Development of a High-Power Capacity Open Source Electrical Stimulation System to Enhance Research into FES-Assisted Devices: Validation of FES Cycling

International audience: Since the first Cybathlon 2016, when twelve teams competed in the FES bike race, we have witnessed a global effort towards the development of stimulation and control strategies to improve FES-assisted devices, particularly for cycling, as a means to practice a recreational physical activity. As a result, a set of technical notes and research paved the way for many other studies and the potential behind FES-assisted cycling has been consolidated. However, engineering research needs instrumented devices to support novel developments and enable precise assessment. Therefore, some researchers struggle to develop their own FES-assisted devices or find it challenging to implement their instrumentation using commercial devices, which often limits the implementation of advanced control strategies and the possibility to connect different types of sensor. In this regard, we hypothesize that it would be advantageous for some researchers in our community to enjoy access to an entire open-source FES platform that allows different control strategies to be implemented, offers greater adaptability and power capacity than commercial devices, and can be used to assist different functional activities in addition to cycling. Hence, it appears to be of interest to make our proprietary electrical stimulation system an open-source device and to prove its capabilities by addressing all the aspects necessary to implement a FES cycling system. The high-power capacity stimulation device is based on a constant current topology that allows the creation of biphasic electrical pulses with amplitude, width, and frequency up to 150 mA, 1000 µs, and 100 Hz, respectively. A mobile application (Android) was developed to set and modify the stimulation parameters of up to eight stimulation channels. A proportional-integral controller was implemented for cadence tracking with the aim to improve the overall cycling performance. A volunteer with complete paraplegia participated in the functional testing of the system. He was able to cycle indoors for 45 min, accomplish distances of more than 5 km using a passive cycling trainer, and pedal 2400 m overground in 32 min. The results evidenced the capacity of our FES cycling system to be employed as a cycling tool for individuals with spinal cord injury. The methodological strategies used to improve FES efficiency suggest the possibility of maximizing pedaling duration through more advanced control techniques

Investigation and Quantification of FES Exercise – Isometric Electromechanics and Perceptions of Its Usage as an Exercise Modality for Various Populations

Functional Electrical Stimulation (FES) is the triggering of muscle contraction by use of an electrical current. It can be used to give paralyzed individuals several health benefits, through allowing artificial movement and exercise. Although many FES devices exist, many aspects require innovation to increase usability and home translation. In addition, the effect of changing electrical parameters on limb biomechanics is not entirely understood; in particular with regards to stimulation duty cycle. This thesis has two distinct components. In the first (public health component), interview studies were conducted to understand several issues related to FES technology enhancement, implementation and home translation. In the second (computational biomechanics component), novel signal processing algorithms were designed that can be used to measure mechanical responses of muscles subjected to electrical stimulation. These experiments were performed by changing duty cycle and measuring its effect on quadriceps-generated knee torque. The studies of this thesis have presented several ideas, toolkits and results which have the potential to guide future FES biomechanics studies and the translatability of systems into regular usage for patients. The public health studies have provided conceptual frameworks upon which FES may be used in the home by patients. In addition, they have elucidated a range of issues that need to be addressed should FES technology reach its true potential as a therapy. The computational biomechanics studies have put forward novel data analysis techniques which may be used for understanding how muscle responds to electrical stimulation, as measured via torque. Furthermore, the effect of changing the electrical stimulation duty cycle on torque was successfully described, adding to an understanding of how electrical stimulation parameter modulation can influence joint biomechanics

Subjective Costs of Movement: Factors beyond Economy in Human Behavior.

Humans tend not to spend excess energy when they perform a task. However, subjective factors, such as comfort, impact how they behave. Some studies use optimization to explain peoples' decision making process. A task’s goal represents a constraint on behavior, and the chosen, or optimal, behavior is that which minimizes some cost function. Researchers often assume energy or kinematic variability costs, which may miss important subjective motivations. In this work, we leverage constrained optimization to predict and control behavior based on a general subjective cost. The ability to quantify and control decisions for behavior could benefit motor learning research, rehabilitation, and strength training. We developed feedback to uncover subjective costs associated with a number of exercise tasks. We alter task constraints, and their associated subjective costs, by unevenly weighting limb power toward a goal sum of this weighted power. The unknown subjective cost function may thus be characterized by sampling the preferred strategies for a range of different constraints. This method can be used to both characterize subjective costs and provide a framework to direct effort toward specific limbs. Results indicate that healthy subjects split effort between limbs based on more than economy alone, such as grip type, or reach length. These factors can alter the subject’s effort distribution between limbs by about 15% of the mean net power performed. Implicitly weighted feedback was used to unveil the trade-off between excess mechanical power generated and factors beyond economy. The relationship between weightings and limb use allowed us to predictably shift effort toward arms or legs up to 37%. We compared implicit feedback with an explicit alternative, which did not weight effort contributions, but rather specified power and effort distribution targets. When they used implicit feedback, subjects displayed 74% less error relative to their feedback goals, and were able to perform simultaneous cognitive tasks 4.2% faster. Finally, subjective costs inform behavior outside of multi-limb exercise. In a drop landing experiment, subjects performed up to 32% less excess work when they dropped on more cushioned surfaces. These experiments allowed us to quantify subjective costs and predict their effect on human behavior.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107287/1/nskinner_1.pd

On treadmill automation and physiological control systems

This thesis deals with a new approach to Treadmill Automation and Physiological Control Systems that will serve as a platform for enhanced rehabilitation therapy, and will open up and facilitate a major new area of research in physiological control systems. On treadmill automation, the investigation focussed on the feasibility of a low-cost non- contact position control system with the aim of maintaining a subject at a prescribed position during a treadmill training exercise in order to ensure safety at all times. The development of an automatic speed control for the treadmill was first carried out using an identified model for the treadmill motor dynamics (response from speed command to actual belt speed). Subsequently, the positioning control system was designed and tested. The fundamental limitations to treadmill automation performance include low bandwidth and long time delays of the hardware (treadmill and ultrasonic sensor). Interactions between natural human control and the position controller, and spontaneous variability of human movement due to body oscillations and swaying, are discussed. On physiological control systems, we considered two key variables - the heart rate and oxygen uptake. The purpose of this is to develop a means of controlling exercise intensity during treadmill exercise. On the control of heart rate, a model of heart rate response to changes in speed was obtained via the system identification method. Thereafter, a heart rate controller was developed, tested, and evaluated on three healthy subjects during treadmill exercise. The results of the experiments demonstrated that the developed heart rate controller is superior to the in-built treadmill heart rate controller. A novel system is developed for the control of oxygen uptake, and is thus presented. The system proved that it is possible to control exercise intensity using the level of oxygen uptake during moderate exercise. The results of this work demonstrate that a linear first order model is able to sufficiently capture the complex dynamics of oxygen uptake during treadmill exercise. A controller was developed using this model and tested on healthy subjects. Six healthy active subjects participated completely in the tests. The results of the tests with these subjects establish the robustness of the controllers to inter-subject variability. Furthermore, the controller was refined and tuned to improve the performance of the control signal (treadmill speed). This was achieved by designing a controller that incorporated a pre-filter in the system. The results of the experiments using this set-up with the same set of healthy subjects show a significant improvement on the control signal (smoother treadmill speed)

A system to provide guidance to stroke patients during independent physiotherapy

Stroke is a serious disease that leaves many sufferers physically disabled. Treatment resources are limited, meaning stroke patients, are in many cases, discharged prior to reaching their full potential of physical recovery. The hypothesis of this research is that a system that enables regular guided and monitored therapeutic exercises in the home can provide a means for stroke patients to achieve a higher level of physical rehabilitation. This research is based on the design, build and testing of an experimental prototype system to allow this, with the aim of investigating the feasibility and potential value for such systems. Any system to assist rehabilitation in the home must clearly be low cost, safe and easy to use. The prototype system therefore aimed to achieve these features as well as focusing on the upper limb. Literature is reviewed in the fields of stroke, human anatomy and mechanisms, motor performance, feedback during motor learning, and existing systems and technology. Interviews are also conducted with stroke physiotherapists to gain input and feedback on concepts that were generated. Although systems exist with similar aims to those mentioned in the hypothesis, there are some areas where investigation is lacking. The prototype system measures movement using a novel combination of gyro sensors and flex sensors. The prototype system is designed with a focus on the method of interaction with patients and the provision of guidance and feedback that simulates that provided by a physiotherapist. The prototype system also provides a unique combination of quantitative information to patients of their personal improvements and graphical feedback of their movements and target movements. Finally, a novel categorisation of movement synergism (a form of movement coordination) is established and a novel method for detecting movement synergism is developed and tested. Performance of the prototype hardware is tested, and it is concluded that identified requirements have been met, although variability of recorded data is high. Tests also indicate that the prototype system is capable of detecting movement synergism. Finally, a controlled test involving healthy participants is performed to investigate the efficacy of the prototype as a whole. It was found that use of the prototype system resulted in a statistically significant improvement in conformance to target movements (ρ < 0.05). Findings are discussed in detail and the hypothesis is concluded as being supported overall. Recommendations for future research are made

Development of a hybrid assist-as-need hand exoskeleton for stroke rehabilitation.

Stroke is one of the leading causes of disability globally and can significantly impair a patient’s ability to function on a daily basis. Through physical rehabilitative measures a patient may regain a level of functional independence. However, required therapy dosages are often not met. Rehabilitation is typically implemented through manual one-to-one assistance with a physiotherapist, which quickly becomes labour intensive and costly. Hybrid application of functional electrical stimulation (FES) and robotic support can access the physiological benefits of direct muscle activation while providing controlled and repeatable motion assistance. Furthermore, patient engagement can be heightened through the integration of a volitional intent measure, such as electromyography (EMG). Current hybrid hand-exoskeletons have demonstrated that a balanced hybrid support profile can alleviate FES intensity and motor torque requirements, whilst improving reference tracking errors. However, these support profiles remain fixed and patient fatigue is not addressed. The aim of this thesis was to develop a proof-of-concept assist-as-need hybrid exoskeleton for post-stroke hand rehabilitation, with fatigue monitoring to guide the balance of support modalities. The device required the development and integration of a constant current (CC) stimulator, stimulus-resistant EMG device, and hand-exoskeleton. The hand exoskeleton in this work was formed from a parametric Watt I linkage model that adapts to different finger sizes. Each linkage was optimised with respect to angular precision and compactness using Differential Evolution (DE). The exoskeleton’s output trajectory was shown to be sensitive to parameter variation, potentially caused by finger measurement error and shifts in coupler placement. However, in a set of cylindrical grasping trials it was observed that a range of movement strategies could be employed towards a successful grasp. As there are many possible trajectories that result in a successful grasp, it was deduced that the exoskeleton can still provide functional assistance despite its sensitivity to parameter variation. The CC stimulator developed in this work has a part cost of USD $145 and allows flexible adjustment of waveform parameters through an on-board micro-controller. The device is designed to output current up to ±30mA given a voltage compliance of ±50V. When applied across a 2kΩ load, the device exhibited a linear output transfer function, with a maximum ramp tracking error of 5 The stimulus-resistant EMG device builds on current designs by using a novel Schmitt trigger based artefact detection channel to adaptively blank stimulation artefacts without stimulator synchronisation. The design has a part cost of USD$150 and has been made open-source. The device demonstrated its ability to record EMG over its predominant energy spectrum during stimulation, through the stimulation electrodes or through separate electrodes. Pearson’s correlation coefficients greater than 0.84 were identified be- tween the normalised spectra of volitional EMG (vEMG) estimates during stimulation and of stimulation-free EMG recordings. This spectral similarity permits future research into applications such as spectral-based monitoring of fatigue and muscle coherence, posing an advantage over current same-electrode stimulation and recording systems, which can- not sample the lower end of the EMG spectrum due to elevated high-pass filter cut-off frequencies. The stimulus-resistant EMG device was used to investigate elicited EMG (eEMG)-based fatigue metrics during vEMG-controlled stimulation and hybrid support profiles. During intermittent vEMG-controlled stimulation, the eEMG peak-to-peak amplitude (PTP) index was the median frequency (MDF) had a negative correlation for all subjects with R > 0:62 during stimulation-induced wrist flexion and R > 0:55 during stimulation-induced finger flexion. During hybrid FES-robotic support trials, a 40% reduction in stimulus intensity resulted in an average 21% reduction in MDF gradient magnitudes. This reflects lower levels of fatigue during the hybrid support profile and indicates that the MDF gradient can provide useful information on the progression of muscle fatigue. A hybrid exoskeleton system was formed through the integration of the CC stimulator, stimulus-resistant EMG device, and the hand exoskeleton developed in this work. The system provided assist-as-need functional grasp assistance through stimulation and robotic components, governed by the user’s vEMG. The hybrid support profile demonstrated consistent motion assistance with lowered stimulation intensities, which in-turn lowered the subjects’ perceived levels of fatigue

The effect of prefabricated wrist-hand orthoses on performing activities of daily living

Wrist-hand orthoses (WHOs) are commonly prescribed to manage the functional deficit associated with the wrist as a result of rheumatoid changes. The common presentation of the wrist is one of flexion and radial deviation with ulnar deviation of the fingers. This wrist position Results in altered biomechanics compromising hand function during activities of daily living (ADL). A paucity of evidence exists which suggests that improvements in ADL with WHO use are very task specific. Using normal subjects, and thus in the absence of pain as a limiting factor, the impact of ten WHOs on performing five ADLs tasks was investigated. The tasks were selected to represent common grip patterns and tests were performed with and without WHOs by right-handed, females, aged 20-50 years over a ten week period. The time taken to complete each task was recorded and a wrist goniometer, elbow goniometer and a forearm torsiometer were used to measure joint motion. Results show that, although orthoses may restrict the motion required to perform a task, participants do not use the full range of motion which the orthoses permit. The altered wrist position measured may be attributable to a modified method of performing the task or to a necessary change in grip pattern, resulting in an increased time in task performance. The effect of WHO use on ADL is task specific and may initially impede function. This could have an effect on WHO compliance if there appears to be no immediate benefits. This orthotic effect may be related to restriction of wrist motion or an inability to achieve the necessary grip patterns due to the designs of the orthoses