Stationary Cycling Induced by Switched Functional Electrical Stimulation Control

Functional electrical stimulation (FES) is used to activate the dysfunctional lower limb muscles of individuals with neuromuscular disorders to produce cycling as a means of exercise and rehabilitation. However, FES-cycling is still metabolically inefficient and yields low power output at the cycle crank compared to able-bodied cycling. Previous literature suggests that these problems are symptomatic of poor muscle control and non-physiological muscle fiber recruitment. The latter is a known problem with FES in general, and the former motivates investigation of better control methods for FES-cycling.In this paper, a stimulation pattern for quadriceps femoris-only FES-cycling is derived based on the effectiveness of knee joint torque in producing forward pedaling. In addition, a switched sliding-mode controller is designed for the uncertain, nonlinear cycle-rider system with autonomous state-dependent switching. The switched controller yields ultimately bounded tracking of a desired trajectory in the presence of an unknown, time-varying, bounded disturbance, provided a reverse dwell-time condition is satisfied by appropriate choice of the control gains and a sufficient desired cadence. Stability is derived through Lyapunov methods for switched systems, and experimental results demonstrate the performance of the switched control system under typical cycling conditions.Comment: 8 pages, 3 figures, submitted to ACC 201

Comparison of stimulation patterns for FES-cycling using measures of oxygen cost and stimulation cost

<b>Aim</b><p></p> The energy efficiency of FES-cycling in spinal cord injured subjects is very much lower than that of normal cycling, and efficiency is dependent upon the parameters of muscle stimulation. We investigated measures which can be used to evaluate the effect on cycling performance of changes in stimulation parameters, and which might therefore be used to optimise them. We aimed to determine whether oxygen cost and stimulation cost measurements are sensitive enough to allow discrimination between the efficacy of different activation ranges for stimulation of each muscle group during constant-power cycling. <p></p> <b>Methods</b><p></p> We employed a custom FES-cycling ergometer system, with accurate control of cadence and stimulated exercise workrate. Two sets of muscle activation angles (“stimulation patterns”), denoted “P1” and “P2”, were applied repeatedly (eight times each) during constant-power cycling, in a repeated measures design with a single paraplegic subject. Pulmonary oxygen uptake was measured in real time and used to determine the oxygen cost of the exercise. A new measure of stimulation cost of the exercise is proposed, which represents the total rate of stimulation charge applied to the stimulated muscle groups during cycling. A number of energy-efficiency measures were also estimated. <p></p> <b>Results</b><p></p> Average oxygen cost and stimulation cost of P1 were found to be significantly lower than those for P2 (paired <i>t</i>-test, <i>p</i> < 0.05): oxygen costs were 0.56 ± 0.03 l min<sup>−1</sup> and 0.61 ± 0.04 l min<sup>−1</sup>(mean ± S.D.), respectively; stimulation costs were 74.91 ± 12.15 mC min<sup>−1</sup> and 100.30 ± 14.78 mC min<sup>−1</sup> (mean ± S.D.), respectively. Correspondingly, all efficiency estimates for P1 were greater than those for P2. <p></p> <b>Conclusion</b><p></p> Oxygen cost and stimulation cost measures both allow discrimination between the efficacy of different muscle activation patterns during constant-power FES-cycling. However, stimulation cost is more easily determined in real time, and responds more rapidly and with greatly improved signal-to-noise properties than the ventilatory oxygen uptake measurements required for estimation of oxygen cost. These measures may find utility in the adjustment of stimulation patterns for achievement of optimal cycling performance. <p></p&gt

Switched Kinematic and Force Control for Lower-Limb Motorized Exoskeletons and Functional Electrical Stimulation

Millions of people experience movement deficits from neurological conditions (NCs) that impair their walking ability and leg function. Exercise-based rehabilitation procedures have shown the potential to facilitate neurological reorganization and functional recovery. Lower-limb powered exoskeletons and motorized ergometers have been combined with functional electrical stimulation (FES) to provide repetitive movement, partially reduce the burden of therapists, improve range of motion, and induce therapeutic benefits. FES evokes artificial muscles contractions and can improve muscle mass and strength, and bone density in people with NCs. Stationary cycling is recommended for individuals who cannot perform load-bearing activities or have increased risks of falling. Cycling has been demonstrated to impart physiological and cardiovascular benefits. Motorized FES-cycling combines an electric motor and electrical stimulation of lower-limb muscles to facilitate coordinated, long-duration exercise, while mitigating the inherent muscle fatigue due to FES. Lower-limb exoskeletons coupled with FES, also called neuroprostheses or hybrid exoskeletons, can facilitate continuous, repetitive motion to improve gait function and build muscle capacity. The human-robot interaction during rehabilitative cycling and walking yield a mix of discrete effects (i.e., foot impact, input switching to engage lower-limb muscles and electric motors, etc.) and continuous nonlinear, uncertain, time-varying dynamics. Switching control is necessary to allocate the control inputs to lower-limb muscle groups and electric motors involved during assisted cycling and walking. Kinematic tracking has been the primary control objective for devices that combine FES and electric motors. However, there are force interactions between the machine and the human during cycling and walking that motivate the design of torque-based controllers (i.e., exploit torque or force feedback) to shape the leg dynamics through controlling joint kinematics and kinetics. Technical challenges exist to develop closed-loop feedback control strategies that integrate kinematic and force feedback in the presence of switching and discontinuous effects. The motivation in this dissertation is to design, analyze and implement switching controllers for assisted cycling and walking leveraging kinematic and force feedback while guaranteeing the stability of the human-robot closed-loop system. In Chapter 1, the motivation to design closed-loop controllers for motorized FES-cycling and powered exoskeletons is described. A survey of closed-loop kinematic and force feedback control methods is also introduced related to the tracking objectives presented in the subsequent chapters of the dissertation. In Chapter 2, the dynamics models for walking and assisted cycling are described. First, a bipedal walking system model with switched dynamics is introduced to control a powered lower-limb exoskeleton. Then, a stationary FES-cycling model with nonlinear dynamics and switched control inputs is introduced based on published literature. The muscle stimulation pattern is defined based on the kinematic effectiveness of the rider, which depends on the crank angle. The experimental setup for lower-limb exoskeleton and FES-cycling are described. In Chapter 3, a hierarchical control strategy is developed to interface a cable-driven lower-limb exoskeleton. A two-layer control system is developed to adjust cable tensions and apply torque about the knee joint using a pair of electric motors that provide knee flexion and extension. The control design is segregated into a joint-level control loop and a low-level loop using feedback of the angular positions of the electric motors to mitigate cable slacking. A Lyapunov-based stability analysis is developed to ensure exponential tracking for both control objectives. Moreover, an average dwell time analysis computes an upper bound on the number of motor switches to preserve exponential tracking. Preliminary experimental results in an able-bodied individual are depicted. The developed control strategy is extended and applied to the control of both knee and hip joints in Chapter 4 for treadmill walking. In Chapter 4, a cable-driven lower-limb exoskeleton is integrated with FES for treadmill walking at a constant speed. A nonlinear robust controller is used to activate the quadriceps and hamstrings muscle groups via FES to achieve kinematic tracking about the knee joint. Moreover, electric motors adjust the knee joint stiffness throughout the gait cycle using an integral torque feedback controller. A Lyapunov-based stability analysis is developed to ensure exponential tracking of the kinematic and torque closed-loop error systems, while guaranteeing that the control input signals remain bounded. The developed controllers were tested in real-time walking experiments on a treadmill in three able-bodied individuals at two gait speeds. The experimental results demonstrate the feasibility of coupling a cable-driven exoskeleton with FES for treadmill walking using a switching-based control strategy and exploiting both kinematic and force feedback. In Chapter 5, input-output data is exploited using a finite-time algorithm to estimate the target desired torque leveraging an estimate of the active torque produced by muscles via FES. The convergence rate of the finite-time algorithm can be adjusted by tuning selectable parameters. To achieve cadence and torque tracking for FES-cycling, nonlinear robust tracking controllers are designed for muscles and motor. A Lyapunov-based stability analysis is developed to ensure exponential tracking of the closed-loop cadence error system and global uniformly ultimate bounded (GUUB) torque tracking. A discrete-time Lyapunov-based stability analysis leveraging a recent tool for finite-time systems is developed to ensure convergence and guarantee that the finite-time algorithm is Holder continuous. The developed tracking controllers for the muscles and electric motor and finite-time algorithm to compute the desired torque are implemented in real-time during cycling experiments in seven able-bodied individuals. Multiple cycling trials are implemented with different gain parameters of the finite-time torque algorithm to compare tracking performance for all participants. Chapter 6 highlights the contributions of the developed control methods and provides recommendations for future research extensions

Switching Adaptive Concurrent Learning Control for Powered Rehabilitation Machines with FES

Interfacing robotic devices with humans presents significant control challenges, as the control algorithms governing these machines must accommodate for the inherent variability among individuals. This requirement necessitates the system’s ability to adapt to changes in the environment, particularly in the context of human-in-the-loop applications, wherein the system must identify specific features of the human interacting with the machine. In the field of rehabilitation, one promising approach for exercise-based rehabilitation involves the integration of hybrid rehabilitation machines, combining robotic devices such as motorized bikes and exoskeletons with functional electrical stimulation (FES) applied on lower-limb muscles. This integrated approach offers the potential for repetitive training, reduced therapist workload, improved range of motion, and therapeutic benefits. However, conducting prolonged rehabilitation sessions to maximize functional recovery using these hybrid machines imposes several difficulties. Firstly, the design and analysis of adaptive controllers are motivated, but challenges exist in coping with the inherent switching effects associated with hybrid machines. Notably, the transitions between gait phases and the dynamic switching of inputs between active lower-limb muscles and electric motors and their incorporation in the control design remain an open problem for the research community. Secondly, the system must effectively compensate for the influence of human input, which can be viewed as an external disturbance in the closed-loop system during rehabilitation. Robust methods for understanding and adapting to the variations in human input are critical for ensuring stability and accurate control of the human-robot closed-loop system. Lastly, FES-induced muscle fatigue diminishes the human torque contribution to the rehabilitation task, leading to input saturation and potential instabilities as the duration of the exercise extends. Overcoming this challenge requires the development of control algorithms that can adapt to variations in human performance by dynamically adjusting the control parameters accordingly. Consequently, the development of rehabilitative devices that effectively interface with humans requires the design and implementation of control algorithms capable of adapting to users with varying muscle and kinematic characteristics. In this regard, adaptive-based control methods provide tools for addressing the uncertainties in human-robot dynamics within exercise-based rehabilitation using FES, while ensuring stability and robustness in the human-robot closed-loop system. This dissertation develops adaptive controllers to enhance the effectiveness of exercise-based rehabilitation using FES. The objectives include the design and evaluation of adaptive control algorithms that effectively handle the switching effects inherent in hybrid machines, adapt to compensate for human input, and account for input saturation due to muscle fatigue. The control designs leverage kinematic and torque feedback and ensure the stability of the human-robot closed-loop system. These controllers have the potential to significantly enhance the practicality and effectiveness of assistive technologies in both clinical and community settings. In Chapter 1, the motivation to design switching adaptive closed-loop controllers for motorized FES-cycling and powered exoskeletons is described. A survey of closed-loop kinematic control methods related to the tracking objectives in the subsequent chapters of the dissertation is also introduced. In Chapter 2, the dynamic models for cycling and bipedal walking are described: (i) a stationary FES-cycling model with nonlinear dynamics and switched control inputs are introduced based on published literature. The muscle stimulation pattern is defined based on the kinematic effectiveness of the rider, which depends on the crank angle. (ii) A phase-dependent bipedal walking system model with switched dynamics is introduced to control a 4-degrees-of-freedom (DoF) lower-limb exoskeleton assuming single stance support. Moreover, the experimental setup of the cycle-rider and lower-limb exoskeleton system are described. Chapter 3 presents a switched concurrent learning adaptive controller for cadence tracking using the cycle-rider model. The control design is decoupled for the muscles and electric motor. An FES controller is developed with minimal parameters, capable of generating bounded muscle responses with an adjustable saturation limit. The electric motor controller employs an adaptive-based method that estimates uncertain parameters in the cycle-rider system and leverages the muscle input as a feedforward term to improve the tracking of crank trajectories. The adaptive motor controller and saturated muscle controller are implemented in able-bodied individuals and people with movement disorders. Three cycling trials were conducted to demonstrate the feasibility of tracking different crank trajectories with the same set of control parameters across all participants. The developed adaptive controller requires minimal tuning and handles rider uncertainty while ensuring predictable and satisfactory performance. This result has the potential to facilitate the widespread implementation of adaptive closed-loop controllers for FES-cycling systems in real clinical and home-based scenarios. Chapter 4 presents an integral torque tracking controller with anti-windup compensation, which achieves the dual objectives of kinematic and torque tracking (i.e., power tracking) for FES cycling. Designing an integral torque tracking controller to avoid feedback of high-order derivatives poses a significant challenge, as the integration action in the muscle loop can induce error buildup; demanding high FES input on the muscle. This can cause discomfort and accelerate muscle fatigue, thereby limiting the practical utility of the power tracking controller. To address this issue, this chapter builds upon the adaptive control for cadence tracking developed in Chapter 3 and integrates a novel torque tracking controller that allows for input saturation in the FES controller. By doing so, the controller achieves cadence and torque tracking while preventing error buildup. The analysis rigorously considers the saturation effect, and preliminary experimental results in able-bodied individuals demonstrate its feasibility. In Chapter 5, a switched concurrent learning adaptive controller is developed to achieve kinematic tracking throughout the step cycle for treadmill-based walking with a 4-DoF lower-limb hybrid exoskeleton. The developed controller leverages a phase-dependent human-exoskeleton model presented in Chapter 2. A multiple-Lyapunov stability analysis with a dwell time condition is developed to ensure exponential kinematic tracking and parameter estimation. The controller is tested in two able-bodied individuals for a six-minute walking trial and the performance of the controller is compared with a gradient descent classical adaptive controller. Chapter 6 highlights the contributions of the developed control methods and provides recommendations for future research directions

Functional electrical stimulation cycling strategies tested during preparation for the First Cybathlon Competition – a practical report from team ENS de Lyon

Whether it is from the patient’s or the physical therapist’s point of view, FES cycling can be considered either as a recreational activity, or an engaging rehabilitation tool. In both cases, it keeps patients with lower-limb paralysis motivated to sustain a regular physical activity. Thus, it is not surprising that it was selected as one of the six disciplines of the first Cybathlon competition held on October 8, 2016. However, many unresolved issues prevent FES cycling from being an activity practiced outdoors on a daily basis; such as, low power production, rapid muscle fatigue, precise electrode positioning, lack of systematic procedures to determine stimulation patterns, and the difficulty of transferring disabled riders from their wheelchair to the tricycle. This article documents the challenges we faced during preparation for the Cybathlon 2016 FES cycling race, and provides results obtained during different phases of the process. A particular specificity of our team was that, unlike most other teams where pilots were mainly paraplegic, both the primary and backup pilots for team ENS de Lyon are C6/C7 tetraplegics, with neither voluntary control of their abdominal muscles nor hand grip, and only partial use of their arms

Feedback control of cycling in spinal cord injury using functional electrical stimulation

This thesis is concerned with the realisation of leg cycling by means of FES in SCI individuals with complete paraplegia. FES lower-limb cycling can be safely performed by paraplegics on static ergometers or recumbent tricycles. In this work, different FES cycling systems were developed for clinical and home use. Two design approaches have been followed. The first is based on the adaptation of commercially available recumbent tricycles. This results in devices which can be used as static trainers or for mobile cycling. The second design approach utilises a commercially available motorised ergometer which can be operated while sitting in a wheelchair. The developed FES cycling systems can be operated in isotonic (constant cycling resistance) or isokinetic mode (constant cadence) when used as static trainers. This represents a novelty compared to existing FES cycling systems. In order to realise isokinetic cycling, an electric motor is needed to assist or resist the cycling movement to maintain a constant cadence. Repetitive control technology is applied to the motor in this context to virtually eliminate disturbance caused by the FES activated musculature which are periodic with respect to the cadence. Furthermore, new methods for feedback control of the patient’s work rate have been introduced. A one year pilot study on FES cycling with paraplegic subjects has been carried out. Effective indoor cycling on a trainer setup could be achieved for long periods up to an hour, and mobile outdoor cycling was performed over useful distances. Power output of FES cycling was in the range of 15 to 20 W for two of the three subjects at the end of the pilot study. A muscle strengthening programme was carried out prior and concurrent to the FES cycling. Feedback control of FES assisted weight lifting exercises by quadriceps stimulation has been studied in this context

Control systems for function restoration, exercise, fitness and health in spinal cord injury

We describe original research contributions to the engineering development of systems which aim to restore function and enable effective exercise for people with spinal cord injury (SCI). Our work utilises functional electrical stimulation (FES) of paralysed muscle. Improving function and general health through participation in exercise is vital to the enhancement of quality of life, well-being and promotion of longevity. Crucial to the development of this research has been judicious use of advanced methods of feedback control engineering; this has been a key enabling factor in many of our original contributions. The consequences of a spinal cord injury can be severe. The primary effects may include; paralysis and loss of sensation in the legs, arms and trunk; disruption of bladder and bowel function; and disruption of the autonomic regulation of blood pressure, heart rate and lung function. If the abdominal and chest muscles are paralysed, breathing will be compromised, and patients with a high-level cervical injury may require mechanical ventilation. These primary effects of a spinal cord injury may, over time, lead to a range of debilitating secondary medical complications. These include reduced cardiovascular fitness, urinary tract infection and an associated risk of kidney disease, reduced bone mineral density, the possible development of pressure sores, and muscle spasticity. People with paralysed chest and abdominal muscles are at increased risk of respiratory infection. Consideration of these factors has led us to focus our research programme in this field on novel engineering solutions which have relevance to the secondary consequences of spinal cord injury, and which may help to alleviate some of their effects. In this thesis we describe our contributions in the following areas: 1. Control of Paraplegic Standing; This work concerns upright stance, and aims to provide; (i) automatic feedback control of balance during stance, with the arms free for functional tasks; (ii) methods and apparatus for dynamic standing therapy, which may help to enhance the individual's retained balance skills. This area of work has successfully demonstrated the automatic control of balance during quiet standing in paraplegic subjects. Further, we have established the feasibility of ankle stiffness control in paraplegic subjects using FES, and we have shown that this can be combined with volitional upper-body inputs to achieve stable, arm-free balance. 2. Lower-limb Cycling: Lower-limb cycling, achieved through electrical stimulation of paralysed leg-actuating muscles, is an effective exercise intervention. We have described refinements to the engineering design of an FES-cycling system, based upon the adaptation of commercially-available recumbent tricycles (of various designs), some of which are equipped with an auxiliary electric motor. We have contributed new methods of feedback control of key variables including cycle cadence and exercise workrate. These contributions have facilitated further detailed study of the effect of the exercise on cardiopulmonary fitness, bone integrity, spasticity, muscle condition, and factors relating to the likelihood of skin breakdown (i.e. the development of pressure sores). 3. Upper-limb Exercise in Tetraplegia; We have developed a new exercise modality for patients with a cervical-level injury and significant loss of arm function. The system allows effective arm ergometry by combining volitional motion with electrical stimulation of the paralysed upper-arm muscles. This work has developed new apparatus and exercise testing protocols, and has examined the effect of the exercise on cardiopulmonary fitness and muscle strength in experiments with tetraplegic subjects. 4. Modelling and Control of Stimulated Muscle; This fundamental area of research has investigated dynamic modelling and feedback control design approaches for electrically-stimulated muscle. This work has been applied in the three areas mentioned above. We identify promising areas for future research. These include extension of work on lower- limb cycling to patients with incomplete injuries, to those with cervical-level injuries, and to children with SCI. We wish to participate in a multi-centre clinical study of implanted nerve- root stimulation technology for restoration of bladder and bowel control, and for lower-limb exercise (including cycling). We have initiated a study of treadmill-based gait therapy for incomplete-lesion patients. The goals of this study are to develop test protocols for accurate characterisation of cardiopulmonary status, and to determine whether this form of cyclical lower-limb exercise has a positive impact on retained voluntary leg function. It is often the case that it is those people most severely affected by neurological impairment who stand to gain the most from these approaches (e.g. high-level tetraplegia, paediatric spinal cord injury, etc.). We must therefore continue to seek ways in which the work can be developed for the maximum benefit of these patients. In conclusion, this thesis has described original research contributions to the engineering development of systems which aim to restore important function and to enable effective exercise for people with spinal cord injury. An important facet of our work has been the application of feedback control methods; this has been an enabling factor in several areas of study. We have focused on areas which promise improved fitness and general health, and which may alleviate some of the secondary consequences of spinal cord injury. This work encompasses fundamental research, clinical studies, and the pursuit of technology transfer into clinical practice. Finally, we recognise the growing awareness of and interest in central nervous system plasticity, and in the broad field of central neural regeneration and repair. It is therefore timely to ask whether cyclical exercise interventions can lead to improvement of volitional function in patients with incomplete or discomplete lesions. Such improvements may, we speculate, result from the strengthening of muscles which retain at least partial volitional control, or from neural plasticity and re-organisation, or from regeneration effects (neurogenesis and functional connectivity). A key requirement in this line of investigation, and a major challenge, will be to develop or to utilise methods which can detect changes in a patient's volitional function and neurological status, and which can isolate the source of such changes. Should reliable methods become available, the way to the study of recovery of function through cyclical exercise would be opened. These considerations will remain, we propose, an indispensable complement to cell-based surgical interventions which may become available in the future

Functional electrical stimulation recumbent bicycle for stroke rehabilitation

Stroke is a severe condition that is one of the leading causes to both disabilities and death in the United States. A stroke occurs when blood stops flowing to the brain. It only takes minutes without blood before the brain cells begin to be damaged and even die. Up to 90 percent of people who survive a stroke suffer from some form of paralysis. It is common among stroke patients to experience hemiparesis which paralyses on one side of the body. Functional remodeling of the brain can improve sensation and motor control. However, muscles and nerves degrade (atrophy) over time with disuse. The more a muscle atrophies the longer it takes to rehabilitate that muscle and the degree of recovery is reduced. Functional Electrical Stimulation (FES) can artificially stimulate these muscles and nerves. FES has been proven to be a viable tool for the rehabilitation of atrophied muscles and nerves. The purpose of this thesis project was to design, build and test a Functional Electrical Stimulation (FES) recumbent bicycle that can be used for stroke rehabilitation. An off the shelf FES device was researched and analyzed to determine its capabilities. A circuit was then designed using a recumbent bicycle as the test bed and a Labview program was written as the control mechanism for the FES device. The data collection was done by an optical encoder mounted onto the recumbent bicycle. The system was programmed using Labview for both control and data collection. After the completion of the recumbent bicycle, the protocol and methods were created to provide guidelines for the testing and data analysis. With these guidelines in place, human subject testing could be conducted. The twelve subjects were tested. Electrodes were attached to their thighs and stimulated using the FES device which was controlled by the Labview program. Each participant performed six trials, three with the FES device operating and three with the FES device switched off. The results showed that there were no statistical difference between the test groups, except for the females only group. The female test group pedalled slower with the FES device switched on then with the FES device off. This research showed that the quality of movement was sufficient to allow cycling assisted by FES on the recumbent bicycle. These results may be encouraging for stroke patients with partial hemiparesis and other forms of paralysis to assist them during rehabilitation. The future for FES systems are continuing to progress in a positive direction. This research in conjunction with other research in the biomedical engineering field are enabling new therapy methods that have the potential to improve the quality of life for stroke patients. The FES research completed for the recumbent bicycle showed that the device was capable of properly controlling the leg and propelling it forward with enough power to push the pedal. The experimental study showed that the quality of movement was sufficient to allow cycling assisted by FES on a Recumbent Bicycle. In fact, no statistical differences were found between normal cycling and FES assisted cycling for most groups studied. Initial testing seems suitable for future studies that assist with stroke patients during rehabilitation