Stroke is a leading cause of long-term disability worldwide. Dropfoot gait, or inability to adequately lift, advance, and land on the foot on the more impaired side, is one of the most common gait impairments following a stroke, and often results in reduced mobility and increased fall risk, severely impacting independence and quality of life. Wearable robotics and exoskeletons have been widely explored for their potential in physical rehabilitation and mobility assistance for individuals with motor disorders. However, despite their promise, few existing systems have demonstrated convincing evidence for use. The specific exoskeleton performance challenges in assisting dropfoot gait that are addressed in this compilation thesis are 1) to simultaneously control both sagittal plane ankle dorsiflexion and frontal plane ankle inversion/eversion motions, and 2) to identify control strategies that are individualized based on gait impairments and subjective preferences. 

Specifically, the aims of the thesis were to design an ankle exoskeleton that is capable of providing biplanar assistance and meets both biomechanical and feasibility requirements, to develop a customized control framework that optimizes multiple performance metrics, and to identify subject-specific assistive parameters that improve gait metrics while aligning with users' subjective assessments as well.

The first two studies focus on the development and feasibility of a soft ankle exoskeleton designed to provide  assistance in both ankle dorsiflexion and eversion, via  cable-driven mechanisms and compliant materials. Initial testing confirmed the exoskeleton's ability to effectively guide ankle joint motion in both planes with minimal resistance. In the second study, the device's feasibility was evaluated in a pilot group of persons with dropfoot gait following a stroke. Improvements in key gait parameters, along with positive user assessment on the device's comfort, usability, and perceived effectiveness, encourage further application of the device in persons in a chronic post-stroke phase.

The third and fourth studies focus on developing personalized exoskeleton control strategies, specifically a multi-objective human-in-the-loop optimization framework that evaluates individual responses to various assistive profiles, then identifies assistive profiles tailored to each individual's gait impairments. The framework was constructed to simultaneously optimize two objectives that describe gait quality. This approach yielded not just one, but a group of good solutions that improve both gait metrics to varying degrees, among which solutions can be selected based on context and preference. The framework was developed and tested on a group of non-disabled subjects with a simulated dropfoot impairment in the third study and on a pilot group of persons with dropfoot following a stroke in the fourth study. In the fourth study, the personalization framework was further advanced by incorporating user preferences, thereby incorporating both objective gait quality metrics and subjective preference in identifying optimal exoskeleton assistance.

This thesis advances the application of exoskeletons for individuals post-stroke by addressing both hardware design and personalized control strategies. The findings highlight the potential of the developed ankle exoskeleton to enhance mobility in this population and underscore the importance of individualized assistance to meet diverse user needs. Together, the exoskeleton design and individualized control framework offer a valuable foundation for future research and practical implementation of assistive technologies.

Stroke är en ledande orsak till långvarig funktionsnedsättning globalt. Droppfot vid gång, eller nedsatt förmåga att lyfta, flytta fram och placera den påverkade foten på ett adekvat sätt är vanligt efter stroke. Droppfot vid gång är ofta förknippad med nedsatt förflyttningsförmåga och ökad fallrisk, samt minskad självständighet och livskvalitet. Potentialen hos bärbar robotik och exoskelett inom rehabilitering och assistans för personer med motoriska funktionsnedsättningar har undersökts brett. Få befintliga system har dock visat övertygande bevis för sin praktiska användning. Denna avhandling fokuserar på två specifika utmaningar i utformningen av ett exoskelettet för assistans vid droppfot under gång: 1) att kontrollera fotens position i både sagittala och frontala plan samtidigt, och 2) att individualisera exoskelettets  assistans för att förbättra både objektiva mått och subjektiv upplevelse.

Syften med avhandlingen är således att utveckla ett exoskelett som kan ge biplanär assistans runt fotleden som uppfyller både biomekaniska och genomförbarhetskrav, att utveckla ett skräddarsytt kontrollramverk som optimerar flera prestationsmått, och att identifiera individuellt anpassad assistans som förbättrar gång och uppfyller subjektiva kriterier.

De två första studierna fokuserar på utvecklingen och genomförbarhet av ett mjukt fotledsexoskelett utformat för att ge assistans vid både dorsalflektion och eversion, via kabeldriven mekanik och följsamma material. Experiment i den första studien bekräftar exoskelettets förmåga att kontrollera fotledsrörelser i båda planen med minimalt motstånd. I den andra studien utvärderades genomförbarheten av exoskelettet i en pilotgrupp av personer med droppfot vid gång i kronisk fas efter stroke. Vi fann förbättringar av viktiga gångparametrar, tillsammans med en positiv användarbedömning av komfort, användbarhet och upplevd effektivitet, vilket ytterligare uppmuntrar tillämpningen för personer i en kronisk fas efter stroke.

Den tredje och fjärde studien fokuserar på att utveckla personliga strategier för exoskelettkontroll, specifikt en multi-objektiv human-in-the-loop optimeringsramverk som utvärderar individuella svar på olika assistansprofiler, och sedan identifierar assistansprofiler skräddarsydda för varje individs gångavvikelse. Ramverket konstruerades för att samtidigt optimera två aspekter av gångkvalitet. Detta tillvägagångssätt gav inte bara en, utan en grupp av bra lösningar vilka förbättrar båda gångparametrarna i varierande grad vilka kan väljas baserat på sammanhang och preferenser. Ramverket utvecklades och testades i den tredje studien på en grupp försökspersoner utan gångavvikelser med en simulerad droppfotsnedsättning och  i den fjärde studien på en pilotgrupp av personer med droppfot efter stroke. I den fjärde studien utvecklades individualiseringsramverket ytterligare genom att ta hänsyn till användarpreferenser, och därigenom identifiera den optimala assistansen utifrån både objektiva gångkvalitetsmått och subjektiva preferenser.

Denna avhandling främjar tillämpningen av exoskelett för individer efter stroke genom både hårdvarudesign och individualiserade kontrollstrategier. Våra resultat stöder utvecklingen av exoskelett för att förbättra förflyttningsförmågan i denna population och belyser vikten av individualiserad assistans för att möta olika användarbehov. Våra resultat och utvecklingen ger en värdefull grund för framtida forskning och praktisk implementering av assisterande hjälpmedel vid gång. 

Wearable robotic exoskeletons are frequently explored for their efficacy in rehabilitation and in assistance in daily activities in people with motor disorders, yet relatively few have convincing evidence for use. Here we describe a cable-driven ankle exoskeleton that provides assistance to the ankle in sagittal and frontal planes simultaneously, aimed for persons with dropfoot and excessive inversion after e.g. stroke. In this study, we propose a multi-objective human-in-the-loop optimization that adjusts exoskeleton control parameters to improve two independent gait quality measures, specifically foot segment kinematics and step length symmetry. We illustrate how the identified solutions represent a balance between the two objectives.

Introduction Wearable exoskeletons are emerging technologies for providing movement assistance and rehabilitation for people with motor disorders. In this study, we focus on the specific gait pathology dropfoot, which is common after a stroke. Dropfoot makes it difficult to achieve foot clearance during swing and heel contact at early stance and often necessitates compensatory movements. Methods We developed a soft ankle exoskeleton consisting of actuation and transmission systems to assist two degrees of freedom simultaneously: dorsiflexion and eversion, then performed several proof-of-concept experiments on non-disabled persons. The actuation system consists of two motors worn on a waist belt. The transmission system provides assistive force to the medial and lateral sides of the forefoot via Bowden cables. The coupling design enables variable assistance of dorsiflexion and inversion at the same time, and a force-free controller is proposed to compensate for device resistance. We first evaluated the performance of the exoskeleton in three seated movement tests: assisting dorsiflexion and eversion, controlling plantarflexion, and compensating for device resistance, then during walking tests. In all proof-of-concept experiments, dropfoot tendency was simulated by fastening a weight to the shoe over the lateral forefoot. Results In the first two seated tests, errors between the target and the achieved ankle joint angles in two planes were low; errors of <1.5 degrees were achieved in assisting dorsiflexion and/or controlling plantarflexion and of <1.4 degrees in assisting ankle eversion. The force-free controller in test three significantly compensated for the device resistance during ankle joint plantarflexion. In the gait tests, the exoskeleton was able to normalize ankle joint and foot segment kinematics, specifically foot inclination angle and ankle inversion angle at initial contact and ankle angle and clearance height during swing. Discussion Our findings support the feasibility of the new ankle exoskeleton design in assisting two degrees of freedom at the ankle simultaneously and show its potential to assist people with dropfoot and excessive inversion.

Integrating mobile eye-tracking and motion capture emerges as a promising approach in studying visual-motor coordination, due to its capability of expressing gaze data within the same laboratory-centered coordinate system as body movement data. In this paper, we proposed an integrated eye-tracking and motion capture system, which can record and analyze temporally and spatially synchronized gaze and motion data during dynamic movement. The accuracy of gaze measurement were evaluated on five participants while they were instructed to view fixed vision targets at different distances while standing still or walking towards the targets. Similar accuracy could be achieved in both static and dynamic conditions. To demonstrate the usability of the integrated system, several walking tasks were performed in three different pathways. Results revealed that participants tended to focus their gaze on the upcoming path, especially on the downward path, possibly for better navigation and planning. In a more complex pathway, coupled with more gaze time on the pathway, participants were also found having the longest step time and shortest step length, which led to the lowest walking speed. It was believed that the integration of eye-tracking and motion capture is a feasible and promising methodology quantifying visual-motor coordination in locomotion.

Research interest in exoskeleton assistance strategies that incorporate the user's torque capacity is rapidly growing, yet uncertainty in predicted torque capacity can significantly impact the user-exoskeleton interface safety. In this paper, we estimated knee flexion/extension torques by using a neuromusculoskeletal (NMS) solver-informed Gaussian process (NMS-GP) model with muscle electromyography signals and joint kinematics as model inputs. The NMS-GP model combined the NMS and GP models by integrating valuable features from an NMS solver into a GP model. The NMSGP model was used to predict knee joint torque in daily activities with uncertainty quantification. The activities included slow walking, self-selected speed walking, fast walking, sit-to-stand, and stand-to-sit. Model performance, defined as low prediction error between the model's predicted torque and measured torques from inverse dynamics computations, of both the NMS-GP and NMS models was analyzed. We found that prediction error was significantly lower in NMS-GP models than in NMS models. We observed relatively high uncertainties in the predicted knee torque at the beginning of each movement, particularly in self-selected speed walking. High uncertainties were also found during terminal stance and swing in self-selected speed walking. Compared to other torque prediction methods, the proposed NMS-GP model not only provides an accurate joint torque prediction but also a measure of the uncertainty. Our study showed that the NMS-GP model has a large potential in control strategy design for rehabilitation exoskeletons and to enhance the overall user experience.

Introduction: Research interest in exoskeleton assistance strategies that incorporate the user's torque capacity is growing rapidly. However, the predicted torque capacity from users often includes uncertainty from various sources, which can have a significant impact on the safety of the exoskeleton-user interface. Methods: To address this challenge, this paper proposes an adaptive control framework for a knee exoskeleton that uses muscle electromyography (EMG) signals and joint kinematics. The framework predicted the user's knee flexion/extension torque with confidence bounds to quantify the uncertainty based on a neuromusculoskeletal (NMS) solver-informed Bayesian Neural Network (NMS-BNN). The predicted torque, with a specified confidence level, controlled the assistive torque provided by the exoskeleton through a TCP/IP stream. The performance of the NMS-BNN model was also compared to that of the Gaussian process (NMS-GP) model. Results: Our findings showed that both the NMS-BNN and NMS-GP models accurately predicted knee joint torque with low error, surpassing traditional NMS models. High uncertainties were observed at the beginning of each movement, and at terminal stance and terminal swing in self-selected speed walking in both NMS-BNN and NMS-GP models. The knee exoskeleton provided the desired assistive torque with a low error, although lower torque was observed during terminal stance of fast walking compared to self-selected walking speed. Discussion: The framework developed in this study was able to predict knee flexion/extension torque with quantifiable uncertainty and to provide adaptive assistive torque to the user. This holds significant potential for the development of exoskeletons that provide assistance as needed, with a focus on the safety of the exoskeleton-user interface.

Dropfoot is a common and persistent impairment following stroke, often leading to reduced mobility and increased fall risk. While assistive exoskeletons have shown potential to enhance mobility in individuals with neurological conditions, there is practically no widespread use. Customized assistance may maximize user-specific benefits, but its application in post-stroke populations remains underexplored. In this study, we propose a customized exoskeleton control framework designed to optimize assistance for individuals with dropfoot gait patterns following a stroke. The approach combines objective metrics from multiobjective human-in-the-loop optimization, with goals related to improving foot kinematics and step length symmetry, with subjective assessment of perceived assistance. Five participants with chronic stroke and dropfoot gait patterns donned our previously developed soft ankle exoskeleton. Three generations of optimization were performed, and a Pareto front of optimal assistive profiles was identified that optimized the objective gait metrics to varying degrees. Subjective scores of perceived assistance were also documented during the experiements. The assistance profiles with the most improved gait metrics agreed to some extent aligned with the most highly rated profiles. These findings support a framework to identify subject-specific exoskeleton assistance profiles based on a combination of objective gait metrics and subjective perception in individuals with gait impairments.

Dropfoot gait pattern during walking commonly persists after stroke and is often associated with muscle weaknessand pathological muscle activation. Exoskeletons have demonstrated the potential to improve mobility inpeople with neurological conditions. We have developed a novel soft ankle exoskeleton and shown its abilityto correct simulated dropfoot and excessive inversion in non-disabled people. In this study, we evaluate its feasibilityin five persons with chronic stroke and dropfoot gait patterns. 3D gait analysis was performed in threeconditions: walking with only shoes, with the exoskeleton unpowered, and powered. Foot and ankle kinematicsand step length asymmetry were evaluated. The participants also reported satisfaction with QUEST 2.0 anda study-specific questionnaire. Compared with only shoes, the powered exoskeleton partially corrected dropfootby increasing dorsiflexion angle and foot clearance height in swing, facilitated heel contact, neutralized ankleinversion, and increased step length symmetry slightly. The participants expressed satisfaction with the exoskeleton’seffectiveness, though some comfort-related issues were identified. This feasibility study suggests that theexoskeleton prototype can improve dropfoot gait patterns and be accepted by individuals in the chronic stage aftera stroke.

Wearable robotic exoskeletons are frequently ex-plored for their efficacy in physical rehabilitation and forassistance in daily activities in people with motor disorders, yetrelatively few have convincing evidence for use. The conceptof human-in-the-loop optimization has been used to identifyideal exoskeleton assistive torques based on measured individualperformance metrics, whereas few studies report optimizing sev-eral performance metrics simultaneously. We developed a cable-driven ankle exoskeleton providing sagittal and frontal planeassistance, aimed at persons with dropfoot and excessive inversionafter stroke. In this study, we propose a multi-objective human-in-the-loop optimization that adjusts exoskeleton assistive profilesto improve several gait quality measures, specifically foot segmentkinematics and step length symmetry. Five non-disabled subjectstested the feasibility of the proposed method. An extra weight wasattached to the top of the right shoe to simulate the gait deviationsassociated with dropfoot and excessive inversion. The gait qualitymetrics improved more for each new generation. With optimalassistive solutions, foot segment kinematics improved (10mmhigher foot clearance height and a 7° increase in inclination angle)and step length became more symmetric (asymmetry reducedfrom 9% to 2%), compared to simulated impairment. Within thisset of solutions, each represents a unique balance between thetwo objectives, wherein a solution that prioritized normalizingstep length symmetry could slightly sacrifice normalizing footsegment kinematics, and vice versa. By taking advantage ofthe trade-off relationship between the two objectives, moreflexible individualized and situation-dependent assistance can beobtained. These results demonstrate the method’s usefulness indetermining subject-specific exoskeleton control parameters thatimprove several measures of gait in persons with motor disability.Future applications include refining this protocol for individualswith actual dropfoot and offering personalized assistance torestore their functional mobility.

Wearable robotic exoskeletons have been explored for their efficacy in physical rehabilitation and for assistance in daily activites in people with motor disorders. The concept of human-in-the-loop optimization has been used to identify ideal exoskeleton assistive torques based on measured individual performance metrics, for instance, metabolic cost, but few studies have attempted to optimize several performance metrics simultaneously.We have previously developed a cable-driven ankle exoskeleton that can provide assistance to the ankle in both sagittal and frontal planes, aimed for persons with dropfoot and excessive inversion. In this study, we propose a multi-objective human-in-the-loop optimization that identifies the ankle exoskeleton properties that improve gait quality, defined here as normal foot segment kinematics and step length symmetry. One able-bodied subject tested the feasibility of the proposed method. The subject wore the exoskeleton while walking on a treadmill. An extra weight was attached on the right foot to simulate the gait deviations associated with dropfoot and excessive inversion.  The Pareto front, comprising six results, was sorted, which illustrated the improvement in both foot segment kinematics and step length symmetry. Within this set of results, each represents a unique compromise or balance between the two objectives, and it was observed that enhancing step length symmetry might lead to an increase in foot segment deviation, showcasing the inherent trade-off relationship between these two aspects.These results suggest that this method can potentially be useful in determining subject-specific exoskeleton control laws that improve several measures of gait in persons with gait disability.