Influence of intermittency and synergy on grasping.

The commentary firstly supports Smeets and Brenner in their choice of a kinematic trajectory, submitting that the challenge posed by the rival torque-change formulation is resolved by consideration of intermittency in human movement control. Second, it examines the choice of optimization criterion for trajectory planning, arguing in favor of minimum acceleration rather than minimum jerk. Third, using the notion of optimized trajectories in task-dependent coordinate space together with synergy generation, it suggests a formulation that reduces the processing load entailed in Smeets and Brenner's proposal of individual trajectories for each digit.

Tonic stretch reflexes in older able-bodied people.

It is a general assumption that, in able-bodied persons, tonic stretch reflex (TSR) activity is not elicited during stretching of relaxed muscles and that the presence of TSR activity following brain damage is, therefore, indicative of spasticity. However, a variety of studies have reported age-related changes in reflex activity, raising the question of whether this assumption is justified in older subjects. The aim of this study was to determine if TSRs were activated in the relaxed elbow flexors of able-bodied people in an age-group at risk of stroke. Electromyographic (EMG) activity was recorded in 30 able-bodied subjects aged 46 to 78 years when their relaxed elbow flexors were subjected to ramp and sinusoidal stretches of different amplitudes and velocities. It was found that these subjects did not exhibit TSR activity under these conditions. Therefore, the practice of measuring TSR activity as a means of quantifying spasticity in stroke patients appears justified.

Spasticity and muscle contracture following stroke.

It has become increasingly recognized that the major functional deficits following brain damage are largely due to "negative' features such as weakness and loss of dexterity rather than spasticity. A variety of studies suggest that spasticity is a distinct problem and separate from the loss of dexterity, but that it may be implicated in the formation of muscle contracture and even in the recovery of strength. In order to address these issues, we examined the relationship between spasticity, contracture, strength and dexterity in the affected upper limb following stroke. Spasticity was measured both as increased tonic stretch reflexes and increased resistance to passive stretch (hypertonia). Twenty-four patients were recruited non-selectively from three rehabilitation units within 13 months of their stroke. Few patients exhibited increased tonic reflexes but half were found to have muscle contracture, the earliest at 2 months following stroke. Hypertonia was associated with contracture but not with reflex hyperexcitability. Increased tonic stretch reflexes were observed only in a subgroup of those with contracture and where present could usually be elicited only at the end of muscle range. This findings suggests that instead of spasticity causing contracture, contracture may actually potentiate spasticity in some patients. However, the majority of patients with contracture did not have increased tonic stretch reflexes. In addition, we found no relationship between spasticity and either weakness or loss of dexterity. Therefore, while hypertonia remains an important problem following cerebral lesions, it would appear that the amount of attention directed to reflex hyperexcitability associated with spasticity is out of proportion with its effects. Consequently, hypertonia needs to be clearly distinguished from reflex hyperexcitability in patients with spasticity.

The problem of redundancy in movement control: the adaptive model theory approach.

The problem of redundancy in movement control is encountered when one attempts to answer the question: How does the central nervous system (CNS) determine the pattern of neural activity required in some 5,000,000 descending motor fibres to control only 100-150 biomechanical degrees of freedom of movement? Mathematically this is equivalent to solving a set of simultaneous equations with many more unknowns than equations. This system of equations is redundant because it has an infinite number of possible solutions. The problem is solved by the neuronal circuitry hypothesized in Adaptive Model Theory (AMT). According to AMT, the CNS includes neuronal circuitry able to compute and maintain adaptively the accuracy of internal models of the reciprocal multivariable relationships between outgoing motor commands and their resulting sensory consequences. To identify these input-output relationships by means of regression analysis, correlations between the input signals have to be taken into account. For example, if the inputs are perfectly correlated, the model reduces to a virtual one-input system. In general, the number of inputs modelled equals the number of degrees of freedom encoded by the signals; that is, the number of independently varying (orthogonal) signals. The adaptive modelling circuitry proposed in AMT automatically tunes itself to extract independently varying sensory and motor signals before computing the dynamic relationships between them. Inverse models are employed during response execution to translate movements pre-planned as desired trajectories of these high-level sensory-feature signals into appropriately co-ordinated motor commands to send to the muscles. Since movement is pre-planned in terms of a number of orthogonalized sensory-feature signals equal to the number of degrees of freedom in the desired response, the problem of redundancy is solved and the correlation or co-ordination between motor-command signals is automatically introduced by the adaptive models.

Control of isometric muscle activity in cerebral palsy.

Voluntary control of muscle contraction was examined in five adults with cerebral palsy, who were required to track a moving target by continuously varying the level of isometric contraction of elbow flexor muscles (measured by EMG). First performance varied from minimal control to almost normal control, depending on the severity of disability. Practice over 12 weeks reduced inappropriate muscle activity in the most disabled patients, but there was no increase in appropriate muscle activity for any patient beyond that observed after the first few minutes of tracking. Thus their ability to translate a visual response into the appropriate motor activity was impaired, and there was no evidence of potential to overcome this. This supports the authors' earlier proposal that impairment of sensory-motor learning is the primary cause of functional disability in cerebral palsy. The EMG tracking task may provide a technique for assessing the ability of individuals with cerebral palsy to control muscle contraction.

Reducing spasticity to control muscle contracture of children with cerebral palsy.

A biofeedback training technique to control spasticity, previously successful with adults with cerebral palsy, was adapted for three children with spastic diplegia at risk for contractures. Visual feedback of muscle stretch reflex sensitivity is provided by video games, which are played by reducing reflex sensitivity. After a 10-week training period two of the three children had significantly reduced spasticity in the gastrocnemius muscle. The technique can be used with children as young as four years, is inexpensive, and can be carried out by parents with supervision by a physiotherapist.

Voluntary muscle control in normal and athetoid dysarthric speakers.

Time domain and frequency domain analyses were performed on smoothed and averaged electromyographic (IEMG) activity recorded intramuscularly from 6 muscles of the lips, tongue and jaw during speech in normal and athetoid cerebral palsy subjects. The speech IEMG waveforms in both groups were composed of slowly changing tonic activity merging with more rapidly changing phasic bursts. Significant increases both in the durations and average levels of IEMG activity in the athetoid subjects resulted in a 5- to 30-fold increase in the speech muscle energy expended by these subjects. The peak-to-peak amplitudes of the IEMG activity were significantly increased in the athetoid subjects, commensurate with their increased average levels, thus demonstrating that they could vary their muscle contraction levels over a wide range. The velocities (rates of change) of muscle IEMG activity did not differ significantly between the two groups. The velocity of the IEMG activity increased linearly with its amplitude in both subject groups, but the durations of the IEMG bursts nevertheless were highly variable. The slope of the velocity-amplitude relation in the athetoid subjects was less than half that in the normal subjects, suggesting that the frequency bandwidth of muscle activity was reduced in the athetoid subjects, despite a normal range of IEMG velocities. The frequency analysis confirmed this suggestion. The upper limit of the average frequency spectrum of voluntary muscle activity for speech was 7 Hz in the normal subjects, whereas this limit was 4 Hz in the athetoid subjects. In the normal subjects each muscle had a different frequency spectrum, whereas the spectra for the 6 muscles were remarkably uniform in the athetoid subjects, implying an abnormality in the functional organization of their muscles. The findings of this study showed clearly that the temporospatial patterns of voluntary muscle activity in the athetoid subjects were grossly abnormal. Since this voluntary activity was reproducible across multiple repetitions of the same speech sample, the dysarthria in these speakers may be attributed to abnormal control of voluntary activity, not to involuntary movement. The results support the view that the primary disability in cerebral palsy is a disruption of the physiological mechanisms which subserve the acquisition of motor skills.

Motor control deficits of orofacial muscles in cerebral palsy.

Voluntary control of the masseter and orbicularis oris superioris muscles was examined in able bodied and cerebral palsied subjects using visual tracking tasks. A smoothed measure of muscle activity (the full-wave rectified and low-pass filtered electromyogram) was presented as a marker on a computer display screen and the subjects could control the vertical position of the marker by voluntarily altering the level of isometric contraction of one of the muscles. A target marker was also displayed on the screen and the subjects were required to follow or "track" the irregular movements of this target with the response marker. Their success in aligning the response marker with the target was analysed for these orofacial muscles. The masseter is influenced by muscle spindle based reflexes, while the orbicularis oris superioris lacks such reflex control. The cerebral palsied subjects displayed similarly poor control over both muscles, implying that their voluntary motor deficits are not related to abnormal muscle spindle based reflexes. It is suggested that the impairment may be related to perceptual-motor integration.

Internal models and intermittency: a theoretical account of human tracking behavior.

This paper concerns the use of tracking studies to test a theoretical account of the information processing performed by the human CNS during control of movement. The theory provides a bridge between studies of reaction time and continuous tracking. It is proposed that the human CNS includes neuronal circuitry to compute inverse internal models of the multiple input, multiple output, dynamic, non-linear relationships between outgoing motor commands and their resulting perceptual consequences. The inverse internal models are employed during movement execution to transform preplanned trajectories of desired perceptual consequences into appropriate outgoing motor commands to achieve them. A finite interval of time is required by the CNS to preplan the desired perceptual consequences of a movement and it does not commence planning a new movement until planning of the old one has been completed. This behavior introduces intermittency into the planning of movements. In this paper we show that the gain and phase frequency response characteristics of the human operator in a visual pursuit tracking task can be derived theoretically from these assumptions. By incorporating the effects of internal model inaccuracy and of speed-accuracy trade-off in performance, it is shown that various aspects of experimentally measured tracking behavior can be accounted for.

Stochastic prediction in pursuit tracking: an experimental test of adaptive model theory.

In this paper we test the proposition that in pursuit tracking, subjects compute stochastic (statistical) models of the temporal variations in position of the target and use these models to forecast target position for at least a response time interval into the future. A computer simulation of a human operator employing stochastic model prediction of target position is used to generate a synthetic pursuit tracking response signal. Actual pursuit tracking response signals are measured from 10 normal subjects using the same stimulus signal. Cross correlation and spectral analysis are employed to compute gain and phase frequency response characteristics for both synthetic and actual tracking data. The similarity of the gain and phase curves for synthetic and actual data provides compelling evidence in support of the proposition.

Reproducibility and variability of speech muscle activity in athetoid dysarthria of cerebral palsy.

Athetoid dysarthria is thought to result from involuntary movements which are variable and irregular in nature. In this study, electromyographic (EMG) activity recorded from six speech muscles was quantified during repetitions of a test sentence by normal and athetoid adult subjects. In the athetoid subjects the articulation of the test sentence was disrupted intermittently by involuntary activity which usually occurred in the time intervals between the syllables in the test sentence, rather than during articulation of the syllables themselves. The EMG activity associated with each syllable in the test sentence was partitioned into reproducible and variable components. The ratio of the reproducible component to the variable component--the signal-to-noise ratio--did not differ significantly between the two subject groups. In the athetoid subjects, however, the reproducible component of the EMG activity was grossly abnormal. We concluded that this abnormal voluntary activity, rather than variable involuntary activity, was the primary cause of athetoid dysarthria.

Control of upper airway structures during nonspeech tasks in normal and cerebral-palsied subjects: EMG findings.

Electromyographic (EMG) recordings taken from 13 orofacial and mandibular muscles during a sequence of nonspeech movements were compared in six normal and six cerebral-palsied adult subjects. Abnormalities in the amplitude of muscle activity and timing of muscle control in the cerebral-palsied subjects were borne out in statistically significant differences between the two subject groups on five measures of muscle activity. The findings do not support hypotheses that weakness affects individual upper airway muscles in cerebral-palsied persons or that a pathological imbalance between positive and negative oral reactions is present in these subjects. A possible mechanism for hypertonicity in facial muscles in cerebral palsy is suggested, based on inappropriate activation patterns across muscles. The results are consistent with a previous proposal that a defect in the specification of motor commands and/or their communication to muscles is a fundamental abnormality in cerebral-palsied individuals which affects both speech and nonspeech motor control.

Self-regulation of spasm and spasticity in cerebral palsy.

Four young adult cerebral palsied subjects with a mixture of spasticity and athetosis attended an experimental reflex training program for three one-hour sessions each week over an 18 month period. During each session on-line measures of contraction level and tonic stretch reflex sensitivity from the biceps brachii muscle were shown to the subject on meter displays. Subjects were asked to attempt to control the displays. They were given goals such as: (1) reduce both contraction level and reflex sensitivity displays to zero and (2) increase the contraction level display to 10% of maximum while keeping the reflex sensitivity display at a minimum. Achievement of goals was automatically sensed and used to activate a cassette tape of the subject's favourite music. Contraction level and reflex sensitivity scores were averaged over one-minute intervals to provide a record of long term progress. Elbow-angle and IEMG data were recorded on FM tape for off-line analysis. All four subjects learned to suppress involuntary muscle activity and resting tonic stretch reflex responses. They also learned to produce a two or three-fold variation in action tonic stretch reflex sensitivity while sustaining 10% maximum voluntary contraction. In other words, subjects learned to self-regulate spasm and spasticity at the elbow and to regulate tonic stretch reflex sensitivity independently of contraction level. A visual tracking task requiring voluntary movement about the elbow was employed to assess improvement in functional control of elbow movement. One athetotic subject improved tracking accuracy as a consequence of reducing the amount of involuntary arm movement while the other three subjects showed negligible improvement in functional control.

Pathophysiology of dysarthria in cerebral palsy.

Electromyograms were recorded with hooked-wire electrodes from sixteen lip, tongue and jaw muscles in six normal and seven cerebral palsied adult subjects during a variety of speech and non-speech tasks. The recorded patterns of muscle activity fail to support a number of theories concerning the pathophysiology of dysarthria in cerebral palsy. There was no indication of weakness in individual articulator muscles. There was no evidence of uncontrolled sustained background activity or of abnormal tonic stretch reflex responses in lip or tongue muscles. Primitive or pathological reflexes could not be elicited by orofacial stimulation. No imbalance between positive and negative oral responses was observed. The view that random involuntary movement disrupts essentially normal voluntary control in athetosis was not supported. Each cerebral palsied subject displayed an idiosyncratic pattern of abnormal muscle activity which was reproduced across repetitions of the same phrase, indicating a consistent defect in motor programming.

Effect of contraction level and magnitude of stretch on tonic stretch reflex transmission characteristics.

Electromyogram tonic stretch reflex responses were recorded from biceps brachii muscles in normal and cerebral palsied subjects sustaining either 10% or 20% of maximum voluntary contraction and attempting to keep the elbow stiff in a fixed position. The muscle was stretch by a sinusoidal perturbation applied by the experimenter to the elbow angle. Five different amplitudes of stretch were employed ranging 1.67 to 10.0 degrees peak to peak variation of elbow angle. Spectral analysis of the rectified and filtered electromyogram revealed "noisy" sinusoidal reflex responses with negligible harmonic distortion but the amplitude of the reflex responses did not increase linearly with the amplitude of stretch. An analysis of variance showed that for both groups of subjects the gain of the tonic stretch reflex increased significantly (p less than 0.001) with contraction level and decreased significantly (p less than 0.001) with magnitude of stretch. This finding illustrates that both magnitude of stretch and level of contraction need to be carefully controlled when measures of tonic stretch reflex responses are used to assess changes of muscle tone.

Procedures for verification of electrode placement in EMG studies of orofacial and mandibular muscles.

Many muscles used in speech are small and intimately interconnected. There is a need for anatomical and physiological data which would allow identification of the particular muscle fibers being recorded in electromyographic (EMG) investigations. EMG recordings were taken from eighteen orofacial and mandibular muscles while gestures believed to be specific to each muscle were performed. The anatomic criteria for the placement of the electrodes, the quality of the EMG spikes and interference patterns obtained, and the degree of differentiation of the temporal sequence of activity from that in neighboring muscles were used to decide on the degree of certainty that a particular muscle was being recorded. The appropriateness of each gesture as a stimulus to any muscle was determined on the basis of the level of activation occurring with the gesture relative to other muscles and its degree of variability between subjects.

Tonic stretch reflexes in lip, tongue and jaw muscles.

Despite considerable speculation it remains unclear as to whether stretch reflexes perform a functional role in speech articulator muscles. Recent research, however, has shown that long loop stretch reflex mechanisms are brought into play during voluntary contraction of limb muscles and a functional role in oscillatory damping has been suggested. It was decided, therefore, to use a method and a technique of analysis similar to that used in limb muscles to search for tonic stretch reflex (TSR) responses in lip, tongue and jaw muscles during sustained voluntary contraction. The term 'action TSR' is used to differentiate stretch reflex responses measured from voluntary activity from those measured at rest. Simultaneous electromyogram (EMG) recordings were taken from the lip, tongue and jaw musculature in normal, stutterer and cerebral spastic subjects. Subjects were instructed to hold the appropriate articulator in a fixed position while the experiment applied an irregular, continuously changing, stretching force. The stretch and EMG signals were analyzed using a cross correlation and spectral analysis technique. This provided a sensitive means of detecting any EMG fluctuations which covaried with applied stretch and might therefore be classified as reflex. No suggestion of such action TSR responses could be found in lip or tongue muscles of any of the subjects tested, including the cerebral spastic subjects with dysarthric speech. It is therefore concluded that action TSR mechanisms are not operative in control of lip and tongue muscles in man. Furthermore, dysarthric speech in cerebral spasticity cannot be attributed to exaggerated tone of lip and tongue muscles resulting from hypersensitivity of TSR mechanisms. In contrast, clear action TSR responses were demonstrable in jaw closing muscles while in jaw opening muscles, small amplitude responses were detected but were not substantial in comparison with background activity. Since the action TSR is present in jaw and limb muscles, but absent in lip and tongue muscles, the suggestion of a functional role of this reflex in damping mechanical oscillations associated with inertial loads is further supported.