Preliminary evaluation of a powered lower limb orthosis to aid walking in paraplegic individuals

This paper describes a powered lower-limb orthosis that is intended to provide gait assistance to spinal cord injured (SCI) individuals by providing assistive torques at both hip and knee joints. The orthosis has a mass of 12 kg and is capable of providing maximum joint torques of 40 Nm with hip and knee joint ranges of motion from 105° flexion to 30° extension and 105° flexion to 10° hyperextension, respectively. A custom distributed embedded system controls the orthosis with power being provided by a lithium polymer battery which provides power for one hour of continuous walking. In order to demonstrate the ability of the orthosis to assist walking, the orthosis was experimentally implemented on a paraplegic subject with a T10 complete injury. Data collected during walking indicates a high degree of step-to-step repeatability of hip and knee trajectories (as enforced by the orthosis) and an average walking speed of 0.8 km/hr. The electrical power required at each hip and knee joint during gait was approximately 25 and 27 W, respectively, contributing to the 117 W overall electrical power required by the device during walking. A video of walking corresponding to the aforementioned data is included in the supplemental material.     

Robot Assisted Gait Training With Active Leg Exoskeleton (ALEX)

Gait training of stroke survivors is crucial to facilitate neuromuscular plasticity needed for improvements in functional walking ability. Robot assisted gait training (RAGT) was developed for stroke survivors using active leg exoskeleton (ALEX) and a force-field controller, which uses assist-as-needed paradigm for rehabilitation. In this paradigm undesirable gait motion is resisted and assistance is provided towards desired motion. The force-field controller achieves this paradigm by effectively applying forces at the ankle of the subject through actuators on the hip and knee joints. Two stroke survivors participated in a 15-session gait training study each with ALEX. The results show that by the end of the training the gait pattern of the patients improved and became closer to a healthy subject's gait pattern. Improvement is seen as an increase in the size of the patients' gait pattern, increased knee and ankle joint excursions and increase in their walking speeds on the treadmill.      

Design and evaluation of a stance-control knee-ankle-foot orthosis knee joint.

Conventional knee-ankle-foot orthoses (KAFOs) are prescribed for people with knee-extensor muscle weakness. However, the orthoses lock the knee in full extension and, therefore, do not permit a natural gait pattern. A new electromechanical stance-control knee-ankle-foot orthosis (SCKAFO) knee joint that employs a novel friction-based belt-clamping mechanism was designed to enable a more natural gait. The SCKAFO knee joint allows free knee motion during swing and other non-weight-bearing activities and inhibits knee flexion while allowing knee extension during weight bearing. A prototype SCKAFO knee joint was mechanically tested to determine the moment at failure, loading behavior, and wear resistance. The mean maximum resisting moment of the SCKAFO knee joint over five loading trials was 69 Nm +/- 4.9 Nm. The SCKAFO knee-joint strength and performance were sufficient to allow testing on a 90 kg subject at normal walking cadence. Proper function of the new electromechanical knee joint was verified in walking trials of an able-bodied subject.     

Preliminary evaluation of a controlled-brake orthosis for FES-aided gait

A hybrid functional-electrical stimulation (FES) gait system that incorporates a computer-controlled orthosis system has been developed to address the problems of rapid muscle fatigue and poor movement control that are characteristic of FES-aided gait. The orthosis is a long-leg brace that contains controllable friction brakes at both hip and knee joints. The system achieves desirable limb trajectories by utilizing the stimulated muscles as a source of unregulated power and regulating the power at each joint by computer control of the friction brakes. Muscle fatigue is reduced by locking the controllable brakes to provide the isometric joint torques necessary during stance. The hybrid gait system was evaluated and compared to conventional four channel FES-aided gait using four subjects with paraplegia. The results demonstrated significant reduction in muscle fatigue and improvement in trajectory control when using the orthosis combined with FES compared to using FES alone. Results for distance and speed improvements varied across subjects. Considerable work remains in the design of the hardware before the system is feasible for use outside the laboratory.     

An EMG-to-force processing approach for determining ankle muscle forces during normal human gait.

Muscle forces move our limbs. These forces must be estimated with indirect techniques, as direct measurements are neither generally possible nor practical. An electromyography (EMG)-to-force processing technique was developed. Ankle joint moments and, by extension, ankle muscle forces were calculated. The ankle moment obtained by inverse dynamics was calculated for ten normal adults during free speed gait. There was close correlation between inverse dynamics ankle moments and moments determined by the EMG-to-force processing approach. Muscle forces were determined. The gait peak Achilles tendon force occurred in late single limb support. Peak force observed (2.9 kN) closely matched values obtained where force transducers were used to obtain in vivo muscle forces (2.6 kN). The EMG-to-force processing model presented here appears to be a practical means to determine in vivo muscle forces.     

Biomechanical evaluation of a prototype foot/ankle prosthesis.

In this paper, we report on our pilot evaluation of a prototype foot/ankle prosthesis. This prototype has been designed and fabricated with the intention of providing decreased ankle joint stiffness during the middle portion of the stance phase of gait, and increased (i.e., more normal) knee range of motion during stance. Our evaluation involved fitting the existing prototype foot/ankle prosthesis, as well as a traditional solid ankle cushioned heel (SACH) foot, to an otherwise healthy volunteer with a below-knee (BK) amputation. We measured this individual's lower extremity joint kinematics and kinetics during walking using a video motion analysis system and force platform. These measurements permitted direct comparison of prosthetic ankle joint stiffness and involved side knee joint motion, as well as prosthetic ankle joint moment and power.     

被动踝足外骨骼对平地行走时运动表现的影响

被动踝关节外骨骼(unpowered-ankle exoskeleton)可实现人体步态行走良好辅助作用,但对其助行增强效果研究多局限于跑步机行走范式,现少有在平地行走下穿戴外骨骼对穿戴者运动生物力学影响的研究。本研究设计组装了一款低成本模块化的被动踝关节外骨骼,首次研究了平地行走下穿戴被动踝关节外骨骼对健康年轻穿戴者肌肉激活与运动表现的影响。实验招募5名穿戴者,在无外骨骼、仅穿戴外骨骼框架和穿戴外骨骼3种实验条件下完成直线行走任务,过程中同步采集人体下肢主要肌群的肌电和关节运动轨迹。提取了肌电激活特征和关节角度特征,采用双因素方差分析方法揭示了不同外骨骼穿戴状态对相关特征的影响。结果表明,外骨骼框架会对人体提供支撑,降低站立阶段腓肠肌内侧肌肉激活。而穿戴外骨骼在站立阶段使腓肠肌内侧的平均肌肉激活显著降低20%,也显著降低了踝关节的峰值力矩及功率。外骨骼会使髋膝关节的运动学和动力学表现发生变化,这可能与下肢关节间的能量交换有关。因此,被动踝关节外骨骼可在平地行走下对踝关节实现支撑相和推离阶段有效辅助,但穿戴者同时也会适应外骨骼穿戴而改变其步态。     

A novel method for automatic treadmill speed adaptation.

Robot-aided treadmill training is an innovative rehabilitation method for patients with locomotor dysfunctions. However, in current rehabilitation systems treadmill speed is restricted to constant values or adjusted by the therapist, whereas self-determined phases of accelerations and decelerations cannot be performed by the patient in an interactive and intuitive way. We present a new approach that allows treadmill walking with intuitive gait speed adaptation. In this approach, the user's trunk position is fixed in walking direction. The horizontal interaction forces applied by the user intending to accelerate or decelerate the gait are measured at the trunk connection and fed to the treadmill controller. The desired gait acceleration is calculated by means of a virtual admittance. Integration yields the desired speed which is fed into the underlying velocity controller of the treadmill. The method was verified by two experimental setups and tested on ten healthy subjects. In one setup, the subject's trunk was rigidly connected by a tether, whereas in the second setup the subject was placed in a robotic gait orthosis. All subjects were able to use both systems immediately and intuitively. The treadmill speed profile during the gait cycle corresponds to that of normal walking. The controller can be extended to simulate different walking conditions, such as slope walking. The method can be used for patient-cooperative control strategies performed with a robotic gait orthosis as well as for any other user-interactive applications in fitness and sports.     

Kinetics of stiff-legged gait: induced acceleration analysis.

Treating spastic paretic stiff-legged gait, defined as reduced knee flexion in swing, holds a high priority in the rehabilitation of patients with upper motor neuron lesions. We propose a method to determine the relative contributions of hip, knee, and ankle impairments to this disability. We analyzed the gait of ten patients with stiff-legged gait (SLG) due to a single stroke and ten healthy, able-bodied controls. Using subject specific models, we analyzed the induced accelerations (IA's) at the knee. Knee IA's throughout the gait cycle were calculated and the sum of the IA's was compared to the knee joint angular acceleration estimated from kinematic data. The preswing and early swing IA's were the focus of our examination as these largely determine knee kinematics in swing. Knee angular accelerations estimated from IA's and kinematic data agreed for both controls and patients. Gait cycle IA analysis of individual patients identified highly variable causes of SLG including ankle and hip joint impairments. Induced acceleration analysis (IAA) suggested that multiple impairments, not just about the knee, but also about the hip and ankle, lead to this disability. Individual subjects are likely to have individual reasons for their stiff-legged gait. Defining the link between the patients specific impairments and their gait disability should be a goal of clinical gait analysis. IAA is a useful tool for this purpose with a strong potential for clinical application.     

Decoding intra-limb and inter-limb kinematics during treadmill walking from scalp electroencephalographic (EEG) signals

Brain-machine interface (BMI) research has largely been focused on the upper limb. Although restoration of gait function has been a long-standing focus of rehabilitation research, surprisingly very little has been done to decode the cortical neural networks involved in the guidance and control of bipedal locomotion. A notable exception is the work by Nicolelis' group at Duke University that decoded gait kinematics from chronic recordings from ensembles of neurons in primary sensorimotor areas in rhesus monkeys. Recently, we showed that gait kinematics from the ankle, knee, and hip joints during human treadmill walking can be inferred from the electroencephalogram (EEG) with decoding accuracies comparable to those using intracortical recordings. Here we show that both intra- and inter-limb kinematics from human treadmill walking can be achieved with high accuracy from as few as 12 electrodes using scalp EEG. Interestingly, forward and backward predictors from EEG signals lagging or leading the kinematics, respectively, showed different spatial distributions suggesting distinct neural networks for feedforward and feedback control of gait. Of interest is that average decoding accuracy across subjects and decoding modes was ~0.68±0.08, supporting the feasibility of EEG-based BMI systems for restoration of walking in patients with paralysis.     

A Method of Estimating the Degree of Active Participation During Stepping in a Driven Gait Orthosis Based on Actuator Force Profile Matching

Visual biofeedback with information about the patients' degree of activity is a valuable adjunct to robot-assisted gait training as means of increasing the motivation and participation of the patients during highly repetitive training sessions. In the driven gait orthosis (DGO) Lokomat, an estimation of the patient's activity level was based on man–machine interaction forces as measured at the hip and knee actuators of the exoskeletal device. In an early approach, theoretical assumptions about the expected man–machine interaction forces, due to the varying behavior of the patients, were formulated for the calculation of quantitative biofeedback. In contrast to this theory-based approach, we have developed a novel method where the biofeedback calculations were based on measured reference man–machine interaction force profiles of healthy subjects when walking with different degrees of activity. To account for intrasubject and intersubject variability, reference force profiles were processed in a model to generate multiple force profiles describing each activity state. To estimate the activity of a subject walking in the DGO, the man–machine interaction force profile was measured, matched to each of the generated force profiles, and the best fitting profile of the different activity states was identified by the smallest Euclidian distance, respectively. By calculating the difference between these Euclidian distances, a quantitative estimate of the patient's degree of activity was obtained. The novel method was evaluated and compared to the conventional approach in a study with 18 healthy subjects. This comparison showed that the novel method was more reliable in detecting different activity states and is, therefore, a promising approach for future biofeedback systems.      

Design and evaluation of the LOPES exoskeleton robot for interactive gait rehabilitation.

This paper introduces a newly developed gait rehabilitation device. The device, called LOPES, combines a freely translatable and 2-D-actuated pelvis segment with a leg exoskeleton containing three actuated rotational joints: two at the hip and one at the knee. The joints are impedance controlled to allow bidirectional mechanical interaction between the robot and the training subject. Evaluation measurements show that the device allows both a "patient-in-charge" and "robot-in-charge" mode, in which the robot is controlled either to follow or to guide a patient, respectively. Electromyography (EMG) measurements (one subject) on eight important leg muscles, show that free walking in the device strongly resembles free treadmill walking; an indication that the device can offer task-specific gait training. The possibilities and limitations to using the device as gait measurement tool are also shown at the moment position measurements are not accurate enough for inverse-dynamical gait analysis.     

Design and quantitative evaluation of a stance-phase controlled prosthetic knee joint for children.

The aims of this study were to demonstrate a theoretical basis for the design of a stance-phase controlled paediatric prosthetic knee joint, clinically test prototypes of the knee, and use a questionnaire to evaluate its efficacy. Biomechanical models were used to analyze the stance-phase control characteristics of the proposed knee, and those of three other commonly prescribed paediatric knee joint mechanisms, which were also the conventional knee joints used by the six participants of this study (mean age 10.8 years). A questionnaire pertaining to stance-phase control was designed and administered twice to each child; once for the evaluation of the prototype knee joint and once for the conventional knee joint. Stance-phase modeling results indicated decreased zones of instability for the new knee as compared to other paediatric knee joints. Questionnaire results revealed a decrease in the frequency of falls with the prototype compared to other knees, especially in highly active children. The children also reported worrying less about the knee collapsing during walking. No differences were evident for stance-phase stability during running, walking on uneven terrain, ambulating up and down stairs and inclines, fatigue, and types of activities performed.     

A Three-Dimensional Biomechanical Evaluation of Quadriceps and Hamstrings Function Using Electrical Stimulation

Neurological disorders such as stroke impair locomotor control and result in abnormal 3-D gait kinematics. Establishment of effective rehabilitation strategies requires an understanding of how individual muscles contribute to pathological movement. Forward dynamic simulations account for complexities of interjoint coupling and can be used to predict dynamic muscle function. However to date, limited experimental validations of dynamic models have been performed. Our objective was to measure 3-D movement induced by the biceps femoris (BF), rectus femoris (RF), and vastus lateralis (VL) in limb configurations corresponding to the swing phase of gait, and to assess the biomechanical factors that affect dynamic function. Subjects were positioned in a robotic gait orthosis that included a compliant interface. Electrical stimulation was introduced into individual muscles while induced hip and knee joint movements were recorded. Measured hip to knee sagittal plane acceleration ratios were consistent with dynamic musculoskeletal model simulations. However RF and VL induced substantially larger frontal plane hip movements than model-based predictions. Sensitivity analyses on musculoskeletal model parameters revealed that muscle function depends primarily on moment arm assumptions. Though generic musculoskeletal models are suitable for predicting sagittal plane muscle function, improvements in moment arm accuracy are essential for investigation of 3-D pathological gait.     

Postural arm control following cervical spinal cord injury

This study used estimates of dynamic endpoint stiffness to quantify postural arm stability following cervical spinal cord injury (SCI) and to investigate how this stability was affected by functional neuromuscular stimulation (FNS). Measurements were made in the horizontal plane passing through the glenohumeral joint on three SCI-impaired arms, which ranged in functional level from a weak C5 to a strong C6. Endpoint stiffness, which characterizes the relationship between externally imposed hand displacements and the resultant forces, was estimated during the application of planar, stochastic perturbations to each arm. These estimates were used in conjunction with voluntary endpoint force measurements to quantify stability and strength during voluntary contractions and during voluntary contractions in the presence of triceps FNS. The primary findings were: (1) the differences in the force generating capabilities of these arms were due primarily to differences in shoulder strength; (2) measurements of strength alone could not be used to predict arm stability; and (3) triceps FNS improved postural arm stability for all tested conditions. These results suggest strategies for improved control of FNS systems designed to restore arm function following cervical SCI and underscore the importance of examining the effects of FNS on both strength and stability     

Adaptive control of a variable-impedance ankle-foot orthosis to assist drop-foot gait

An active ankle-foot orthoses (AAFO) is presented where the impedance of the orthotic joint is modulated throughout the walking cycle to treat drop-foot gait. During controlled plantar flexion, a biomimetic torsional spring control is applied where orthotic joint stiffness is actively adjusted to minimize forefoot collisions with the ground. Throughout late stance, joint impedance is minimized so as not to impede powered plantar flexion movements, and during the swing phase, a torsional spring-damper control lifts the foot to provide toe clearance. To assess the clinical effects of variable-impedance control, kinetic and kinematic gait data were collected on two drop-foot participants wearing the AAFO. For each participant, zero, constant, and variable impedance control strategies were evaluated and the results were compared to the mechanics of three age, weight, and height matched normals. We find that actively adjusting joint impedance reduces the occurrence of slap foot allows greater powered plantar flexion and provides for less kinematic difference during swing when compared to normals. These results indicate that a variable-impedance orthosis may have certain clinical benefits for the treatment of drop-foot gait compared to conventional ankle-foot orthoses having zero or constant stiffness joint behaviors.     

Assessment of motion of a swing leg and gait rehabilitation with a gravity balancing exoskeleton.

The gravity balancing exoskeleton, designed at University of Delaware, Newark, consists of rigid links, joints and springs, which are adjustable to the geometry and inertia of the leg of a human subject wearing it. This passive exoskeleton does not use any motors but is designed to unload the human leg joints from the gravity load over its range-of-motion. The underlying principle of gravity balancing is to make the potential energy of the combined leg-machine system invariant with configuration of the leg. Additionally, parameters of the exoskeleton can be changed to achieve a prescribed level of gravity assistance, from 0% to 100%. The goal of the results reported in this paper is to provide preliminary quantitative assessment of the changes in kinematics and kinetics of the walking gait when a human subject wears such an exoskeleton. The data on kinematics and kinetics were collected on four healthy and three stroke patients who wore this exoskeleton. These data were computed from the joint encoders and interface torque sensors mounted on the exoskeleton. This exoskeleton was also recently used for a six-week training of a chronic stroke patient, where the gravity assistance was progressively reduced from 100% to 0%. The results show a significant improvement in gait of the stroke patient in terms of range-of-motion of the hip and knee, weight bearing on the hemiparetic leg, and speed of walking. Currently, training studies are underway to assess the long-term effects of such a device on gait rehabilitation of hemiparetic stroke patients.     

Optimal control of walking with functional electrical stimulation: a computer simulation study.

Bipedal locomotion was simulated to generate a pattern of activating muscles for walking using electrical stimulation in persons with spinal cord injury (SCI) or stroke. The simulation presented in this study starts from a model of the body determined with user-specific parameters, individualized with respect to the lengths, masses, inertia, muscle and joint properties. The trajectory used for simulation was recorded from an able-bodied subject while walking with ankle-foot orthoses. A discrete mathematical model and dynamic programming were used to determine the optimal control. A cost function was selected as the sum of the squares of the tracking errors from the desired trajectories, and the weighted sum of the squares of agonist and antagonist activations of the muscle groups acting around the hip and knee joints. The aim of the simulation was to study plausible trajectories keeping in mind the limitations imposed by the spinal cord injury or stroke (e.g., spasticity, decreased range of movements in some joints, limited strength of paralyzed, externally activated muscles). If the muscles were capable of generating the movements required and the trajectory was achieved, then the simulation provided two kinds of information: 1) timing of the onset and offset of muscle activations with respect to the various gait events and 2) patterns of activation with respect to the maximum activation. These results are important for synthesizing a rule-based controller.     

The use of selective electrical stimulation of the quadriceps to improve standing function in paraplegia.

Persons with spinal cord injury (SCI) can benefit significantly from functional neuromuscular stimulation (FNS) systems for standing if manual tasks can be performed while upright. Using FNS to sufficiently activate the knee extensors to rise from a sitting position often results in inadvertent activation of the rectus femoris and/or sartorius, which flex the hip. In this study, intramuscular electrodes implanted in the vastus lateralis and medialis of four subjects with SCI were used to activate these muscles individually and simultaneously to measure knee extension moment. Support forces applied to the arms and feet were measured while upright to quantify the effects of recruiting rectus femoris and/or sartorius. In three of the four subjects, vastus lateralis, by itself, generated adequate knee extension moment for rising from a chair and to maintain static standing. Simultaneous activation of the vastus lateralis and medialis using a bifurcated electrode generated adequate knee extension moment in one subject, and was within 10% of the required moment in another. While upright, activation of the rectus femoris resulted in arm support force increases of 4-11% body weight, while deactivation resulted in arm support force decreases of 6-9% body weight. The results indicate that selective activation of the vastus lateralis, individually or in combination with vastus medialis, can improve current FNS standing systems by reducing the arm support forces required to remain upright.