Erwin Aertbeliën

Constraint-based Task Specification and Estimation for Sensor-Based Robot Systems in the Presence of Geometric Uncertainty

This paper introduces a systematic constraint-based approach to specify complex tasks of general sensor-based robot systems consisting of rigid links and joints. The approach integrates both instantaneous task specification and estimation of geometric uncertainty in a unified framework. Major components are the use of feature coordinates, defined with respect to object and feature frames, which facilitate the task specification, and the introduction of uncertainty coordinates to model geometric uncertainty. While the focus of the paper is on task specification, an existing velocity- based control scheme is reformulated in terms of these feature and uncertainty coordinates. This control scheme compensates for the effect of time varying uncertainty coordinates. Constraint weighting results in an invariant robot behavior in case of conflicting constraints with heterogeneous units. The approach applies to a large variety of robot systems (mobile robots, multiple robot systems, dynamic human-robot interaction, etc.), various sensor systems, and different robot tasks. Ample simulation and experimental results are presented.

Human-inspired robot assistant for fast point-to-point movements

A first step towards truly versatile robot assistants consists of building up experience with simple tasks such as the cooperative manipulation of objects. This paper extends the state-of-the-art by developing an assistant which actively cooperates during the point-to-point transportation of an object. Besides using admittance control to react to interaction forces generated by its operator, the robot estimates the intended human motion and uses this identified motion to move along with the operator. The offered level of assistance can be scaled, which is vital to give the operator the opportunity to gradually learn how to interact with the system. Experiments revealed that, while the robot is programmed to adapt to the human motion, the operator also adapts to the offered assistance. When using the robot assistant the required forces to move the load are greatly reduced and the operators report that the assistance feels comfortable and natural.

Three-Dimensional Upper Limb Movement Characteristics in Children with Hemiplegic Cerebral Palsy and Typically Developing Children.

The aim of this study was to measure which three-dimensional spatiotemporal and kinematic parameters differentiate upper limb movement characteristics in children with hemiplegic cerebral palsy (HCP) from those in typically developing children (TDC), during various clinically relevant tasks. We used a standardized protocol containing three reach tasks (forwards, upwards, and sideways), two reach-to-grasp tasks (with objects requiring different hand orientations), and three gross motor tasks. Spatiotemporal (movement duration, trajectory straightness, maximum velocity, and timing of maximum velocity), as well as kinematic parameters (discrete angles and waveforms of the trunk, scapula, shoulder, elbow and wrist), were compared between 20 children with HCP (age 10.9 ± 2.9 years) and 20 individually age-matched TDC (age 10.9 ± 3.0 years). Kinematic calculations followed the recommendations from the International Society of Biomechanics. Results showed that children with HCP had longer movement durations, less straight hand trajectories, and lower maximum velocities compared to the TDC. Timing of maximum velocity did not differ between both groups. The movement pathology in children with HCP was highlighted by increased trunk movements and reduced shoulder elevation during reaching and reach-to-grasp. We also measured an increased anterior tilting and protraction of the scapula in children with HCP, although differences were not significant for all tasks. Finally, compared to the TDC, children with HCP used less elbow extension and supination and more wrist flexion to execute all tasks. This study reported distinct 3D upper limb movement characteristics in children with HCP and age-matched TDC, establishing the discriminative ability of the measurement procedure. From a clinical perspective, combining spatiotemporal and kinematic parameters may facilitate the identification of the pathological movement patterns seen in children with HCP and thereby add to a well-targeted upper limb treatment planning.

A Clinical Measurement to Quantify Spasticity in Children with Cerebral Palsy by Integration of Multidimensional Signals

Most clinical tools for measuring spasticity, such as the Modified Ashworth Scale (MAS) and the Modified Tardieu Scale (MTS), are not sufficiently accurate or reliable. This study investigated the clinimetric properties of an instrumented spasticity assessment. Twenty-eight children with spastic cerebral palsy (CP) and 10 typically developing (TD) children were included. Six of the children with CP were retested to evaluate reliability. To quantify spasticity in the gastrocnemius (GAS) and medial hamstrings (MEH), three synchronized signals were collected and integrated: surface electromyography (sEMG); joint-angle characteristics; and torque. Muscles were manually stretched at low velocity (LV) and high velocity (HV). Spasticity parameters were extracted from the change in sEMG and in torque between LV and HV. Reliability was determined with intraclass-correlation coefficients and the standard error of measurement; validity by assessing group differences and correlating spasticity parameters with the MAS and MTS. Reliability was moderately high for both muscles. Spasticity parameters in both muscles were higher in children with CP than in TD children, showed moderate correlation with the MAS for both muscles and good correlation to the MTS for the MEH. Spasticity assessment based on multidimensional signals therefore provides reliable and clinically relevant measures of spasticity. Moreover, the moderate correlations of the MAS and MTS with the objective parameters further stress the added value of the instrumented measurements to detect and investigate spasticity, especially for the GAS.

Spasticity and Its Contribution to Hypertonia in Cerebral Palsy

Spasticity is considered an important neural contributor to muscle hypertonia in children with cerebral palsy (CP). It is most often treated with antispasticity medication, such as Botulinum Toxin-A. However, treatment response is highly variable. Part of this variability may be due to the inability of clinical tests to differentiate between the neural (e.g., spasticity) and nonneural (e.g., soft tissue properties) contributions to hypertonia, leading to the terms “spasticity” and “hypertonia” often being used interchangeably. Recent advancements in instrumented spasticity assessments offer objective measurement methods for distinction and quantification of hypertonia components. These methods can be applied in clinical settings and their results used to fine-tune and improve treatment. We reviewed current advancements and new insights with respect to quantifying spasticity and its contribution to muscle hypertonia in children with CP. First, we revisit what is known about spasticity in children with CP, including the various definitions and its pathophysiology. Second, we summarize the state of the art on instrumented spasticity assessment in CP and review the parameters developed to quantify the neural and nonneural components of hypertonia. Lastly, the impact these quantitative parameters have on clinical decision-making is considered and recommendations for future clinical and research investigations are discussed.

Geometric Relations Between Rigid Bodies (Part 1): Semantics for Standardization

This tutorial explicitly states the semantics of all coordinate-invariant properties and operations, and, more importantly, all the choices that are made in coordinate representations of these geometric relations. This results in a set of concrete suggestions for standardizing terminology and notation, allowing programmers to write fully unambiguous software interfaces, including automatic checks for semantic correctness of all geometric operations on rigid-body coordinate representations. A concrete proposal for community-driven standardization via the Robot Engineering Task Force [4] is accepted as a Robotics Request for Comment.

Is an Instrumented Spasticity Assessment an Improvement Over Clinical Spasticity Scales in Assessing and Predicting the Response to Integrated Botulinum Toxin Type A Treatment in Children With Cerebral Palsy

OBJECTIVE To compare responsiveness and predictive ability of clinical and instrumented spasticity assessments after botulinum toxin type A (BTX) treatment combined with casting in the medial hamstrings (MEHs) in children with spastic cerebral palsy (CP). DESIGN Prospective cohort study. SETTING Hospital. PARTICIPANTS Consecutive sample of children (N=31; 40 MEH muscles) with CP requiring BTX injections. INTERVENTION Clinical and instrumented spasticity assessments before and on average ± SD 53±14 days after BTX. MAIN OUTCOME MEASURES Clinical spasticity scales included the Modified Ashworth Scale and the Modified Tardieu Scale. The instrumented spasticity assessment integrated biomechanical (position and torque) and electrophysiological (surface electromyography) signals during manually performed low- and high-velocity passive stretches of the MEHs. Signals were compared between both stretch velocities and were examined pre- and post-BTX. Responsiveness of clinical and instrumented assessments was compared by percentage exact agreement. Prediction ability was assessed with a logistic regression and the area under the receiver operating characteristic (ROC) curves of the baseline parameters of responders versus nonresponders. RESULTS Both clinical and instrumented parameters improved post-BTX (P≤.005); however, they showed a low percentage exact agreement. The baseline Modified Tardieu Scale was the only clinical scale predictive for response (area under the ROC curve=0.7). For the instrumented assessment, baseline values of root mean square (RMS) electromyography and torque were better predictors for a positive response (area under the ROC curve=.82). Baseline RMS electromyography remained an important predictor in the logistic regression. CONCLUSIONS The instrumented spasticity assessment showed higher responsiveness than the clinical scales. The amount of RMS electromyography is considered a promising parameter to predict treatment response.

The relation between spasticity and muscle behavior during the swing phase of gait in children with cerebral palsy

There is much debate about how spasticity contributes to the movement abnormalities seen in children with spastic cerebral palsy (CP). This study explored the relation between stretch reflex characteristics in passive muscles and markers of spasticity during gait. Twenty-four children with CP underwent 3D gait analysis at three walking velocity conditions (self-selected, faster and fastest). The gastrocnemius (GAS) and medial hamstrings (MEHs) were assessed at rest using an instrumented spasticity assessment that determined the stretch-reflex threshold, expressed in terms of muscle lengthening velocity. Muscle activation was quantified with root mean square electromyography (RMS-EMG) during passive muscle stretch and during the muscle lengthening periods in the swing phase of gait. Parameters from passive stretch were compared to those from gait analysis. In about half the children, GAS peak muscle lengthening velocity during the swing phase of gait did not exceed its stretch reflex threshold. In contrast, in the MEHs the threshold was always exceeded. In the GAS, stretch reflex thresholds were positively correlated to peak muscle lengthening velocity during the swing phase of gait at the faster (r = 0.46) and fastest (r = 0.54) walking conditions. In the MEHs, a similar relation was found, but only at the faster walking condition (r = 0.43). RMS-EMG during passive stretch showed moderate correlations to RMS-EMG during the swing phase of gait in the GAS (r = 0.46-0.56) and good correlations in the MEHs (r = 0.69-0.77) at all walking conditions. RMS-EMG during passive stretch showed no correlations to peak muscle lengthening velocity during gait. We conclude that a reduced stretch reflex threshold in the GAS and MEHs constrains peak muscle lengthening velocity during gait in children with CP. With increasing walking velocity, this constraint is more marked in the GAS, but not in the MEHs. Hyper-activation of stretch reflexes during passive stretch is related to muscle activation during the swing phase of gait, but has a limited contribution to reduced muscle lengthening velocity during swing. Larger studies are required to confirm these results, and to investigate the contribution of other impairments such as passive stiffness and weakness to reduced muscle lengthening velocity during the swing phase of gait.

Identification of the neural component of torque during manually-applied spasticity assessments in children with cerebral palsy.

Clinical assessment of spasticity is compromised by the difficulty to distinguish neural from non-neural components of increased joint torque. Quantifying the contributions of each of these components is crucial to optimize the selection of anti-spasticity treatments such as botulinum toxin (BTX). The aim of this study was to compare different biomechanical parameters that quantify the neural contribution to ankle joint torque measured during manually-applied passive stretches to the gastrocsoleus in children with spastic cerebral palsy (CP). The gastrocsoleus of 53 children with CP (10.9 ± 3.7 y; females n = 14; bilateral/unilateral involvement n = 28/25; Gross Motor Functional Classification Score I-IV) and 10 age-matched typically developing (TD) children were assessed using a manually-applied, instrumented spasticity assessment. Joint angle characteristics, root mean square electromyography and joint torque were simultaneously recorded during passive stretches at increasing velocities. From the CP cohort, 10 muscles were re-assessed for between-session reliability and 19 muscles were re-assessed 6 weeks post-BTX. A parameter related to mechanical work, containing both neural and non-neural components, was compared to newly developed parameters that were based on the modeling of passive stiffness and viscosity. The difference between modeled and measured response provided a quantification of the neural component. Both types of parameters were reliable (ICC > 0.95) and distinguished TD from spastic muscles (p < 0.001). However, only the newly developed parameters significantly decreased post-BTX (p = 0.012). Identifying the neural and non-neural contributions to increased joint torque allows for the development of individually tailored tone management.

Manually controlled instrumented spasticity assessments: a systematic review of psychometric properties

The first aim of this study was to systematically review and critically assess manually controlled instrumented spasticity assessment methods that combine multidimensional signals. The second aim was to extract a set of quantified parameters that are psychometrically sound to assess spasticity in a clinical setting.