Vibration-induced post-effects: a means to improve postural asymmetry in lower leg amputees?

Muscle vibration has been shown to induce long-lasting and oriented alteration of standing posture in healthy individuals. The postural alterations can last several minutes following the end of vibration and are called post-effects. The goal of this study was to determine whether persons with lower leg amputation that show persistent postural asymmetry after usual rehabilitation experience these postural post-effects and if this could improve their weight bearing on the prosthesis. Centre of pressure (CP) position during stance was recorded prior to and up to 13 min after a 30s unilateral vibration applied during sitting to lateral neck (trapezius) or hip (gluteus medius) muscles in 14 individuals with unilateral lower leg amputation and 18 controls. The amputees' postural asymmetry was confirmed prior to the vibration intervention. A CP displacement, without an increase in CP velocity, was observed in both groups of participants over the 13 min post-vibration. For both the neck or hip vibration sites, the CP shifts were directed in the medio-lateral plane and were oriented either towards the vibrated side or the opposite side across subjects. This led to a decrease of postural asymmetry in half of the group of amputees. Within subject, the orientation of the post-effect was constant and changed to the opposite direction with vibration of the opposite body side. It is suggested that the post-effects are produced by a change of the postural reference consequent to the sustained proprioceptive message induced during the muscle vibration period. The orientation of the post-effects is discussed in relation to the notion of reference frame preference. All in all, because post-effect orientation is constant within subject and adaptive, future studies should investigate if individuals with lower leg amputation could benefit from postural post-effects induced by muscle vibration to improve function.

Electromyographic studies of congenital mirror movements.

We studied two patients with congenital mirror movements. In one, the movements were associated with impaired sensation and a cervicodorsal meningocele. In the other, no abnormality of the nervous system was found. In an EMG study, the normal temporal characteristics, response latency, duration, and amount of EMG on the normal and mirror sides suggest that similar motor commands are responsible for both voluntary and mirror movements.

Prolonged orbicularis oculi activity: a major factor in apraxia of lid opening.

BACKGROUND: Apraxia of lid opening (ALO) is a nonparalytic inability to open the eyes at will in the absence of visible contraction of the orbicularis oculi (OOc) muscle. OBJECTIVE: To test the hypothesis that sustained activity of the OOc is a major factor in the difficulty in opening the eyes in this condition. METHODS: Lid movement detected in an electromagnetic field and electromyogram activity of the septal and pretarsal portions of the OOc were recorded in 12 healthy control subjects and 12 patients with a clinical diagnosis of ALO. The latencies to onset and to complete eye opening and the time during which eye opening was sustained were measured and analyzed in relation to OOc activity. RESULTS: The lid opening latencies and the lid movement duration were significantly increased in patients compared with control values. An abnormal persistence of OOc activity was present in 10 of the 11 patients with a delay in complete lid opening. The complete lid opening delay showed a strong positive correlation with the time it took to inhibit the OOc activity. This relationship fit the same linear regression in both groups. CONCLUSIONS: Patients with ALO show significant delays in eye opening. An abnormal persistence of OOc activity, detectable electromyographically but not clinically, could be the main factor contributing to the delay in lid opening in these patients.

Mirror drawing in a deafferented patient and normal subjects: visuoproprioceptive conflict.

Results on a mirrow drawing task showed that a deafferented patient had no problem completing the pattern, whereas normal subjects needed more than four trials to attain a similar performance. The results suggest the presence of integrated visual and proprioceptive maps. The inversion of visual coordinates requires the need for a recalibration. Without proprioception, the task is more like a simple visual tracking task.

Gait of a deafferented subject without large myelinated sensory fibers below the neck.

We evaluated the gait pattern of a deafferented subject who suffered a permanent loss of large sensory myelinated fibers below the neck following an acute episode of purely sensory neuropathy 21 years ago. The subject has developed several strategies to achieve a secure gait, namely: (1) a reduction of the degrees of freedom by freezing the knee articulations during the stance phase, (2) a preservation of body balance by enlarging his base of support, and (3) visual monitoring of his step by stabilizing the head-trunk linkage together with a characteristic forward tilt. As a result, the gait of the deafferented subject lacks the fluidity of normal gait. Compared with normal subjects, the gait pattern of the deafferented subjects is characterized by a shorter cycle length, a longer cycle duration, a slower speed, and a lower cadence. Using a dual-task paradigm, the attentional demands for walking were particularly important (as indexed by longer probe reaction times) during the double-support phase, suggesting that the deafferented subject uses the double-support phase as a transitory stable phase to update cognitively the postural features necessary for generating his next step.

Role of afferent information in the timing of motor commands: a comparative study with a deafferented patient.

The accuracy of the motor system in synchronizing simultaneous movements initiations was tested in two conditions: (1) when the motor commands were triggered by an external signal (reactive condition), and (2) when subjects self-paced their movement onsets (self-paced condition). The task consisted of initiating simultaneously ipsilateral finger extension and heel raising. Eight normal subjects and a deafferented patient were tested. In the reactive condition, both normal subjects and the deafferented patient exhibited a precession of finger initiation over heel raising. This delay corresponds to the difference observed in the reaction time of the two limbs when measured independently. It reflects the difference in conduction times of the efferent pathways, as if the two motor commands were released simultaneously through a common triggering signal in the motor cortex. In contrast, in the self-paced condition normal subjects showed precession of heel over finger onsets, suggesting that synchrony is based upon the evaluation of afferent information. Unlike normal subjects, the patient showed no heel precession in the self-paced condition. These findings suggest that reactive and self-paced responses are produced through two different control modes and that afferent information contributes to the timing of motor commands in the self-paced mode.

Production of short timing responses: a comparative study with a deafferented patient.

How far can proprioception contribute to time keeping? To answer this question, a deafferented patient and neurologically normal subjects produced 1- or 5-sec durations in a sustained (continuous finger press) or discrete (two successive finger taps) manner, with and without Knowledge of Results. The findings were that: (1) proprioceptive afferents contribute to timing regulation in motor production of short durations; (2) this role can be rapidly compensated by Knowledge of Results; (3) the proprioceptive contribution is more important for sustained than for discrete performances; (4) stable performances are produced even when KR is withdrawn, suggesting familiarization with KR leads to the establishment of a mnemonic trace.

Prevalence and characteristics of chronic sensory problems in burn patients.

Problems of pain and paresthesia in the healed wounds of burn patients are an understudied and poorly documented phenomenon. This descriptive study was designed to examine the prevalence and characteristics of these chronic sensory problems 1 year or more postburn. Four hundred and thirty patients were sent questionnaires which assessed the frequency and intensity of the problems, influencing factors and impact on patients' lives. These problems were assessed by rating scales (visual analogue and categorical scales) and the McGill Pain Questionnaire (MPQ). The response rate was 67%. Over one-third of the participants (36.4%) complained of pain while the prevalence of paresthetic sensations was 71.2%. More than half of the symptomatic patients experienced sensory problems every week sufficient to interfere with daily living. No relationships were found between these sensory problems and the patients' age or sex, burn etiology, or length of time elapsed since injury. Burn severity was related to the frequency of the problems. Discussion emphasizes the need for adequate treatment of these problems and suggests further research issues.

Anticipatory postural adjustment in the absence of normal peripheral feedback.

The response to voluntary unloading (by the subject's contralateral arm) and imposed unloading (by the experimenter) of a 1 kg weight supported at the wrist were studied in normal volunteers (controls) and in a deafferented subject (patient). The patient had no touch, pressure or kinesthetic sensations in either of the arms, but the motor nerve fibers were unaffected. The reflex activity generated by imposed unloading in the controls was never observed in the patient. The displacement's amplitude of the unloaded forearm was 3 x smaller with voluntary than with imposed unloading. In both the controls and the patient, the displacement was of similar amplitude and preceded by an anticipatory postural adjustment. It is concluded that this postural adjustment is of central origin since it can be generated in the absence of peripheral feedback.

The role of proprioceptive information for the production of isometric forces and for handwriting tasks.

A patient showing a total loss of all the large sensory myelinated fibers but intact peripheral motor system produced simple isometric force pulses and more complex tasks like handwriting and drawing. Overall, the patient was able to perform the isometric force task with an accuracy that approached that of normal subjects. The writing tasks, however, proved to be more challenging. In absence of vision, the different forms and cursive trajectories forming letters (morphocinetic components) were preserved but their localization within the constraints of the graphic space (topocinetic components) were severely impaired. These results demonstrate that, in absence of visual information, proprioceptive information is necessary to calibrate the hand in space.

Rapid elbow flexion in the absence of proprioceptive and cutaneous feedback.

Rapid goal-directed movements of elbow flexion were studied in normal human subjects and in patients deprived of proprioceptive and cutaneous feedback. All normal subjects showed a burst of electromyographic (EMG) activity in the extensor muscle (antagonist) that served to arrest the limb precisely in the target zone. The magnitude of this burst co-varied with the magnitude of the initial accelerating burst in the flexor muscle (agonist). In patients, there was a small decelerating burst poorly correlated with the agonist activity. All patients had difficulty to control the amplitude of their movements due to improper adjustment of the size and time of onset of the decelerating burst. It is concluded that the central nervous system can generate a sequence of commands to accelerate and decelerate a limb in the absence of peripheral feedback. However, information from the moving limb is required to adjust the magnitude and time of onset of deceleration.

Postural adjustments associated with different unloadings of the forearm: effects of proprioceptive and cutaneous afferent deprivation.

Postural adjustments to imposed (passive) and voluntary (active) unloading conditions of the forearm were studied in normal subjects and a deafferented patient. The latency of the postural behaviour (deactivation of the biceps supporting the weight) was linearly related to the displacement amplitude of the unloaded forearm, independent of the unloading conditions. The postural behaviour consisted of an anticipatory postural adjustment (APA) occurring prior to active unloading (in both normals and the patient) and conversely in an unloading reflex response following passive unloading (only in the normals). In both the deafferented and the normal subjects, the amplitudes of the displacement during active unloadings were much smaller (3x) than in the passive conditions and an APA was present in both the deafferented and the normal subjects. The APA could not be triggered by some types of active movement and was absent when the movement was not directly producing the unloading. The EMG latencies of the APA and of the contralateral muscles used to unload were tightly coupled. However, the latency would sometimes be decoupled, particularly when a temporal delay was introduced between the active movement and the unloading in normal subjects. In contrast to the normal subjects, who were able to adapt quickly to an unusual unloading condition (produced by voluntary knee flexion), the deafferented patient did not show an APA in this task. It was concluded that, although the APA is of central origin, it cannot be generated only on the basis of internal timing cues and must rely on afferent information for its generation during unfamiliar unloading conditions.

A re-examination of the effects of instruction on the long-latency stretch reflex response of the flexor pollicis longus muscle.

We re-examined the issue of how a subject's intention to react to a joint perturbation may modulate the long-latency M2 stretch reflex response. The experiments were done on the flexor pollicis longus muscle (FPL) of the human thumb, for which there is evidence that its M2 reflex response is mediated, at least in part, by a pathway that traverses the motor cortex. The participation of the cerebral cortex in the genesis of the M2 reflex response may allow for a modulation of its amplitude, based on the intention of the subject. To test whether the M2 response is genuinely modulated by the subject's intention, we examined the magnitude of this response as a function of the FPL background level of activation, measured by the surface rectified and filtered EMG. The subject was instructed either to oppose the perturbation as quickly as possible, not to react, or to relax as quickly as possible after the onset of the perturbation. The time integral of the long latency FPL EMG response, computed between 40 and 70 ms following the onset of stretch, was plotted against the mean torque produced by the distal inter-phalangeal joint of the thumb, or against the mean background FPL EMG. There were no significant differences in the FPL M2 EMG responses for different instructions. The amplitude of the reflex response was dependent only--in an approximately linear manner--on the background level of muscle activation. The total joint stiffness (intrinsic plus reflex) was also calculated for each combination of instruction and background torque.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of cutaneous and proprioceptive sensation in children: a reliability study.

Forty-three healthy children aged between six and 12 years were tested to determine the intra-rater reliability of four clinical cutaneous and proprioceptive sensory assessment tools: touch pressure, vibration perception, thermal discrimination and kinaesthesis. The tests were carried out bilaterally on proximal and distal upper- and lower- extremity sites. The mean intraclass coefficients represented good to excellent reliability, which suggests that these assessment tools allow objective and reproducible measurements of cutaneous and proprioceptive sensation in children. The results were compared with existing data in the literature: in general, children and adults obtain similar sensory scores, and sex, age and laterality have no significant effect on the results for any test. However, the touch-pressure and vibration scores were significantly influenced by the site tested, the index fingerpad being the most sensitive area in both tests.

Evaluation of two-point discrimination in children: reliability, effects of passive displacement and voluntary movements.

Two-point discrimination (2-PD) thresholds were studied at three sites (tip of index finger, thenar eminence, external malleolus) and under various testing conditions (dynamic vs static, with and without displacement of instrument on skin; and active vs passive, with and without voluntary movement of subject) in 47 healthy children between 6 and 13 years of age. Reliability was fair to moderate, with slightly better perception (learning effect) during the second testing session. The threshold values of these children were similar to those of adults. For the passive conditions, the threshold values were lower for dynamic than for static tests at all sites. For the active condition at the index finger, there were no differences between dynamic and static values and there was no active/passive main effect. It is concluded that: (1) the hand is more sensitive than the ankle, with the finger being the most sensitive area, (2) the 2-PD test procedure used was most reliable at the index finger and least reliable at the external malleolus and (3) displacement of the instrument on the skin can improve 2-PD threshold values in children but the discrimination thresholds are not changed by active movement of the subject.

Motor performance following a mild traumatic brain injury in children: an exploratory study.

Mild Traumatic Brain Injury (TBI) is a common occurrence in the paediatric population and, as the concept of motor performance has not been assessed specifically in this population, the purpose of this study was to determine if motor performance deficits are present and can be objectively identified in a sample of children having sustained a mild TBI (Glasgow Coma Scale score 13-15). Twenty-eight children aged between 5 and 15 years were recruited immediately post-trauma. Subjects were considered normal on standard neurological exam at the time of discharge. They were assessed 13-18 days post-trauma using the Bruininks-Oseretsky Test of Motor Proficiency, a norm referenced clinical standardized assessment tool. Compared to published norms, motor performance was significantly lower in domains of balance, response speed and running speed an agility (t-test p < 0.01), and significantly higher in domains of upper extremity coordination and visual motor control (t-test p < 0.01). Although excellent performance can be observed in domains requiring upper limb coordination, motor planning and execution of motor tasks, deficits in balance and response speed can be identified in a significant number of children even after mild TBI. More specific and sensitive evaluations are necessary to identify the exact nature of the problems and evaluate their functional impact on daily activities.

Initial agonist burst duration changes with movement amplitude in a deafferented patient.

Changes in the duration of the initial agonist burst were studied in a deafferented human. The patient had been functionally deafferented for five years, having no touch, vibration, pressure or kinesthetic sensation nor any tendon reflexes in the four limbs. Pain and temperature sensation were intact and motor fibres were unaffected. The subject made visually guided step-tracking movements using flexion/extension movements about the elbow. Initial agonist burst duration increased with movement amplitude. Burst duration was approximately 65 ms in small movements (6-12 deg) increasing to 136 ms in intermediate (36 deg) and 200 ms in large (54 and 60 deg) movements. Similar changes in initial burst duration with movement amplitude were seen when the subject made non-visually guided movements. It is concluded that the duration of the initial agonist burst is centrally determined.

Long-term adjustment in burn victims: a matched-control study.

BACKGROUND: To date, there is little information about how severely burned patients compare to unburned healthy individuals in terms of psychological profile and quality of life. As part of a larger study on the sensory consequences of burns, we assessed psychological functioning and quality of life in burned patients and unburned healthy control subjects. We also examined whether burn patients experiencing pain and/or paresthetic sensations (i.e. symptomatic patients) present a profile that is different from those who are asymptomatic. METHODS: Forty-nine burned patients (% total body surface area = 34 59 % +/- 13.40; 82% males/18% females) were evaluated 63 59 +/- 28.1 months post-burn. They were matched with 49 unburned healthy volunteers on age, sex, and education level. All subjects were administered the Symptom Checklist 90-Revised (SCL-90-R) to assess psychological functioning and the 36-item Short-Form Health Survey (SF-36) to assess quality of life. RESULTS: Approximately 25% of the burn patients presented clinically-significant psychological disturbances compared to 12% in the control group. Burn patients enjoyed a quality of life comparable to that of the control subjects, although they perceived some deterioration in their general health. More symptomatic than asymptomatic patients suffered from clinically-relevant somatization and obsessive-compulsive disturbances. CONCLUSIONS: Severely burned patients adjust relatively well, although some develop clinically-significant psychological disturbances such as somatization and phobic anxiety. Burn patients experiencing abnormal sensations in their healed wounds (i.e. symptomatic patients) do not suffer from maladjustment to a greater extent than their asymptomatic counterparts, although more symptomatic patients experience somatization and obsessive-compulsive behaviours.

Evidence for a contribution of the motor cortex to the long-latency stretch reflex of the human thumb.

1. In normal subjects, transcranial magnetic stimulation of the hand region of the motor cortex evokes motor responses only in contralateral hand muscles at a latency of about 19-24 ms. In contrast, stimulation of the motor cortex of three mirror movement subjects evoked, nearly simultaneously, motor responses in hand muscles on both sides of the body at latencies similar to those of normal subjects. In these subjects no other neuroanatomical pathways appear to be abnormally directed across the mid-line. Thus, their mirror movements are probably due to a projection of the corticospinal tract to homologous motoneurone pools on each side of the body. 2. We reasoned that if the motor cortex contributes to the generation of long-latency stretch reflex responses then in these mirror movement subjects stretching a muscle on one side of the body should produce long-latency reflex responses in the ipsilateral and the homologous contralateral muscle. 3. To test this idea experiments were done on normal human subjects and on the subjects with mirror movements. The electromyographic (EMG) activity of the flexor pollicis longus muscle (FPL) on each side of the body was recorded. Stretch of the distal phalanx of the thumb of one hand produced a series of distinct reflex EMG responses in the ipsilateral FPL. The earliest response, when present, began at 25 ms (S.D. = 3.5 ms) and was followed by responses at 40 (S.D. = 3.9 ms) and 56 ms (S.D. = 4.3 ms). There was no difference, either in timing or intensity, between the ipsilateral FPL EMG responses of normal subjects and those of the mirror movement subjects. 4. No response of any kind was observed in the contralateral (unstretched) FPL of normal subjects. In contrast, we observed in all three mirror movement subjects EMG responses in the contralateral (unstretched) FPL beginning at 45-50 ms. The latency of this response is considerably shorter than the fastest voluntary kinaesthetic reaction time, which was on average 130 ms (S.D. = 11 ms). The contralateral long-latency EMG response observed in the mirror movement subjects was on average 30% (range 5-60%) of that on the ipsilateral side. No short-latency response (25 ms) was ever observed in the contralateral FPL of these subjects. 5. These observations are quite consistent with the idea that the long-latency stretch reflex responses of hand and finger muscles are produced, at least in part, by the motor cortex.

The modulation of somatosensory evoked potentials during the foreperiod of a forewarned reaction time task.

During the foreperiod of a forewarned reaction time (RT) task reflexes in the executing limb increase to a lesser extent than those in the contralateral limb. This is possibly due to input modulation. The present study investigates the possibility of cutaneous sensory modulation during motor preparation by studying the amplitudes of somatosensory evoked potentials (SEPs). Eighteen subjects performed a forewarned RT task with the same fingers as the ones which were electrically stimulated. SEPs evoked during the 4 sec preparatory period were compared to those evoked during movement execution and during the resting period after the motor response respectively. During response execution most SEP components showed smaller amplitudes, i.e., they were gated, which agrees with other studies. In the first part of the foreperiod no SEP modulation was observed. Towards the end of the foreperiod, 500 msec before the response stimulus (RS), the amplitude of the contralateral parietal N70-P100 was significantly decreased, while the P45-N70 showed a similar tendency. However, at the same time the P100-N140 was increased in amplitude. The decrease of the intermediate latency components towards the end of the foreperiod is discussed in terms of gating, while the increase in the long latency component is discussed with respect to a decrease in RT on trials where the fingers were stimulated just before the RS, pointing to the role of attentional mechanisms.