Peripheral vs. central determinants of vibrotactile adaptation

Long-lasting mechanical vibrations applied to the skin induce a reversible decrease in the perception of vibration at the stimulated skin site. This phenomenon of vibrotactile adaptation has been studied extensively, yet there is still no clear consensus on the mechanisms leading to vibrotactile adaptation. In particular, the respective contributions of 1) changes affecting mechanical skin impedance, 2) peripheral processes, and 3) central processes are largely unknown. Here we used direct electrical stimulation of nerve fibers to bypass mechanical transduction processes and thereby explore the possible contribution of central vs. peripheral processes to vibrotactile adaptation. Three experiments were conducted. In the first, adaptation was induced with mechanical vibration of the fingertip (51- or 251-Hz vibration delivered for 8 min, at 40× detection threshold). In the second, we attempted to induce adaptation with transcutaneous electrical stimulation of the median nerve (51- or 251-Hz constant-current pulses delivered for 8 min, at 1.5× detection threshold). Vibrotactile detection thresholds were measured before and after adaptation. Mechanical stimulation induced a clear increase of vibrotactile detection thresholds. In contrast, thresholds were unaffected by electrical stimulation. In the third experiment, we assessed the effect of mechanical adaptation on the detection thresholds to transcutaneous electrical nerve stimuli, measured before and after adaptation. Electrical detection thresholds were unaffected by the mechanical adaptation. Taken together, our results suggest that vibrotactile adaptation is predominantly the consequence of peripheral mechanoreceptor processes and/or changes in biomechanical properties of the skin.

Adaptive control of grip force to compensate for static and dynamic torques during object manipulation

Manipulating a cup by the handle requires compensating for the torque induced by the moment of the mass of the cup relative to the location of the handle. In the present study, we investigated the control strategy of subjects asked to perform grip-lift movements with an object with center of mass located away from the grip axis. Participants were asked to lift the manipulandum with a two-fingers precision grip and stabilize it in front of a visual target. Subjects showed a gradual and slow adaptation of the grip-force scaling across trials: the grip force tended to decrease slowly, and the temporal coordination between grip-force and load-torque rates displayed gradually, better-coordinated patterns. Importantly, this adaptation was much slower than the stabilization of the same parameters measured either when no torque came into play or after previous adaptation to the presence of a torque. In contrast, the maximum rotation induced by the torque was controlled efficiently after only few trials, and an unexpected decrease in the tangential torque produced significant overcompensation. An unexpected increase in torque produced a consistent opposite effect. This shows that the compensation for the dynamic torque was based on an anticipatory, dynamic counter-torque produced by the arm and wrist motor commands. The comparatively slow stabilization of grip-force control suggests a specific adaptation process engaged by the presence of the torque. This paradigm, including tangential torques, clearly constitutes a powerful tool to extract the adaptive component of grip control during object manipulation.

Movement Stability Under Uncertain Internal Models of Dynamics

Sensory noise and feedback delay are potential sources of instability and variability for the on-line control of movement. It is commonly assumed that predictions based on internal models allow the CNS to anticipate the consequences of motor actions and protect the movements from uncertainty and instability. However, during motor learning and exposure to unknown dynamics, these predictions can be inaccurate. Therefore a distinct strategy is necessary to preserve movement stability. This study tests the hypothesis that in such situations, subjects adapt the speed and accuracy constraints on the movement, yielding a control policy that is less prone to undesirable variability in the outcome. This hypothesis was tested by asking subjects to hold a manipulandum in precision grip and to perform single-joint, discrete arm rotations during short-term exposure to weightlessness (0 g), where the internal models of the limb dynamics must be updated. Measurements of grip force adjustments indicated that the internal predictions were altered during early exposure to the 0 g condition. Indeed, the grip force/load force coupling reflected that the grip force was less finely tuned to the load-force variations at the beginning of the exposure to the novel gravitational condition. During this learning period, movements were slower with asymmetric velocity profiles and target undershooting. This effect was compared with theoretical results obtained in the context of optimal feedback control, where changing the movement objective can be directly tested by adjusting the cost parameters. The effect on the simulated movements quantitatively supported the hypothesis of a change in cost function during early exposure to a novel environment. The modified optimization criterion reduces the trial-to-trial variability in spite of the fact that noise affects the internal prediction. These observations support the idea that the CNS adjusts the movement objective to stabilize the movement when internal models are uncertain.

Functional assessment in physiotherapy. A literature review

The present literature review on functional assessment in physiotherapy was carried out for the following reasons: 1) to identify the functional instruments used in the field of physiotherapy that were supported by published evidence of their psychometric qualities; 2) to investigate how these instruments relate to the International Classification of Functioning, Disability and Health (ICF); and 3) to investigate the use of functional instruments in the financing of physiotherapy. A search of Medline from 1990 to December 2005, in the domains of functional evaluation, psychometric qualities, functional classification, and health policy in relation to physiotherapy resulted in a list of 1,567 studies. Two reviewers examined the resulting references on the basis of their title and abstract, in order to select the studies that presented data on the psychometric qualities of functional evaluation tests, leading to a final selection of 44 such studies. A selection of functional tests was identified in four major diagnostic groups treated in community physiotherapy: musculoskeletal disorders (including lower back pain), stroke, the elderly, and traumatic brain injuries. The functional tests authors identified essentially cover the body and activities dimension of the ICF. The selected tests could be used as a basis for the standardisation of functional evaluation of the major diagnostic groups treated in community physiotherapy. This means that standards are available for reporting and following the evolution of patients both longitudinally and transversally. Nevertheless, in the current literature review no attempt at using functional outcomes as a rationale for financing physiotherapy could be found to date.

The use of outcome measures in physical medicine and rehabilitation within Europe

The aim of the study was to survey the use of outcome measures in rehabilitation within Europe. It was envisaged that this would provide the basis for further studies on the cross-cultural validity of outcome measures. A postal questionnaire was distributed in November 1998 to 866 units providing rehabilitation. In total, 418 questionnaires were returned, corresponding to a response rate of 48%. These 418 centres treated an estimated 113,000 patients annually, undertaking 360,000 assessments. The survey focused on nine diagnostic groups: hip and knee replacement, low back pain, lower limb amputees, multiple sclerosis, neuromuscular disorders, rheumatoid arthritis, spinal cord lesions, stroke and traumatic brain injury. It identified a relatively small number of dominant outcome assessments for each diagnostic group and some variation in the preference for measures across regions. A large number of measures, however, are being used in one or a small number of locations and with relatively few patients. For rehabilitation of orthopaedic patients the majority of assessments undertaken are at the impairment level. For patients with neurological disorders the emphasis is mostly upon measures of disability.

The ABILHAND questionnaire as a measure of manual ability in chronic stroke patients: Rasch-based validation and relationship to upper limb impairment

BACKGROUND AND PURPOSE

Chronic hemiparetic patients often retain the ability to manage activities requiring both hands, either through the use of the affected arm or compensation with the unaffected limb. A measure of this overall ability was developed by adapting and validating the ABILHAND questionnaire through the Rasch measurement model. ABILHAND measures the patient's perceived difficulty in performing everyday manual activities.

METHODS

One hundred three chronic (>6 months) stroke outpatients (62% men; mean age, 63 years) were assessed (74 in Belgium, 29 in Italy). They lived at home and walked independently and were screened for the absence of major cognitive deficits (dementia, aphasia, hemineglect). The patients were administered the ABILHAND questionnaire, the Brunnström upper limb motricity test, the box-and-block manual dexterity test, the Semmes-Weinstein tactile sensation test, and the Geriatric Depression Scale. The brain lesion type and site were recorded. ABILHAND results were analyzed with the use of Winsteps Rasch software.

RESULTS

The Rasch refinement of ABILHAND led to a change from the original unimanual and bimanual 56-item, 4-level scale to a bimanual 23-item, 3-level scale. The resulting ability scale had sufficient sensitivity to be clinically useful. Rasch reliability was 0.90, and the item-difficulty hierarchy was stable across demographic and clinical subgroups. Grip strength, motricity, dexterity, and depression were significantly correlated with the ABILHAND measures.

CONCLUSIONS

The ABILHAND questionnaire results in a valid person-centered measure of manual ability in everyday activities. The stability of the item-difficulty hierarchy across different patient classes further supports the clinical application of the scale.

Chronic demyelinating hypertrophic brachial plexus neuropathy

A patient with unilateral, painless, chronic progressive upper limb sensorimotor deficit showed electrophysiological evidence of a focal demyelinating neuropathy with almost complete conduction block across the brachial plexus. Magnetic resonance imaging disclosed marked brachial plexus hypertrophy. Intravenous immunoglobulin led to fast and complete recovery, maintained by intermittent perfusions. Hypertrophic brachial plexus neuropathy can be a presentation of focal chronic inflammatory demyelinating polyradiculoneuropathy. Objective and quantitative assessment of hand function is useful to evaluate treatment results and to optimize treatment regimens.

Impact of the surface slipperiness of grasped objects on their subsequent acceleration

Seven subjects were asked to reach and grasp an object between the thumb and index finger, lift it about 30 cm high and 25 cm forward from one table to another, at their preferred speed. The perpendicular grip force and the tangential load force applied to the contact surface were digitized at 500 Hz and stored on a laboratory computer. The trajectory of the wrist and of the object was recorded using four infrared cameras tracking the movement of reflective markers attached to the distal styloid process of the radius and on the top of the object. The aim of this study was to demonstrate the influence of low friction (i.e. surface slipperiness) on the acceleration of the wrist. Friction was reduced by coating the smooth brass grasping surface with talc. The seven subjects had skin to surface coefficients of friction which ranged from 0.52-1.18 for dry brass and 0.24 0.34 for talc-coated brass. Two weights (418 and 1070 g) were used with each surface. The results indicated that with the slippery surface the necessary higher grip force/load force ratio was produced by an increase in the grip force and by a decrease in the wrist acceleration and a consequent reduction in the load force. This strategy was observed for both weights over a range of grip strengths between 21-98% of the individual's maximum voluntary contraction (MVC). This implies that even with adequate grip force reserves the reduction in acceleration is an acceptable and probable alternative solution to the force control problem. Our results also suggested that the loading rate and the object acceleration were planned and controlled together which emphasizes the role played by a predictive mechanism in organizing the kinematics of movements involving hand-held objects. This study shows that friction of the grasping surface not only affects the prehensile force dynamics, but it also influences the kinematics of the entire upper limb.

Long-term functional results of wrist arthrodesis in rheumatoid arthritis

The purpose of this study was to evaluate the long-term effectiveness of wrist fusion on the relief of pain and also the functional capacities of the upper limbs in patients with rheumatoid arthritis (RA). Eighteen patients were assessed at a mean of 7 years after wrist arthrodesis and a mean of 17 years after the onset of RA. Radiological measurements, pain assessment and impairment rating of the upper limbs were made of the fused and non-fused sides. The average position of arthrodesis was 8 degrees of extension and 9 degrees of ulnar deviation. All patients were pleased with the procedure and had satisfactory pain relief. Impairment ratings did not detect any significant difference in the sensory and motor function of the hand when the fused and non-fused groups were compared. We conclude that in patients with rheumatoid arthritis, wrist arthrodesis is a reliable procedure that provides predictable pain relief and a high degree of satisfaction without additional functional loss in the upper limb.

ABILHAND: a Rasch-built measure of manual ability

OBJECTIVE

To apply the Rasch measurement model to the development of a clinical tool for measuring manual (dis)ability (ABILHAND).

DESIGN

Manual ability was evaluated in terms of the difficulty perceived by a hand-impaired patient on 57 representative unimanual or bimanual activities.

SETTING

A clinical laboratory.

PATIENTS

Eighteen rheumatoid arthritis patients (14 women, 4 men) were interviewed after wrist arthrodesis (10 right, 4 left, and 4 both wrists). Their ages ranged from 38 to 77 years, time since diagnosis ranged from 7 to 41 years, and time since surgery ranged from 0.5 to 17 years.

MAIN OUTCOME MEASURE

ABILHAND, administered at a mean duration of 7 years after arthrodesis.

RESULTS

Forty-six of the 57 items define a common, single manual ability continuum with widespread measurement range and regular item distribution. Items relating to feeding, grooming, and dressing upper body worked consistently with their counterparts in other disability scales. More difficult items extend the measurement range beyond that of most existing manual ability scales.

CONCLUSION

Even in a small sample of patients, using the Rasch methodology enabled the investigators to produce a useful scale of manual (dis)ability and to define manual ability as a unique construct, at least in patients with rheumatoid arthritis.

Assessment of hand function in a patient with chronic sensory demyelinating neuropathy

A 60-year-old man presented with progressive large fiber sensory loss in the right first three fingers and, to a lesser extent, in both fourth and fifth fingers. Electrophysiologic studies were characteristic of chronic sensory demyelinating polyneuropathy, a variant of chronic inflammatory demyelinating polyneuropathy. Plasma exchange was unsuccessful, but intravenous immunoglobulin (IVIG) led to complete recovery of sensation for 2 months, although neurophysiologic abnormalities persisted. A battery of noninvasive tests to measure hand grip strength, tactile sensation at the fingertips, and motor control of prehension during precision grip revealed marked abnormalities in the right hand before IVIG. One month after IVIG, all test results had normalized, but they returned to pretreatment levels after 3 months. Functional evaluation of the hand may be a sensitive method to objectively quantify loss of and changes in cutaneous mechanoreceptor function of the fingers in large fiber sensory neuropathy.

Short-term effect of hand-arm vibration exposure on tactile sensitivity and manual skill

OBJECTIVES

The present study investigated whether the impairment of tactile sensitivity after exposure to vibration disturbs the motor control of precision handling and, if so, whether it can result in an increased risk of injury during or after tasks involving the use of vibrating tools.

METHODS

Twelve men were manually exposed to vibration from an electric sander for 30 min. Cutaneous sensitivity was quantified to measuring the pressure perception threshold and vibration perception threshold (125 Hz) on the pulp of the second finger. Manipulative skill was evaluated by grip-lift movements and the Purdue pegboard test.

RESULTS

The vibration perception threshold increased very significantly from 94.0 dB (0.06 m/s2) before the vibration exposure to 127.5 dB (3.2 m/s2) immediately after the exposure. The pressure perception threshold tended to increase after vibration exposure, although not significantly, but manipulative skill was not altered.

CONCLUSIONS

Change in vibration perception threshold was not associated with a significant increase in the pressure perception threshold or a perturbation of manual skill. Therefore, in conditions similar to those of our experiment, the safety of a precision task does not appear to be reduced after such vibration exposure.

Stability of the braced ankle. A biomechanical investigation

We measured the bare ankle and the braced angle-torque relationships in 12 uninjured volunteers under static and dynamic conditions within the full range of inversion motion. These relationships were measured with a specially designed mechanical device that allowed inversion movements with angular velocities up to 850 deg/sec. In testing the bare ankle under static conditions, the torque showed a 10-fold increase within the full range of motion (average, from 0.9 N-m at 7 degrees to about 8 N-m at 48 degrees of inversion). The slope of the angle-torque relationship increased under dynamic conditions giving higher torque values (up to 18 N-m on average). Both orthoses induced similar additional torques that increased linearly, up to about 6 N-m at 45 degrees, with higher angles of inversion. These additional torques are small compared with the amount of stress applied to the foot during a typical ankle sprain situation, such as recovering from a jump. Therefore, we propose that orthotic devices increase the ankle torque, counteracting the inversion movement, and also prevent the start of the inversion movement by preloading and maintaining the ankle in a proper anatomic position with optimal contact between the articular surfaces.