with selective Purkinje cell distal axonal degeneration.

The Equine Movement Disorder "Shivers" Is Associated With Selective Cerebellar Purkinje Cell Axonal Degeneration.

"Shivers" is a progressive equine movement disorder of unknown etiology. Clinically, horses with shivers show difficulty walking backward, assume hyperflexed limb postures, and have hind limb tremors during backward movement that resembles shivering. At least initially, forward movements are normal. Given that neither the neurophysiologic nor the pathologic mechanisms of the disease is known, nor has a neuroanatomic locus been identified, we undertook a detailed neuroanatomic and neuropathologic analysis of the complete sensorimotor system in horses with shivers and clinically normal control horses. No abnormalities were identified in the examined hind limb and forelimb skeletal muscles nor the associated peripheral nerves. Eosinophilic segmented axonal spheroids were a common lesion. Calretinin-positive axonal spheroids were present in many regions of the central nervous system, particularly the nucleus cuneatus lateralis; however, their numbers did not differ significantly from those of control horses. When compared to controls, calretinin-negative, calbindin-positive, and glutamic acid decarboxylase-positive spheroids were increased 80-fold in Purkinje cell axons within the deep cerebellar nuclei of horses with shivers. Unusual lamellar or membranous structures resembling marked myelin decompaction were present between myelin sheaths of presumed Purkinje cell axons in the deep cerebellar nuclei of shivers but not control horses. The immunohistochemical and ultrastructural characteristics of the lesions combined with their functional neuroanatomic distribution indicate, for the first time, that shivers is characterized by end-terminal neuroaxonal degeneration in the deep cerebellar nuclei, which results in context-specific hypermetria and myoclonus.

Robot-assisted training to improve proprioception does benefit from added vibro-tactile feedback.

Proprioception is central for motor control and its role must also be taken into account when designing motor rehabilitation training protocols. This is particularly important when dealing with motor deficits due to proprioceptive impairment such as peripheral sensory neuropathy. In these cases substituting or augmenting diminished proprioceptive sensory information might be beneficial for improving motor function. However it still remains to be understood how proprioceptive senses can be improved by training, how this would translate into motor improvement and whether additional sensory modalities during motor training contribute to the sensorimotor training process. This preliminary study investigated how proprioceptive/haptic training can be augmented by providing additional sensory information in the form of vibro-tactile feedback. We tested the acuity of the wrist proprioceptive position sense before and after robotic training in two groups of healthy subjects, one trained only with haptic feedback and one with haptic and vibro-tactile feedback. We found that only the group receiving the multimodal feedback significantly improved proprioceptive acuity. This study demonstrates that non-proprioceptive position feedback derived from another somatosensory modality is easily interpretable for humans and can contribute to an increased precision of joint position. The clinical implications of this finding will be outlined.

Location and restoration of function after cerebellar tumor removal-a longitudinal study of children and adolescents.

Sequelae in children following cerebellar tumor removal surgery are well defined, and predictors for poor recovery include lesions of the cerebellar nuclei and the inferior vermis. Dynamic reorganization is thought to promote functional recovery in particular within the first year after surgery. Yet, the time course and mechanisms of recovery within this critical time frame are elusive and longitudinal studies are missing. Thus, a group of children and adolescents (n = 12, range 6-17 years) were followed longitudinally after cerebellar surgery and compared to age- and gender-matched controls (n = 11). Patients were examined (1) within the first days, (2) 3 months, and (3) 1 year after surgery. Each time behavioral tests of balance and upper limb motor function, ataxia rating, and a MRI scan were performed. Data were used for subsequent lesion-symptom mapping of cerebellar function. Behavioral improvements continued beyond 3 months, but were not complete in all patients after 1 year. At that time, remaining deficits were mild. Within the first 3 months, cerebellar lesion volumes were notably reduced by vanishing edema. Reduction in edema affecting the deep cerebellar nuclei but not reduction of total cerebellar lesion volume was a major predictor of early functional recovery. Persistent impairment in balance and upper limb function was linked to permanent lesions of the inferior vermis and the deep cerebellar nuclei.

Current advances in lesion-symptom mapping of the human cerebellum.

While high-resolution structural magnetic resonance imaging (MRI) combined with newer analysis methods has become a powerful tool in human cerebral lesion studies, comparatively few studies have used these advanced imaging techniques to study lesions of the human cerebellum and their associated symptoms. This review will summarize the methodology of MRI-based lesion-symptom mapping of the human cerebellum and discuss its potential for gaining insights into cerebellar function. The investigation of patients with defined focal lesions yields the greatest potential for obtaining meaningful correlations between lesion site and behavioral deficits. In smaller groups of patients overlay plots and subtraction analysis are good options. If larger groups of patients are available, different statistical techniques have been introduced to compare behavior and lesion site on a voxel-by-voxel basis. Although localization in degenerative cerebellar disorders is less accurate because of the diffuse nature of the disease, certain information about the supposed function of larger subdivisions of the cerebellum can be gained. Examples are given which show that lesion-symptom mapping allows to investigate the function of the intermediate zone and cerebellar nuclei. We conclude that meaningful correlations between lesion site and behavioral data can be obtained in patients with degenerative as well as focal cerebellar disorders.

Lesion-symptom mapping of the human cerebellum.

High-resolution structural magnetic resonance imaging (MRI) has become a powerful tool in human cerebellar lesion studies. Structural MRI is helpful to analyse the localisation and extent of cerebellar lesions and to determine possible extracerebellar involvement. Functionally meaningful correlations between a cerebellar lesion site and behavioural data can be obtained both in subjects with degenerative as well as focal cerebellar disorders. In this review, examples are presented which demonstrate that MRI-based lesion-symptom mapping is helpful to study the function of cerebellar cortex and cerebellar nuclei. Behavioural measures were used which represent two main areas of cerebellar function, that is, motor coordination and motor learning. One example are correlations with clinical data which are in good accordance with the known functional compartmentalisation of the cerebellum in three sagittal zones: In patients with cerebellar cortical degeneration ataxia of stance and gait was correlated with atrophy of the medial (and intermediate) cerebellum, oculomotor disorders with the medial, dysarthria with the intermediate and limb ataxia with atrophy of the intermediate and lateral cerebellum. Similar findings were obtained in patients with focal lesions. In addition, in patients with acute focal lesions, a somatotopy in the superior cerebellar cortex was found which is in close relationship to animal data and functional MRI data in healthy control subjects. Finally, comparison of data in patients with acute and chronic focal lesions revealed that lesion site appears to be critical for motor recovery. Recovery after lesions to the nuclei of the cerebellum was less complete. Another example which extended knowledge about functional localisation within the cerebellum is classical conditioning of the eyeblink response, a simple form of motor learning. In healthy subjects, learning rate was related to the volume of the cortex of the posterior cerebellar lobe. In patients with focal cerebellar lesions, acquisition of eyeblink conditioning was significantly reduced in lesions including the cortex of the superior posterior lobe, but not the inferior posterior lobe. Disordered timing of conditioned eyeblink responses correlated with lesions of the anterior lobe. Findings are in good agreement with the animal literature. Different parts of the cerebellar cortex may be involved in acquisition and timing of conditioned eyeblink responses in humans. These examples demonstrate that MRI-based lesion-symptom mapping is helpful to study the contribution of functionally relevant cerebellar compartments in motor control and recovery in patients with cerebellar disease. In addition, information about the function of cerebellar cortex and nuclei can be gained.

Increased basal-ganglia activation performing a non-dystonia-related task in focal dystonia.

BACKGROUND AND PURPOSE: We tried to determine whether altered sensorimotor cortex and basal-ganglia activation in blepharospasm (BSP) and cervical dystonia (CD) are restricted to areas directly responsible for the innervation of dystonic muscles, or whether impairment in focal dystonia reaches beyond these direct associations supporting a more global disturbance of sensory and motor control in focal dystonia. METHODS: Twenty patients with focal dystonia (11 BSP, 9 CD) and 14 healthy controls were investigated with functional magnetic resonance imaging (fMRI) performing a simple grip force forearm contraction task. RESULTS: BSP and CD patients and healthy controls showed similar activation in the pre-motor, primary motor and primary sensory cortex, whilst basal-ganglia activation was increased in BSP and CD with related activation patterns compared with controls. BSP patients had increased activation in the thalamus, caudate nucleus, putamen and lateral globus pallidus, whilst CD patients showed increased activation in the caudate nucleus, putamen and thalamus. No differences in applied grip force were detected between groups. CONCLUSIONS: In both, BSP and CD, increased basal-ganglia activation could be demonstrated in a task not primarily involving the dystonic musculature affected by these disorders. Comparable activation changes may also indicate a common pathway in the pathophysiology in BSP and CD.

Impact of surgery and adjuvant therapy on balance function in children and adolescents with cerebellar tumors.

OBJECTIVES: This study examined the effects of posterior fossa tumor surgery and concomitant irradiation and/or chemotherapy on the long-term recovery of balance function in children and adolescent patients. SUBJECTS AND METHODS: 22 patients, treated during childhood for a benign (n = 14) or malignant cerebellar tumor (n = 8), were examined in chronic state (mean latency between surgery and testing: 7.7 years, range 3 - 17 years). Postural impairments were assessed with static and dynamic posturography. All cerebellar lesions were documented by standardized and normalized MRI data. Healthy age- and gender-matched subjects served as a control group. RESULTS: Comparing the balance function of (i) children with or without affected cerebellar nuclei and (ii) children with and without adjuvant chemotherapy and/or radiotherapy revealed that damage to the cerebellar nuclei had more impact on neurological impairment than concomitant tumor therapy. Balance abnormalities were most pronounced when a lesion affected the fastigial nucleus. Chemotherapy with its neurological side effect was associated with enhanced postural sway in only two children with malignant tumors. CONCLUSIONS: The study results indicate that the sparing of the deep cerebellar nuclei had the greatest impact on the recovery of balance function in pediatric patients treated for both a benign or malignant cerebellar tumor.

The effect of subthalamic nucleus stimulation on kinaesthesia in Parkinson's disease.

BACKGROUND: Parkinson's disease is accompanied by deficits in passive motion and limb position sense. OBJECTIVE: To investigate whether deep brain stimulation of the subthalamic nucleus (STN-DBS) reverses these proprioceptive deficits. METHODS AND RESULTS: A passive movement task was applied to nine patients with Parkinson's disease and bilateral chronic STN-DBS and to seven controls. Thresholds for 75% correct responses were 0.9 degrees for controls, 2.5 degrees for Parkinson's disease patients when stimulation was OFF, and 2.0 degrees when stimulation was ON. CONCLUSIONS: STN-DBS improves kinaesthesic deficits in Parkinson's disease, but does not lead to a full recovery of proprioceptive function.

Predicting children's overarm throw ball velocities from their developmental levels in throwing.

This study examined the movement process-product relationship from a developmental perspective. The authors used multiple regression to investigate the changing relationship between qualitative movement descriptions of the overarm throw and the throwing outcome, horizontal ball velocity. Seventeen girls and 22 boys were filmed longitudinally at ages 6, 7, 8, and 13 years. Their movements were assessed using Roberton's (Roberton & Halverson, 1984) developmental sequences for action of the humerus, forearm, trunk, stepping, and stride length. The sequences accounted for 69-85% (adjusted) of the total velocity variance each year. The components that best predicted ball velocity changed over time, although humerus or forearm action always accounted for considerable variance. Gender was a good predictor of ball velocity, but if the developmental descriptions were entered first in a stepwise regression, gender then explained no more than 2% additional variance.

The effect of the Ebbinghaus illusion on grasping behaviour of children.

Within the context of the Ebbinghaus illusion, adults regularly misjudge the physical size of a centre disc, yet scale their hand aperture according to its actual size. Separate visual pathways for perception and action are assumed to account for this finding. The dorsal visual stream is said to elaborate on egocentric (visuomotor), while the ventral stream is involved in allocentric transformations (object recognition). This study examines the ontogenetic development of this dissociation between perception and action in 35 children between the ages of 5 and 12 years. We report four major results. First, when children judged object size without grasping the disc, their judgements were deceived by the illusion to the same extent as adults. However, when asked to estimate size and then to grasp the disc, young children's (5-7 years) perceptual judgements became unreliable, while adults were still reliably deceived by the illusion in 80% of their trials. Second, the younger the children, the more their aperture was affected by the illusional surround. Discs of the same size were grasped with a smaller aperture when surrounded by a small annulus, although they were perceived as being larger. Third, young children used the largest safety margin during grasping. Fourth, the reliance on visual feedback decreased with increasing age, which was documented by shorter movement times and earlier maximum hand opening during grasping in the older children (feedforward control). Our results indicate that grasping behaviour in children is subject to an interaction between ventral and dorsal processes. Both pathways seem not to be functionally segregated in early and middle childhood. The data are inconclusive about whether young children predominantly use a specific visual stream for either a perceptual or motor task. However, our data demonstrate that children were relying on both visual processing streams during perceptual as well as visuomotor tasks. We found that children used egocentric cues to make perceptual judgements, while their grasping gestures were not exclusively shaped by viewer-centred but also by object-centred information.

Effect of prolonged neck muscle vibration on lateral head tilt in severe spasmodic torticollis.

UNLABELLED: Short term vibration of the dorsal neck muscles (10-35 s) is known to induce involuntary movements of the head in patients with spasmodic torticollis. To investigate whether neck muscle vibration might serve as a therapeutic tool when applied for a longer time interval, we compared a vibration interval of 5 seconds with a 15 minute interval in a patient with spasmodic torticollis with an extreme head tilt to the right shoulder. Head position was recorded with a two camera optoelectronic motion analyzer in six different test conditions. Vibration regularly induced a rapid change of head position that was markedly closer to a normal, upright posture. After 5 seconds of vibration, head position very quickly returned to the initial position within seconds. During the 15 minute interval, head position remained elevated. After terminating vibration in this condition, the corrected head position remained stable at first and then decreased slowly within minutes to the initial tilted position. CONCLUSIONS: (1) In this patient, muscle vibration was the specific sensory input that induced lengthening of the dystonic neck muscles. Neither haptic stimulation nor transcutaneous electrical stimulation had more than a marginal effect. (2) The marked difference in the change of head position after short and prolonged stimulation supports the hypothesis that spasmodic torticollis might result from a disturbance of the central processing of the afferent input conveying head position information-at least in those patients who are sensitive to sensory stimulation in the neck region. (3) Long term neck muscle vibration may provide a convenient non-invasive method for treating spasmodic torticollis at the central level by influencing the neural control of head on trunk position.

Predictive control of muscle responses to arm perturbations in cerebellar patients.

OBJECTIVES: To examine changes in predictive control of early antagonist responses to limb perturbations in patients with defined lesions of the cerebellum. METHODS: Eight cerebellar patients and eight sex and age matched control subjects participated. Subjects held a handle that was rotated around the elbow joint. They were instructed to hold the forearm at 90 degrees flexion against a mechanical perturbation. Extensor torque (5 Nm) was applied for 140 ms (pulse), or for 1400 ms (step) through an external motor. Motor responses were tested under two different conditions of anticipatory information. In the expected condition, subjects anticipated and received a pulse. Under the unexpected condition, subjects expected steps, but received unexpected pulses. Biceps and triceps EMG as well as angular kinematics were compared between expected and unexpected pulse perturbations to quantify possible effects of prediction. RESULTS: In all healthy subjects, the degree of overshoot in the return flexion movement was significantly less in expected pulse perturbations compared with unexpected trials. The degree of amplitude reduction was significantly smaller in the patient group than in the control group (22.8% v 40.0%). During the expected trials, latency of peak triceps activity was on average 20% shorter in the control group, but 4% larger in the cerebellar patients. CONCLUSIONS: In the expected condition, controls achieved a significant reduction in angular amplitude by generating triceps activity earlier, whereas the ability to use prediction for adjusting early antagonist responses after limb perturbation was impaired in cerebellar patients.

Identification of time-varying stiffness, damping, and equilibrium position in human forearm movements.

Knowledge of how stiffness, damping, and the equilibrium position of specific limbs change during voluntary motion is important for understanding basic strategies of neuromotor control. Presented here is an algorithm for identifying time-dependent changes in joint stiffness, damping, and equilibrium position of the human forearm. The procedure requires data from only a single trial. The method relies neither on an analysis of the resonant frequency of the arm nor on the presence of an external bias force. Its validity was tested with a simulated forward model of the human forearm. Using the parameter estimations as forward model input, the angular kinematics (model output) were reconstructed and compared to the empirically measured data. Identification of mechanical impedance is based on a least-squares solution of the model equation. As a regularization technique and to improve the temporal resolution of the identification process, a moving temporal window with a variable width was imposed. The method's performance was tested by (a) identifying a priori known hypothetical time-series of stiffness, damping, and equilibrium position, and (b) determining impedance parameters from recorded single-joint forearm movements during a hold and a goal-directed movement task. The method reliably reconstructed the original angular kinematics of the artificial and human data with an average positional error of less than 0.05 rad for movement amplitudes of up to 0.9 rad, and did not yield hypermetric trajectories like previous procedures not accounting for damping.

Do patients with neglect show abnormal hand velocity profiles during tactile exploration of peripersonal space?

It has been suggested that the movement impairments experienced by patients with neglect are not restricted to spatial disorders, but also affect higher-order kinematics (velocity and acceleration) to the extent that movements towards the neglected side are slower than movements away from it. In a recent study, we could not confirm this hypothesis, but found that patients with unilateral neglect exhibited no distinct direction-specific deficits in hand velocity when performing goal-directed reaching movements. Here we investigated whether neglect patients might reveal direction-specific deficits during exploratory hand movements. Six patients with left-sided neglect and six age-matched healthy control subjects scanned with their right hands the surface of a large table searching for a (non-existent) tactile target. Movements were performed in darkness. Time-position data of the hand were recorded with an optoelectronic camera system. Median activity of the patients' exploratory hand movements was shifted to the right (Karnath and Perenin 1998). Hand trajectories were partitioned into sections of leftward/rightward or, along the sagittal plane, into sections of near/far movements. For each movement section average and peak velocities were computed. The patients' hand movements were bradykinetic when compared with the control group. However, we found no evidence that average or peak velocities of leftward intervals were systematically lower than during rightward motion. Direction-specific deficits in velocity were also not observed for movements to and away from the body (sagittal plane). In conclusion, we found evidence for general bradykinesia in neglect patients but not for a direction-specific deficit in the control of hand velocity during exploratory hand movements.

A developmental approach to visually-guided reaching in artificial systems.

The aim of the present paper is to propose that the adoption of a framework of biological development is suitable for the construction of artificial systems. We will argue that a developmental approach does provide unique insights on how to build highly complex and adaptable artificial systems. To illustrate our point, we will use as an example the acquisition of goal-directed reaching. In the initial part of the paper we will outline (a) how mechanisms of biological development can be adapted to the artificial world, and (b) how this artificial development differs from traditional engineering approaches to robotics. An experiment performed on an artificial system initially controlled by motor reflexes is presented, showing the acquisition of visuo-motor maps for ballistic control of reaching without explicit knowledge of the system's kinematic parameters.

Kinematics of goal-directed arm movements in neglect: control of hand velocity.

Do patients with unilateral neglect exhibit direction-specific deficits in the control of movement velocity when performing goal-directed arm movements? Five patients with left-sided neglect performed unrestrained three-dimensional pointing movements to visual targets presented at body midline, the left and right hemispace. A group of healthy adults and a group of patients with right-hemispheric brain damage but no neglect served as controls. Pointing was performed under normal room light or in darkness. Time-position data of the hand were recorded with an opto-electronic camera system. We found that compared to healthy controls, movement times were longer in both patient groups due to prolonged acceleration and deceleration phases. Tangential peak hand velocity was lower in both patient groups, but not significantly different from controls. Single peak, bell-shaped velocity profiles of the hand were preserved in all right hemispheric patients and in three out of five neglect patients. Most important, the velocity profiles of neglect patients to leftward targets did not differ significantly from those to targets in the right hemispace. In summary, we found evidence for general bradykinesia in neglect patients, but not for a direction-specific deficit in the control of hand velocity. We conclude that visual neglect induces characteristic changes in exploratory behavior, but not in the kinematics of goal-directed movements to objects in peripersonal space.

Interaction of finger representation in the human first somatosensory cortex: a neuromagnetic study.

Neuromagnetic responses to separate tactile stimulation of digits I, II and V and simultaneous stimulation of digit pairs II and I, and II and V, were recorded in six healthy adult subjects using a 122-channel whole-head neuromagnetometer in order to investigate functional overlap of finger representations in primary somatosensory cortex (SI). Evoked responses to single digit stimulation were explained by time-varying equivalent current dipoles (ECDs) located in SI. These ECDs were then used to explain responses to stimulation of digit pairs. A cortical interaction ratio (IR) was defined as the vector sum of peak source amplitudes to separate stimulations of two fingers divided by the vector sum of source amplitudes to simultaneous stimulation of the two digits. Mean IR was significantly higher (P<0.05; Wilcoxon test) for digit pair II + I (1.69+/- 0.15) compared to digit pair II + V (1.14+/- 0.12). These results indicate that there is an overlap of finger representations in human SI which differs between anatomically adjacent and non-adjacent digit pairs.

Coordination of multi-joint arm movements in cerebellar ataxia: analysis of hand and angular kinematics.

Kinematic abnormalities of fast multijoint movements in cerebellar ataxia include abnormally increased curvature of hand trajectories and an increased hand path and are thought to originate from an impairment in generating appropriate levels of muscle torques to support normal coordination between shoulder and elbow joints. Such a mechanism predicts that kinematic abnormalities are pronounced when fast movements are performed and large muscular torques are required. Experimental evidence that systematically explores the effects of increasing movement velocities on movement kinematics in cerebellar multijoint movements is limited and to some extent contradictory. We, therefore, investigated angular and hand kinematics of natural multijoint pointing movements in patients with cerebellar degenerative disorders and healthy controls. Subjects performed self-paced vertical pointing movements with their right arms at three different target velocities. Limb movements were recorded in three-dimensional space using a two-camera infrared tracking system. Differences between patients and healthy subjects were most prominent when the subjects performed fast movements. Peak hand acceleration and deceleration were similar to normals during slow and moderate velocity movements but were smaller for fast movements. While altering movement velocities had little or no effect on the length of the hand path and angular motion of elbow and shoulder joints in normal subjects, the patients exhibited overshooting motions (hypermetria) of the hand and at both joints as movement velocity increased. Hypermetria at one joint always accompanied hypermetria at the neighboring joint. Peak elbow angular deceleration was markedly delayed in patients compared with normals. Other temporal movement variables such as the relative timing of shoulder and elbow joint motion onsets were normal in patients. Kinematic abnormalities of multijoint arm movements in cerebellar ataxia include hypermetria at both the elbow and the shoulder joint and, as a consequence, irregular and enlarged paths of the hand, and they are marked with fast but not with slow movements. Our findings suggest that kinematic movement abnormalities that characterize cerebellar limb ataxia are related to an impairment in scaling movement variables such as joint acceleration and deceleration normally with movement speed. Most likely, increased hand paths and decomposition of movement during slow movements, as described earlier, result from compensatory mechanisms the patients may employ if maximum movement accuracy is required.

Multijoint arm movements in cerebellar ataxia: abnormal control of movement dynamics.

In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients' deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients' difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements.