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Assessment of the sodium/potassium (Na/K) pump functioning can play an important role in the early diagnosis of several polyneuropathies. However, currently no method exists that allows in vivo testing of the Na/K pump. Therefore, this study developed a test based on repetitive nerve stimulation at single motor units. In order to evaluate the Na/K pump functioning, measurements were taken at various frequencies. Low frequency stimulation was applied to record the recovery cycle of a single motor unit at which no Na/K pump influence is expected. Stimulation trains of physiological frequency perturbed the Na/K pump and led to hyperpolarization of the membrane potential. Hence hyperpolarization leads to a decreased neural excitability, a threshold change can be recorded. Since both stimulation at physiological and low frequency led to a threshold change, a model was introduced to discriminate the influence of the Na/K pump. During the recordings at physiological stimulation frequency a remarkable phenomenon was noticed which was identified as neural bifurcation.

The goal of this study is to quantify the recovery of spinal reflexes at the elbow after neurosurgical intervention in patients with brachial plexus injuries. So far, main focus was on the recovery of muscle force and little on sensory- and reflex system. As reflexes play an important role during normal movement, it is of interest to determine to what extend reflexes have been restored after surgery. Arm admittance (dynamic relationship between displacements in a response to forces) at endpoint level (hand) was estimated using force perturbations in two directions (horizontal) applied by a two-joint robotic manipulator. Three different task instructions were used to provoke different intrinsic and reflexive behavior, being a position task (PT), a relax task (RT) and a force task (FT) where the subject was required to minimize hand displacements, not react to the perturbations and minimize force deviations (being compliant) respectively. Ten patients with brachial plexus lesions participated in this experiment and were suffering from varying degrees of arm dysfunction. All had successful recovery for the biceps (MRC grade 3 and higher) after surgical nerve repair of the n. musculocutaneous. Estimated intrinsic and reflexive parameters were compared to those of a control group (n = 10, age and sex matched). The task instructions had great influence on the admittance, especially between the PT and RT. In all patients, reflexive activity was found corresponding to assumed muscle spindles (velocity- and position feedback) and Golgi tendon organ (force feedback) function. For the PT, the difference in parameters between patients and control subjects was largest. Overall, patients exhibit more intrinsic stiffness at the shoulder and elbow than the control subjects, an indication of co-contraction, and less reflexive feedback at the elbow. There are two possible explanations for this: 1) The intrinsic and reflexive properties did not recover to the combination as before the injury and are not cooperating correctly, and 2) The patients are relying more on intrinsic control than on reflexive control (different control strategy). Whereas the control group uses an energy efficient approach, i.e. less intrinsic and more reflex activity, the patients appear to use a more maximal activation approach resulting in co-contraction. It is possible that the exercises performed during rehabilitation which focus on muscle force do not provoke enough reflexive behavior. More research, e.g. experiments that are designed to disable co-contraction, is needed to verify if the use of co-contraction is learnt or a necessity. Conclusion: reflexes do recover after surgery to severe Brachial plexus injuries, the amount of reflex function is less or less effective than for the control group.

Assessing mechanisms of peripheral reflex control is important for understanding movement disorders after suprapsinal nerve lesions like stroke. In the present study, reflex provocation by ramp and hold rotations (R&H) was combined with Transcranial Magnetic Stimulation (TMS). In four subjects, subthreshold single pulses TMS were applied to the primary motor cortex at carefully timed intervals, while short and long latency EMG responses of the m. flexor carpi radialis were elicited by R&H rotations around the wrist joint. TMS was found to inhibit the long latency response with a maximum inhibition when TMS was calculated to arrive at 45ms after stretch onset in all subjects. Excitation was found at 60 ms in all subjects. An involvement of the primary motor cortex in peripheral reflex loop operation was demonstrated. This involvement may be either exictatory or inhibititory on the stretch reflex.

When performing minimally invasive interventions, surgeons use grippers to grip and manipulate tissue. These grippers generally rely on their toothed profile and require pinching of the tissue for sufficient grip, entailing a risk of tissue damage. An solution should be found for this risk of damage. An alternative could be to manipulate the tissue without pinching or even touching it. Contactless gripping exists in the industrial field by using the principle of Bernoulli. This study explores the possibility of applying Bernoulli gripping for tissue manipulation during minimally invasive surgery. A medical gripper using the principle of Bernoulli was developed. Increasing the air flow, the radius of the gripper face and the radius of the nozzle has a positive effect on the lifting force. In order to prevent tissue damage, different variants are tested that change the direction of the air flow. In an exploring experiment, a deflector was selected out of seven variants as best solution to prevent damage from the air flow. The Bernoulli gripper was made expandable and collapsible for insertion in the body of the patient with a system of living hinges. The effect of Venturi channels and the position of a membrane for an airtight surface on the lifting force were tested. The lifting force generated on the object during gripping was measured using a tensile-strength tester. Bernoulli’s theory was compared to the results of the experiments and a discrepancy was found between theory and results. This study shows that a Bernoulli gripper is feasible to lift flexible tissue.

A bi-phasic force response is found in muscle stretch. Force response is high at the start of the stretch and drops at certain length. The high initial force response is referred to as Short Range Stiffness and the moment at which the force response drops is referred to as the elastic limit. It is likely that this bi-phasic torque response is caused by the properties of the contractile elements of the muscle. The first part of the force response is attributed to the elastic stretch of attached cross-bridges and the stiffness reduction is attributed to the breakage of cross bridges. The aim of this study was to estimate Short-range stiffness in eccentric and concentric joint loading. Flexion and extension rotations were imposed to the wrist at 3 different velocities and at 4 voluntary contraction levels using a servo controlled electrical motor. A nonlinear model-based identification procedure was used to identify Short-range stiffness and the elastic limit from in vivo recordings of the human wrist joint. The results showed that Short-range stiffness was equal in concentric and eccentric loading. The decrease in stiffness after Short-range stiffness is greater in concentric loading compared to eccentric loading. The results corresponded well to the muscle force velocity relation.

Functional Electrical Stimulation (FES) is used to help patient to recover or improve movement of their limbs after a spinal cord injury, a stroke or other neurological disorders effecting their motor system. The advance in technology in recent years has provided new possibilities for more accurate and better applications of FES. One of the goals of this technology is to develop a device which can stimulate the muscles of a patient to achieve natural and dynamic movements. However in order to achieve this goal there still is more investigation to be done. In this thesis the combination of FES and surface electromyography (sEMG) is used to evaluate a method that could be used to stimulate the muscles to achieve a dynamic movement. In order to achieve this dynamic and natural motion two characteristics are looked for: the position of the limb that has is being stimulated and the best position on the skin where to apply the surface electrical stimulation. FES is used to stimulate synchronously a group of motor fibres. The resulting signal from this stimulation is then recorded by the sEMG and analyzed. This thesis investigates whether this signal contains enough information to obtain the position of the limb that is being stimulated and the best position on the skin where to apply the electrical stimulation. The worked realized in this thesis consists in creating the experimental set-up and developing the adequate analysis to show the feasibility of this concept. Different set-ups are tested on the biceps of 4 healthy subjects while the analysis of the signal is done offline. Experiments using this technique in static conditions are presented to demonstrate accuracies as high as 97% for certain configurations and for subjects with certain physiological characteristics. Also positive results in the detection of the best location where to apply the electrical stimulation are presented. Now that there is a proof-of-concept, a series of next steps are defined with the purpose to develop this technology: test other muscles, increase the population under test, test this technology in real time and dynamic conditions and increase the accuracy of the algorithms.

In active, toddler sized hand prostheses the cosmetic glove introduces a stiffness which causes the required operation forces to be too high to be generated by toddlers. The Wilmer WHD-4 was measured with 3 different cosmetic gloves. Despite using gloves of identical size and brand, different glove stiffness characteristic curves were measured. The goal of this study is to design a voluntary closing toddler sized hand prosthesis using an adjustable glove compensation mechanism. A prototype of the glove compensated hand prosthesis (GCHP) was designed and manufactured. High friction locked the compensation mechanism over the range of motion and prevented it from functioning. To investigate the causes of the higher than predicted friction, the prototype was examined in assembled and disassembled state. Imperfections of the manufactured GCHP hand basis were causing the mechanism to lock. The GCHP hand basis is a complicated part to manufacture and proved to be susceptible to precision errors. A test block, based on the GCHP hand basis, was designed and manufactured to measure the standalone compensation mechanism. Springs with lower stiffness are used in the measurement to reduce the forces on the axes. The mechanism did not lock in place and the behavior was measured. It was concluded that the mechanism does not result in a proper compensation characteristic due to the high frictional forces. The solution lies in heat treatment of the cosmetic gloves. Heat treatment will decrease the glove stiffness significantly. The lower glove stiffness will be accompanied with two phenomena: the required spring volume will decrease and the forces on the axes of the mechanism will decrease. Both these aspects will decrease the friction present in the system. The conclusion of this research is that to make glove compensation possible, the stiffness and hysteresis of the cosmetic gloves has to be reduced. Heat treatment of cosmetic gloves presented by Herder et al. showed promising results and should be further researched.

Galvanic vestibular stimulation (GVS) alters the firing rates of vestibular afferents and consequently provokes the illusion of movement. In standing balance GVS is used to assess the contribution of the vestibular system, where it has been shown to elicit coherent responses in lower extremity muscles involved in maintaining balance. However, to date no information exists regarding the influence of GVS on neck muscles or head-neck stabilization. This study aims to test the hypothesis that GVS can be used as a technique to investigate the vestibular contribution to head and neck stabilization. Sinusoidal stimuli of 0.5 – 2 mA within the bandwidth of 0.4 – 5.2 Hz were used as GVS signals and applied to eleven healthy subjects using a bilateral bipolar configuration. Subjects were blindfolded and stimulated while seated on and restrained to a chair. Measurements of natural sway (without stimulation) were included as control trials. Displacements of head and torso were recorded using a motion capture system. System identification techniques were used to identify the relationship between the input (GVS) and the output (motion) of the head and neck. The results show significant coherence between GVS and the head-neck kinematics and modulation of these responses was observed across frequency and not amplitude, demonstrating the linear range of the vestibular feedback. Furthermore, the vestibular origin of the responses was demonstrated using non-vestibular stimulation tests. EMG measurements were used to characterize the relationship between the vestibular input and muscle activities in neck. Based on the findings of this study we propose that using GVS with system identification techniques provides a viable approach to quantify small motions (~ 0.1 mm) in neck and understand motion control of the head-neck.

Corticomuscular communication during wrist motor tasks was investigated in this study. EEG signals from the sensorimotor cortex and EMG data from the reflexive carpi radialis and extensor carpi radialis muscles were recorded from five healthy subjects while performing visual-motor force tasks, with and without perturbation on the wrist. Different continuous perturbation signals with different frequency content (multisines), as well as perturbation resulting in rapid angular displacements of the wrist were applied to study the existence of synchronization on corticomuscular communication, as well as the possible trancortical contribution to the late reflexes on the muscle. Corticomuscular, perturbation- EEG and perturbation - EMG coherences were calculated for all tasks. Three out of five subjects did show high coherence results in beta band when applying multisine perturbation and decreased in base task and in tasks with rapid angular displacements of the wrist, implying an Ia afferent contribution from muscle spindles to beta EEG. The connection of the perturbation to the brain and the muscles is considered non-linear due to high corticomuscular coherence found in harmonics of the excited frequencies. Current source density was applied on frequencies with high corticomuscular coherence. Contralateral supplementary motor cortex is more likely to cause corticomuscular communication at high frequencies of the beta band. Moreover, proprioceptive-evoked potentials were calculated from tasks with continuous rapid angular displacements of the wrist. The basal ganglia is more likely to be involved in the generation of early proprioceptive-evoked activity.

The goal of this project is to model, fabricate and characterize a multi microchamber biochip, which allows for crosstalk between two types of living cells. This biochip uses laminar flow to separate two microchambers for cell co-culture. A study of fluid dynamics through finite element analysis was implemented to optimize two methods for crosstalk between the microchambers. The fabrication process that consists of multilayer soft lithography was also optimized for this particular biochip. The knowledge acquired from the theoretical modeling and preliminary experiments with a single microchamber biochip led to the production of a working prototype ready to be tested for cell culture experiments.

Thermal spread is the extend of lateral damage resulting from soft tissue dissection by means of an energetic surgical instrument. Both a true definition of this factor, and a consistent means to quantify it, are still lacking. This study attempts to standardize thermal spread by 1) listing the relevant medical, technical, and statistical parameters, 2) selecting a soft tissue substitute to reduce biological influences, and 3) testing the protocol by determining the thermal spread of the Lotus grasper. Thermal spread was measured in a meat replacement product (quorn) by means of eight thermocouples. Spread of heat was determined as the distance between the instrument shaft and the 7:5_C isotherm. It was measured at 5 mm depth in the left (3:09 _ 0:637 mm), and right flank (2:70 _ 0:966 mm). In addition, two more depth levels were measured in the left flank, 2:5 mm (2:11_ 0:732 mm), and 7:5 mm (2:95_0:959 mm). The thermal distribution of the Lotus obtained by this measuring technique was comprehensible and educative. The use of thermocouples seemed adequate, as spread could be measured in the deep direction, and sufficiently precise. The selected test material (quorn) was sub- optimal from a mechanical viewpoint. This may have led to experimental uncertainties and an increased measurement variance.

Aim: Inspect the MRI image sequence to see if it can image the components of an atherosclerotic plaque and investigate if pressure induced deformation measurements can be done on these images. Methods: Four porcine iliac arteries with atherosclerotic plaques were scanned with a T2 imaging sequence in a 7T MRI scanner. One high resolution scan with an in-plane resolution of 56x56mm and three low resolution scans with an in-plane resolution of 98x98mm were obtained. The pressure inside the artery for the high-resolution scan was 100mmHg and for the low resolution scans the pressure was 80, 100 and 120mmHg, respectively. For each artery one high resolution MRI slice was compared with two histology staining to see if the lumen, intima, media, adventitia, calcium, and lipid and collagen poor/rich regions could be indentified. The surfaces of the lumen and external elastic lamina (EEL) were compared for the three different pressures to see what the global deformations were. Also a pilot has been performed to see if local deformations could be indentified with the program Elastix [1]. Results: In all four MRI slices of the porcine arteries the lumen, intima, media, adventitia and calcium could clearly been seen. The lipid poor/rich and collagen poor/rich regions were not always indentified correctly. The surface area of the lumen and EEL seem to increase as the pressure increases. However, the increase in surface area for these diseased arteries seem to be smaller then expected. The local deformations obtained with the program Elastix showed a logical deformation pattern for the artery with the largest deformations, but for the smaller deformations the deformation field was contradictory to expectations. Conclusion: With the high-resolution T2 sequence it is possible to clearly delineate the lumen, intima, media, adventitia and calcium, but not lipid or collagen poor/rich regions. Global deformations can be measured from the low resolution MRI scans, but the local deformations are hard to indentify with the program Elastix if the displacements are close to the resolution of the MRI scans.

Purpose: An accurate needle or catheter placement is essential for the success of many procedures like regional anesthesia or biopsy taking. The accuracy of the needle or catheter placement could be increased by application of a rotational motion, which reduces the tissue indentation and required insertion force. Materials and methods: A catheter (5F/1.67 mm Pig angiographic, Boston Scientific) was pulled through muscle tissue (pork tenderloin) over a distance of 125 mm in directions parallel or perpendicular to the muscle fibers. Insertion velocities of 1, 10 and 50 mm/s were applied in combination with 0,114 and 572 RPM. The friction force between the tissue and catheter was measured by the use of a force sensor (Futek Lsb200, 2.5 N). In total 576 measurements were obtained using 16 tissue samples. Results: The results show that the fiber orientation has no effect on the amount of friction. Also the insertion velocity on its own has little effect on the steady friction force. However, a higher insertion velocity normally result in higher maximum insertion force. An increased rotational velocity normally results in a reduced maximum. The steady insertion force is reduced when the tangential velocity is higher than the insertion velocity. A larger ratio results in a lower amount of friction. Conclusion: We conclude that friction forces between instrument and tissue can be reduced by application of a rotational. Hence, rotation of needles and catheters might reduce the effect of indentation and tissue movements and therefore increase the precision of needle or catheter placement.

The voluntary opening prostheses have a limited pinch force, limiting the performance of the users in their daily activities. The proprioceptive feedback provided by these prostheses is inverse and very counterintuitive. This is caused by the voluntary opening operating principle of these prostheses. In order to improve the functional performance of the WILMER appealing prehensor for toddlers, the Delft Institute of Prosthetics and Orthotics suggested to transform the operating principle of this split-hook from voluntary opening to voluntary closing. Conform this suggestion a new mechanism is designed and prototyped. Three concepts are proposed and the best among them is further elaborated into a prototype. This prototype is tested using an existing test bench. The energy dissipation is lowered with a factor of 2.5 in comparison with the best performing commercially available voluntary closing prosthesis, which shows the higher efficiency of the proposed design as compared to the currently available prostheses. The force transmission ratio of the prototype is 0.5, representing the ratio between activation force and pinch force. This is comparable with the best available voluntary closing prosthesis. Whereas the new design has no protruding mechanical parts out of the frame compared to the other voluntary closing prostheses, reducing damage to the clothing considerably. Conform the benefits of this new prosthetic device, it is a better performing voluntary closing prosthesis than the commercially available ones. Its performance together with its appearance makes it a promising alternative for the commercially available VC prostheses for the unilateral below-elbow amputees.

Galvanic vestibular stimulation (GVS) alters the firing rates of vestibular afferents and consequently provokes the illusion of movement. In standing balance GVS is used to assess the contribution of the vestibular system, where it has been shown to elicit coherent responses in lower extremity muscles involved in maintaining balance. However, to date no information exists regarding the influence of GVS on neck muscles or head-neck stabilization. This study aims to test the hypothesis that GVS can be used as a technique to investigate the vestibular contribution to head and neck stabilization. Sinusoidal stimuli of 0.5 – 2 mA within the bandwidth of 0.4 – 5.2 Hz were used as GVS signals and applied to eleven healthy subjects using a bilateral bipolar configuration. Subjects were blindfolded and stimulated while seated on and restrained to a chair. Measurements of natural sway (without stimulation) were included as control trials. Displacements of head and torso were recorded using a motion capture system. System identification techniques were used to identify the relationship between the input (GVS) and the output (motion) of the head and neck. The results show significant coherence between GVS and the head-neck kinematics and modulation of these responses was observed across frequency and not amplitude, demonstrating the linear range of the vestibular feedback. Furthermore, the vestibular origin of the responses was demonstrated using non-vestibular stimulation tests. EMG measurements were used to characterize the relationship between the vestibular input and muscle activities in neck. Based on the findings of this study we propose that using GVS with system identification techniques provides a viable approach to quantify small motions (~ 0.1 mm) in neck and understand motion control of the head-neck.

Vertebroplasty is a minimally invasive procedure for treating spine. Cement leakage is problem occurring from usage of low viscosity bone cements. High viscosity bone cements are difficult to inject with current design of cement gun due to very high injection forces. A new design of cement gun is developed for injection of high viscosity bone cements with low injection forces.

Objective: Injuries caused by falls of elderly people are a common worldwide problem and ageing of population will even further increase related burdens and costs. Recent technology using active monitoring systems have proven their success in order to analyze human actions. What is lacking in these researches is implementation in real elderly home environments. Most of the healthcare researches are focusing on the detection of falls and not on the detection of normal daily actions. We present a single camera with a fisheye lens which is capable of monitoring an entire room. The use of only one camera reduces the costs and simplifies the computational burden which results in a real time system. While different research is done on the detection of such actions, none of these is done using real data by elderly people in their own living environment. Using this data will increase the difficulty level of the action recognition, because every living environment will have different settings and noise factors. Main: We developed an action detection system which monitors the actions of elderly people in their homes during normal daily activities with the idea to raise the alarm in the case of danger. Our system is equipped with a single wide angle camera mounted on the ceiling of an elderly home. This gives a topview image of the environment resulting in a clear map of household objects without any occlusions. The main idea is to monitor the motion information of elderly and to model actions as a change of motion or poses in time that leads to a specific action. After background subtraction using Gaussian Mixture Models, the motion information is extracted using the Motion History Images method and analyzed to detect important actions. We propose to model actions as the shape deformations of the motion history image in time. Every action is defined at several moments in time, called “Action peaks” using different features, the holistic area, contour and location measurements as well as the Fourier shape descriptors. We combine all the measurements into the Bag of Word model and create unique action representations called „Action Signatures‟. These action signatures are then transformed and combined using feature fusion in order to learn the optimal combination of features for each action. Learning the optimal feature fusion is performed using Support Vector Machines. The final trained system is used to classify each new action. Results: the result section is divided into 2 sections. First the scientific data is used which is recorded in a testing room, simulating elderly home, with colleagues and students. We recorded and detected multiple actions: Bending, Walking, Falling, Collapsing, all with very high accuracy rates, above 93%. Finally real data is recorded in real elderly homes observing 4 elderly people. Different actions are monitored: Walking, Sitting, Open Door, and Eating. Results in a real environment depict high detection rates and prove that the system is able to detect multiple human actions using only one single camera.

The available shoulder harness controlled prostheses do no fulfill the requirements of the users. It is unknown for which operating forces and cable displacements the user can make good use of proprioceptive feedback without feeling pain, discomfort or fatigue. These operating forces and cable displacements can be related to grasping forces and opening widths for activities of daily living to result in force and displacement transmission ratios for improved prosthetic design. The purpose of this research was to find operating force and cable displacement combinations that could be produced best without visual feedback. The force-displacement combinations were realized by a force task and interchangeable spring in the experimental setup. Thirty participants without arm defect wore a prosthesis simulator. They were first asked to produce a reference force with direct visual feedback of their operating force, next they were asked to reproduce this reference force without visual feedback. The error between visual produced value and blind reproduced value was used to evaluate the results. Best reproduced operating forces were found between 24 and 32 N. Best reproduced cable displacements were dependent on the spring that was used; the larger the cable displacement was, the better the result. For larger forces it was more difficult to repeat the reference force-displacement. A perfect prosthesis should be operated by forces between 24 N and 32 N. It is important to take the amputee and their individual abilities into account as the results differ largely between participants.

Usability issues have been observed in luminaire repositioning, during surgical operations. These difficulties were confirmed to be related to the kinematics of the translational subsystem of the suspension mechanism, specifically the possibility of singularity. Due to which the required force for luminaire repositioning depends on the spatial arrangement of the mechanism. Based on these findings, the goal of this research is to design a surgical luminaire suspension system that improves luminaire repositioning. With the hypothesis that a suspension mechanism without the possibility of singularity will improve luminaire repositioning. Within this research framework, the task was specified to the design of a passive serial suspension mechanism for the surgical luminaire that improves luminaire repositioning and can be easily actuated. This design project exists of two parts, a conceptual design process and a validation of the design. Based on a function analysis of the suspension it was chosen to focus the conceptual design process on the translational subsystem of the suspension. A computer aided method was devised to optimise the mechanism kinematics to the required movement space in the operating room. Which resulted in 13900 serial combination of revolute joints, prismatic joints and links. Based on a scoring routine, a selection of concepts was made and further assessed. The resulting design is an adaptation of the translational subsystem of the conventional suspension mechanism and is considered most feasible. The adaptations consist of a rail system from which the mechanism is suspended and a wrapping pair that couples the two vertical rotations of the pendant-type mechanism. As a result, the horizontal movement space is described without singularity and the movement space is improved. To validate the design and test the hypothesis, a full scale prototype has been build and tested in a user experiment with 14 participants. The prototype is based on a donated conventional suspension system that is adapted to easily switch between the (new) coupled state and the (conventional) uncoupled state. All participants completed an equal movement sequence in both states, thus creating paired measurements of input forces and the position of the mechanism’s end-effector. Based on the consistency in work between opposite movements in one state it was determined that the movement forces in the new design are indeed independent of spatial arrangement, whereas the reverse is true for the uncoupled state. Further analysis between states shows significant improvement in movement duration, work and jerk cost for the coupled mechanism. Also, qualitative data collected during the experiment show that participants significantly favour the coupled mechanism. With these results it can be concluded that the hypothesis is valid and that the research goal has been accomplished.

This study quantifies and explores the nonlinearities of human arm responses to large force perturbations while subjects (n=10) performed either a position or relax task. Continuous perturbations with large variations of amplitude levels (RMS values of 2.5, 7.5, 22.5 mm displacements) and pulse perturbations with large amplitudes (average of 16 cm displacements) were applied at the hand by a 2-DOF robotic manipulator. Linear multivariable identification techniques were used to estimate the endpoint mechanical admittance from the continuous perturbations. The admittance is the relationship between force input and displacement output. Reflexive and intrinsic parameters of a 2-DOF linear arm model were fitted onto the estimated endpoint admittance. This model was used to predict the pulse perturbations. In particular, we determined to what extent human arm displacements in response to large amplitude force pulses can be predicted from identification of continuous perturbations with small amplitudes. Results showed that the estimated admittance for the relax task is a factor 18 larger compared to the estimated admittance for the position task. For the position and relax task, the estimated admittance respectively decreased with a factor 1.5 and increased with a factor 3.8 at the largest perturbation amplitude compared to the lowest amplitude. For the position task, this effect probably resulted from adaptation to the perturbation. The nonlinearity observed for the relax task might be well explained by nonlinear muscle properties such as the short range stiffness. On average, model predictions underestimated the peak displacements in response to the pulse stimulus by a factor 1.7. This shows the need to include nonlinearities in models for pulse shaped loading conditions.