Cerebellar ataxia: quantitative assessment and cybernetic interpretation

Does Proprioceptive Impairment Affect Feedforward Motor Control? A Cross-Sectional Study on Patients With Brain Damage

Sensory ataxia and cerebellar ataxia share common manifestations including dysmetria, intentional tremor and lack of smoothness. We formulated a theoretical framework to describe the patients' sensory and cerebellar ataxic behavior as consequences of a forward model impairment. To test this framework, the present study aimed to compare upper limb movement kinematics in an index-to-nose task between three groups: healthy controls, people with CNS focal lesions and cerebellar deficits and people with CNS focal lesions and somatosensory impairment. We recruited 12 healthy controls (age years, female = 5) and 20 participants with focal CNS lesions. We divided the sample according to the lesion site in participants with lesions in areas involved in the somatosensory information processing (n = 12, age years, female = 5) and participants with lesions in the cerebellum or cerebellar peduncle (n = 8, age years, female = 1). Movement features concerning movement efficiency (average velocity, peak velocity), accuracy (spatial error when pointing to the nose) and motor planning (timing and spatial occurrence of velocity peak, velocity and deviation from ideal trajectory at 150ms after the movement onset) were computed. Both the groups of participants with CNS lesions performed the movement slower than healthy controls. When comparing results from the two groups of patients, we showed that participants with cerebellar lesions were characterized by greater trial-to-trial variability of the velocity peak (repeated measure ANOVA group effect: F = 5.242, p = 0.012) and its timing (condition*group interaction: F = 5.38, p = 0.011). Our findings suggested that both participants with cerebellar and somatosensory deficits showed signs of anticipatory motor control impairment.

Multifeature quantitative motor assessment of upper limb ataxia including drawing and reaching

OBJECTIVE

Voluntary upper limb movements are an ecologically important yet insufficiently explored digital-motor outcome domain for trials in degenerative ataxia. We extended and validated the trial-ready quantitative motor assessment battery "Q-Motor" for upper limb movements with clinician-reported, patient-focused, and performance outcomes of ataxia.

METHODS

Exploratory single-center cross-sectional assessment in 94 subjects (46 cross-genotype ataxia patients; 48 matched controls), comprising five tasks measured by force transducer and/or position field: Finger Tapping, diadochokinesia, grip-lift, and-as novel implementations-Spiral Drawing, and Target Reaching. Digital-motor measures were selected if they discriminated from controls (AUC >0.7) and correlated-with at least one strong correlation (rho ≥0.6)-to the Scale for the Assessment and Rating of Ataxia (SARA), activities of daily living (FARS-ADL), and the Nine-Hole Peg Test (9HPT).

RESULTS

Six movement features with 69 measures met selection criteria, including speed and variability in all tasks, stability in grip-lift, and efficiency in Target Reaching. The novel drawing/reaching tasks best captured impairment in dexterity (|rho9HPT| ≤0.81) and FARS-ADL upper limb items (|rhoADLul| ≤0.64), particularly by kinematic analysis of smoothness (SPARC). Target hit rate, a composite of speed and endpoint precision, almost perfectly discriminated ataxia and controls (AUC: 0.97). Selected measures in all tasks discriminated between mild, moderate, and severe impairment (SARA upper limb composite: 0-2/>2-4/>4-6) and correlated with severity in the trial-relevant mild ataxia stage (SARA ≤10, n = 20).

INTERPRETATION

Q-Motor assessment captures multiple features of impaired upper limb movements in degenerative ataxia. Validation with key clinical outcome domains provides the basis for evaluation in longitudinal studies and clinical trial settings.

Efficacy of levetiracetam on upper limb movement in multiple sclerosis patients with cerebellar signs: a multicenter double-blind, placebo-controlled, crossover study

BACKGROUND AND PURPOSE

The literature provides contrasting results on the efficacy of levetiracetam (LEV) in multiple sclerosis (MS) patients with cerebellar signs. It was sought to evaluate the efficacy of LEV on upper limb movement in MS patients.

METHODS

In this multicenter double-blind placebo-controlled crossover study, MS patients with prevalently cerebellar signs were randomly allocated into two groups: LEV followed by placebo (group 1) or placebo followed by LEV (group 2). Clinical assessments were performed by a blinded physician at T0 (day 1), T1 (day 22), T2 (2-week wash-out period, day 35) and T3 (day 56). The primary outcome was dexterity in the arm with greater deficit, assessed by the nine-hole peg test (9HPT). Secondary clinical outcomes included responders on the 9HPT (∆9HPT >20%), tremor activity of the daily living questionnaire and self-defined upper limb impairment, through a numeric rating scale. Kinematic evaluation was performed using a digitizing tablet, providing data on normalized jerk, aiming error and centripetal acceleration.

RESULTS

Forty-eight subjects (45.2 ± 10.4 years) were randomly allocated into two groups (n = 24 each). 9HPT significantly improved in the LEV phase in both groups (P < 0.001). The LEV treatment phase led to a significant improvement (P < 0.01) of all clinical outcomes in group 1 and in dexterity in group 2. No significant changes were reported during both placebo phases in the two groups. Considering the kinematic analysis, only normalized jerk significantly improved after treatment with LEV (T0-T1) in group 1.

CONCLUSIONS

Levetiracetam treatment seems to be effective in improving upper limb dexterity in MS patients with cerebellar signs.

An Affordable Method for Evaluation of Ataxic Disorders Based on Electrooculography

Ataxias are a group of neurodegenerative disorders characterized by cerebellar dysfunction that cause irregularities in the rate, rhythm, amplitude, and force of voluntary movements. The electrooculogram (EOG) may provide clues about ataxic disorders because most of these patients have difficulty with visual tracking and fixing their gaze. Using electrodes, EOG records the biopotentials generated by eye movements. In this paper, three surface electrodes are placed around the eye socket, and the biopotentials generated by eye movements are acquired using a commercial bioamplifier device. Next, the signals are sent to the computer to be digitally processed to extract the rate of saccades as well as the delay and deviation of the gaze in response to a stimulus. These features are analysed in a novel software application designed to help physicians in evaluating ataxia. After applying several tests to both healthy and ataxia-affected patients, differences in EOG results were found. The evaluation of the reliability of the designed software application is made according to three metrics: sensitivity, specificity, and accuracy. The results indicate the proposed system's viability as an affordable method for evaluation of ataxic disorders.

Contribution of the Cerebellum to Predictive Motor Control and Its Evaluation in Ataxic Patients

Goal-directed movements are predictive and multimodal in nature, especially for moving targets. For instance, during a reaching movement for a moving target, humans need to predict both motion of the target and movement of the limb. Recent computational studies show that the cerebellum predicts current and future states of the body and its environment using internal forward models. Sensory feedback signals from the periphery have delays in reaching the central nervous system, ranging between tens to hundreds of milliseconds. It is well known in engineering that feedback control based on time-delayed inputs can result in oscillatory and often unstable movements. In contrast, the brain predicts a current state from a previous state using forward models. This predictive mechanism most likely underpins stable and dexterous control of reaching movements. Although the cerebro-cerebellum has long been suggested as loci of various forward models, few methods are available to evaluate accuracy of the forward models in patients with cerebellar ataxia. Recently, we developed a non-invasive method to analyze receipt of motor commands in terms of movement kinematics for the wrist joint (B_r/K_r ratio). In the present study, we have identified two components (F1 and F2) of the smooth pursuit movement. We found that the two components were in different control modes with different B_r/K_r ratios. The major F1 component in a lower frequency range encodes both velocity and position of the moving target (higher B_r/K_r ratio) to synchronize movement of the wrist joint with motion of the target in a predictive manner. The minor F2 component in a higher frequency range is biased to position control in order to generate intermittent small step-wise movements. In cerebellar patients, the F1 component shows a selective decrease in the B_r/K_r ratio, which is correlated with decrease in accuracy of the pursuit movement. We conclude that the B_r/K_r ratio of the F1 component provides a unique parameter to evaluate accuracy of the predictive control. We also discuss the pathophysiological and clinical implications for clinical ataxiology.

Gait, Balance, and Coordination Impairments in Niemann Pick Disease, Type C1

This is the first study to objectively measure gait, balance, and upper limb coordination in a group of patients with NPC1 and compare the results to age and gender matched controls. This is also the first study to report effect sizes in these measures. Spatiotemporal gait analysis, static and dynamic posturography, and upper limb reaching motion analysis were performed. The findings showed that the NPC1 subjects had statistically significant deficits on 12 out of the 16 parameters investigated compared to controls, and large effect sizes for all but 1 parameter. When ranking the variables in terms of the effect sizes, the top 5 included at least 1 parameter from each of the 3 motor domains investigated. These results can provide insight to clinical researchers on the selection of outcome measures for longitudinal and interventional studies.

Body Sway Increases After Functional Inactivation of the Cerebellar Vermis by cTBS

Balance stability correlates with cerebellar vermis volume. Furthermore, the cerebellum is involved in precise timing of motor processes by fine-tuning the sensorimotor integration. We tested the hypothesis that any cerebellar action in stance control and in timing of visuomotor integration for balance is impaired by continuous theta-burst stimulation (cTBS) of the vermis. Ten subjects stood quietly and underwent six sequences of 10-min acquisition of center of foot pressure (CoP) data after cTBS, sham stimulation, and no stimulation. Visual shifts from eyes closed (EC) to eyes open (EO) and vice versa were presented via electronic goggles. Mean anteroposterior and mediolateral CoP position and oscillation, and the time delay at which body sway changed after visual shift were calculated. CoP position under both EC and EO condition was not modified after cTBS. Sway path length was greater with EC than EO and increased in both visual conditions after cTBS. CoP oscillation was also larger with EC and increased under both visual conditions after cTBS. The delay at which body oscillation changed after visual shift was longer after EC to EO than EO to EC, but unaffected by cTBS. The time constant of decrease or increase of oscillation was longer in EC to EO shifts, but unaffected by cTBS. Functional inactivation of the cerebellar vermis is associated with increased sway. Despite this, cTBS does not detectably modify onset and time course of the sensorimotor integration process of adaptation to visual shifts. Cerebellar vermis normally controls oscillation, but not timing of adaptation to abrupt changes in stabilizing information.

A New Method for Functional Evaluation of Motor Commands in Patients with Cerebellar Ataxia

Quantitative evaluation of motor functions of patients with cerebellar ataxia is vital for evidence-based treatments and has been a focus in previous investigations of movement kinematics. Due to redundancy of the musculoskeletal system, muscle activities contain more information than the movement kinematics. Therefore, it is preferable to analyze causal anomalies of muscle activities to evaluate motor functions in patients. Here we propose a new method to evaluate the motor functions at the level of muscle activities and movement kinematics. Nineteen patients and 10 control subjects performed two movement tasks of the wrist joint, a step-tracking task and a pursuit task, with a manipulandum. The movements of the wrist joint and activities of the four wrist prime movers were recorded. We developed a linear model for the wrist joint to approximate the causal relationship between muscle activities and movement kinematics in terms of the wrist joint torque. We used a canonical correlation analysis to verify the causality between the muscle activities and the movement kinematics in the model. We found that the activities of the four muscles were related almost entirely to the position and velocity, with negligible correlation with the acceleration of the wrist joint. Moreover, the ratio of the weights for position- and velocity-related torque components characterized the contents of the muscle activities in terms of the movement kinematics. Next, we compared the ratios for the two movement tasks between the controls and patients. In control subjects, the ratios indicated clear task-specific changes that conformed to the functional requirements of the tasks. In contrast, in patients, the task-specific changes diminished highly significantly. The present results indicate that this ability to accommodate motor commands to the task requirements provides a novel quantitative parameter to characterize motor functions in patients with cerebellar ataxia.

Aging and limb alter the neuromuscular control of goal-directed movements

The purpose of this study was to determine whether the neuromuscular control of goal-directed movements is different for young and older adults with the upper and lower limbs. Twenty young (25.1 ± 3.9 years) and twenty older adults (71.5 ± 4.8 years) attempted to accurately match the displacement of their limb to a spatiotemporal target during ankle dorsiflexion or elbow flexion movements. We quantified neuromuscular control by examining the movement endpoint accuracy and variability, and the antagonistic muscle activity using surface electromyography (EMG). Our results indicate that older adults exhibit impaired endpoint accuracy with both limbs due to greater time variability. In addition, older adults exhibit greater EMG burst and lower EMG burst variability as well as lower coactivation of the antagonistic muscles. The impaired accuracy of older adults during upper limb movements was related to lower coactivation of the antagonistic muscles, whereas their impaired accuracy during lower limb movements was related to the amplified EMG bursts. The upper limb exhibited greater movement control than the lower limb, and different neuromuscular parameters were related to the accuracy and consistency for each limb. Greater endpoint error during upper limb movements was related to lower coactivation of the antagonistic muscles, whereas greater endpoint error during lower limb movements was related to the amplified EMG bursts. These findings indicate that the age-associated impairments in movement control are associated with altered activation of the involved antagonistic muscles. In addition, independent of age, the neuromuscular control of goal-directed movements is different for the upper and lower limbs.

Quantitative evaluation of cerebellar ataxia based on pathological patterns of the muscle activities

Quantitative evaluation of cerebellar ataxia is crucial for precise evaluation of cerebellar diseases. In particular, it is essential to capture anomaly of the causal motor commands as well as the resultant movement for the ataxia. In this paper, we propose a new method to make a quantitative evaluation of the cerebellar ataxia based on EMG signals. As an experimental task, we asked subjects to perform step-tracking wrist movements with a manipulandum, and recorded wrist joint movements and muscle activities of four wrist prime movers with surface electrodes. The subjects included fourteen patients with cerebellar diseases and thirteen normal controls. We succeeded to extract two parameters from the EMG signals of the four wrist prime movers, which characterize the pathological patterns of muscle activities for the cerebellar ataxia, Total Co-contraction Level (TCL) and Directionality of Muscle Activity (DMA). We found that the two parameters were useful to characterize pathological patterns of muscle activities in cerebellar ataxia. Consequently, it is expected that our proposed method is useful not only in tracking condition of cerebellar patients but also in evaluating the effects of a treatment or neuro-rehabilitation aiming at the normalization of motor commands.

Assessment of upper limb function in young Friedreich ataxia patients compared to control subjects using a new three-dimensional kinematic protocol

BACKGROUND

The assessment of Friedreich ataxia effects on upper limb function in clinical follow-up remains a challenging issue. To complete the usual clinical scales, an upper limb kinematic protocol adapted to Friedreich ataxia children and young adults has been developed and applied to both patients and control subjects.

METHODS

Nineteen Friedreich ataxia patients (7-24 years old) and fifteen healthy controls (9-24) were examined twice during three tasks (drawing, pointing, pro-supination) inspired from the "International Cooperative Ataxia Rating Scale". A custom-made and adjustable device allowed standardized positioning of the subject (in a seated position) and task execution. A three-dimensional kinematic analysis of the whole upper limb was performed using an electromagnetic device. The between session reliability and measurement errors of spatiotemporal and angular kinematic parameters were quantified before the analysis of their discriminative ability between healthy subjects and patients.

FINDINGS

Most of the parameters were significantly different between ataxia patients and controls, showing the discriminative ability between these two populations. In particular, the task duration, the drawing and pointing errors were higher for ataxia patients. In most of the cases, the between session reliability was found good to excellent for the spatiotemporal parameters and moderate to excellent for the kinematic parameters.

INTERPRETATION

Kinematic differences have been pointed out between Friedreich ataxia patients and controls, leading to a better understanding of the effect of this pathology on upper limb function. Discriminative ability and reliability of the developed protocol were demonstrated for many parameters, making it a relevant tool for clinical follow-up.

Passive motion paradigm: an alternative to optimal control

IN THE LAST YEARS, OPTIMAL CONTROL THEORY (OCT) HAS EMERGED AS THE LEADING APPROACH FOR INVESTIGATING NEURAL CONTROL OF MOVEMENT AND MOTOR COGNITION FOR TWO COMPLEMENTARY RESEARCH LINES: behavioral neuroscience and humanoid robotics. In both cases, there are general problems that need to be addressed, such as the "degrees of freedom (DoFs) problem," the common core of production, observation, reasoning, and learning of "actions." OCT, directly derived from engineering design techniques of control systems quantifies task goals as "cost functions" and uses the sophisticated formal tools of optimal control to obtain desired behavior (and predictions). We propose an alternative "softer" approach passive motion paradigm (PMP) that we believe is closer to the biomechanics and cybernetics of action. The basic idea is that actions (overt as well as covert) are the consequences of an internal simulation process that "animates" the body schema with the attractor dynamics of force fields induced by the goal and task-specific constraints. This internal simulation offers the brain a way to dynamically link motor redundancy with task-oriented constraints "at runtime," hence solving the "DoFs problem" without explicit kinematic inversion and cost function computation. We argue that the function of such computational machinery is not only restricted to shaping motor output during action execution but also to provide the self with information on the feasibility, consequence, understanding and meaning of "potential actions." In this sense, taking into account recent developments in neuroscience (motor imagery, simulation theory of covert actions, mirror neuron system) and in embodied robotics, PMP offers a novel framework for understanding motor cognition that goes beyond the engineering control paradigm provided by OCT. Therefore, the paper is at the same time a review of the PMP rationale, as a computational theory, and a perspective presentation of how to develop it for designing better cognitive architectures.

Effect of long-term climbing training on cerebellar ataxia: a case series

Background. Efficient therapy for both limb and gait ataxia is required. Climbing, a complex task for the whole motor system involving balance, body stabilization, and the simultaneous coordination of all 4 limbs, may have therapeutic potential. Objective. To investigate whether long-term climbing training improves motor function in patients with cerebellar ataxia. Methods. Four patients suffering from limb and gait ataxia underwent a 6-week climbing training. Its effect on ataxia was evaluated with validated clinical balance and manual dexterity tests and with a kinematic analysis of multijoint arm and leg pointing movements. Results. The patients increased their movement velocity and achieved a more symmetric movement speed profile in both arm and leg pointing movements. Furthermore, the 2 patients who suffered the most from gait ataxia improved their balance and 2 of the 4 patients improved manual dexterity. Conclusion. Climbing training has the potential to serve as a new rehabilitation method for patients with upper and lower limb ataxia.

Jerk analysis of active body-weight-transfer

Recent studies have shown that whole-body vibration improves posture and gait control in stroke patients. Patients with degenerative cerebellar disease suffer from ataxic gait also which is characterised by the variation of gait pattern. Our interest is to test whole-body vibration as a method for rehabilitation treatment in cerebellar patients and to assess the success of the treatment using dynamic tests. The aim of this study was to introduce a method for quantifying movement dynamics during an active voluntary sidestep that results in a body-weight-transfer. Subjects had to perform a step from a feet-apart-position to a feet-together-position and back again. The algorithms presented in this study allow automatic identification of the timing of the dynamic phases by analysing the centre of pressure trajectory. For this study the time flow of averaged speed, acceleration, and jerk was calculated for the active movement only. This study demonstrates that jerk provides a sensitive measure for the improvement in gait in rehabilitation and during training.

Quantitative evaluation of movement disorders in neurological diseases based on EMG signals

In this paper, we propose a new method to make a quantitative evaluation for movement disorders. Based on the EMG signals, we analyzed the movement disorders for cerebellar patients at the motor command level. As an experimental task, we asked subjects to perform step-tracking wrist movements with a manipulandum, and simultaneously recorded wrist joint movements and muscle activities of four wrist prime movers with surface electrodes. In order to quantitatively evaluate the correspondence between the movement kinematics and the activities of the four muscles, we approximated the relationship between the wrist joint torque calculated from the kinematics and the four EMG signals using a dynamics model of wrist joint. Our surprising observation was that there was very high correlation between the wrist joint torque and the EMG signals. In fact, we identified causal abnormality of muscle activities for movement disorders of cerebellar patients, confirming effectiveness of our proposed method for analysis of movement disorders at the level of the motor command.

Activity, tolerability and efficacy of levetiracetam on cerebellar symptoms in multiple sclerosis patients: a pilot kinematic study

BACKGROUND AND PURPOSE

The aim of this study was to evaluate the activity measured by kinematic analysis, tolerability and efficacy of levetiracetam (LEV) in multiple sclerosis (MS) patients affected by cerebellar symptoms, in a randomized single-blind, placebo-controlled cross-over study.

METHODS

Eight MS subjects with cerebellar signs (five female and three male; mean EDSS: 4.77; mean disease duration 9.2) performed a reaching task on a digitizing tablet and their trajectories went through a kinematic analysis. The subjects were assessed at baseline, after 21 days of treatment, after wash-out period (day 35) and after 21 days of treatment (day 56). LEV was used at the maximum dosage of 1500 mg daily. The primary outcome was the modification on smoothness (JERK) whilst aiming error (AAI) and centripetal acceleration (CA) were considered as secondary outcomes.

RESULTS

Two subjects were excluded from the final analysis. Primary outcome (i.e. JERK) was significantly affected by the administration of LEV overtime (nine arms in active treatment versus three arms in placebo decreased the mean values of their JERK). Regarding secondary outcomes CA was significantly affected by the administration of LEV. No statistical significant results were found comparing clinical scales during the four assessments.

DISCUSSION

The results indicate that LEV was able to modify kinematic parameter so the medication was active but no improvement in clinical scales was observed. LEV needs to be tested in a larger group of subjects designed to verify treatment efficacy using higher dosage of the medication.

Disruption of state estimation in the human lateral cerebellum

The cerebellum has been proposed to be a crucial component in the state estimation process that combines information from motor efferent and sensory afferent signals to produce a representation of the current state of the motor system. Such a state estimate of the moving human arm would be expected to be used when the arm is rapidly and skillfully reaching to a target. We now report the effects of transcranial magnetic stimulation (TMS) over the ipsilateral cerebellum as healthy humans were made to interrupt a slow voluntary movement to rapidly reach towards a visually defined target. Errors in the initial direction and in the final finger position of this reach-to-target movement were significantly higher for cerebellar stimulation than they were in control conditions. The average directional errors in the cerebellar TMS condition were consistent with the reaching movements being planned and initiated from an estimated hand position that was 138 ms out of date. We suggest that these results demonstrate that the cerebellum is responsible for estimating the hand position over this time interval and that TMS disrupts this state estimate.

Motor control depends on the brain's awareness of the current state of the body. Knowing the current position and movement of the arm, for example, allows one to reach rapidly and accurately towards a target. However, sensory information reaches the brain only after a short delay, and the arm may already be in motion. Therefore, it has been proposed that the brain must calculate a “state estimate”—by combining sensory information about the last known position of the arm with predictions of its responses to recent movement commands—which it uses to accurately plan and control a reaching movement. To test this idea, we used transcranial magnetic stimulation to briefly disrupt several separate areas in the brain as participants reached to a target. We show that stimulation over the cerebellum caused reaching errors consistent with movements planned on the arm's position about 140 ms previously, whereas stimulation of other brain areas did not disrupt reaching direction. These results add weight to the hypothesis that the cerebellum predicts the state of the motor system. This hypothesis can explain the loss of movement control experienced by cerebellar patients and supports computational theories that the cerebellum is a predictive model of the motor system.

Transcranial magnetic stimulation of the human cerebellum causes errors in reaching movements that are consistent with a temporary disruption in estimating the arm's current state.

Subtle upper limb impairment in asymptomatic multiple sclerosis subjects

We evaluated upper limb function in multiple sclerosis (MS) subjects (11 clinically definite MS patients and seven clinically isolated syndrome (CIS) subjects), with a normal upper limb standard neurological examination. Subjects performed center-out reaching movements under visual control, with and without vision of the hand. Their movements were recorded through a digitizing tablet. Motor performance was also related to lesion load, estimated from magnetic resonance imaging (MRI). We found that in MS and CIS subjects, under the hand vision condition, movements were significantly less smooth, and had a less symmetric speed profile. However, the observed impairment did not correlate with MRI findings. This result may be interpreted as evidence of a compensatory strategy, elicited by subtle alterations in sensorimotor control. Multiple Sclerosis 2007; 13: 428-432. http://msj.sagepub.com

Procedure for the quantitative evaluation of motor disturbances in cerebellar ataxic patients

Cerebellar ataxia is a complex motor disturbance that involves the planning and execution of movements and reduces movement accuracy and co-ordination. The quantification of ataxic signs is commonly realised through visual examination of motor tasks performed by the patient and assignment of scores to specific items composing the international co-operative ataxia rating scale (ICARS). The present work studied an experimental procedure to characterise specific aspects of motor disturbances in ataxia objectively. Four tests belonging to the ICARS were considered: walking, knee-tibia test, finger-to-nose and finger-to-finger test. Through a kinematic analysis performed during the above tests, specific indices were defined to quantify velocity, linearity, asymmetry, tremor, instability and smoothness of movement or posture. The procedure was applied to five patients with cerebellar ataxia and to ten healthy adult subjects. Results demonstrated that the patients moved significantly more slowly than the healthy subjects (0.67 against 0.97m s(-1) and 0.81 against 1.02 m s(-1), respectively, for straight walk and finger-to-nose tests) and showed poorer linearity and smoothness behaviour. Velocity, linearity, tremor, smoothness and instability indices showed moderate to good correlation with the corresponding ICARS score. Some of these indices can separately evaluate aspects that are combined in single ICARS subscores. It is concluded that the combination of clinical assessments and instrumental evaluations allows a better insight into ataxic patients' motor disturbances and is a useful tool for the definition and follow-up of rehabilitation programmes.

Body sway during quiet standing: Is it the residual chattering of an intermittent stabilization process?

This paper reviews different approaches for explaining body sway while quiet standing that directly address the instability of the human inverted pendulum. We argue that both stiffness control [Winter, D. A., Patla, A. E., Riedtyk, S., & Ishac, M. (2001). Ankle muscle stiffness in the control of balance during quiet standing. Journal of Neurophysiology, 85, 2630-2633] and continuous feedback control by means of a PID (Proportional, Integral, Derivative) mechanism [Peterka, R. J. (2000). Postural control model interpretation of stabilogram diffusion analysis. Biological Cybernetics, 83, 335-343.] can guarantee asymptotic stability of controlled posture at the expense of unrealistic assumptions: the level of intrinsic muscle stiffness in the former case, and the level of background noise in the latter, which also determines an unrealistic level of jerkiness in the sway. We show that the decomposition of the control action into a slow and a fast component (rambling and trembling, respectively, as proposed by [Zatsiorsky, V. M., & Duarte, M. (1999). Instant equilibrium point and its migration in standing tasks: Rambling and trembling components of the stabilogram. Motor Control, 4, 185-200; Zatsiorsky, V. M., & Duarte, M. (2000). Rambling and trembling in quiet standing. Motor Control, 4, 185-200.]) is useful but must be modified in order to take into account that rambling is not a stable equilibrium trajectory. We address the possibility of a form of stability weaker than asymptotic stability in light of the intermittent stabilization mechanism outlined by [Loram, I. D., & Lakie, M. (2002a). Human balancing of an inverted pendulum: position control by small, ballistic-like, throw and catch movements. Journal of Physiology, 540, 1111-1124.], and propose an indicator of intermittent stabilization that is related to the phase portrait of the human inverted pendulum. This indicator provides a further argument against the plausibility of PID-like control mechanisms. Finally, we draw attention to the sliding mode control theory that provides a useful theoretical framework for formulating realistic intermittent, stabilization models.

Human postural sway results from frequent, ballistic bias impulses by soleus and gastrocnemius

It has been widely assumed for nearly a century, that postural muscles operate in a spring-like manner and that muscle length signals joint angle (the mechano-reflex mechanism). Here we employ automated analysis of ultrasound images to resolve calf muscle (soleus and gastrocnemius) length changes as small as 10 mum in standing subjects. Previously, we have used balancing of a real inverted pendulum to make predictions about human standing. Here we test and confirm these predictions on 10 subjects standing quietly. We show that on average the calf muscles are actively adjusted 2.6 times per second and 2.8 times per unidirectional sway of the body centre of mass (CoM). These alternating, small (30-300 microm) movements provide impulsive, ballistic regulation of CoM movement. The timing and pattern of these adjustments are consistent with multisensory integration of all information regarding motion of the CoM, pattern recognition, prediction and planning using internal models and are not consistent with control solely by local reflexes. Because the system is unstable, errors in stabilization provide a perturbation which grows into a sway which has to be reacted to and corrected. Sagittal sway results from this impulsive control of calf muscle activity rather than internal sources (e.g. the heart, breathing). This process is quite unlike the mechano-reflex paradigm. We suggest that standing is a skilled, trial and error activity that improves with experience and is automated (possibly by the cerebellum). These results complement and extend our recent demonstration that paradoxical muscle movements are the norm in human standing.