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10:30   Motor Control I Chair: Sabine Van Huffel 10:30 15 mins DISTURBED INTEGRATION OF SENSORY FORCE FEEDBACK IN CRPS-RELATED DYSTONIA Winfred Mugge, Frans van der Helm, Alfred Schouten Abstract: 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, a movement disorder of sustained muscle contraction and abnormal postures. The origin of fixed dystonia is poorly understood, yet recent insights involve 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. Position and force are physically related, allowing translation from one modality into the other. When stiffness is known, combining the sensory feedback of position and force (sensory integration) provides increased accuracy of the estimate of either modality. 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. CRPS-patients with dystonia (n=10) and age and gender-matched healthy subjects blindly reproduced a trained force against a linear spring which on occasion was covertly replaced by a non-linear spring, revealing the sensory weighting between force and position feedback [1]. The current study provides experimental evidence for dysfunctional sensory integration in fixed dystonia, showing that CRPS-patients with fixed dystonia do not reweight force and position feedback as controls do. The study shows that patients always favor position feedback, making it the first to demonstrate disturbed integration of force feedback in fixed dystonia, an important step towards understanding the pathophysiology of fixed dystonia. 10:45 15 mins SENSORY REWEIGHTING OF PROPRIOCEPTIVE INPUT DURING BALANCE CONTROL IN HEALTHY ELDERLY Jantsje Pasma, Denise Engelhart, Alfred Schouten, Andrea Maier, Carel Meskers, Herman van der Kooij Abstract: Sensory (re)weighting is the automated and unconscious process of dynamically combining sensory inputs, e.g. proprioception, graviception and vision, during balance control [1]. Typically, reliable sensory inputs are weighted more than unreliable and noisy sensory inputs, to prevent deterioration of balance control. Malfunctioning of sensory reweighting may be an important determinant of balance deficits in elderly with the consequence of physical impairment and falls. In this study, we compared sensory weight and reweighting of proprioceptive input of the ankle in healthy young versus healthy old adults during upright stance. Ten healthy young (aged 20-30 years) and ten healthy old adults (aged 75-80 years) were asked to maintain balance while proprioceptive input of each ankle was perturbed by rotation of the support surface around the ankle axes. Support surface rotations were applied with specific frequency content and perturbation amplitude increased over trials. Body sway and reactive ankle torque were recorded. The sensitivity function of the ankle torque to the perturbation amplitude was determined using system identification techniques [2]. The gain of the sensitivity function describes the ratio of perturbation amplitude and response amplitude as function of frequency. Parameters describing the sensitivity functions were estimated using optimized model fits. Old adults had a significant higher gain of the sensitivity function compared with young (p<0.001). Both groups showed a decrease in gain with increasing perturbation amplitude (p<0.001). The estimated proprioceptive weight was significantly higher in old adults compared with young (p<0.001). In both groups, the proprioceptive weight decreased with increasing perturbation amplitude (p<0.001). There was an interaction effect (p=0.038) between perturbation amplitude and age group. Results indicate that old and young adults had an equal capability to sensory reweight proprioceptive input during balance control, in which old adults rely more strongly on propriceptive input. These results are important in understanding the interplay between available sensory inputs in balance and falling. 11:00 15 mins A WAY TO ASSESS THE ROLE OF IMPROPER JOINT STIFFNESS AND ITS CONTRIBUTORS IN PATIENTS WITH CENTRAL NEUROLOGICAL DISEASE DURING FUNCTIONAL MOVEMENT Karin de Gooijer-van de Groep, Stijn Van Eesbeek, Lizeth Sloot, Hans Arendzen, Jurriaan de Groot, Jaap Harlaar, Carel Meskers, Erwin de Vlugt Abstract: Movement disorders of central neurological origin are characterized by increased joint stiffness, i.e. resistance to movement by improper muscle activation (neural) and/or changes in viscoelastic properties of connective tissues (non-neural). Differentiation between neural and non-neural components is hard to achieve by common manual tests. Most importantly, current passive clinical tests are not able to assess functional effects of therapy. This leaves the clinician with uncertainties in treatment selection and doses assignment. The ROBIN project (ROBot aided system Identification: novel tools for diagnosis and assessment in Neurological rehabilitation) aims to understand the role of improper joint stiffness in movement (walking) and the effects of intervention. The assessment of causal relations between neuromechanical contributors to joint stiffness, requires combined biomechanical and control engineering approach using open and closed loop system identification and parameter estimation (SIPE) techniques under different experimental (task, environment) conditions. The ROBIN assessment goes from identification of single ankle joint stiffness during passive conditions all the way up to identification of ankle joint stiffness during walking. We designed a hierarchical assessment protocol, consisting of the following elements: 1) Neural and non-neural parameter estimation over the whole range of motion (RoM) under passive conditions using the Achilles, a 1-DOF ankle perturbator (MOOG, The Netherlands). This part most closely resembles current clinical tests like the Ashworth test. 2) Instantaneous neural and non-neural parameter estimation during tasks requiring active changes of muscle force and joint angle, resembling walking activity using the Achilles and time variant identification. 3) Identification of ankle joint stiffness during actual walking using small amplitude belt perturbations delivered by a high end dual belt treadmill (Forcelink, the Netherlands). The first part of the ROBIN protocol was successfully applied on patients with stroke and cerebral palsy (CP). In CP, calf muscle reflexive torque was on average 5.7 times larger (p = 0.002) and tissue stiffness 2.1 times larger (p = 0.018) compared to controls. High tissue stiffness was associated with reduced RoM (p < 0.001). The next step is to integrate ROBIN into current clinical care to assess sensitivity, specificity and repeatability of the ROBIN assessment compared to current clinical assessment with respect to specific current therapy for either non-neural and neural contributors to enhanced joint stiffness. ROBIN is a step forward in determining patient and component specific therapy to tune improper joint stiffness and to optimize mobility in patient with movement disorders after central neurological diseases. 11:15 15 mins ASSESSING CORTICAL INVOLVEMENT IN STRETCH REFLEX RESPONSE USING SUBTHRESHOLD TMS Matthijs Perenboom, Mark van de Ruit, Jurriaan de Groot, Alfred Schouten, Carel Meskers Abstract: Understanding mechanisms of reflex control provides insight into movement disorders following supraspinal nerve lesions, for example after stroke. To assess cortical involvement in reflex modulation, subthreshold transcranial magnetic stimulation (TMS) is used to alter motoneuron pool excitability. Subthreshold TMS does not elicit direct muscle response, thereby allowing to define the effect of supraspinal input without afferent contributions. This study combined mechanically applied ramp-and-hold perturbations (stretch duration: 40 ms) with TMS (Magstim Rapid2 with figure-8 coil, Magstim Co, Whitland, UK) at interstimulus intervals (ISI) ranging from 35-80 ms, with random non-TMS and non-reflex trials mixed in. Eleven participants (48±13yrs (M±SD), 3 female) had to maintain a target torque of 10% maximum voluntary torque. Subthreshold TMS pulses (97% of active motor threshold) were applied over the primary motor cortex, at the individually assessed hotspot for m. flexor carpi radialis using neuronavigation (ANT, Enschede, The Netherlands). Electromyographic (EMG) muscle response was measured using high-density EMG (TMSi, Enschede, The Netherlands), with an afterwards constructed bipolar bar electrode configuration. Magnitude of resulting EMG peak activity, i.e. short (20-50 ms) and long (55-100) latency reflex responses were calculated by taking mean of 40% highest samples per peak time slot at all ISIs and compared to non-TMS trials. The effect of ISI on reflex response was tested using a linear mixed model (SPSS Version 17, alpha = 0.05). Subthreshold magnetic stimulation was found to nearly double the stretch evoked EMG response (p < 0.001) when TMS pulses were timed to arrive at the muscle in the time slot of the long latency response. Involvement of the primary motor cortex in peripheral stretch reflex loop was demonstrated for the human wrist flexor muscle. This involvement indicates the existence of supraspinal pathways in the long latency reflex component. 11:30 15 mins INFLUENCE OF FORCE LEVEL ON THE FORCE REPRODUCTION ERROR IN HUMANS Bram Onneweer, Winfred Mugge, Alfred Schouten Abstract: In haptic tele-manipulation systems, the human operator manipulates objects and uses tools in remote environments using a master-slave system. The human controls the slave (e.g. a remote robot arm), via the master (e.g. a joystick) while haptic information of the forces at the slave is fed back to the human operator. For optimal system design, it is important to understand the accuracy and limitations of human force perception. Previous research demonstrated that humans generate higher forces when asked to reproduce an externally applied target force [1,2]. It has been proposed that the nervous system attenuates feedback from self-generated forces, i.e. humans perceive self-generated forces lower than externally applied forces [1]. Walsh et al. 2011 found that the force difference diminished when matching higher externally applied force levels or when the target was self-generated with the other hand [2]. The goal of our study was to determine how accurately subjects reproduce self-generated forces with the same hand over a broad range of force levels. Subjects (n=10) were instructed to generate an onscreen target force with visual support and subsequently reproduce the same force without visual support with their right hand against a static handle equipped with a force sensor. In the experiment, six force levels (10, 40, 70, 100, 130, 160N) were presented in random order; each with eight repetitions. Subjects generated too high forces for lower force levels (<70N) and too low forces for higher force levels (>100N). The results for low force levels are in accordance to previous findings using externally applied target forces [1,2], but contradicts with the findings using self-generated target forces [2]. Our results indicate that attenuated feedback of self-generated forces is not the key factor in force reproduction. Instead we suggest that the integration of tactile and proprioceptive feedback plays an important role in force estimation and varies with force level. 11:45 15 mins BALANCE TASK DOES NOT INFLUENCE VESTIBULAR INPUT TO CERVICAL MUSCLES Patrick Forbes, Gunter Seigmund, Alfred Schouten, Riender Happee, Jean-Sébastien Blouin Abstract: Vestibular sensory input is critical to facilitate system wide effective balance control in the face of unpredictable perturbations as observed in responses of the lower limb, lumbar and neck muscles. Neck muscles in particular possess strong connections to the vestibular organ composed of primarily disynaptic pathways innervated by cranial or high level spinal nerves, which elicit short latency response (~10 msec) during natural or electrical stimulation. Defined as vestibulocollic reflexes (VCR), these responses can be studied in humans by perturbing the system through acoustic, vibratory or electrical stimulation of the vestibular organ. Here we use electrical stimulation to elicit vestibulomuscular responses in neck muscles and aim to show that their responses are independent of the balance task. We compare standing and sitting conditions with the head-free, as well as head-free and head-fixed conditions during sitting, where different directions of force were applied in the head-fixed sitting conditions. In head-free conditions (standing and sitting) subjects (n = 8) were asked to rotate their head 60º to the left while bipolar stochastic vestibular signals (0-75 Hz) were delivered. In head fixed conditions (sitting) subjects performed isometric neck contraction in twist at orientations of 0º and 60º, as well as flexion, extension and right lateral flexion at an orientation of 0º. EMG levels were controlled in all conditions to mimic the contraction measured in the head free conditions. Intramuscular EMG was collected bilaterally in the sternocleidomastoid (SCM) and splenius capitis (SPL) muscles. Significant muscles responses (P < 0.05) correlated to the input stimuli were observed for all conditions provided the muscle was actively used in the contraction task. For both muscles no significant differences were observed between standing and seated conditions, or between the head free and head fixed conditions at 60º. The force direction had a significant effect (P < 0.05) on the magnitude of the muscle responses where the highest responses were observed in conditions where muscle contraction was most imbalanced relative to the perceived vestibular roll stimulation, i.e. in the SCM the magnitudes from highest to lowest were lateral flexion, twist and forward flexion. This study demonstrates neck muscle responses to vestibular input are irrespective of the balance state (i.e. seated vs standing and head-free vs head fixed). Unlike lower limb muscles which limit vestibular input when balance control is not required [1], the central nervous system continues delivers vestibular input to activate neck motoneurons in head fixed conditions. REFERENCES 1. Luu, B.L., et al., Human standing is modified by an unconscious integration of congruent sensory and motor signals. Journal of Physiology-London, 2012. Accepted Article.