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This study aimed to identify the functional contribution of reflexes to human motor control during posture maintenance. Continuous random force disturbances were applied at the hand while the subjects were instructed to minimize the deviation resulting from the force disturbances. The results were analyzed in the frequency domain with frequency response functions (FRFs). Two FRFs were evaluated: 1) the mechanical admittance and 2) the reflexive impedance, expressing the dynamic relation between position and muscle activation (assessed via electromyography, EMG). The reflexive impedance is a direct measure of the proprioceptive reflexes. To record all relevant dynamical characteristics of the arm, wide bandwidth signals were used as force disturbance. Distributing the power of the signal over fewer frequencies within the bandwidth improved the signal-to-noise-ratio SNR of the EMG recordings, facilitating reliable estimation of the reflexive impedance. The coherence indicated that the relation between force disturbance and EMG is linear under the given conditions and improved with the SNR. The method of designing disturbance signals and the estimation of the reflexive impedance are useful for studies aiming to quantify proprioceptive reflexes and to investigate its functionality. Index Terms--Arm admittance, electromyography (EMG), identification, proprioceptive reflexes, reflexive impedance.

This study aimed to analyse the dynamic properties of the muscle spindle feedback system of shoulder muscles during a posture task. External continuous force disturbances were applied at the hand while subjects had to minimize their hand displacements. The results were analysed using two frequency response functions (FRFs) from which the model parameters were derived, being 1) the mechanical admittance and 2) the reflexive impedance. These FRFs were analysed by a neuromusculoskeletal model that implicitly separates the reflexive feedback properties (position, velocity and acceleration feedback gains) from intrinsic muscle visco-elasticity. The results show substantial changes in estimated reflex gains under conditions of variable bandwidth of the applied force disturbance or variable degrees of external damping. Position and velocity feedback gains were relatively larger when the force disturbance contained only low frequencies. With increasing damping of the environment, acceleration feedback gain decreased, velocity feedback gain remained almost constant and position feedback gain increased. It is concluded that under the aforementioned circumstances, the reflex system increases its gains to maximize the mechanical resistance to external force disturbances while preserving sufficient stability. Index Terms--Arm admittance, electromyography, identification, proprioceptive reflexes, reflexive impedance.

Abstract Background Quantifying increased joint resistance into its contributing factors i.e. stiffness and viscosity ("hypertonia") and stretch reflexes ("hyperreflexia") is important in stroke rehabilitation. Existing clinical tests, such as the Ashworth Score, do not permit discrimination between underlying tissue and reflexive (neural) properties. We propose an instrumented identification paradigm for early and tailor made interventions. Methods Ramp-and-Hold ankle dorsiflexion rotations of various durations were imposed using a manipulator. A one second rotation over the Range of Motion similar to the Ashworth condition was included. Tissue stiffness and viscosity and reflexive torque were estimated using a nonlinear model and compared to the Ashworth Score of nineteen stroke patients and seven controls. Results Ankle viscosity moderately increased, stiffness was indifferent and reflexive torque decreased with movement duration. Compared to controls, patients with an Ashworth Score of 1 and 2+ were significantly stiffer and had higher viscosity and patients with an Ashworth Score of 2+ showed higher reflexive torque. For the one second movement, stiffness correlated to Ashworth Score (r2 = 0.51, F = 32.7, p < 0.001) with minor uncorrelated reflexive torque. Reflexive torque correlated to Ashworth Score at shorter movement durations (r2 = 0.25, F = 11, p = 0.002). Conclusion Stroke patients were distinguished from controls by tissue stiffness and viscosity and to a lesser extent by reflexive torque from the soleus muscle. These parameters were also sensitive to discriminate patients, clinically graded by the Ashworth Score. Movement duration affected viscosity and reflexive torque which are clinically relevant parameters. Full evaluation of pathological joint resistance therefore requires instrumented tests at various movement conditions.

Abstract Background Instead of hyper-reflexia as sole paradigm, post-stroke movement disorders are currently considered the result of a complex interplay between neuronal and muscular properties, modified by level of activity. We used a closed loop system identification technique to quantify individual contributors to wrist joint stiffness during an active posture task. Methods Continuous random torque perturbations applied to the wrist joint by a haptic manipulator had to be resisted maximally. Reflex provoking conditions were applied i.e. additional viscous loads and reduced perturbation signal bandwidth. Linear system identification and neuromuscular modeling were used to separate joint stiffness into the intrinsic resistance of the muscles including co-contraction and the reflex mediated contribution. Results Compared to an age and sex matched control group, patients showed an overall 50% drop in intrinsic elasticity while their reflexive contribution did not respond to provoking conditions. Patients showed an increased mechanical stability compared to control subjects. Conclusion Post stroke, we found active posture tasking to be dominated by: 1) muscle weakness and 2) lack of reflex adaptation. This adds to existing doubts on reflex blocking therapy as the sole paradigm to improve active task performance and draws attention to muscle strength and power recovery and the role of the inability to modulate reflexes in post stroke movement disorders.

Abstract Background Main claims of the literature are that functional recovery of the paretic upper limb is mainly defined within the first month post stroke and that rehabilitation services should preferably be applied intensively and in a task-oriented way within this particular time window. EXplaining PLastICITy after stroke (acronym EXPLICIT-stroke) aims to explore the underlying mechanisms of post stroke upper limb recovery. Two randomized single blinded trials form the core of the programme, investigating the effects of early modified Constraint-Induced Movement Therapy (modified CIMT) and EMG-triggered Neuro-Muscular Stimulation (EMG-NMS) in patients with respectively a favourable or poor probability for recovery of dexterity. Methods/design 180 participants suffering from an acute, first-ever ischemic stroke will be recruited. Functional prognosis at the end of the first week post stroke is used to stratify patient into a poor prognosis group for upper limb recovery (N = 120, A2 project) and a group with a favourable prognosis (N = 60, A1 project). Both groups will be randomized to an experimental arm receiving respectively modified CIMT (favourable prognosis) or EMG-NMS (poor prognosis) for 3 weeks or to a control arm receiving usual care. Primary outcome variable will be the Action Research Arm Test (ARAT), assessed at 1,2,3,4,5, 8, 12 and 26 weeks post stroke. To study the impact of modified CIMT or EMG-NMS on stroke recovery mechanisms i.e. neuroplasticity, compensatory movements and upper limb neuromechanics, 60 patients randomly selected from projects A1 and A2 will undergo TMS, kinematical and haptic robotic measurements within a repeated measurement design. Additionally, 30 patients from the A1 project will undergo fMRI at baseline, 5 and 26 weeks post stroke. Conclusion EXPLICIT stroke is a 5 year translational research programme which main aim is to investigate the effects of early applied intensive intervention for regaining dexterity and to explore the underlying mechanisms that are involved in regaining upper limb function after stroke. EXPLICIT-stroke will provide an answer to the key question whether therapy induced improvements are due to either a reduction of basic motor impairment by neural repair i.e. restitution of function and/or the use of behavioural compensation strategies i.e. substitution of function. EXPLICIT is registered at the Netherlands Trial Register (NTR, http://www.trialregister.nl., TC 1424)

An emerging and innovative way of organizing projects in health technology and innovation is the so-called "living lab". Because of their characteristics, living labs may provide a solution to a very old problem: how to facilitate the meaningful participation of stakeholders in science and technology? In this article, I (we use a first-person perspective in the paper) aim to contribute to the literature by providing an account of my experiences as a participation researcher with stakeholder participation in a living lab in the Netherlands. I participated in a yearlong project on ensuring freedom for residents in a closed psychogeriatric ward. Using three key moments from that experience, I illustrate why participation was the intention, but was harder to achieve in practice. Participation processes and living labs are situated in specific social and physical contexts. I discuss the "situatedness" of living labs and propose to reconceptualize them as "situated practices": the value of a living lab lies in the processes of work it conducts on specific innovations situated in its local context. A key conclusion is that providing narrative descriptions of living lab projects, with attention to situatedness and stakeholder participation, can provide invaluable examples, insights, and inspirations for other researchers in the field. [ABSTRACT FROM AUTHOR]

The dynamic behavior of the wrist joint is governed by nonlinear properties, yet applied mathematical models, used to describe the measured input-output (perturbation-response) relationship, are commonly linear. Consequently, the linearly estimated model parameters will depend on properties of the applied perturbation properties (such perturbation amplitude and velocity). We aimed to systematically address the effects of perturbation velocity on linearly estimated neuromechanical parameters. Using a single axis manipulator ramp and hold perturbations were applied to the wrist joint. Effects of perturbation velocity (0.5, 1 and 3 rad/s) were investigated at multiple background torque levels (0, 0.5 and 1 N·m). With increasing perturbation velocity, estimated joint stiffness remained constant, while damping and reflex gain decreased. This variation in model parameters is dependent on background torque levels, i.e. muscle contraction. These observations support the future development of nonlinear models that are capable of describing wrist joint behavior over a larger range of loading conditions, exceeding the restricted range of operation that is required for linearization. [ABSTRACT FROM AUTHOR]

We suggest short range stiffness (SRS) at the elbow joint as an alternative diagnostic for EMG to assess cocontraction. Elbow SRS is compared between obstetric brachial plexus lesion (OBPL) patients and healthy subjects (cross-sectional study design). Seven controls (median 28 years) and five patients (median 31 years) isometrically flexed and extended the elbow at rest and three additional torques [2.1, 4.3, 6.4 N m] while a fast stretch stimulus was applied. SRS was estimated in silico using a neuromechanical elbow model simulating the torque response from the imposed elbow angle. SRS was higher in patients (250 ± 36 N m/rad) than in controls (150 ± 21 N m/rad, p = 0.014), except for the rest condition. Higher elbow SRS suggested greater cocontraction in patients compared to controls. SRS is a promising mechanical alternative to assess cocontraction, which is a frequently encountered clinical problem in OBPL due to axonal misrouting. [ABSTRACT FROM AUTHOR]

Progress in diagnosis and treatment of movement disorders after neurological diseases like stroke, cerebral palsy (CP), dystonia and at old age requires understanding of the altered capacity to adequately respond to physical obstacles in the environment. With posture and movement disorders, the control of muscles is hampered, resulting in aberrant force generation and improper impedance regulation. Understanding of this improper regulation not only requires the understanding of the role of the neural controller, but also attention for: (1) the interaction between the neural controller and the "plant", comprising the biomechanical properties of the musculaskeletal system including the viscoelastic properties of the contractile (muscle) and non-contractile (connective) tissues: neuromechanics; and (2) the closed loop nature of neural controller and biomechanical system in which cause and effect interact and are hence difficult to separate. Properties of the neural controller and the biomechanical system need to be addressed synchronously by the combination of haptic robotics, (closed loop) system identification (SI), and neuro-mechanical modeling. In this paper, we argue that assessment of neuromechanics in response to well defined environmental conditions and tasks may provide for key parameters to understand posture and movement disorders in neurological diseases and for biomarkers to increase accuracy of prediction models for functional outcome and effects of intervention. [ABSTRACT FROM AUTHOR]

System identification techniques have been used to separate intrinsic muscular and reflexive contributions to joint impedance, which is an essential step in the proper choice of patient specific treatment. These techniques are, however, only well developed for linear systems. Assuming linearity prescribes the neuromuscular system to be perturbed only around predefined operating points. In this study, we test the validity of a commonly used linear model by analyzing the effects of flexion–extension displacement amplitude (2^\circ, 4^\circ, and 8^\circ) on damping, stiffness, and reflex gain of the wrist joint, at different background torque levels (0, 1, and 2 N\cdotm). With displacement amplitude, intrinsic damping increased, while intrinsic stiffness and reflex gains decreased. These changes were dependent on the level of wrist torque. The dependency of the neuromuscular system properties on even small variations in angular displacement is evident and has to be accounted for when comparing different studies and clinical interpretations using linear identification techniques. Knowledge of the behavior of the neuromuscular system around operating points is an essential step toward the development of nonlinear models that allow for discrimination between patients and controls in a larger range of loading conditions. [ABSTRACT FROM PUBLISHER]

Background: Spastic paresis in cerebral palsy (CP) is characterized by increased joint stiffness that may be of neural origin, i.e. improper muscle activation caused by e.g. hyperreflexia or non-neural origin, i.e. altered tissue viscoelastic properties (clinically: "spasticity" vs. "contracture"). Differentiation between these components is hard to achieve by common manual tests. We applied an assessment instrument to obtain quantitative measures of neural and non-neural contributions to ankle joint stiffness in CP. Methods: Twenty-three adolescents with CP and eleven healthy subjects were seated with their foot fixated to an electrically powered single axis footplate. Passive ramp-and-hold rotations were applied over full ankle range of motion (RoM) at low and high velocities. Subject specific tissue stiffness, viscosity and reflexive torque were estimated from ankle angle, torque and triceps surae EMG activity using a neuromuscular model. Results: In CP, triceps surae reflexive torque was on average 5.7 times larger (p = .002) and tissue stiffness 2.1 times larger (p = .018) compared to controls. High tissue stiffness was associated with reduced RoM (p < .001). Ratio between neural and non-neural contributors varied substantially within adolescents with CP. Significant associations of SPAT (spasticity test) score with both tissue stiffness and reflexive torque show agreement with clinical phenotype. Conclusions: Using an instrumented and model based approach, increased joint stiffness in CP could be mainly attributed to higher reflexive torque compared to control subjects. Ratios between contributors varied substantially within adolescents with CP. Quantitative differentiation of neural and non-neural stiffness contributors in CP allows for assessment of individual patient characteristics and tailoring of therapy. [ABSTRACT FROM AUTHOR]

Linear system identification methods combined with neuromechanical modeling enable the quantification of reflex gains from recorded joint angular perturbation, torque, and/or electromyography (EMG). However, the stretch reflex response as recorded by EMG consists of multiple consecutive activation volleys (M1 and M2 responses) separated by a period of reduced activity and is nonlinearly related to joint perturbation. The goal of this study is to assess to what extent linear assumptions hold when quantifying these reflexive responses. Series of ramp-and-hold angular perturbations with fixed velocity but different ramp durations (and, therefore, different amplitudes) were applied to the wrist joint of seven healthy volunteers. Evoked EMG responses were compared to the reflex response estimated from a common linear reflex model relating EMG to perturbation velocity. Model fits described the measured EMG responses best when the perturbation and M1 response durations were equivalent. With increasing perturbation duration, i.e., amplitude, EMG response increased but reflex gain decreased due to the inert period after M1, which is believed to be related to alignment of the refractory period of the motoneurons. For angular joint perturbations exceeding the M1 duration (coinciding with 2^\circ of wrist joint rotation in this study), reflex gain variation may be largely explained from a shortcoming of the linear model in describing the nonlinear reflex response, and in particular the period of low reflexive activity after M1. [ABSTRACT FROM PUBLISHER]

Background: Quantifying increased joint resistance into its contributing factors i.e. stiffness and viscosity ("hypertonia") and stretch reflexes ("hyperreflexia") is important in stroke rehabilitation. Existing clinical tests, such as the Ashworth Score, do not permit discrimination between underlying tissue and reflexive (neural) properties. We propose an instrumented identification paradigm for early and tailor made interventions. Methods: Ramp-and-Hold ankle dorsiflexion rotations of various durations were imposed using a manipulator. A one second rotation over the Range of Motion similar to the Ashworth condition was included. Tissue stiffness and viscosity and reflexive torque were estimated using a nonlinear model and compared to the Ashworth Score of nineteen stroke patients and seven controls. Results: Ankle viscosity moderately increased, stiffness was indifferent and reflexive torque decreased with movement duration. Compared to controls, patients with an Ashworth Score of 1 and 2+ were significantly stiffer and had higher viscosity and patients with an Ashworth Score of 2+ showed higher reflexive torque. For the one second movement, stiffness correlated to Ashworth Score (r² = 0.51, F = 32.7, p < 0.001) with minor uncorrelated reflexive torque. Reflexive torque correlated to Ashworth Score at shorter movement durations (r² = 0.25, F = 11, p = 0.002). Conclusion: Stroke patients were distinguished from controls by tissue stiffness and viscosity and to a lesser extent by reflexive torque from the soleus muscle. These parameters were also sensitive to discriminate patients, clinically graded by the Ashworth Score. Movement duration affected viscosity and reflexive torque which are clinically relevant parameters. Full evaluation of pathological joint resistance therefore requires instrumented tests at various movement conditions. [ABSTRACT FROM AUTHOR]

Background: Instead of hyper-reflexia as sole paradigm, post-stroke movement disorders are currently considered the result of a complex interplay between neuronal and muscular properties, modified by level of activity. We used a closed loop system identification technique to quantify individual contributors to wrist joint stiffness during an active posture task.Methods: Continuous random torque perturbations applied to the wrist joint by a haptic manipulator had to be resisted maximally. Reflex provoking conditions were applied i.e. additional viscous loads and reduced perturbation signal bandwidth. Linear system identification and neuromuscular modeling were used to separate joint stiffness into the intrinsic resistance of the muscles including co-contraction and the reflex mediated contribution.Results: Compared to an age and sex matched control group, patients showed an overall 50% drop in intrinsic elasticity while their reflexive contribution did not respond to provoking conditions. Patients showed an increased mechanical stability compared to control subjects.Conclusion: Post stroke, we found active posture tasking to be dominated by: 1) muscle weakness and 2) lack of reflex adaptation. This adds to existing doubts on reflex blocking therapy as the sole paradigm to improve active task performance and draws attention to muscle strength and power recovery and the role of the inability to modulate reflexes in post stroke movement disorders. [ABSTRACT FROM AUTHOR]

This study aimed to analyse the dynamic properties of the muscle spindle feedback system of shoulder muscles during a posture task. External continuous force disturbances were applied at the hand while subjects had to minimize their hand displacements. The results were analysed using two frequency response functions (FRFs) from which the model parameters were derived, being 1) the mechanical admittance and 2) the reflexive impedance. These FRFs were analysed by a neuromusculoskeletal model that implicitly separates the reflexive feedback properties (position, velocity and acceleration feedback gains) from intrinsic muscle visco-elasticity. The results show substantial changes in estimated reflex gains under conditions of variable bandwidth of the applied force disturbance or variable degrees of external damping. Position and velocity feedback gains were relatively larger when the force disturbance contained only low frequencies. With increasing damping of the environment, acceleration feedback gain decreased, velocity feedback gain remained almost constant and position feedback gain increased. It is concluded that under the aforementioned circumstances, the reflex system increases its gains to maximize the mechanical resistance to external force disturbances while preserving sufficient stability. [ABSTRACT FROM AUTHOR]

Background: Main claims of the literature are that functional recovery of the paretic upper limb is mainly defined within the first month post stroke and that rehabilitation services should preferably be applied intensively and in a task-oriented way within this particular time window. EXplaining PLastICITy after stroke (acronym EXPLICIT-stroke) aims to explore the underlying mechanisms of post stroke upper limb recovery. Two randomized single blinded trials form the core of the programme, investigating the effects of early modified Constraint-Induced Movement Therapy (modified CIMT) and EMG-triggered Neuro-Muscular Stimulation (EMG-NMS) in patients with respectively a favourable or poor probability for recovery of dexterity.Methods/design: 180 participants suffering from an acute, first-ever ischemic stroke will be recruited. Functional prognosis at the end of the first week post stroke is used to stratify patient into a poor prognosis group for upper limb recovery (N = 120, A2 project) and a group with a favourable prognosis (N = 60, A1 project). Both groups will be randomized to an experimental arm receiving respectively modified CIMT (favourable prognosis) or EMG-NMS (poor prognosis) for 3 weeks or to a control arm receiving usual care. Primary outcome variable will be the Action Research Arm Test (ARAT), assessed at 1,2,3,4,5, 8, 12 and 26 weeks post stroke. To study the impact of modified CIMT or EMG-NMS on stroke recovery mechanisms i.e. neuroplasticity, compensatory movements and upper limb neuromechanics, 60 patients randomly selected from projects A1 and A2 will undergo TMS, kinematical and haptic robotic measurements within a repeated measurement design. Additionally, 30 patients from the A1 project will undergo fMRI at baseline, 5 and 26 weeks post stroke.Conclusion: EXPLICIT stroke is a 5 year translational research programme which main aim is to investigate the effects of early applied intensive intervention for regaining dexterity and to explore the underlying mechanisms that are involved in regaining upper limb function after stroke. EXPLICIT-stroke will provide an answer to the key question whether therapy induced improvements are due to either a reduction of basic motor impairment by neural repair i.e. restitution of function and/or the use of behavioural compensation strategies i.e. substitution of function. [ABSTRACT FROM AUTHOR]

Is human balance control dominated by time invariant continuous feedback mechanisms or do noncontinuous mechanisms play a significant role like intermittent control? The goal of this paper is to quantify how much of the postural responses evoked by pseudorandom external periodic perturbations can be explained by continuous time invariant feedback control. Nine healthy subjects participated in this study. Center of mass and ankle torque responses were elicited by periodic platform perturbations in forward-backward directions containing energy in the 0.06- to 4.5-Hz frequency band. Subjects had their eyes open (EO) or eyes closed (EC). Responses were decomposed into a periodic component and a remnant (stochastic) component using spectral analysis. It is concluded that periodic responses can explain most of the evoked responses, although the remnant power spectral densities (PSDs) were significant especially for slow responses (<0.2 Hz) and largest for EC. The found remnant PSD did depend on the sensory condition but not on the platform perturbation amplitude. The ratio of the body sway and ankle torque remnant PSD reflects the body dynamics. Both findings are consistent with the idea that estimation of body orientation is part of a continuous feedback loop and that (stochastic) estimation errors increase when one source of sensory information is removed. The findings are not consistent with the idea that discrete or discontinuous intermittent feedback mechanisms significantly shape postural responses. [ABSTRACT FROM AUTHOR]

Abstract: This study estimates intrinsic and reflexive properties of single- and two-joint muscles acting around the human shoulder, elbow and wrist joint during posture maintenance. External force disturbances were applied to the hand while subjects (n =5) were instructed to minimize their hand displacement amplitude in a horizontal plane. To examine how the nervous system modifies intrinsic and reflexive muscle properties, we varied external damping, disturbance power and arm configuration. A new identification method is introduced to quantify a large set of model parameters describing intrinsic, activation and reflexive properties, the latter representing afferent feedback gains from muscle spindles and Golgi tendon organs. Consistent estimates were found for the gains of the different types of reflex pathways. The results showed that intrinsic visco-elasticity of shoulder muscles was minimal, whereas reflexive feedback was largest compared to the elbow and wrist joint. Intrinsic and reflexive properties of the two-joint shoulder–elbow muscles were larger than the two-joint elbow–wrist muscles. Contrasting to previous single-joint studies, the reflex gains did not vary with the experimental conditions. It is concluded that during redundant multijoint posture maintenance, the mechanical properties are efficiently controlled on the joint level and not on the endpoint level. [Copyright &y& Elsevier]

Abstract: This paper describes the design of a torque-controlled manipulator to identify the dynamics of the wrist joint. With torque disturbances, the subject can actively control the joint angle, giving a natural task. The application of a hybrid haptic controller guarantees linearity over a large bandwidth and adjustable virtual dynamics. The haptic controller has a bandwidth of 50 Hz, meaning that the virtual dynamics are realistically felt at up to 50 Hz. To let the subject ‘feel’ the torque, disturbances as well as possible the apparent, or virtual, dynamics of the device must be small. The minimal apparent inertia of the device is 1.6 g m2, which is of the same order as for a normal wrist, and the minimal damping and stiffness are negligible. To judge the accuracy of the manipulator, loads of known physical properties were attached and their parameters were quantified. The parameters of the loads were estimated with a maximum error of 5%. As the eigenfrequency of a (co)-contracted human wrist is approximately 15 Hz, the 50-Hz bandwidth of the haptic device is sufficient to measure all relevant dynamics of the human wrist. With this device, the dynamics of the human wrist joint can be measured under varying virtual dynamics, as well as the effect of neurological dysfunction on human motor control, for example. [Copyright &y& Elsevier]

This paper describes the design and application of a haptic device to study the mechanical properties of the human arm during interaction with compliant environments. Estimates of the human endpoint admittance can be obtained by recording position deviations as a result of force perturbations. Previous studies attempted to estimate the impedance by recording force as a result of position perturbations, but these experiments do not require a feasible task of human beings. A general problem of force-controlled haptic devices is the occurrence of contact instability, especially where a small virtual mass is required. This negative effect is reduced by the use of a lightweight but stiff manipulator and a robust servo-based admittance controller. The virtual admittance is accurate to at least 13 Hz, attaining a minimum virtual mass of 1.7 kg (isotropic, without damping and stiffness). The properties of known test loads were estimated with an accuracy higher than 98%, up to 20 Hz. The application of the manipulator is evaluated by an experiment with a subject performing a position maintenance task. With this device it is possible to study the adaptability of the neuromuscular system to a variety of environments, enabling a new and functional approach to human motion research. [Copyright &y& Elsevier]

This study presents a new multivariable closed-loop identification technique for estimating the dynamic compliance of the multijoint human arm during posture maintenance. The method is designed for the application of continuous force disturbances that facilitate interaction of the limb with the environment. The dynamic compliance of the arm arises from different physiological mechanisms and is important for maintaining stable postures and to suppress disturbances. Estimates can be useful to analyze the ability of the nervous system to adapt the arm compliance to different types of disturbances and environments. The technique is linear and requires no a priori knowledge of the system. Linear system behavior is justified for posture tasks where the hand position deviates slightly from a reference position. Interaction results in a closed-loop configuration of arm and environment. The problem with previous methods is the restriction to open-loop systems. With the current technique, the dynamic arm compliance is separately estimated from the closed-loop. The accuracy of the identification technique is tested by simulations for different values of the dynamic compliance of the arm and environment and for different methodological parameters. It is concluded that the identification technique is accurate, even for short observation periods and severe noise. [Copyright &y& Elsevier]

Background: Understanding movement disorder after stroke and providing targeted treatment for post stroke patients requires valid and reliable identification of biomechanical (passive) and neural (active and reflexive) contributors. Aim of this study was to assess test-retest reliability of passive, active and reflexive parameters and to determine clinical responsiveness in a cohort of stroke patients with upper extremity impairments and healthy volunteers.Methods: Thirty-two community-residing chronic stroke patients with an impairment of an upper limb and fourteen healthy volunteers were assessed with a comprehensive neuromechanical assessment protocol consisting of active and passive tasks and different stretch reflex-eliciting measuring velocities, using a haptic manipulator and surface electromyography of wrist flexor and extensor muscles (Netherlands Trial Registry number NTR1424). Intraclass correlation coefficients (ICC) and Standard Error of Measurement were calculated to establish relative and absolute test-retest reliability of passive, active and reflexive parameters. Clinical responsiveness was tested with Kruskal Wallis test for differences between groups.Results: ICC of passive parameters were fair to excellent (0.45 to 0.91). ICC of active parameters were excellent (0.88-0.99). ICC of reflexive parameters were fair to good (0.50-0.74). Only the reflexive loop time of the extensor muscles performed poor (ICC 0.18). Significant differences between chronic stroke patients and healthy volunteers were found in ten out of fourteen parameters.Conclusions: Passive, active and reflexive parameters can be assessed with high reliability in post-stroke patients. Parameters were responsive to clinical status. The next step is longitudinal measurement of passive, active and reflexive parameters to establish their predictive value for functional outcome after stroke. [ABSTRACT FROM AUTHOR]

In this study a new methodology to quantify reflexive feedback gains from the mechanical behavior of the human arm during posture maintenance is proposed. Disturbance experiments were carried out on human subjects using continuous random force inputs. The task instruction was ‘minimize displacements’, prescribing a maximum performance task. For the separation of intrinsic and reflexive components, system identification in the frequency domain is applied. From the time records of position and force, frequency response functions (FRFs) are estimated. Given a model structure and an appropriate estimate of the intrinsic component, an estimate of the reflex gains for length and velocity are obtained from the FRFs. The feedback gains vary considerably with the frequency content of the disturbance signal. The results show that reflexive dynamics are substantial for narrow-band and especially low-frequency input signals. It is likely that high reflex gains are most effective for low-frequency inputs (<3 Hz) that do not excite the closed-loop system''s eigenfrequency. Also significant negative reflex gains are estimated for near-sinusoidal inputs (>1.5 Hz). It is concluded that this new methodology can offer interesting insights into the ability of the central nervous system to modulate reflexive feedback gains. [Copyright &y& Elsevier]

Proper understanding of motor control requires insight into the extent and manner in which task performance and control strategy are influenced by various aspects of visual information. We therefore systematically manipulated the visual presentation (i.e., scaling factor and optical flow density) of a visuomotor tracking task without changing the task itself, and investigated the effect on performance, effort, motor control strategy (i.e., anticipatory or corrective steering) and underlying neuromechanical parameters (i.e., intrinsic muscle stiffness and damping, and proprioceptive and visual feedback). Twenty healthy participants controlled the left-right position of a virtual car (by means of wrist rotations in a haptic robot) to track a slightly curved virtual road (presented on a 60” LED screen), while small torque perturbations were applied to the wrist (1.25–20 Hz multisine) for quantification of the neuromechanical parameters. This visuomotor tracking task was performed in conditions with low/medium/high scaling factor and low/high optical flow density. Task performance was high in all conditions (tracking accuracy 96.6%–100%); a higher scaling factor was associated with slightly better performance. As expected, participants did adapt their control strategy and the use of proprioceptive and visual feedback in response to changes in the visual presentation. These findings indicate that effects of visual representation on motor behavior should be taken into consideration in designing, interpreting and comparing experiments on motor control in health and disease. In future studies, these insights might be exploited to assess the sensory-motor adaptability in various clinical conditions. [ABSTRACT FROM AUTHOR]

Spastic cerebral palsy (CP) is characterized by increased joint resistance, caused by a mix of increased tissue stiffness, as well as involuntary reflex and background muscle activity. These properties can be quantified using a neuromechanical model of the musculoskeletal complex and instrumented assessment. The construct validity of the neuromechanical parameters was examined (i.e. the internal model validity, effect of knee angle, speed and age, sensitivity to patients versus controls, spasticity severity and treatment), together with the repeatability. We included 38 children with CP and 35 controls. A motor driven footplate applied two slow (15°/s) and two fast (100°/s) rotations around the ankle joint, at two different knee angles. Ankle angle, torque and EMG of the gastrocnemius (GA), soleus (SO) and tibialis anterior (TA) muscle were used to optimize a nonlinear neuromuscular model. Outcome measures were tissue stiffness, reflex and background activity for GA, SO and TA. The internal model validity showed medium to high parameter confidence and good model fits. All parameter could discriminate between patients with CP and controls according to CP pathology. Other measures of external model validity (effect of test position, speed and age) showed behaviour along the lines of current knowledge of physiology. GA/SO background activity was sensitive to spasticity severity, but reflex activity was not. Preliminary data indicated that reflex activity was reduced after spasticity treatment. The between-trial and -day repeatability was moderate to good. The large variance between patients in the ratio of stiffness and neural resistance indicates that the method could potentially contribute to patient-specific treatment selection. [ABSTRACT FROM AUTHOR]