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When sliding our fingertip against a textured surface, complex vibrations are produced in the skin. It is increasingly recognised that the neural transduction and processing of these vibrations plays an important role in the dynamic tactile perception of textures. The aim of the present study was to develop a novel means to tag the cortical activity related to the processing of these vibrations, by periodically modulating the amplitude of texture exploration-induced vibrations such as to record a steady-state evoked potential (SS-EP). The EEG was recorded while the right index fingertip was scanned against four different textures using a constant exploration velocity. Amplitude modulation of the elicited vibrations was achieved by periodically modulating the force applied against the finger. Frequency analysis of the recorded EEG signals showed that modulation of the vibrations induced by the fingertip-texture interactions elicited an SS-EP at the frequency of modulation (3 Hz) as well as its second harmonic (6 Hz), maximal over parietal regions contralateral to the stimulated side. Textures generating stronger vibrations also generated SS-EPs of greater magnitude. Our results suggest that frequency tagging using SS-EPs can be used to isolate and explore the brain activity related to the tactile exploration of natural textures.

The temporal evolution of surface strain, resulting from a combination of normal and tangential loading forces on the fingerpad, was calculated from high-resolution images. A customized robotic device loaded the fingertip with varying normal force, tangential direction and tangential speed. We observed strain waves that propagated from the periphery to the centre of the contact area. Consequently, different regions of the contact area were subject to varying degrees of compression, stretch and shear. The spatial distribution of both the strains and the strain energy densities depended on the stimulus direction. Additionally, the strains varied with the normal force level and were substantial, e.g. peak strains of 50% with a normal force of 5 N, i.e. at force levels well within the range of common dexterous manipulation tasks. While these observations were consistent with some theoretical predictions from contact mechanics, we also observed substantial deviations as expected given the complex geometry and mechanics of fingertips. Specifically, from in-depth analyses, we conclude that some of these deviations depend on local fingerprint patterns. Our data provide useful information for models of tactile afferent responses and background for the design of novel haptic interfaces., (© 2016 The Authors.)

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., (Copyright © 2016 the American Physiological Society.)

Background: The predictive value of an individual's attitude towards painful situations and the status of his immune system for postoperative analgesic requirements are not well understood. These may help the clinician to anticipate individual patient's needs., Methods: Sixty patients, who underwent a laparoscopic cholecystectomy under standardised general anaesthesia, were included. The total analgesic requirements during the first 48 h were the primary endpoint (unitary dosage, UD). The individual's attitude towards imaginary painful situations was measured with the Situational Pain Scale (SPS). The emotional status was assessed by the Hospital Anxiety and Depression Scale (HADS) and the inflammatory status by the neutrophil-to-lymphocyte ratio (NLR)., Results: Univariate analyses revealed a significant association between UD and SPS, HADS and NLR. A negative relationship between SPS and NLR (NLR = 0.820-0.180*SPS;R(2) = 0.211;P < 0.001) and a positive relationship between SPS and HADS (HADS = 14.8 + 1.63*SPS; R(2) = 0.159;P = 0.002) were observed. A multiple linear regression analysis showed that the contribution of NLR to the UD was the most effective. A mediation analysis showed a complete mediation of the effect of SPS on UD (R(2) = 0.103;P = 0.012), by the NLR (SPS on NLR: R(2) = 0.211;P = <0.001), the HADS (SPS on HADS: R(2) = 0.159;P = 0.002). The variance in UD explained by the SPS was indirect and amounts to 46% through NLR and to 34% through HADS., Conclusions: In this series, preoperative pain-related attitudes (SPS) were associated with the postoperative analgesic requirements (UD) after a cholecystectomy. Eighty per cent of this effect was mediated by the HADS and the NLR.

A hallmark of movement control expressed by healthy humans is the ability to gradually improve motor performance through learning. In the context of object manipulation, previous work has shown that the presence of a torque load has a direct impact on grip-force control, characterized by a significantly slower grip-force adjustment across lifting movements. The origin of this slower adaptation rate remains unclear. On the one hand, information about tangential constraints during stationary holding may be difficult to extract in the presence of a torque. On the other hand, inertial torque experienced during movement may also potentially disrupt the grip-force adjustments, as the dynamical constraints clearly differ from the situation when no torque load is present. To address the influence of inertial torque loads, we instructed healthy adults to perform visually guided reaching movements in weightlessness while holding an unbalanced object relative to the grip axis. Weightlessness offered the possibility to remove gravitational constraints and isolate the effect of movement-related feedback on grip force adjustments. Grip-force adaptation rates were compared with a control group who manipulated a balanced object without any torque load and also in weightlessness. Our results clearly show that grip-force adaptation in the presence of a torque load is significantly slower, which suggests that the presence of torque loads experienced during movement may alter our internal estimates of how much force is required to hold an unbalanced object stable. This observation may explain why grasping objects around the expected location of the center of mass is such an important component of planning and control of manipulation tasks.

Outcome measures are considered the most important tools to monitor patients' outcome in both clinical and research settings. Measuring the clinical state of patients is a fundamental part of our daily clinical practice and research that sometimes is taken for granted. In peripheral neuropathies, there are many scales available, but most of these are at the ordinal level. This paper will systematically address the types of scales available (being nominal, ordinal, interval, or ratio data-based) in terms of their strengths and weaknesses. The differences between classical test theory-based and modern test method-based outcome measures will be addressed with emphasis on Rasch methodology. Various steps will be highlighted as part of the evaluation and construction of outcome measures using the Rasch method, with the aim to increase the knowledge and utility of this technique. We argue that Rasch-built outcome measures should be used for future studies in neuromuscular disorders and their method of construction could be easily extrapolated to other neurological illnesses., (© 2015 Peripheral Nerve Society.)

Background and Purpose: To determine the efficacy of the ACTIVLIM-Stroke questionnaire in detecting changes in functional status of stroke patients., Method: Sixty-eight Benin and Belgian stroke patients participated in this 2-month longitudinal study, involving baseline and follow-up assessments. Outcome measures combined ACTIVLIM-Stroke questionnaire, Barthel Index (BI), 6-minute-walk test (6MWT) and modified Rankin scale (MRS). Responsiveness of ACTIVLIM-Stroke was investigated through different methodological approaches and compared with BI, 6MWT and MRS. Statistical analyses were performed using the paired t tests, effect size (ES) and correlation tests., Results: ACTIVLIM-Stroke detected changes in the whole sample (p<0.001, ES=0.78) and even in a subgroup of patients (p<0.001, ES=0.29) that were classified as stable according to the MRS. Moreover, ACTIVLIM-Stroke permitted the classification of patients into more discriminative groups, including those showing an important improvement (p<0.001, ES=1.87), a slight but non-clinically meaningful improvement (p<0.001, ES=0.38), and no improvement (p=0.1, ES=0.11), demonstrating its high sensitivity to change. Furthermore, there were concordant relationships between ACTIVLIM-Stroke change and any observed changes in BI, 6MWT and MRS scores (r≥0.50, p<0.001), confirming the external responsiveness of ACTVLIM-Stroke., Conclusions: ACTIVLIM-Stroke showed good responsiveness and can detect accurately clinical changes in the functional status of stroke patients. The BI and the 6MWT were also responsive and may provide complementary information while investigating change in functional status. However, in addition to being highly sensitive to change, ACTIVLIM-Stroke presents significant methodological advantages for quantifying functional changes in stroke patients., (Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.)

The grip-lift task enables a quantitative assessment of grasping ability. Patients are regularly assessed in a supine position, which offers a different view of the grasped object from that in the sitting position. To our knowledge, no data are currently available on the influence of posture and vision on grip-lift task parameters. We therefore aimed to determine the effects of posture and vision on these parameters. Twenty-six healthy right-handed adults performed grip-lift tasks with a manipulandum that measured different temporal and dynamic parameters in four conditions: sitting eyes open, sitting blindfolded, lying down eyes open and lying down blindfolded. A repeated-measures analysis of variance with two factors (vision and position) showed that the absence of vision affected all the parameters measured. The lying down position increased the time between the first contact with the object and the modification of the vertical force as well as the delay between the first increase of the horizontal force and the increase of the vertical force. In addition, there was a lower adaption of the horizontal force, required to squeeze the object, to the vertical force. Finally, the interaction of position and vision was associated with significant differences in the delay between the contact of each digit with the object, the maximum horizontal force and the ratio between the horizontal and vertical force during a static holding period. Both position and vision appear to affect the grip-lift task. Consequently, sequential assessments should be performed in the same condition to obtain reliable data.

Through highly precise perceptual and sensorimotor activities, the human tactile system continuously acquires information about the environment. Mechanical interactions between the skin at the point of contact and a touched surface serve as the source of this tactile information. Using a dedicated custom robotic platform, we imaged skin deformation at the contact area between the finger and a flat surface during the onset of tangential sliding movements in four different directions (proximal, distal, radial and ulnar) and with varying normal force and tangential speeds. This simple tactile event evidenced complex mechanics. We observed a reduction of the contact area while increasing the tangential force and proposed to explain this phenomenon by nonlinear stiffening of the skin. The deformation's shape and amplitude were highly dependent on stimulation direction. We conclude that the complex, but highly patterned and reproducible, deformations measured in this study are a potential source of information for the central nervous system and that further mechanical measurement are needed to better understand tactile perceptual and motor performances.

Objective: To calibrate and validate the Benin version of ABILOCO, a Rasch-built scale developed to assess locomotion ability in stroke patients., Design: Prospective study and questionnaire development., Setting: Rehabilitation centers., Participants: Stroke patients (N=230; mean age ± SD, 51.1±11.6 y; 64.3% men)., Intervention: Not applicable., Main Outcome Measures: Participants completed a preliminary list of 36 items including the 13 items of ABILOCO. Items were scored as "impossible," "difficult," or "easy." The mobility subdomain of FIM (FIM-mobility), the Functional Ambulation Classification (FAC), the 6-minute walk test (6MWT), and the 10-meter walk test (10MWT) were used to evaluate and elucidate the validity of the ABILOCO-Benin scale., Results: Successive Rasch analyses led to the selection of 15 items that define a unidimensional, invariant, and linear measure of locomotion ability in stroke patients. This modified version of the ABILOCO scale, named ABILOCO-Benin, showed an excellent internal consistency, with a Person Separation Index of .93, and excellent test-retest reliability with high intraclass correlation coefficients of .95 (P<.001) for item difficulty and .93 (P<.001) for subject measures. It also presented good construct validity compared with FAC, FIM-mobility, 6MWT, and 10MWT (r≥.75, P<.001)., Conclusions: ABILOCO-Benin presents good psychometric properties. It allows valid, reliable, and objective measurements of locomotion ability in stroke patients., (Copyright © 2014 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Moving requires handling gravitational and inertial constraints pulling on our body and on the objects that we manipulate. Although previous work emphasized that the brain uses internal models of each type of mechanical load, little is known about their interaction during motor planning and execution. In this report, we examine visually guided reaching movements in the horizontal plane performed by naive participants exposed to changes in gravity during parabolic flight. This approach allowed us to isolate the effect of gravity because the environmental dynamics along the horizontal axis remained unchanged. We show that gravity has a direct effect on movement kinematics, with faster movements observed after transitions from normal gravity to hypergravity (1.8g), followed by significant movement slowing after the transition from hypergravity to zero gravity. We recorded finger forces applied on an object held in precision grip and found that the coupling between grip force and inertial loads displayed a similar effect, with an increase in grip force modulation gain under hypergravity followed by a reduction of modulation gain after entering the zero-gravity environment. We present a computational model to illustrate that these effects are compatible with the hypothesis that participants partially attribute changes in weight to changes in mass and scale incorrectly their motor commands with changes in gravity. These results highlight a rather direct internal mapping between the force generated during stationary holding against gravity and the estimation of inertial loads that limb and hand motor commands must overcome., (Copyright © 2014 the American Physiological Society.)

Objective: Tactile explorations with the fingertips provide information regarding the physical properties of surfaces and their relative pleasantness. Previously, we performed an investigation in the active touch domain and linked several surface properties (i.e. frictional force fluctuations and net friction) with their pleasantness levels. The aim of the present study was to investigate physical factors being important for pleasantness perception during passive fingertip stimulation. Specifically we were interested to see whether factors, such as surfaces' topographies or their frictional characteristics could influence pleasantness. Furthermore, we ascertained how the stimulus pleasantness level was impacted by (i) the normal force of stimulus application (FN) and (ii) the stimulus temperature (TS)., Methods and Results: The right index fingertips of 22 blindfolded participants were stimulated using 27 different stimuli, which varied in average roughness (Ra) and TS. A 4-axis robot moved the stimuli horizontally under participants' fingertips with three levels of FN. The robot was equipped with force sensors, which recorded the FN and friction force (FT) during stimulation. Participants rated each stimulus according to a three-level pleasantness scale, as very pleasant (scored 0), pleasant (scored 1), or unpleasant (scored 2). These ordinal pleasantness ratings were logarithmically transformed into linear and unidimensional pleasantness measures with the Rasch model. Statistical analyses were conducted to investigate a possible link between the stimulus properties (i.e. Ra, FN, FT, and TS) and their respective pleasantness levels. Only the mean Ra and FT values were negatively correlated with pleasantness. No significant correlation was detected between FN or TS and pleasantness., Conclusion: Pleasantness perception, resulting from passive fingertip stimulation, seems to be influenced by the surfaces' average roughness levels and average FT occurring during fingertip stimulation.

Skilled hand movements require a precise coordination between the grip force and the load force. To coordinate those forces, we rely on both a predictive and a reactive control. On the basis of specific impairments observed previously in children with hemiplegic cerebral palsy, we aimed to assess the predictive or reactive nature of hand deficits in stroke patients. This case-control study was carried out with eight stroke patients and eight control participants. The load of a handheld object was rapidly increased by dropping a mass attached to the object. We tested predictive and reactive aspects of the movement in the same task as the drop was triggered either unexpectedly by the examiner (reactive condition) or by the patient himself (predictive condition). Deficits observed in the paretic hand were similar to those highlighted previously in children with hemiplegic cerebral palsy. Under predictive conditions, temporal deficits were observed after impact. Under reactive conditions, the reflex latency was slightly increased in the paretic hand. The nonparetic hand showed similar results to controls. The predictive mechanism is present but altered in the paretic hand. These alterations suggest an inability to anticipate the consequences of dynamic perturbations in the paretic hand only.

Brain lesions may disturb hand functioning in children with cerebral palsy (CP), making it difficult or even impossible for them to perform several manual activities. Most conventional treatments for hand dysfunction in CP assume that reducing the hand dysfunctions will improve the capacity to manage activities (i.e., manual ability, MA). The aim of this study was to investigate the directional relationships (direct and indirect pathways) through which hand skills influence MA in children with CP. A total of 136 children with CP (mean age: 10 years; range: 6-16 years; 35 quadriplegics, 24 diplegics, 77 hemiplegics) were assessed. Six hand skills were measured on both hands: touch-pressure detection (Semmes-Weinstein esthesiometer), stereognosis (Manual Form Perception Test), proprioception (passive mobilization of the metacarpophalangeal joints), grip strength (GS) (Jamar dynamometer), gross manual dexterity (GMD) (Box and Block Test), and fine finger dexterity (Purdue Pegboard Test). MA was measured with the ABILHAND-Kids questionnaire. Correlation coefficients were used to determine the linear associations between observed variables. A path analysis of structural equation modeling was applied to test different models of causal relationships among the observed variables. Purely sensory impairments did seem not to play a significant role in the capacity to perform manual activities. According to path analysis, GMD in both hands and stereognosis in the dominant hand were directly related to MA, whereas GS was indirectly related to MA through its relationship with GMD. However, one-third of the variance in MA measures could not be explained by hand skills. It can be concluded that MA is not simply the integration of hand skills in daily activities and should be treated per se, supporting activity-based interventions.

Background: After stroke, deregulated interhemispheric interactions influence residual paretic hand function. Anodal or cathodal transcranial direct current stimulation (tDCS) can rebalance these abnormal interhemispheric interactions and improve motor function., Objective: We explored whether dual-hemisphere tDCS (dual-tDCS) in participants with chronic stroke can improve fine hand motor function in 2 important aspects: precision grip and dexterity., Methods: In all, 19 chronic hemiparetic individuals with mild to moderate impairment participated in a double-blind, randomized trial. During 2 separate cross-over sessions (real/sham), they performed 10 precision grip movements with a manipulandum and the Purdue Pegboard Test (PPT) before, during, immediately after, and 20 minutes after dual-tDCS applied simultaneously over the ipsilesional (anodal) and contralateral (cathodal) primary motor cortices., Results: The precision grip performed with the paretic hand improved significantly 20 minutes after dual-tDCS, with reduction of the grip force/load force ratio by 7% and in the preloading phase duration by 18% when compared with sham. The dexterity of the paretic hand started improving during dual-tDCS and culminated 20 minutes after the end of dual-tDCS (PPT score +38% vs +5% after sham). The maximal improvements in precision grip and dexterity were observed 20 minutes after dual-tDCS. These improvements correlated negatively with residual hand function quantified with ABILHAND., Conclusions: One bout of dual-tDCS improved the motor control of precision grip and digital dexterity beyond the time of stimulation. These results suggest that dual-tDCS should be tested in longer protocols for neurorehabilitation and with moderate to severely impaired patients. The precise timing of stimulation after stroke onset and associated training should be defined.

Background: When scanning surfaces, humans perceive some of their physical attributes. These percepts are frequently accompanied by a sensation of (un)pleasantness. We therefore hypothesized that aspects of the mechanical activity induced by scanning surfaces with fingertips could be objectively associated with a pleasantness sensation. Previously, we developed a unidimensional measure of pleasantness, the Pleasant Touch Scale, quantifying the pleasantness level of 37 different materials. Findings of this study suggested that the sensation of pleasantness was influenced by the average magnitude of the frictional forces brought about by sliding the finger on the surface, and by the surface topography. In the present study, we correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness., Results: Eight blindfolded participants tactually explored twelve materials of the Pleasant Touch Scale through lateral sliding movements of their index fingertip. During exploration, the normal and tangential interaction force components, fN and fT , as well as the fingertip trajectory were measured. The effect of the frictional force on pleasantness sensation was investigated through the analysis of the ratio fT to fN , i.e. the net coefficient of kinetic friction, μ. The influence of the surface topographies was investigated through analysis of rapid fT fluctuations in the spatial frequency domain. Results showed that high values of μ were anticorrelated with pleasantness. Furthermore, surfaces associated with fluctuations of fT having higher amplitudes in the low frequency range than in the high one were judged to be less pleasant than the surfaces yielding evenly distributed amplitudes throughout the whole spatial frequency domain., Conclusion: Characteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.

In daily life, object manipulation is usually performed concurrently to the execution of cognitive tasks. The aim of the present study was to determine which aspects of precision grip require cognitive resources using a motor-cognitive dual-task paradigm. Eighteen healthy participants took part in the experiment, which comprised two conditions. In the first condition, participants performed a motor task without any concomitant cognitive task. They were instructed to grip, lift and hold an apparatus incorporating strain gauges allowing a continuous measurement of the force perpendicular to each contact surface (grip force, GF) as well as the total tangential force applied on the object (load force, LF). In the second condition, participants performed the same motor task while concurrently performing a cognitive task consisting in a complex visual search combined with counting. In the dual-task condition, we found a significant increase in the duration of the preload phase (time between initial contact of the fingers with the apparatus and onset of the load force), as well as a significant increase of the grip force during the holding phase, indicating that the cognitive task interfered with the initial force scaling performed during the preload phase and the fine-tuning of grip force during the hold phase. These findings indicate that these aspects of precision grip require cognitive resources. In contrast, other aspects of the precision grip, such as the temporal coupling between grip and load forces, were not affected by the cognitive task, suggesting that they reflect more automatic processes. Taken together, our results suggest that assessing the dynamic and temporal parameters of precision grip in the context of a concurrent cognitive task may constitute a more ecological and better-suited tool to characterize motor dysfunction in patients.

Objective: To determine whether regular brisk walking can promote functional recovery in community-dwelling stroke patients., Patients: A total of 44 chronic stroke patients, recruited in Belgium and Benin, respectively European high-income and African low-income countries., Methods: This longitudinal, single-cohort, observational study with 1 intervention period and 4 time-points of assessments (2 baseline, 1 post-intervention and 1 follow-up) was structured in 3 periods: pre-intervention period (1 month), intervention period (3 months) and follow-up period (3 month). Intervention consisted of a 3 times/week group-based brisk walking programme. Primary outcome measures were ACTIVLIM-Stroke questionnaire and the 6-minute walk test (6MWT). Secondary outcome measures were the Stroke Impairment Assessment Set (SIAS), the Hospital Anxiety and Depression Scale (HADS), and the Berg Balance Scale (BBS)., Results: All outcome measures were stable during the pre-intervention period (p ≥ 0.16). They all improved significantly after intervention (p ≤ 0.01), except the HADS (p = 0.058). However, during the follow-up period, SIAS (p = 0.002) and BBS (p = 0.001) decreased, while ACTIVLIM-Stroke, 6MWT and HADS showed no significant change (p ≥ 0.13)., Conclusion: This study suggests regular brisk walking as an effective approach to promote functional recovery in chronic stroke survivors. However, further studies are required before generalizing these results to the whole stroke population.

Most chronic stroke patients present with difficulty in the manipulation of objects. The aim of this study was to test whether an intensive program of precision grip training could improve hand functioning of patients at more than 6 months after a stroke. This was a cross-over study; hence, at inclusion, the patients were randomly divided into two groups: one group started with the bilateral movement therapy and the other group started with the unilateral movement therapy. The patients were assessed on four separate occasions across a 12-week period: (a) at inclusion in the study, (b) 4 weeks later, immediately before the first rehabilitation session, (c) after 4 weeks of one therapy, and (d) after a further 4 weeks of the other therapy. Ten patients completed two consecutive 4-week sessions (1 h, 3 days/week) of therapy. The therapy comprised unilateral and bilateral repetitive grip-lift task-oriented rehabilitation with rhythmic auditory cueing. The grip-lift force coordination, digital dexterity, manual ability, and the level of satisfaction (with activities and participation) were assessed. A one-way repeated-measure analysis of variance across the four evaluations did not detect any objective improvement in the measured variables after 8 weeks of specific intensive training. Precision grip training was shown to not generate significant improvement in the grip-lift task, digital dexterity, manual ability, or satisfaction in chronic stroke patients.

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick-slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function.

Objectives: Several ABILHAND Rasch-built manual ability scales were previously developed for chronic stroke (CS), cerebral palsy (CP), rheumatoid arthritis (RA), systemic sclerosis (SSc) and neuromuscular disorders (NMD). The present study aimed to explore the applicability of a generic manual ability scale unbiased by diagnosis and to study the nature of manual ability across diagnoses., Design: Cross-sectional study., Setting: Outpatient clinic homes (CS, CP, RA), specialised centres (CP), reference centres (CP, NMD) and university hospitals (SSc)., Participants: 762 patients from six diagnostic groups: 103 CS adults, 113 CP children, 112 RA adults, 156 SSc adults, 124 NMD children and 124 NMD adults., Primary and Secondary Outcome Measures: Manual ability as measured by the ABILHAND disease-specific questionnaires, diagnosis and nature (ie, uni-manual or bi-manual involvement and proximal or distal joints involvement) of the ABILHAND manual activities., Results: The difficulties of most manual activities were diagnosis dependent. A principal component analysis highlighted that 57% of the variance in the item difficulty between diagnoses was explained by the symmetric or asymmetric nature of the disorders. A generic scale was constructed, from a metric point of view, with 11 items sharing a common difficulty among diagnoses and 41 items displaying a category-specific location (asymmetric: CS, CP; and symmetric: RA, SSc, NMD). This generic scale showed that CP and NMD children had significantly less manual ability than RA patients, who had significantly less manual ability than CS, SSc and NMD adults. However, the generic scale was less discriminative and responsive to small deficits than disease-specific instruments., Conclusions: Our finding that most of the manual item difficulties were disease-dependent emphasises the danger of using generic scales without prior investigation of item invariance across diagnostic groups. Nevertheless, a generic manual ability scale could be developed by adjusting and accounting for activities perceived differently in various disorders.

Humans can detect and discriminate between fine variations of surface roughness using active touch. It is hitherto believed that roughness perception is mediated mostly by cutaneous and subcutaneous afferents located in the fingertips. However, recent findings have shown that following abolishment of cutaneous afferences resulting from trauma or pharmacological intervention, the ability of subjects to discriminate between textures roughness was not significantly altered. These findings suggest that the somatosensory system is able to collect textural information from other sources than fingertip afference. It follows that signals resulting of the interaction of a finger with a rough surface must be transmitted to stimulate receptor populations in regions far away from the contact. This transmission was characterized by measuring in the wrist vibrations originating at the fingertip and thus propagating through the finger, the hand and the wrist during active exploration of textured surfaces. The spectral analysis of the vibrations taking place in the forearm tissues revealed regularities that were correlated with the scanned surface and the speed of exploration. In the case of periodic textures, the vibration signal contained a fundamental frequency component corresponding to the finger velocity divided by the spatial period of the stimulus. This regularity was found for a wide range of textural length scales and scanning velocities. For non-periodic textures, the spectrum of the vibration did not contain obvious features that would enable discrimination between the different stimuli. However, for both periodic and non-periodic stimuli, the intensity of the vibrations could be related to the microgeometry of the scanned surfaces.

Objectives: To develop a fitness index unlinked to resting heart rate and suitable for clinical use, and to obtain reference values of this new index for healthy subjects., Design: Cross-sectional study., Setting: Research laboratory., Participants: A volunteer sample of healthy subjects (N=100; 50 men; age range, 20-70y) randomly recruited from the general community., Interventions: Not applicable., Main Outcome Measures: Participants performed a submaximal, multistage cycle ergometer test. A new fitness index, the physical working capacity at 75% of the predicted maximal heart rate per kilogram of body weight (PWC(75%)/kg), was calculated. Its concordance with a previously described fitness index and its relationship with age were examined, as well as differences attributable to sex and lifestyle. Reference values of the PWC(75%)/kg (mean ± SD and 95% confidence interval) were calculated and categorized by age classes of 10 years and by sex., Results: The intraclass correlation coefficient (ICC) between PWC(75%)/kg and the working capacity index at 65% of the heart rate reserve per kilogram of body weight (WCI(65%HRreserve)/kg) was very high (ICC=.96, P<.001), indicating that the fitness index can be estimated without measuring the resting heart rate. PWC(75%)/kg decreased as age increased. The average PWC(75%)/kg was significantly higher in men than in women (P<.001), and in active than in inactive subjects (P<.01)., Conclusions: This study presents a new fitness index, the PWC(75%)/kg, which is suitable for measuring fitness in active and sedentary people aged 20 to 70 years. It may also be a suitable fitness index for selected chronically ill individuals. This study also provides reference values of the PWC(75%)/kg obtained from healthy men and women., (Copyright © 2012 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Background: Evidence suggests that somatic sensation has a modality for pleasant touch., Objective: To investigate pleasant touch at the fingertip level (i.e., glabrous skin site) through the elaboration of a linear unidimensional scale that measures (i) various materials according to the level of pleasantness they elicit through active fingertip explorations and (ii) subjects according to their pleasantness leniency levels., Subjects: We enrolled 198 healthy subjects without any neurological disease., Methods: Blindfolded subjects actively explored 48 materials with their index fingertips and reported the perceived pleasantness of each on a 4-level scale. The fingertip moisture levels on each subject were measured before the experimental session. Data were analyzed using the Rasch model., Results: We elaborated unidimensional linear scale that included 37 materials according to their pleasantness of touch. The pleasantness level of 21 materials was perceived differently, depending on the fingertip moisture levels of the subjects., Conclusion: Based on our findings, we formulated a Pleasant Touch Scale. Fingertip moisture levels appeared to be a major factor for (un)pleasant feelings during active exploration.

This study aims to investigate the relative contribution of remote mechanoreceptors to perception of roughness and spatial acuity. We examined two unilateral pathological conditions affecting differently innervation of the index finger: unilateral carpal tunnel syndrome (n=12) and surgically repaired complete traumatic median nerve section at the wrist following surgical repair (n=4). We employed a control condition consisting of ring-block anesthesia of the entire index in 10 healthy subjects to model pathological denervation of the fingertip. Spatial acuity and the ability to discern roughness were assessed using a grating orientation task and a roughness discrimination task, respectively. In patients with carpal tunnel syndrome, we observed a significant reduction of spatial resolution acuity but an intact ability to discriminate roughness with the fingertip. For patients with traumatic median nerve section there was no recovery with the grating orientation task up to 20 months post surgery but a progressive and full recovery with the roughness discrimination task between 6 and 9 months. Finally, in the anesthetic ring bloc group, the nerve block completely disrupted performances in grating orientation task, but unexpectedly left unaffected performances in the roughness discrimination task. Taken together, these lines of evidence support the view that the neural mechanisms underlying tactile roughness discrimination differ from those involved in spatial resolution acuity. Vibrotaction is necessary and sufficient for the perception of fine textures and, when the innervation of the fingerpad is compromised, information about textures can be captured and encoded by remote mechanoreceptors located in more proximal tissues where the innervation is intact., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Background and Purpose: This study describes the development of a Rasch-built scale measuring activity limitations in stroke patients, named ACTIVLIM-Stroke., Method: This new Rasch-built measure was constructed based on stroke patients' perceptions of difficulty in performing daily activities. Patients were recruited from inpatient and outpatient rehabilitation departments in Belgium and Benin. A 73-item questionnaire was completed by 204 participants. A random subsample of 83 subjects was given the questionnaire a second time. Data were analyzed using RUMM2030 software., Results: After successive Rasch analyses, the ACTIVLIM-Stroke questionnaire, a unidimensional and linear 20-item measure of activity limitations, was constructed. All 20 items fulfilled Rasch requirements (overall and individual item fit, category discrimination, invariance, local response independence, and nonredundancy in item difficulty). This simple patient-based scale encompasses a large range of activities related to self-care, transfer, mobility, manual ability, and balance. The ACTIVLIM-Stroke questionnaire exhibited high internal validity, excellent internal consistency, and good crosscultural validity. The test-retest reliability of item difficulty hierarchy (intraclass correlation coefficient=0.99) and patient location (intraclass correlation coefficient=0.92) were both excellent. Furthermore, it showed good external construct validity using correlations with the Functional Independence Measure motor and the Barthel Index and a higher discriminating capacity than either of these widely used indices., Conclusions: The ACTIVLIM-Stroke questionnaire has good psychometric qualities and provides accurate measures of activity limitations in patients with stroke. It is recommended for evaluating clinical and research interventions in patients with stroke, because it provides a higher discrimination and might be more sensitive to change.

Most object manipulation tasks involve a series of actions demarcated by mechanical contact events, and gaze is usually directed to the locations of these events as the task unfolds. Typically, gaze foveates the target 200 ms in advance of the contact. This strategy improves manual accuracy through visual feedback and the use of gaze-related signals to guide the hand/object. Many studies have investigated eye-hand coordination in experimental and natural tasks; most of them highlighted a strong link between eye movements and hand or object kinematics. In this experiment, we analyzed gaze strategies in a collision task but in a very challenging dynamical context. Participants performed collisions while they were exposed to alternating episodes of microgravity, hypergravity and normal gravity. First, by isolating the effects of inertia in microgravity, we found that peak hand acceleration marked the transition between two modes of grip force control. Participants exerted grip forces that paralleled load force profiles, and then increased grip up to a maximum shifted after the collision. Second, we found that the oculomotor strategy adapted visual feedback of the controlled object around the collision, as demonstrated by longer durations of fixation after collision in new gravitational environments. Finally, despite large variability of arm dynamics in altered gravity, we found that saccades were remarkably time-locked to the peak hand acceleration in all conditions. In conclusion, altered gravity allowed light to be shed on predictive mechanisms used by the central nervous system to coordinate gaze, hand and grip motor actions during a mixed task that involved transport of an object and high impact loads.

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.

The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.

Objectives: To report the body functions, activities, and participation of stroke patients, and to investigate the relationships over time with the patients' perceived satisfaction with their level of activity and participation., Design: Descriptive., Setting: Hospital, home., Participants: Stroke patients (N = 45; mean age ± SD, 69 ± 10.7y) assessed by the same examiner., Interventions: Not applicable., Main Outcome Measures: The Stroke Impairment Assessment Set was used to measure body functions, the Mini-Mental State Examination was used to measure cognitive level, and the Depression subscale of the Hospital Anxiety and Depression Scale was used to measure mood level. The ABILHAND and ABILOCO scales were used to measure activity, and the SATIS-Stroke questionnaire was used to measure satisfaction with activities and participation. Social and physical environmental factors were assessed by the World Health Organization International Classification of Functioning, Disability and Health Checklist Version 2.1a Clinician form. checklist. Patients were assessed during the acute (1 wk), postacute (3 mo), and chronic (6 mo) phases after stroke onset., Results: Significant changes were observed over time in stroke body functions, cognitive status, manual and locomotion abilities, and satisfaction with activity and participation. At 1 week, satisfaction with activity and participation was not related to any body functions, activities, or environmental factors. At 3 months, manual ability was the only variable that was significantly related to satisfaction. During the chronic phase, manual ability and body functions were the best predictors of the stroke patient's perceived satisfaction. However, this combination of factors predicted only 43% of the variation in the SATIS-Stroke measures., Conclusions: Satisfaction with activity and participation cannot simply be inferred from body functions and activities, because it depends on complex interactions between functional, personal, and environmental factors., (Copyright © 2011 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Anticipatory grip force adjustments are a prime example of the predictive nature of motor control. An object held in precision grip is stabilized by fine adjustments of the grip force against changes in tangential load force arising from inertia during acceleration and deceleration. When an object is subject to sudden impact loads, prediction becomes critical as the time available for sensory feedback is very short. Here, we investigated the control of grip force when participants performed a targeted tapping task with a hand-held object. During the initial transport phase of the movement, load force varied smoothly with acceleration. In contrast, in the collision, load forces sharply increased to very large values. In the transport phase, grip force and load force were coupled in phase, as expected. However, in the collision, grip force did not parallel load force. Rather, it exhibited a stereotyped profile with maximum ∼65 ms after peak load at contact. By using catch trials and a virtual environment, we demonstrate that this peak of grip force is pre-programmed. This observation is validated across experimental manipulations involving different target stiffness and directions of movement. We suggest that the central nervous system optimizes stability in object manipulation-as in catching-by regulating mechanical parameters including stiffness and damping through grip force. This study provides novel insights about how the brain coordinates grip force in manipulation involving an object interacting with the environment., (Copyright © 2011 IBRO. Published by Elsevier Ltd. All rights reserved.)

Objective: ABILHAND is a Rasch-built questionnaire that measures manual ability in rheumatoid arthritis (RA) patients. This study aimed to examine the test-retest reliability and the responsiveness of ABILHAND in RA patients., Methods: Eighty-eight patients underwent 3 evaluations: the first evaluation was at baseline (time 1), the second was 2 weeks later (time 2), and the third was 1 year later (time 3). Disease activity was assessed using the Disease Activity Score in 28 joints using the C-reactive protein level (DAS28-CRP). Patients rated the intensity of their RA-related pain using a 100-mm visual analog scale for pain and completed questionnaires based on their activity limitations (ABILHAND and the Health Assessment Questionnaire) and quality of life., Results: The responsiveness analyses were conducted by using global, group, and individual approaches. The global approach showed significant differences between the time 1 and time 3 scores of the DAS28-CRP (P = 0.04) and ABILHAND (P = 0.04). Based on the changes in disease activity scores and the European League Against Rheumatism response criteria, the sample was divided into 3 groups: deteriorated, stable, and improved. The mean ± SD changes in manual ability were higher in the deteriorated (-1.23 ± 1.53 logit) and in the improved (1.22 ± 2.06 logits) groups than in the stable group (0.48 ± 1.09 logit). The effect size and standardized response mean confirmed that observation. The minimal clinically important difference was assessed in each group of patients., Conclusion: The ABILHAND questionnaire exhibited responsiveness in detecting slight changes in RA patients. Therefore, the ABILHAND tool can be used to evaluate the functional status of RA patients in clinical trials and settings., (Copyright © 2011 by the American College of Rheumatology.)

During object manipulation, predictive grip force modulation allows compensation for inertial forces induced by the object's acceleration. This coupling between grip force (GF) and load force (LF) during voluntary movements has demonstrated high levels of complexity, adaptability, and flexibility under many loading conditions in a broad range of experimental studies. The association between GF and LF indicates the presence of internal models underlying predictive GF control. The present experiment sought to identify the variables taken into account during GF modulation at the initiation of a movement. Twenty subjects performed discrete point-to-point movements under normal and hypergravity conditions induced by parabolic flights. Two control experiments performed under normal gravitational conditions compared the observed effect of the increase in gravity with the effects of a change in movement kinematics and a change in mass. In hypergravity, subjects responded accurately to the increase in weight during stationary holding but overestimated inertial loads. During dynamic phases, the relationship between GF and LF under hypergravity varied in a manner similar to the control test in which object mass was increased, whereas a change in movement kinematics could not reproduce this result. We suggest that the subjects' strategy for anticipatory GF modulation is based on sensorimotor mapping that combines the perception of the weight encoded during stationary holding with an internal representation of the movement kinematics. In particular, such a combination reflects a prior knowledge of the unequivocal relationship linking mass, weight, and loads under the invariant gravitational context experienced on Earth.

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.

Background: Neuromuscular disorders (NMDs) can lead to specific manual disabilities due to hand muscle weakness and atrophy, myotonia or loss of sensory function. The aim of this study was to adapt and validate the ABILHAND questionnaire in children and adults with NMDs using the Rasch model., Methods: This questionnaire contained specific manual activities for children and for adults, as well as common manual activities. 124 adult patients and the parents of 124 paediatric patients were asked to provide their perceived difficulty in performing each manual activity on a three level scale: impossible (0), difficult (1) or easy (2). Items were selected from well established psychometric criteria (ordered categories, equal item discrimination, adequate fit to the Rasch model, lack of redundancy) using the Rasch Unidimensional Measurement Models (RUMM2020) computer programme., Results: The 22 selected items contain four children specific items, four adult specific items and 14 items commonly applicable to both children and adults. They define a unidimensional and linear measure of manual ability and demonstrate continuous progression in their difficulty. The item hierarchy of difficulty was invariant across six patient related factors. The scale exhibited good precision (r=0.95) and the 22 items were well targeted to the patients' locations. The ABILHAND measures were strongly related to the ACTIVLIM measures (r=0.76) and poorly related to grip strength (r=0.36 for the right hand and r=0.40 for the left hand)., Conclusion: This scale can be used for adults and children, allowing manual ability to be assessed from childhood to adulthood.

Background and Objectives: Grasping an object between the thumb and index finger requires precise coordination between grip force (GF) and tangential load force (LF), which is impaired in children with congenital hemiplegia (CH). This study aimed to determine the respective contributions of predictive and reactive control in the impaired precision grip of 12 children with CH between 10 and 16 years of age when compared with age- and gender-matched controls., Methods: The load of a handheld object was increased rapidly by generating an impact through the drop of a mass attached to the object. The drop was triggered by the participant (predictive conditions) or unexpectedly by the examiner (reactive conditions). In both conditions, participants aimed to prevent the object from falling. Both hands of children with CH and controls were tested., Results: During our task, no differences in the GF levels were observed between paretic, nonparetic, and control hands. Under predictive conditions, the temporal variables related to the GF were preserved before impact in children with CH but altered after impact. Under reactive conditions, the reactive delays were longer in the paretic hand. Predictive and reactive control were preserved on the nonparetic hand., Conclusions: Deficits were observed in both predictive and reactive control for the paretic hand. The predictive control exists but is altered after the impact, suggesting an inability to anticipate the consequences of a dynamic perturbation. The authors suggest that the abilities of the nonparetic side could be used in neurorehabilitation to improve motor control of the paretic side.

The manipulation of small objects requires continuous contributions from both predictive and reactive mechanisms. The authors aimed to study the development of predictive and reactive mechanisms used by children from 6 to 14 years of age to manage impulsive loading. The load of a handheld object was increased rapidly by the drop of a weight hung on the object. The drop was triggered either by the child (predictive condition) or by the examiner (reactive condition). Regardless of the condition, the control strategy was refined with age. Younger children were unable to adapt their grip force (GF) to the friction of their fingers, whereas the older children provided GF that was well adapted to their variable coefficient of friction, thereby producing a secure grip. This reflected either an inadequate amount of force or an inability to integrate cutaneous information from the fingers in younger children. Additionally, a modulation with age for both predictive and reactive mechanisms was observed. All together, the better predictive abilities and the more secure grip exhibited by older children allow decreased slipping and improved performance in an impulsive loading task.

The present study examined the relationship between the tactile roughness discrimination threshold (TRDT) and the tactile spatial resolution threshold (TSRT) at the index fingertip in humans. A new device was built for measuring TRDT, allowing pair-wise presentations of two sets of six different sandpaper grits. The smoothest grits ranged from 18 to 40 microm and the roughest grits ranged from 50 to 195 microm particle size. The reference sandpaper had a 46 microm particle size. A two-alternative forced choice paradigm and a double interlaced adaptive staircase procedure yielding a 75% just noticeable difference (75%jnd) was used according to Zwislocki and Relkin. Contact force and scanning velocity were measured at the fingertip with a built-in sensor. The TSRT was assessed with an extended set of grating domes. Fifty-three male and female subjects, spanning a wide age range participated in this study. The JND75% or TRDT was lower for the smoothest sandpapers (15+/-8.5 microm) compared to the roughest sandpapers (44+/-32.5 microm). TRDT performance was unrelated to age or gender. Additionally, grit size had no effect on the mean forces (normal and tangential) exerted at the fingertip or the mean scan velocities. In contrast, there was a significant degradation of TSRT performance with age. Lastly, there was no significant correlation between TRDT and TRST performance. Results of this study support the theory that the neural mechanisms underlying the perception of tactile roughness discrimination for fine textures differ from those involved in spatial resolution acuity often associated with the SA1 afferents., (Copyright 2009 Elsevier B.V. All rights reserved.)

Coordination between the normal force exerted by fingers on a held object and the tangential constraints at the fingertips helps to successfully manipulate objects. It is well established that the minimal grip force required to prevent an object from slipping strongly depends on the frictional properties at the finger-object interface. Moreover, interindividual variation in the modulation of grip force suggests that the moisture level of the skin could influence grip force strategy. In the present study we asked subjects to perform a horizontal point-to-point task holding an object with a precision grip. The object was equipped with a moisture sensor. We found large inter- and intraindividual moisture level variations. There was a strong correlation between grip force exerted and moisture level at the fingertips. Indeed, the grip force was minimal when the fingertip moisture was optimal with respect to friction. Furthermore, fingertip moisture tended toward this optimal level at which grip force is minimal. In conclusion, we showed a modulation of the grip force with moisture level and hypothesized novel mechanisms of moisture regulation that tend to stabilize the moisture level toward the value that minimizes grip force.

The aim of this study was to determine whether tactile spatial resolution and digital dexterity performance are associated during childhood. A total of 104 healthy children (52 boys and 52 girls) were assessed. The tactile spatial resolution threshold was determined using an enlarged set of JVP domes with a procedure adapted for children. Digital dexterity was tested with the Purdue Pegboard Test. While tactile spatial resolution and digital dexterity improved with age following a parallel evolution, no correlation was found between the scores of the children for both tests. We suggest that central maturational processes are responsible for a global refinement of sensory and motor function between the ages of 4 and 10 years, reaching the range of adult values at this age.

Impulsive loadings during object grasping are common in everyday life. In predictable conditions, the grip force (GF) increases before the impact to anticipate the perturbation and reaches a maximum after the perturbation. In the present study, the authors addressed the predictive or reactive nature of this late GF component. The load of a handheld object was briskly increased by dropping a mass attached to the object (impact trials). The drop was self-induced, but for one third of the trials, the mechanism was blocked and no impact occurred (blank trials). Evidence that the late GF component is programmed as a predictive action emerged from a systematic comparison between impact and blank trials. The authors conclude that the GF increase occurring after a predictable impulsive loading is essentially of a predictive nature.

The planning and control of motor actions requires knowledge of the dynamics of the controlled limb to generate the appropriate muscular commands and achieve the desired goal. Such planning and control imply that the CNS must be able to deal with forces and constraints acting on the limb, such as the omnipresent force of gravity. The present study investigates the effect of hypergravity induced by parabolic flights on the trajectory of vertical pointing movements to test the hypothesis that motor commands are optimized with respect to the effect of gravity on the limb. Subjects performed vertical pointing movements in normal gravity and hypergravity. We use a model based on optimal control to identify the role played by gravity in the optimal arm trajectory with minimal motor costs. First, the simulations in normal gravity reproduce the asymmetry in the velocity profiles (the velocity reaches its maximum before half of the movement duration), which typically characterizes the vertical pointing movements performed on Earth, whereas the horizontal movements present symmetrical velocity profiles. Second, according to the simulations, the optimal trajectory in hypergravity should present an increase in the peak acceleration and peak velocity despite the increase in the arm weight. In agreement with these predictions, the subjects performed faster movements in hypergravity with significant increases in the peak acceleration and peak velocity, which were accompanied by a significant decrease in the movement duration. This suggests that movement kinematics change in response to an increase in gravity, which is consistent with the hypothesis that motor commands are optimized and the action of gravity on the limb is taken into account. The results provide evidence for an internal representation of gravity in the central planning process and further suggest that an adaptation to altered dynamics can be understood as a reoptimization process.

Faces scales are one of the most commonly used instruments to assess pain intensity in children. Most available faces scales present five to seven faces. The present research was conducted to investigate the ability of 4- to 7-year-old children to distinguish the response categories of different faces scales. In the first study, 121 children were asked to rate painful situations on a 3- and a 6-level faces scale commencing with a smiling 'no pain' face. Children were divided into two age groups (4-5 and 6-7 years). Investigations of the category functioning were performed with a rating scale Rasch model for each age group. Results revealed the low performances of the 6-level faces scale as compared to the 3-level faces scale and also the difficulty children experienced in scoring the imaginary painfulness of items. Consequently, a second study was conducted. In this second study, 76 children were asked to rate pictures depicting painful situations on a 3-level faces scale beginning with a neutral 'no pain' face. Results of this second study confirmed an improvement in the ability to distinguish the three response categories with age. The 4-5 year-old children could only distinguish two response categories and the 6-7 year-old children were able to discern the three levels of the 3-level faces scale. In conclusion, young children do not distinguish as many faces as proposed by the majority of available faces scales. These results strongly recommend a reduction in the number of response levels of faces scales for pain assessment in children.

In this experiment, we investigated whether the CNS uses internal forward models of inertial loads to maintain the stability of a precision grip when manipulating objects in the absence of gravity. The micro-gravity condition causes profound changes in the profile of tangential constraints at the finger-object interface. In order to assess the ability to predict the micro-gravity-specific variation of inertial loads, we analyzed the grip force adjustments that occurred when naive subjects held an object in a precision grip and performed point-to-point movements under the weightless condition induced by parabolic flight. Such movements typically presented static and dynamic phases, which permitted distinction between a static component of the grip force (measured before the movement) and a dynamic component of the grip force (measured during the movement). The static component tended to gradually decrease across the parabolas, whereas the dynamic component was rapidly modulated with the micro-gravity-specific inertial loads. In addition, the amplitude of the modulation significantly correlated with the amplitude of the tangential constraints for the dynamic component. These results strongly support the hypothesis that the internal representation of arm and object dynamics adapts to new gravitational contexts. In addition, the difference in time scales of adaptation of static and dynamic components suggests that they can be processed independently. The prediction of self-induced variation of inertial loads permits fine modulation of grip force, which ensures a stable grip during manipulation of an object in a new environment.

When humans manipulate an object, the minimal grip force (GF) required to avoid slipping depends on the frictional properties between the fingers and the object. As a consequence, fingertip skin friction plays a critical role during object manipulation. Here, the effects of the normal force and moisture content on the skin's static coefficient of friction (CF) for human fingertips were studied. Ten subjects were asked to pinch an object with a given normal force. Slippage of the object on the fingertips was generated for different ranges of normal force using a linear translation stage. The exerted forces and moisture of the fingertips were then measured, and the static coefficient of friction was calculated as the ratio between the tangential force and normal force at slippage. These results demonstrate that the effects of the normal force and moisture content on the CF exhibit a complex interaction. For a given moisture condition, the CF varies as a power function of the normal force; in contrast, for a given normal force, the CF is described by a "bell-shaped" function of moisture. A global expression of the CF as a function of the normal force and moisture content is derived, and a method is proposed for a continuous measure of the CF. This new method shall be of particular interest in investigating dexterous manipulation.

Objective: To adapt and validate a manual ability questionnaire, the ABILHAND, developed through the Rasch methodology in patients with systemic sclerosis (SSc)., Methods: The original version of the ABILHAND, which includes 81 manual daily activities, was presented to 156 patients with SSc. They were asked to provide their perceived difficulty in performing each manual activity on a 3-level scale: impossible, difficult, or easy. Items were selected from well-established psychometric criteria. The patients were reassessed 1 month later to test the reproducibility. Concomitantly, they were clinically evaluated for their disease activity/severity, and their functional ability was tested with the Health Assessment Questionnaire (HAQ)., Results: The 26 selected items defined a unidimensional and linear measure of manual ability and showed a continuous progression in their difficulty. The item difficulty hierarchy was invariant across 12 patient-related factors and the manual ability score was reproducible over time. Finally, the manual ability was significantly poorer in SSc patients with more severe disease, and was negatively correlated with the HAQ score (rho = -0.733)., Conclusion: The SSc-adapted ABILHAND questionnaire is a reliable, valid, reproducible, linear, and unidimensional measure to assess and followup on the manual ability of patients with SSc; therefore, it could become a useful additional tool in clinical trials to assess treatment efficacy.

Aim: The strength and nature of the relationships between motor impairments and activity limitations assessed by the ACTIVLIM questionnaire were investigated in 245 patients with neuromuscular disorders., Methods: Measures of motor impairments consisted of: (1) a grip strength test using a Jamar dynamometer, (2) a Manual Muscle Testing bilaterally performed in 18 muscle groups and (3) a gait speed spontaneously adopted by the patients using the 10 m timed walking test., Results: Activity limitations were poorly correlated with grip strength in both hands (r = 0.3 and 0.36) and moderately correlated with gait speed (r = 0.53). Spearman's coefficients of correlation between the manual muscle testing and activity limitations were moderate to very poor (rho = 0.5 to 0.17)., Conclusion: The relationships between motor impairments and activity limitations are not straightforward in patients with neuromuscular disorders, indicating that the activity limitations should be separately assessed and cannot be simply inferred from motor impairment measures.

Recently, a self-reported scale of activity limitations, the ACTIVLIM questionnaire, was developed and validated in patients with neuromuscular disorders (NMD). The purpose of this study was to investigate its sensitivity to change. One hundred thirty-two patients with NMD (mean age, range: 31, 6-80) were assessed twice, with 21+/-4 months in between, using the ACTIVLIM questionnaire. Mean score change, effect size, standardized response, mean paired t-test and an individual-level statistical approach were calculated for groups of patients according to their self-rated functional status evolution and for three main diagnostic groups (Ambulant and wheelchair-bound Duchenne muscular dystrophy patients, myotonic dystrophy patients, and patients with Charcot-Marie-Tooth neuropathy). The responsiveness indices showed that the change in activity measures was higher in patients who reported deteriorated functional status and in patients with Duchenne muscular dystrophy. The ACTIVLIM questionnaire showed a good sensitivity to change and could be useful in research settings to characterize the disease course of NMD.

Fine prehension is ubiquitous in everyday skilled hand manipulations. The anticipatory nature of the control of normal grip forces exerted against tangential loads has been extensively used to provide insights into the working of neural control of movements. We designed a new versatile device to measure the three dimensional forces and torques during a broad panel of precision grip tasks. The instrument measures constraints exerted independently on two grip surfaces by the thumb and the opposing fingers. In addition, the device can be loaded to increase the weight and/or induce torques and can be easily integrated in a variety of experimental contexts. Its compactness improves its stability during movement and allows an ergonomic manipulation for impaired persons or children.

Objective: To develop a questionnaire (ABILOCO-Kids) based on the Rasch measurement model that assesses locomotion ability in children with cerebral palsy., Design: Prospective study and questionnaire development., Subjects: A total of 113 children with cerebral palsy (10 (standard deviation 2.5) years old)., Methods: A 41-item questionnaire was developed based on existing scales and on the clinical experience of professionals in the field of rehabilitation. This questionnaire was tested separately on the 113 children with cerebral palsy and their parents. Their responses were analysed using the Rasch model (RUMM-2020) to select items that had an ordered rating scale and that fit a unidimensional model., Results: The final ABILOCO-Kids scale consisted of 10 locomotion activities, of which difficulty was rated by the parents. The parents gave a more precise assessment of their children's ability than the children themselves, leading to a wider range of measurement that was well-targeted on the sample population and that had good reliability (r=0.97) and reproducibility (intraclass correlation coefficient=0.96). Item calibration did not vary with age, sex or clinical presentation (hemiplegia, diplegia, quadriplegia). The concurrent validity of the ABILOCO-Kids questionnaire was also shown by its correlation with the Gross Motor Function Classification System., Conclusion: The ABILOCO-Kids questionnaire has good psychometric qualities for measuring a wide range of locomotion abilities in children with cerebral palsy.