Grip force behavior during object manipulation in neurological disorders: Toward an objective evaluation of manual performance deficits

The effect of type 2 diabetes and diabetic peripheral neuropathy on predictive grip force control

This study investigated the impact of type 2 diabetes and diabetic peripheral neuropathy on grip force control during object manipulation. The study included three age-matched groups: type 2 diabetes alone (n = 11), type 2 diabetes with neuropathy (n = 13), and healthy controls (n = 12). Grip force control variables derived from lifting and holding an experimental cup were the ratio between grip force and load forces during lifting (GFR), latency 1 and latency 2, which represented the time between the object's grip and its lift-off from the table, and the period between object's lift-off and the grip force peak, respectively; time lag, which denoted the time difference between the grip and load force peaks during the lifting phase, and finally static force, which was the grip force average during the holding phase. Grip force control variables were compared between groups using one-way ANOVA and Kruskal-Wallis test. Post-hoc analysis was used to compare differences between groups. GFR and latency 1 showed significant differences between groups; the type 2 diabetes with neuropathy group showed larger GFR than the type 2 diabetes alone and healthy control groups. The latency 1was longer for the group with neuropathy in comparison with the health control group. There were no significant differences between groups for latency 2, time lag, and static force. Our results showed impaired GFR and latency 1 in participants with type 2 diabetes with neuropathy while the time lag was preserved. People with type 2 diabetes alone might not have any deficits in grip force control. Higher grip forces might expose people with type 2 diabetes and diabetic peripheral neuropathy to the risk of fatigue and injuring their hands. Future studies should investigate strategies to help people with type 2 diabetes with neuropathy adjust grip forces during object manipulation.

Characterizing Touch Discrimination Impairment from Pooled Stroke Samples Using the Tactile Discrimination Test: Updated Criteria for Interpretation and Brief Test Version for Use in Clinical Practice Settings

Somatosensory loss post-stroke is common, with touch sensation characteristically impaired. Yet, quantitative, standardized measures of touch discrimination available for clinical use are currently limited. We aimed to characterize touch impairment and re-establish the criterion of abnormality of the Tactile Discrimination Test (TDT) using pooled data and to determine the sensitivity and specificity of briefer test versions. Baseline data from stroke survivors (n = 207) and older neurologically healthy controls (n = 100) assessed on the TDT was extracted. Scores were re-analyzed to determine an updated criterion of impairment and the ability of brief test versions to detect impairment. Updated scoring using an area score was used to calculate the TDT percent maximum area (PMA) score. Touch impairment was common for the contralesional hand (83%) but also present in the ipsilesional hand (42%). The criterion of abnormality was established as 73.1 PMA across older adults and genders. High sensitivity and specificity were found for briefer versions of the TDT (25 vs. 50 trials; 12 or 15 vs. 25 trials), with sensitivity ranging between 91.8 and 96.4% and specificity between 72.5 and 95.0%. Conclusion: Updated criterion of abnormality and the high sensitivity and specificity of brief test versions support the use of the TDT in clinical practice settings.

Concomitant sensory stimulation during therapy to enhance hand functional recovery post stroke

Background

Post-stroke hand impairment is prevalent and persistent even after a full course of rehabilitation. Hand diminishes stroke survivors’ abilities for activities of daily living and independence. One way to improve treatment efficacy is to augment therapy with peripheral sensory stimulation. Recently, a novel sensory stimulation, TheraBracelet, has been developed in which imperceptible vibration is applied during task practice through a wrist-worn device. The objective of this trial is to determine if combining TheraBracelet with hand task practice is superior to hand task practice alone.

Methods

A double-blind randomized controlled trial will be used. Chronic stroke survivors will undergo a standardized hand task practice therapy program (3 days/week for 6 weeks) while wearing a device on the paretic wrist. The device will deliver TheraBracelet vibration for the treatment group and no vibration for the control group. The primary outcome is hand function measured by the Wolf Motor Function Test. Other outcomes include the Box and Block Test, Action Research Arm Test, upper extremity use in daily living, biomechanical measure of the sensorimotor grip control, and EEG-based neural communication.

Discussion

This research will determine clinical utility of TheraBracelet to guide future translation. The TheraBracelet stimulation is delivered via a wrist-worn device, does not interfere with hand motion, and can be easily integrated into clinical practice. Enhancing hand function should substantially increase stroke survivors' independence and quality of life and reduce caregiver burden.

Trial registration

NCT04569123. Registered on September 29, 2020

Objective Assessment of Progression and Disease Characterization of Friedreich Ataxia via an Instrumented Drinking Cup: Preliminary Results

The monitoring of disease progression in certain neurodegenerative conditions can significantly be quantified with the help of objective assessments. The severity assessment of diseases like Friedreich ataxia (FRDA) are usually based on different subjective measures. The ability of a participant with FRDA to perform standard neurological tests is the most common way of assessing disease progression. In this feasibility study, an Ataxia Instrumented Measurement-Cup (AIM-C) is proposed to quantify the disease progression of 10 participants (mean age 39 years, onset of disease 16.3 years) in longitudinal timepoints. The device consists of a sensing system with the provision of extracting both kinetic and kinematic information while engaging in an activity closely associated with activities of daily living (ADL). A common functional task of simulated drinking was used to capture features that possesses disease progression information as well as certain other features which intrinsically correlate with commonly used clinical scales such as the modified Friedreich Ataxia Rating Scale (mFARS), the Functional Staging of Ataxia score and the ADL scale. Frequency and time-frequency domain features allowed the longitudinal assessment of participants with FRDA. Furthermore, both kinetic and kinematic measures captured clinically relevant features and correlated 85% with clinical assessments.

Bradykinesia in motoneuron diseases

OBJECTIVE

Only few studies investigated voluntary movement abnormalities in patients with motoneuron diseases (MNDs) or their neurophysiological correlates. We aimed to kinematically assess finger tapping abnormalities in patients with amyotrophic lateral sclerosis (ALS) and primary lateral sclerosis (PLS), as compared to healthy controls (HCs), and their relationship with motoneuron involvement.

METHODS

Fourteen ALS and 5 PLS patients were enrolled. Finger tapping was assessed by a motion analysis system. Patients underwent a central motor conduction time assessment, a motor nerve conduction study, and needle electromyography. Data were compared to those of 79 HCs using non-parametric tests. Possible relationships between clinical, kinematic, and neurophysiological data were assessed in patients.

RESULTS

As a major finding, ALS and PLS patients performed finger tapping slower than HCs. In both conditions, movement slowness correlated with muscle strength. In ALS, movement slowness also correlated with the amplitude of the compound muscle action potential recorded from the muscles involved in the task and with denervation activity. No correlations were found between slowness, measures of upper motoneuron involvement, and other clinical and neurophysiological data.

CONCLUSIONS

This study provides novel information on voluntary movement abnormalities in MNDs.

SIGNIFICANCE

The results highlight the pathophysiological role of motoneurons in generating movement slowness.

Anticipatory and Reactive Grip Force Control in Patients with Alzheimer's Disease: A Pilot Study

BACKGROUND

Alzheimer's disease (AD) affects several cognitive functions and causes altered motor function. Fine motor deficits during object manipulation are evident in other neurological conditions, but have not been assessed in dementia patients yet.

OBJECTIVE

Investigate reactive and anticipatory grip force control in response to unexpected and expected load force perturbation in AD.

METHODS

Reactive and anticipatory grip force was investigated using a grip-device with force sensors. In this pilot study, fifteen AD patients and fourteen healthy controls performed a catching task. They held the device with one hand while a sandbag was dropped into an attached receptacle either by the experimenter or by the participant.

RESULTS

In contrast to studies of other neurological conditions, the majority of AD patients exerted lower static grip force levels than controls. Interestingly, patients who were slow in the Luria's three-step test produced normal grip forces. The timing and magnitude of reactive grip force control were largely preserved in patients. In contrast, timing and extent of anticipatory grip forces were impaired in patients, although anticipatory control was generally preserved. These deficits were correlated with decreasing Mini-Mental State Examination scores. Apraxia scores, assessed by pantomime of tool-use, did not correlate with performance in the catching task.

CONCLUSION

We interpreted the decreased grip force in AD in the context of loss of strength and lethargy, typical for patients with AD. The lower static grip force during object manipulation may emerge as a potential biomarker for early stages of AD, but more studies with larger sample sizes are necessary.

A robot goes to rehab: a novel gamified system for long-term stroke rehabilitation using a socially assistive robot-methodology and usability testing

Background

Socially assistive robots (SARs) have been proposed as a tool to help individuals who have had a stroke to perform their exercise during their rehabilitation process. Yet, to date, there are no data on the motivating benefit of SARs in a long-term interaction with post-stroke patients.

Methods

Here, we describe a robot-based gamified exercise platform, which we developed for long-term post-stroke rehabilitation. The platform uses the humanoid robot Pepper, and also has a computer-based configuration (with no robot). It includes seven gamified sets of exercises, which are based on functional tasks from the everyday life of the patients. The platform gives the patients instructions, as well as feedback on their performance, and can track their performance over time. We performed a long-term patient-usability study, where 24 post-stroke patients were randomly allocated to exercise with this platform—either with the robot or the computer configuration—over a 5–7 week period, 3 times per week, for a total of 306 sessions.

Results

The participants in both groups reported that this rehabilitation platform addressed their arm rehabilitation needs, and they expressed their desire to continue training with it even after the study ended. We found a trend for higher acceptance of the system by the participants in the robot group on all parameters; however, this difference was not significant. We found that system failures did not affect the long-term trust that users felt towards the system.

Conclusions

We demonstrated the usability of using this platform for a long-term rehabilitation with post-stroke patients in a clinical setting. We found high levels of acceptance of both platform configurations by patients following this interaction, with higher ratings given to the SAR configuration. We show that it is not the mere use of technology that increases the motivation of the person to practice, but rather it is the appreciation of the technology’s effectiveness and its perceived contribution to the rehabilitation process. In addition, we provide a list of guidelines that can be used when designing and implementing other technological tools for rehabilitation.

Trial registration: This trial is registered in the NIH ClinicalTrials.gov database. Registration number NCT03651063, registration date 21.08.2018. https://clinicaltrials.gov/ct2/show/NCT03651063.

Quantitative Assessment of Friedreich Ataxia via Self-Drinking Activity

Effective monitoring of the progression of neurodegenerative conditions can be significantly improved by objective assessments. Clinical assessments of conditions such as Friedreich's Ataxia (FA), currently rely on subjective measures commonly practiced in clinics as well as the ability of the affected individual to perform conventional tests of the neurological examination. In this study, we propose an ataxia measuring device, in the form of a pressure canister capable of sensing certain kinetic and kinematic parameters of interest to quantify the impairment levels of participants particularly when engaged in an activity that is closely associated with daily living. In particular, the functional task of simulated drinking was utilised to capture characteristic features of disability manifestation in terms of diagnosis (separation of individuals with FA and controls) and severity assessment of individuals diagnosed with the debilitating condition of FA. Time and frequency domain analysis of these biomarkers enabled the classification of individuals with FA and control subjects to reach an accuracy of 98% and a correlation level reaching 96% with the clinical scores.

Grip force anticipation of nonlinear, underactuated load force

Feedforward internal model-based control enabled by efference copies of motor commands is the prevailing theoretical account of motor anticipation. Grip force control during object manipulation-a paradigmatic example of motor anticipation-is a key line of evidence for that account. However, the internal model approach has not addressed the computational challenges faced by the act of manipulating mechanically complex objects with nonlinear, underactuated degrees of freedom. These objects exhibit complex and unpredictable load force dynamics which cannot be encoded by efference copies of underlying motor commands, leading to the prediction from the perspective of an efference copy-enabled feedforward control scheme that grip force should either lag or fail to coordinate with changes in load force. In contrast to that prediction, we found evidence for strong, precise, anticipatory grip force control during manipulations of a complex object. The results are therefore inconsistent with the internal forward model approach and suggest that efference copies of motor commands are not necessary to enable anticipatory control during active object manipulation.NEW & NOTEWORTHY From the perspective of feedforward internal model-based control, precise, anticipatory grip force (GF) control when manipulating a complex object should not be possible as the object's changing load forces (LFs) cannot be encoded by efference copies of the underlying movements. However, we observed that GF exhibited strong, precise, anticipatory coupling with LF during extended manipulations of a complex object. These findings suggest that an alternative theoretical framework is needed to account for anticipatory GF control.

Grip force control during object manipulation in cervical myelopathy

STUDY DESIGN

Cross-sectional research.

OBJECTIVES

To objectively evaluate grip force (GF) control while holding a freely movable object in individuals with cervical myelopathy (CM).

SETTING

Harunaso Hospital, Takasaki, Japan.

METHODS

We studied 52 hands from 26 individuals with CM. Participants performed a grip-and-lift task by pulp pinch using the thumb and index finger before surgery. We monitored individual finger GF (N) during the first 3 s while lifting and holding an object. Correlations between the GF and other clinical tests were evaluated. A multiple stepwise regression analysis was used to examine the contribution of the GF to the severity of clinical symptoms.

RESULTS

Thumb GF was negatively correlated with the 10-s test (rs = -0.32), and index finger GF was positively correlated with its cutaneous pressure threshold (rs = 0.34). Multiple regression for the severity of upper extremity symptoms revealed that the model including the GF had a larger adjusted R² and a lower AIC value than that of conventionally used clinical tests.

CONCLUSIONS

These results suggested that the assessment of individual finger GF control could provide an indicator of the clinical severity of upper extremity in individuals with CM.

Effect of Sensory Deprivation on Maximal Force Abilities from Local to Non-local Digits

The present study investigates the effect of sensory deprivation of the index and middle finger on motor function of all digits during maximal voluntary force production tasks. A total of 27 subjects performed maximal isometric pressing tasks by using different instructed finger combinations. Subjects completed the same tasks in two visits: a control visit when they had normal sensory feedback in all fingers, and an anesthesia visit when digital nerve blocks were performed on their right index and middle fingers. We evaluated three aspects of motor adaptation on both local (anesthetized) and non-local (non-anesthetized) digits during maximal force production: (1) task-relevant and overall force magnitude, (2) force directional application, and (3) digital individuation and force sharing. Our results indicate that selective digital anesthesia resulted in decreased maximal force magnitude, changed direction of force production, and significant changes extended to non-local digits. The motor weakness and inefficiency revealed in the non-local digits implies that sensory information from each digit can be shared across the digits to assist motor execution within the same hand.

The impact of hand proportions on tool grip abilities in humans, great apes and fossil hominins: A biomechanical analysis using musculoskeletal simulation

Differences in grip techniques used across primates are usually attributed to variation in thumb-finger proportions and muscular anatomy of the hand. However, this cause-effect relationship is not fully understood because little is known about the biomechanical functioning and mechanical loads (e.g., muscle or joint forces) of the non-human primate hand compared to that of humans during object manipulation. This study aims to understand the importance of hand proportions on the use of different grip strategies used by humans, extant great apes (bonobos, gorillas and orangutans) and, potentially, fossil hominins (Homo naledi and Australopithecus sediba) using a musculoskeletal model of the hand. Results show that certain grips are more challenging for some species, particularly orangutans, than others, such that they require stronger muscle forces for a given range of motion. Assuming a human-like range of motion at each hand joint, simulation results show that H. naledi and A. sediba had the biomechanical potential to use the grip techniques considered important for stone tool-related behaviors in humans. These musculoskeletal simulation results shed light on the functional consequences of the different hand proportions among extant and extinct hominids and the different manipulative abilities found in humans and great apes.

Increased center of pressure trajectory of the finger during precision grip task in stroke patients

The aim of this study was to assess the spatial stability of stroke patients while holding a freely movable object. Twenty-two acute stroke patients with mild hand impairment performed a grip and lift task using the thumb and index finger. The displacement of the center of pressure (COP) trajectory, the grip force (GF) and several clinical parameters were monitored. Although the GF was not different between paretic and nonparetic hands, the COP trajectory of the paretic index finger was increased. Moreover, the COP trajectories of the thumb and index finger in hemorrhagic patients were longer than those in ischemic patients. These discrepancies between kinetic parameters suggest that different aspects of grip force control may be considered in patients with mild stroke.

Effects of the Surface Texture and Weight of a Pinch Apparatus on the Reliability and Validity of a Hand Sensorimotor Control Assessment

OBJECTIVES

To investigate the reliability and validity of a modified pinch apparatus devised with 3 surface textures and 2 different weights for clinical application.

DESIGN

Case-controlled study.

SETTING

A university hospital.

PARTICIPANTS

The participants (N=32) included carpal tunnel syndrome (CTS) patients (n=16) with 20 sensory neuropathy hands, and an equal number of age-sex matched volunteers without CTS, as well as young volunteers without CTS (n=16 with 20 hands) used to analyze both the testing validity and reliability of the modified device.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The Semmes-Weinstein monofilament (SWM) and two-point discrimination (2PD) tests were conducted, and the force ratio between the FP_peak (peak pinch force during lifting phase) and FL_max (maximum load force at maximum upward acceleration onset) detected from a pinch-holding-up activity (PHUA) under various testing conditions was obtained.

RESULTS

The range of the intraclass correlation coefficient of this pinch device was 0.369-0.952. The CTS patients exhibited poorer force modulation ability according to the inertial change in a dynamic lifting task when compared to the controls under all testing conditions (P<.001). The area under the receiver operating characteristic force ratio curve was 0.841, revealing high accuracy of the test for diagnosing CTS neuropathic hands under the testing condition in which the 125-g coarse texture device was used. In addition, the weight factor was shown to have significant effects on the sensitivity and accuracy of the PHUA assessment.

CONCLUSIONS

This study showed that the PHUA test via the modified pinch apparatus is a sensitive tool that can be used in clinical practice for detecting neuropathic CTS hands. In addition, changing the weight of the pinch device has a significant effect on the sensitivity and accuracy of the PHUA assessment.

Effects of Somatosensory Impairment on Participation After Stroke

OBJECTIVE

Our objective was to determine the effect of loss of body sensation on activity participation in stroke survivors.

METHOD

Participants (N = 268) were assessed at hospital admission for somatosensory and motor impairment using the National Institutes of Health Stroke Scale. Participation was assessed using the Activity Card Sort (ACS) in the postacute phase. Between-group differences in activity participation were analyzed for participants with and without somatosensory impairment and with or without paresis.

RESULTS

Somatosensory impairment was experienced in 33.6% of the sample and paresis in 42.9%. ACS profiles were obtained at a median of 222 days poststroke. Somatosensory loss alone (z = 1.96, p = .048) and paresis in upper and lower limbs without sensory loss (z = 4.62, p < .001) influenced activity participation.

CONCLUSION

Somatosensory impairment is associated with reduced activity participation; however, paresis of upper and lower limbs can mask the contribution of sensory loss.

Testing rTMS-Induced Neuroplasticity: A Single Case Study of Focal Hand Dystonia

Focal hand dystonia in musicians is a neurological motor disorder in which aberrant plasticity is caused by excessive repetitive use. This work's purposes were to induce plasticity changes in a dystonic musician through five daily thirty-minute sessions of 1 Hz repetitive transcranial magnetic stimulation (rTMS) applied to the left M1 by using neuronavigated stimulation and to reliably measure the effect of these changes. To this aim, the relationship between neuroplasticity changes and motor recovery was investigated using fine-grained kinematic analysis. Our results suggest a statistically significant improvement in motor coordination both in a task resembling the dystonic-inducing symptoms and in a reach-to-grasp task. This single case study supports the safe and effective use of noninvasive brain stimulation in neurologic patients and highlights the importance of evaluating outcomes in measurable ways. This issue is a key aspect to focus on to classify the clinical expression of dystonia. These preliminary results promote the adoption of kinematic analysis as a valuable diagnostic tool.

A preliminary study on characterisation of finger interface kinetics using a pressure and shear sensor system

BACKGROUND

Our hands constantly handle objects throughout our lives, where a crucial component of this interaction is the detection of grasping (pressure) and slipping (shear) of the object. While there have been a large amount of studies using pressure sensors for grasping detection, synchronised pressure and shear detection at the finger/object interface is still needed.

OBJECTIVES

This study aims to assess the feasibility of a sensor system designed to detect both pressure and shear at the fingertip/object interface via a single subject test.

STUDY DESIGN

Descriptive study, proof of concept.

METHODS

One healthy subject participated in the study and was asked to perform a single finger test protocol and a simple hand test protocol. The corresponding multidirectional loads at the fingertip/object interface were measured in real time using a pressure and shear sensor system.

RESULTS

Results from the finger test protocol show peak values of up to approximately 50 kPa (5 N) and 30 kPa (3 N) of pressure for each test, respectively. Results from the hand test protocol show a pressure and shear profile that shows a large increase in grip force during the initial grasping of the object, with a peak pressure of approximately 50 kPa (5 N). The pressure and shear profile demonstrates that the load is not evenly distributed across all digits.

CONCLUSION

This study provides evidence that the reported sensor system has sufficient resolution, dynamic response and load capability to capture biomechanical information during basic protocols and hand-grasping tasks. Clinical relevance The presented sensor system could be potentially used as a tool for measuring and evaluating hand function and could be incorporated into a prosthetic hand as a tactile feedback system.

Quantitative Assessment of Hand Dysfunction in Patients with Early Parkinson's Disease and Focal Hand Dystonia

Objective

Motor impairments related to hand function are common symptoms in patients with movement disorders, such as Parkinson’s disease (PD) and focal hand dystonia (FHD). However, hand dysfunction has not been quantitatively assessed as a clinical tool for screening patient groups from healthy controls (HCs). The aim of our study was 1) to quantitatively assess hand dysfunction in patients with PD and FHD and its usefulness as a screening tool 2) to grade disease severity in PD and FHD based on hand dysfunction.

Methods

The current case-control study included HCs (n = 50) and patients with known history of PD (n = 25) or FHD (n = 16). Hand function was assessed by a precision grip task while participants lifted objects of 1.3 N and 1.7 N under dry skin conditions, followed by very wet skin conditions (VWSCs). Receiver operating characteristic and summative scoring analyses were performed.

Results

In PD, the combination of loading phase duration and lifting phase duration at quantitative cutoffs of 0.36 and 0.74 seconds identified 21/25 patients as diseased and 49/50 subjects as HCs with 1.7 N under VWSCs. In PD, 5/21 was graded as “mild” and 16/21 as “moderate cases.” In FHD, slip force at a cutoff of 1.2 N identified 13/16 patients as diseased and 41/50 subjects as HC with 1.7 N under VWSCs, but disease severity could not be graded.

Conclusion

Our results demonstrate the use of precision grip task as an important clinical tool in assessment of hand dysfunction in movement disorder patients. Use of quantitative cutoffs may improve diagnostic accuracy and serve as a valuable adjunct to existing clinical assessment methods.

Effects of cervical manipulation on pain, grip force control, and upper extremity muscle activity: a randomized controlled trial

Objectives

Individuals with neck pain experience disrupted grip force control when performing manipulative tasks. Manipulative physical therapy might decrease pain and change the activity of surrounding muscles; however, its effect on upper limb motor control remains undetermined. This study aims to analyze the effects of cervical manipulation on pressure pain threshold (PPT), upper extremity muscle activity along with grip force control in individuals with neck pain.

Methods

Thirty subjects with neck pain were instructed to grasp and lift an object before and after cervical (n = 15) or sham (n = 15) manipulation. The patients' PPT, electromyographic (EMG) activity of the upper extremity/scapular muscles, and grip force control were analyzed before and after one session of manipulation.

Results

No significant differences were found in the grip force control, PPT and EMG activity variables between groups.

Discussion

These results suggest that a single session of cervical manipulation may not modify upper limb motor control, more specifically grip force control and EMG activity, in patients with cervical pain. Future studies should investigate potential changes in grip force control in patients with different features of neck pain and/or by applying long-term treatment.

Level of Evidence

1b.

Effects of transcranial direct current stimulation on grip force control in patients with cerebellar degeneration

BACKGROUND

The control of grip forces when moving a hand held object is impaired in patients with cerebellar degeneration. We asked the question whether after-effects of anodal transcranial direct current stimulation (tDCS) applied to the lateral cerebellum or M1 improved grip force control in cerebellar patients.

METHODS

Grip force control while holding an object during cyclic arm movements was assessed in patients with pure cerebellar degeneration (n = 14, mean age 50.2 years ± SD 8.8 years) and age- and sex-matched control participants (n = 14, mean age 50.7 years ± SD 9.8 years). All subjects were tested before and after application of tDCS (2 mA, 22 min) in a within-subject design. Each subject received anodal tDCS applied to the cerebellum, anodal tDCS applied to M1 or sham-stimulation with a break of 1 week between the three experimental sessions.

RESULTS

There were no clear after-effects of tDCS on grip force control neither in control participants nor in cerebellar patients. Cerebellar patients showed typical impairments with higher grip forces, a higher variability of movements.

CONCLUSION

In the present study, deficits in grip force control were neither improved by tDCS applied over the cerebellum nor M1 in cerebellar degeneration.

Pathophysiology of writer's cramp: an exploratory study on task-specificity and non-motor symptoms using an extended fine-motor testing battery

BACKGROUND

Writer's cramp (WC) is a task-specific focal dystonia which manifests itself as abnormal postures interfering with motor performance. As the spread of motor symptoms remains controversial and non-motor symptoms are widely discussed, in this exploratory study, we explore the pathophysiology of WC, focusing on task-specificity and the psychological profiles of WC patients.

METHODS

In 14 right-handed WC patients and matched controls, we assessed motor control by applying motor performance tests (Vienna Test Series), as well as using writing analysis and grip-force measurements. Moreover, detailed psychological factors were assessed. Classification trees were used to distinguish patients from controls.

RESULTS

The total duration of writing and the vertical writing frequency of the pen are the most important variables to split the data set successfully into patients and controls. No other variables concerning motor performance tests, grip-force measurements or psychological factors correctly separated patients and controls.

CONCLUSIONS

Only variables from the writing tasks successfully separated patients and controls, indicating a strong task-specificity of WC in our patient group. Future research should be performed with larger samples of untreated WC patients in early stages of impairment, without any secondary motor disturbances, to verify our findings.

Influence of force feedback on grasp force modulation in prosthetic applications: a preliminary study

In typical movement, humans use a combination of feed-forward and feedback motor control strategies to interact with the world around them. However, when sensory input is impaired or absent, as in the case of various neuropathies or amputation, the ability to perform everyday tasks, like modulating grip force to object weight, can be affected. In this study, we show the results of a preliminary study using a pressure cuff-like force feedback device (CUFF) with the SoftHand Pro (SHP) prosthetic hand. Subjects lifted an object of various weights using their own hand, with the SHP without feedback, and the SHP with force feedback. As expected, significant differences were found between the two SHP conditions and the native hand, but surprisingly not between the SHP conditions. A closer look at the data suggests the feedback may help diminish the overall grip force used during grasping even if it does not alter the grip force modulation to object weight. The lack of significance may be due in part to high intra- and inter-subject variability. Additional training with the CUFF and/or customization of the feedback may enhance the effects and warrants further study.

The biology of skin wetness perception and its implications in manual function and for reproducing complex somatosensory signals in neuroprosthetics

Our perception of skin wetness is generated readily, yet humans have no known receptor (hygroreceptor) to signal this directly. It is easy to imagine the sensation of water running over our hands or the feel of rain on our skin. The synthetic sensation of wetness is thought to be produced from a combination of specific skin thermal and tactile inputs, registered through thermoreceptors and mechanoreceptors, respectively. The present review explores how thermal and tactile afference from the periphery can generate the percept of wetness centrally. We propose that the main signals include information about skin cooling, signaled primarily by thinly myelinated thermoreceptors, and rapid changes in touch, through fast-conducting, myelinated mechanoreceptors. Potential central sites for integration of these signals, and thus the perception of skin wetness, include the primary and secondary somatosensory cortices and the insula cortex. The interactions underlying these processes can also be modeled to aid in understanding and engineering the mechanisms. Furthermore, we discuss the role that sensing wetness could play in precision grip and the dexterous manipulation of objects. We expand on these lines of inquiry to the application of the knowledge in designing and creating skin sensory feedback in prosthetics. The addition of real-time, complex sensory signals would mark a significant advance in the use and incorporation of prosthetic body parts for amputees in everyday life.NEW & NOTEWORTHY Little is known about the underlying mechanisms that generate the perception of skin wetness. Humans have no specific hygroreceptor, and thus temperature and touch information combine to produce wetness sensations. The present review covers the potential mechanisms leading to the perception of wetness, both peripherally and centrally, along with their implications for manual function. These insights are relevant to inform the design of neuroengineering interfaces, such as sensory prostheses for amputees.

Processing of probabilistic information in weight perception and motor prediction

We studied the effects of probabilistic cues, i.e., of information of limited certainty, in the context of an action task (GL: grip-lift) and of a perceptual task (WP: weight perception). Normal subjects (n = 22) saw four different probabilistic visual cues, each of which announced the likely weight of an object. In the GL task, the object was grasped and lifted with a pinch grip, and the peak force rates indicated that the grip and load forces were scaled predictively according to the probabilistic information. The WP task provided the expected heaviness related to each probabilistic cue; the participants gradually adjusted the object's weight until its heaviness matched the expected weight for a given cue. Subjects were randomly assigned to two groups: one started with the GL task and the other one with the WP task. The four different probabilistic cues influenced weight adjustments in the WP task and peak force rates in the GL task in a similar manner. The interpretation and utilization of the probabilistic information was critically influenced by the initial task. Participants who started with the WP task classified the four probabilistic cues into four distinct categories and applied these categories to the subsequent GL task. On the other side, participants who started with the GL task applied three distinct categories to the four cues and retained this classification in the following WP task. The initial strategy, once established, determined the way how the probabilistic information was interpreted and implemented.

Simultaneous assessment of hand function and neuromuscular quickness through a static object manipulation task in healthy adults

Both hand function [as seen through the coordination between grip force (GF) and load force (LF)] and the ability to produce a submaximal force quickly (i.e., neuromuscular quickness) are two important qualities of motor function that could be seriously affected by the presence of neurological diseases. Therefore, their quantitative assessment is very important in clinical settings. Within this study, we aimed to develop, standardize, and measure the within-session reliability of a clinically meaningful test that assesses both hand function and neuromuscular quickness simultaneously. Thirteen healthy young adults produced around 90 rapid isometric LF pulses to varying submaximal magnitudes by either pulling down or pulling up on an externally fixed GF- and LF-measuring device. Results revealed high indices of force coordination (i.e., GF scaling as assessed by GF/LF and GF coupling as assessed by maximum cross-correlation between GF and LF) in both force directions, while GF coupling was higher in downward than in upward direction (p < 0.001). Regarding the indices of neuromuscular quickness (i.e., the regression parameters obtained from the relationship between peak force and it's rate of development and half-relaxation time), results, in general, revealed a higher slope (named as rate of force development scaling factor; p < 0.01), similar R ² (p > 0.05), and shorter half-relaxation time (p < 0.05) for LF than for GF. Furthermore, all of the selected variables showed moderate to excellent within-session reliability with only 45 pulses. Findings suggest that brief force production tasks should be further evaluated as clinical tests of hand function and neuromuscular quickness in various populations.

Fast-adapting mechanoreceptors are important for force control in precision grip but not for sensorimotor memory

Sensory feedback from cutaneous mechanoreceptors in the fingertips is important in effective object manipulation, allowing appropriate scaling of grip and load forces during precision grip. However, the role of mechanoreceptor subtypes in these tasks remains incompletely understood. To address this issue, psychophysical tasks that may specifically assess function of type I fast-adapting (FAI) and slowly adapting (SAI) mechanoreceptors were used with object manipulation experiments to examine the regulation of grip force control in an experimental model of graded reduction in tactile sensitivity (healthy volunteers wearing 2 layers of latex gloves). With gloves, tactile sensitivity decreased significantly from 1.9 ± 0.4 to 12.3 ± 2.2 μm in the Bumps task assessing function of FAI afferents but not in a grating orientation task assessing SAI afferents (1.6 ± 0.1 to 1.8 ± 0.2 mm). Six axis force/torque sensors measured peak grip (PGF) and load (PLF) forces generated by the fingertips during a grip-lift task. With gloves there was a significant increase of PGF (14 ± 6%), PLF (17 ± 5%), and grip and load force rates (26 ± 8%, 20 ± 8%). A variable-weight series task was used to examine sensorimotor memory. There was a 20% increase in PGF when the lift of a light object was preceded by a heavy relative to a light object. This relationship was not significantly altered when lifting with gloves, suggesting that the addition of gloves did not change sensorimotor memory effects. We conclude that FAI fibers may be important for the online force scaling but not for the buildup of a sensorimotor memory.

Adaptive grip force is modulated by subthalamic beta activity in Parkinson's disease patients

Introduction

Healthy subjects scale grip force to match the load defined by physical object properties such as weight, or dynamic properties such as inertia. Patients with Parkinson's disease (PD) show an elevated grip force in dynamic object handling, but temporal aspects of anticipatory grip force control are relatively preserved. In PD patients, beta frequency oscillatory activity in the basal ganglia is suppressed prior to externally paced movements. However, the role of the subthalamic nucleus (STN) in anticipatory grip force control is not known.

Methods

After implantation of deep brain stimulation (DBS) electrodes in the STN, PD patients performed adaptive and voluntary grip force tasks, while we recorded subthalamic local field potentials (LFP) and scalp EEG.

Results

During adaptive grip force control (Shake), we found event related desynchronization (ERD) in the beta frequency band, which was time-locked to the grip force. In contrast, during voluntary grip force control (Press) we recorded a biphasic ERD, corresponding to peak grip force and grip force release. Beta synchronization between STN and cortical EEG was reduced during adaptive grip force control.

Conclusion

The time-locked suppression of beta oscillatory activity in the STN is in line with previous reports of beta ERD prior to voluntary movements. Our results show that the STN is involved in anticipatory grip force control in PD patients. The difference in the phasic beta ERD between the two tasks and the reduction of cortico-subthalamic synchronization suggests that qualitatively different neuronal network states are involved in different grip force control tasks.

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Subthalamic nucleus is involved in anticipatory grip force control in PD patients.

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Subthalamic beta activity is the first derivative of grip force in a shaking task.

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Synchronization between STN and cortex is reduced during the shaking task.

A novel method for the quantification of key components of manual dexterity after stroke

BACKGROUND

A high degree of manual dexterity is a central feature of the human upper limb. A rich interplay of sensory and motor components in the hand and fingers allows for independent control of fingers in terms of timing, kinematics and force. Stroke often leads to impaired hand function and decreased manual dexterity, limiting activities of daily living and impacting quality of life. Clinically, there is a lack of quantitative multi-dimensional measures of manual dexterity. We therefore developed the Finger Force Manipulandum (FFM), which allows quantification of key components of manual dexterity. The purpose of this study was (i) to test the feasibility of using the FFM to measure key components of manual dexterity in hemiparetic stroke patients, (ii) to compare differences in dexterity components between stroke patients and controls, and (iii) to describe individual profiles of dexterity components in stroke patients.

METHODS

10 stroke patients with mild-to-moderate hemiparesis and 10 healthy subjects were recruited. Clinical measures of hand function included the Action Research Arm Test and the Moberg Pick-Up Test. Four FFM tasks were used: (1) Finger Force Tracking to measure force control, (2) Sequential Finger Tapping to measure the ability to perform motor sequences, (3) Single Finger Tapping to measure timing effects, and (4) Multi-Finger Tapping to measure the ability to selectively move fingers in specified combinations (independence of finger movements).

RESULTS

Most stroke patients could perform the tracking task, as well as the single and multi-finger tapping tasks. However, only four patients performed the sequence task. Patients showed less accurate force control, reduced tapping rate, and reduced independence of finger movements compared to controls. Unwanted (erroneous) finger taps and overflow to non-tapping fingers were increased in patients. Dexterity components were not systematically related among each other, resulting in individually different profiles of deficient dexterity. Some of the FFM measures correlated with clinical scores.

CONCLUSIONS

Quantifying some of the key components of manual dexterity with the FFM is feasible in moderately affected hemiparetic patients. The FFM can detect group differences and individual profiles of deficient dexterity. The FFM is a promising tool for the measurement of key components of manual dexterity after stroke and could allow improved targeting of motor rehabilitation.

The extent of altered digit force direction correlates with clinical upper extremity impairment in chronic stroke survivors

Many stroke survivors suffer from impaired hand function. Biomechanics of hand grip suggests that abnormally directed grip force can hamper gripping abilities and hand function. This study examined the relation between the ability to precisely direct fingertip force and clinical hand function scores among individuals affected by stroke. Specifically, clinical hand function tests of the Fugl-Meyer, Chedoke McMaster, and Box and Block Test were used, since they involve various hand movements required for activities of daily living. Digit force direction during static grip was recorded using multiaxial load cells. Data for 59 chronic stroke survivors were analyzed. We found that larger angular deviation of digit force from the normal direction was significantly associated with lower hand functional levels (p<.001 for all three clinical tests). Particularly, stroke survivors whose digit force deviated more than 21° from the normal direction could not achieve the normal level of Fugl-Meyer or Chedoke or move more than 4 blocks in a minute. The biomechanics of the way digit force direction affects hand grip function is described. In addition, underlying mechanisms for altered digit force direction post stroke are postulated, including impaired somatosensation and abnormal neural input to muscles. In summary, this study identifies a new biomechanical marker for hand functional level and recovery. Future interventions may focus on correcting digit force direction to improve hand functional outcome.

Computerised pinch dynamometry in the assessment of adult hand spasticity

BACKGROUND/AIM

The hand engages with the environment through the grasp, stabilisation, manipulation and release of objects during everyday tasks, activities and routines. Upper motor neuron syndrome following acquired brain injury may negatively impact hand function, reducing strength, range of motion and motor control. It is important for clinicians to reliably measure such impacts, particularly for the impact of intervention and to monitor change in performance over time. Therefore, the aim of this study was to investigate the test-retest reliability and construct validity of Dynamic Computerised pinch Dynamometry for measuring fine hand motor performance following acquired brain injury.

METHODS

The Dynamic Computerised pinch Dynamometry protocol was completed by 36 community dwelling adults and 27 healthy adults using a simulated pinch and release task in lateral and pincer grip positions. Measurements were conducted over two testing occasions approximately five weeks apart. Dynamic Computerised pinch Dynamometry output was evaluated to determine the test-retest reliability and construct validity of the measure.

RESULTS

Test-retest reliability scores using Kendall coefficient of concordance ranged from W = 0.61-0.94. Dynamic Computerised pinch Dynamometry discriminated between participants with and without acquired brain injury (z = 4.97-6.50, P < 0.05) and between the affected and non-affected hand of participants with acquired brain injury (z = 3.37-5.22, P < 0.001).

CONCLUSIONS

Dynamic Computerised pinch Dynamometry in both lateral and pincer positions had fair to excellent test-retest reliability, and had good construct validity for discrimination between participants with and without acquired brain injury as well as between the affected and non-affected hand of participants with acquired brain injury.

Motor output variability, deafferentation, and putative deficits in kinesthetic reafference in Parkinson's disease

Parkinson’s disease (PD) is a neurodegenerative disorder defined by motor impairments that include rigidity, systemic slowdown of movement (bradykinesia), postural problems, and tremor. While the progressive decline in motor output functions is well documented, less understood are impairments linked to the continuous kinesthetic sensation emerging from the flow of motions. There is growing evidence in recent years that kinesthetic problems are also part of the symptoms of PD, but objective methods to readily quantify continuously unfolding motions across different contexts have been lacking. Here we present evidence from a deafferented subject (IW) and a new statistical platform that enables new analyses of motor output variability measured as a continuous flow of kinesthetic reafferent input. Systematic increasing similarities between the patterns of motor output variability in IW and the participants with increasing degrees of PD severity suggest potential deficits in kinesthetic sensing in PD. We propose that these deficits may result from persistent, noisy, and random motor patterns as the disorder progresses. The stochastic signatures from the unfolding motions revealed levels of noise in the motor output fluctuations of these patients bound to decrease the kinesthetic signal’s bandwidth. The results are interpreted in light of the concept of kinesthetic reafference ( Von Holst and Mittelstaedt, 1950). In this context, noisy motor output variability from voluntary movements in PD leads to a returning stream of noisy afference caused, in turn, by those faulty movements themselves. Faulty efferent output re-enters the CNS as corrupted sensory motor input. We find here that severity level in PD leads to the persistence of such patterns, thus bringing the statistical signatures of the subjects with PD systematically closer to those of the subject without proprioception.

A method to study precision grip control in viscoelastic force fields using a robotic gripper

Instrumented objects and multipurpose haptic displays have commonly been used to investigate sensorimotor control of grasping and manipulation. A major limitation of these devices, however, is the extent to which the experimenter can vary the interaction dynamics to fully probe sensorimotor control mechanisms. We propose a novel method to study precision grip control using a grounded robotic gripper with two moving, mechanically coupled finger pads instrumented with force sensors. The device is capable of stably rendering virtual mechanical properties with a wide dynamic range of achievable impedances. Eight viscoelastic force fields with different combinations of stiffness and damping parameters were implemented, and tested on eight healthy subjects performing 30 consecutive repetitions of a grasp, hold, and release task with time and position constraints. Rates of thumb and finger force were found to be highly correlated (r>0.9) during grasping, revealing that, despite the mechanical coupling of the two finger pads, subjects performed grasping movements in a physiological fashion. Subjects quickly adapted to the virtual dynamics (within seven trials), but, depending on the presented force field condition, used different control strategies to correctly perform the task. The proof of principle presented in this paper underscores the potential of such a one-degree-of-freedom robotic gripper to study neural control of grasping, and to provide novel insights on sensorimotor control mechanisms.

Impact of distal median neuropathy on handwriting performance for patients with carpal tunnel syndrome in office and administrative support occupations

PURPOSE

This study investigates the handwriting performance of patients with carpal tunnel syndrome (CTS) and healthy controls in office and administrative support occupations, adopting both biomechanical and functional perspectives. This work also explores how surgical intervention altered the performance of the CTS patients.

METHODS

Fourteen CTS patients and 14 control subjects were recruited to complete a self-reported survey and participate in sensory tests, hand strength, dexterity and handwriting tasks using a custom force acquisition pen along with motion capture technology. Based on the results of these, the sensory measurements, along with functional and biomechanical parameters, were used to determine the differences between the groups and also reveal any improvements that occurred in the CTS group after surgical intervention.

RESULTS

The CTS patients showed significantly poorer hand sensibility and dexterity than the controls, as well as excessive force exertion of the digits and pen tip, and less efficient force adjustment ability during handwriting. After surgery and sensory recovery, the hand dexterity and pen tip force of the CTS patients improved significantly. The force adjustment abilities of the digits also increased, but these changes were not statistically significant.

CONCLUSIONS

This study provides the objective measurements and novel apparatus that can be used to determine impairments in the handwriting abilities of office or administrative workers with CTS. The results can also help clinicians or patients to better understand the sensory-related deficits in sensorimotor control of the hand related to CTS, and thus develop and implement more suitable training or adaptive protocols.

Carpal tunnel syndrome impairs sustained precision pinch performance

OBJECTIVE

The purpose of this study was to investigate effects of carpal tunnel syndrome (CTS) on digit force control during a sustained precision pinch.

METHODS

Eleven CTS individuals and 11 age- and gender-matched healthy volunteers participated in the study. The subjects were instructed to isometrically pinch an instrumented apparatus for 60s with a stable force output. Visual feedback of force output was provided for the first 30s but removed for the remaining 30s. Pinch forces were examined for accuracy, variability, and inter-digit correlation.

RESULTS

CTS led to a decrease in force accuracy and an increase in amount of force variability, particularly without visual feedback (p<0.001). However, CTS did not affect the structure of force variability or force correlation between digits (p>0.05). The force of the thumb was less accurate and more variable than that of the index finger for both the CTS and healthy groups (p<0.001).

CONCLUSIONS

Sensorimotor deficits associated with CTS lead to inaccurate and unstable digit forces during sustained precision pinch.

SIGNIFICANCE

This study shed light on basic and pathophysiological mechanisms of fine motor control and aids in development of new strategies for diagnosis and evaluation of CTS.

How the impact of median neuropathy on sensorimotor control capability of hands for diabetes: an achievable assessment from functional perspectives

To comprehend the sensorimotor control ability in diabetic hands, this study investigated the sensation, motor function and precision pinch performances derived from a pinch-holding-up activity (PHUA) test of the hands of diabetic patients and healthy subjects. The precision, sensitivity and specificity of the PHUA test in the measurements of diabetic patients were also analyzed. We hypothesized that the diabetic hands would have impacts on the sensorimotor functions of the hand performances under functionally quantitative measurements. One hundred and fifty-nine patients with clinically defined diabetes mellitus (DM) and 95 age- and gender-matched healthy controls were included. Semmes-Weinstein monofilament (SWM), static and moving two-point discrimination (S2PD and M2PD), maximal pinch strength and precision pinch performance tests were conducted to evaluate the sensation, motor and sensorimotor status of the recruited hands. The results showed that there were significant differences (all p<0.05) in SWM, S2PD, M2PD and maximum pinch strength between the DM and control groups. A higher force ratio in the DM patients than in the controls (p<0.001) revealed a poor ability of pinch force adjustment in the DM patients. The percentage of maximal pinch strength was also significantly different (p<0.001) between the DM and control groups. The sensitivity, specificity and area under the receiver operating characteristic curve were 0.85, 0.51, and 0.724, respectively, for the PHUA test. Statistically significant degradations in sensory and motor functions and sensorimotor control ability were observed in the hands of the diabetic patients. The PHUA test could be feasibly used as a clinical tool to determine the sensorimotor function of the hands of diabetic patients from a functional perspective.

Excessive Sensitivity to Uncertain Visual Input in L-DOPA-Induced Dyskinesias in Parkinson's Disease: Further Implications for Cerebellar Involvement

When faced with visual uncertainty during motor performance, humans rely more on predictive forward models and proprioception and attribute lesser importance to the ambiguous visual feedback. Though disrupted predictive control is typical of patients with cerebellar disease, sensorimotor deficits associated with the involuntary and often unconscious nature of l-DOPA-induced dyskinesias in Parkinson’s disease (PD) suggests dyskinetic subjects may also demonstrate impaired predictive motor control.

Methods: We investigated the motor performance of 9 dyskinetic and 10 non-dyskinetic PD subjects on and off l-DOPA, and of 10 age-matched control subjects, during a large-amplitude, overlearned, visually guided tracking task. Ambiguous visual feedback was introduced by adding “jitter” to a moving target that followed a Lissajous pattern. Root mean square (RMS) tracking error was calculated, and ANOVA, robust multivariate linear regression, and linear dynamical system analyses were used to determine the contribution of speed and ambiguity to tracking performance.

Results: Increasing target ambiguity and speed contributed significantly more to the RMS error of dyskinetic subjects off medication. l-DOPA improved the RMS tracking performance of both PD groups. At higher speeds, controls and PDs without dyskinesia were able to effectively de-weight ambiguous visual information.

Conclusion: PDs’ visually guided motor performance degrades with visual jitter and speed of movement to a greater degree compared to age-matched controls. However, there are fundamental differences in PDs with and without dyskinesia: subjects without dyskinesia are generally slow, and less responsive to dynamic changes in motor task requirements, but in PDs with dyskinesia, there was a trade-off between overall performance and inappropriate reliance on ambiguous visual feedback. This is likely associated with functional changes in posterior parietal–ponto–cerebellar pathways.

Task specific grip force control in writer's cramp

OBJECTIVE

Writer's cramp is defined as a task specific focal dystonia generating hypertonic muscle co-contractions during handwriting resulting in impaired writing performance and exaggerated finger force. However, little is known about the generalisation of grip force across tasks others than writing. The aim of the study was to directly compare regulation of grip forces during handwriting with force regulation in other fine-motor tasks in patients and control subjects.

METHODS

Handwriting, lifting and cyclic movements of a grasped object were investigated in 21 patients and 14 controls. The applied forces were registered in all three tasks and compared between groups and tasks. In addition, task-specific measures of fine-motor skill were assessed.

RESULTS

As expected, patients generated exaggerated forces during handwriting compared to control subjects. However there were no statistically significant group differences during lifting and cyclic movements. The control group revealed a generalisation of grip forces across manual tasks whereas in patients there was no such correlation.

CONCLUSION

We conclude that increased finger forces during handwriting are a task-specific phenomenon that does not necessarily generalise to other fine-motor tasks.

SIGNIFICANCE

Force control of patients with writer's cramp in handwriting and other fine-motor tasks is characterised by individualised control strategies.

Characterizing coordination of grasp and twist in hand function of healthy and post-stroke subjects

The goal of this study was to characterize the coordination of grasp and twist in hand function of normal and post-stroke subjects using a two degree of freedom hand robot. Results of the analysis of data from eight control subjects indicated that normal grip coordination involves the linear modulation of grip force with load torque. Thus, there was a high correlation between grip force and load torque. Also, the force generated by the thumb was highly correlated with the force generated by the index, middle and ring fingers. Finally, the safety margin used to stabilize grasp and avoid slip was consistent across normal subjects. In contrast, results from chronic post-stroke subjects indicated that they generally: (1) exerted excessive grip force to stabilize grasp using their ipsilesional hand; (2) lost the close amplitude coupling between grip force and load torque; and (3) lost the close modulation of the thumb force with finger force. These results suggest that our methods may provide objective, quantitative means of characterizing coordination problems following stroke.

Integration of sensory force feedback is disturbed in CRPS-related dystonia

Complex regional pain syndrome (CRPS) is characterized by pain and disturbed blood flow, temperature regulation and motor control. Approximately 25% of cases develop fixed dystonia. The origin of this movement disorder is poorly understood, although recent insights suggest involvement of disturbed force feedback. Assessment of sensorimotor integration may provide insight into the pathophysiology of fixed dystonia. Sensory weighting is the process of integrating and weighting sensory feedback channels in the central nervous system to improve the state estimate. It was hypothesized that patients with CRPS-related dystonia bias sensory weighting of force and position toward position due to the unreliability of force feedback. The current study provides experimental evidence for dysfunctional sensory integration in fixed dystonia, showing that CRPS-patients with fixed dystonia weight force and position feedback differently than controls do. The study shows reduced force feedback weights in CRPS-patients with fixed dystonia, making it the first to demonstrate disturbed integration of force feedback in fixed dystonia, an important step towards understanding the pathophysiology of fixed dystonia.

Two-dimensional static manipulation tasks: does force coordination depend on change of the tangential force direction?

Coordination of the grip force (GF) with a tangential force (TF, often referred to as load force) exerted along a certain line in space (i.e., one-dimensional tasks) during object manipulation has proved both to be high and based on feed-forward neural control mechanisms. However, GF-TF coordination deteriorates when the TF of one-dimensional task consecutively switches its direction (bidirectional task). In the present study, we aimed to explore GF-TF coordination in the generally neglected multi-dimensional manipulations. We hypothesized that the coordination would depend on the number of unidirectional and bidirectional orthogonal components of a two-dimensional TF exertion. Fourteen subjects traced various circular TF patterns and their orthogonal diameters shown on a computer screen by exerting a static TF. As expected, the unidirectional tasks revealed higher GF-TF coordination than the bidirectional ones (e.g., higher GF-TF correlations and GF gains, and lower GF/TF ratio). Regarding the circular tasks, most of the data were in line with the hypothesis revealing higher coordination associated with higher number of unidirectional components. Of particular importance could be that the circular tasks also revealed prominent time lags of GF with respect to TF, suggesting involvement of feedback mechanisms. We conclude that the force coordination in bidirectional static manipulations could be affected by changes in TF direction along either of its orthogonal components. The time lags observed from the circular tasks could be a consequence of the activity of sensory afferents, rather than of the visual feedback provided or the task complexity.

Design of a robotic device for assessment and rehabilitation of hand sensory function

This paper presents the design and implementation of the Robotic Sensory Trainer, a robotic interface for assessment and therapy of hand sensory function. The device can provide three types of well controlled stimuli: (i) angular displacement at the metacarpophalangeal (MCP) joint using a remote-center-of-motion double-parallelogram structure, (ii) vibration stimuli at the fingertip, proximal phalange and palm, and (iii) pressure at the fingertip, while recording position, interaction force and feedback from the user over a touch screen. These stimuli offer a novel platform to investigate sensory perception in healthy subjects and patients with sensory impairments, with the potential to assess deficits and actively train detection of specific sensory cues in a standardized manner. A preliminary study with eight healthy subjects demonstrates the feasibility of using the Robotic Sensory Trainer to assess the sensory perception threshold in MCP angular position. An average just noticeable difference (JND) in the MCP joint angle of 2.46° (14.47%) was found, which is in agreement with previous perception studies.

Grip surface affects maximum pinch force

OBJECTIVE

The aim of this study was to investigate whether people change their isometric pinch grip generation depending on the surface they gripped. Specifically, the effect of grip surface friction condition on (a) maximum force produced in the direction normal to the contact surface, (b) fluctuation of normal force, and (c) the digit force's angular deviation from the direction normal to the grip surface was quantified.

BACKGROUND

Isometric pinch grip has been traditionally thought to be independent from the friction condition between the finger and gripped surface, which may be questionable.

METHOD

For this study, 12 healthy participants performed maximum isometric pinch grip exertion on high-friction rubber and low-friction paper surfaces. Maximum normal force, normal force variance,and digit force's angular deviation from the normal direction were quantified.

RESULTS

Pinch grip on the high-friction rubber surface was associated with 10% greater maximum normal force and 50% reduced normal force variance, compared with the low-friction paper surface (p < .05). Digit force's angular deviation was not significantly different between the two surface friction conditions.

CONCLUSION

The data support that people do change their pinch grip generation (maximum normal force and normal force variance) depending on the surface they gripped, potentially by using sensory feedback. The results of this study demonstrate that even a simple isometric pinch grip (no lifting associated) is affected by grip surface friction.

APPLICATION

Grip surface condition should be considered for clinical assessments, biomechanical investigation, and motor control studies to ensure consistency in measurements and validity of comparisons.

Correlation of digital sensibility and precision of pinch force modulation in patients with nerve repair

The outcome measures for patients following peripheral nerve repairs commonly include muscle strength and sensory assessments. However, no significant discussion exists on the impact of nerve injury on sensorimotor control. The objective of this longitudinal study was to explore the effects of nerve regeneration on the control of pinch force in executing functional tasks. Seven patients with digital or median nerve repairs were assessed by a custom-designed pinch device and conventional sensory tools at monthly intervals following nerve repair. These tools measured sensibility, maximum pinch strength, and anticipated pinch force adjustments to movement-induced load fluctuations in a pinch-holding-up activity (PHUA). Six force-related and temporal parameters for sensory measurement were used to determine improvements in pinch performance over time following sensory recovery. The results revealed significant differences in the parameters of peak pinch force, baseline pinch force, force ratio, and the percentage of maximal pinch force output at different points in the course of nerve regeneration. A strong relationship was also found between kinetic data from the PHUA test and the traditional sensibility tests for the nerve repair patients in the present study.

Impaired prehension is associated with lesions of the superior and inferior hand representation within the human cerebellum

Impairment of patients with cerebellar disease in prehension is well recognized. So far specific localizations within the human cerebellum associated with the impairment have rarely been assessed. To address this question we performed voxel-based lesion symptom mapping (VLSM) in patients with chronic focal cerebellar lesions in relation to specific deficits in prehensile movements. Patients with stroke within the posterior inferior cerebellar artery territory (n = 13) or the superior cerebellar artery (SCA) territory (n = 7) and corresponding control subjects were included in the study. Participants reached out, grasped, and lifted an object with either the left or right hand and with fast or normal movement speed. Both kinematic and grip-force parameters were recorded. Magnetic resonance imaging anatomical scans of the cerebellum were acquired, and lesions were marked as regions of interest. For VLSM analysis, a nonparametric test (Brunner-Munzel) was applied. Cerebellar patients showed clear abnormalities in hand transport (impaired movement speed and straightness) and, to a lesser degree, in hand shaping (increased finger touch latencies) while grip function was preserved. Deficits were most prominent in patients with SCA lesions and for ipsilesional, fast movements. Disorders in hand transport may be more difficult to compensate than deficits in hand shaping and grip-force control in chronic focal lesions of the cerebellum because of higher demands on predictive control of interaction torques. Lesions of the superior cerebellar cortex (lobules IV, V, VI) were associated with slower hand transport, whereas lesions of both superior (lobules VI, V, VI) and inferior cerebellar cortex (lobules VII, VIII) were associated with impaired movement straightness. These findings show that both the superior and inferior hand representations within the cerebellum contribute to hand transport during prehensile movements; however, they may have a different functional role.