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Adaptive Ankle Exoskeleton Control: Validation Across Diverse Walking Conditions
Abstract
Ankle exoskeletons hold potential to augment human walking ability, yet their use in free-living environments has been limited by the absence of practical and effective control strategies that can appropriately adapt to variable terrain. To address this challenge, we derived a novel analytical ankle joint moment estimation model using custom wearable sensors and developed an exoskeleton control scheme to adapt assistance proportional to the biological plantar-flexor moment in real-time for unimpaired individuals and individuals with disabilities who are able to ambulate independently. We validated the controller during level, 5circ incline and decline walking, each at multiple speeds; stair ascent; stair descent; and 90circ turning (88 1 3% average accuracy, R = 0.96 1 0.01 average correlation coefficient). This study demonstrates the ability of the controller to accurately adapt assistance in unimpaired individuals across a wide variety of walking conditions without the need for walking condition classification or real-time assessment of muscle activity. Clinical feasibility testing in four individuals with cerebral palsy suggests that this control method holds promise for incline and stair walking in individuals with mild-to-severe impairment. This ankle-moment-adaptive control system can be used to prescribe ankle exoskeleton assistance that adapts in real-time across the tested conditions.
... Of the eight exoskeletons reported in the table, only one is a knee exoskeleton [65], whereas the remaining seven are ankle exoskeletons. Five of them use high precision motors from Maxon™ [58], [60], [61], [63], [64], one employs a linear servo actuator [59], and two do not specify the details of the motors [62], [65]. In addition, Maxon motors with varying power outputs are used, including 90, 120, and 150 W, each connected to different gearboxes and harmonic drives to optimize performance. ...
... For example, Orekhov et al. [63] employed an angle sensor to monitor the movements of the ankle joint, providing direct feedback on the angular position, a crucial parameter for joint control. Similarly, Bishe et al. [64] integrated a torque sensor, focusing on the force applied to the ankle joint, offering valuable information on the level of assistance provided by the exoskeleton during gait. Tagoe et al. [60] also relied on torque sensors, further highlighting the importance of tracking force output in joint actuation. ...
... Control strategy Different control strategies have been developed for use in exoskeleton systems, with hierarchical control being one of the most commonly applied frameworks. In hierarchical con- [64] trol, the system is divided into three levels: supervisory, highlevel, and low-level control [70], [71]. At the supervisory level, which is the highest, task-specific control is executed based on pre-defined objectives and system state. ...
Cerebral palsy (CP) is the most common physical disability affecting children, often resulting in reduced quality of life due to challenges with movement control, posture, balance, and increased energy expenditure required to walk. Lower-limb exoskeletons (LLEs) offer a promising alternative to traditional rehabilitation methods, with the potential to improve mobility in children with CP. This review aims to provide an updated overview of mechanical characteristics, control strategies, technological advancements, clinical efficacy, and limitations associated with pediatric LLEs for gait rehabilitation in children with CP. The main objectives were to evaluate the current state of technology, review the clinical outcomes of exoskeleton use, and identify emerging trends and challenges in this field. To achieve this, a comprehensive search was conducted using Scopus and Web of Science databases to identify relevant recent studies published from 2020 onward. A total of 11 studies were included based on pre-defined inclusion and exclusion criteria. The findings suggest that pediatric exoskeletons have shown promise in improving gait parameters, such aswalking speed and joint kinematics while reducing the metabolic cost in children with CP. Despite these positive outcomes, challenges remain regarding device customization, long-term adherence, and integration into clinical practice. The review underscores the potential of pediatric exoskeletons to significantly improve mobility and quality of life in children with CP but also emphasizes the need for further research to optimize device design, long-term outcomes, and standardized guidelines and protocols to conduct and evaluate clinical trials. These findings have important implications for clinicians, researchers, and developers who seek to advance the field of pediatric rehabilitation technologies.
... Another approach is to place a rigid structure near the target assistive joint to support the cables [55,57,[66][67][68]71,72], thereby preventing unnecessary friction and slippage of the cables and reducing power loss during transmission. This method is referred to as the "hybrid cable-driven" type. ...
... Therefore, the articles [41,57,84] were excluded. Additionally, the articles [36,39,40,45,55,59,60,67,69,87,89,94] were excluded due to the lack of information on system weight. ...
... For studies that reported reduced muscle activation in multiple muscles, we selected the maximum value for analysis. The final statistical results are shown in Figure 8. [33,37,38,42,44,46,58,[58][59][60][61][62]64,64,66,67,[71][72][73][83][84][85][86][87][88][93][94][95]. (b) The comparison of EMG muscle activity changes [31,36,40,41,43,45,57,62,68,70,72,85,88,89,92]. ...
Exoskeletons can track the wearer’s movements in real time, thereby enhancing physical performance or restoring mobility for individuals with gait impairments. These wearable assistive devices have demonstrated significant potential in both rehabilitation and industrial applications. This review focuses on the major advancements in exoskeleton technology published since 2020, with particular emphasis on the development of structural designs for lower-limb exoskeletons employed in locomotion assistance. We employed a systematic literature review methodology, categorizing the included studies into three main types: rigid exoskeleton, soft exoskeleton, and tethered platform. The current development status of robotic exoskeletons is analyzed based on publication year, system weight, target assistive joints, and main effects. Furthermore, we examine the factors driving these advancements and their implications for the field. The key challenges and opportunities that may influence the future development of exoskeleton technologies are also highlighted in this review.
... The exoskeleton had a total mass between 2.4 and 2.6 kg depending on the size of footplates and cuffs, was untethered, and had most of the mass placed on the pelvis to minimize metabolic detriment [19]. The device provided adaptive plantar-flexor assistance during stance based on a proportional joint moment controller (PJMC, [22], [23], Fig. 1D). The peak plantar-flexor torque that could be generated by the device was 30 Nm [6]. ...
... The Institutional Review Board of Northern Arizona University approved this protocol (#986744). For this experiment, we used an unmodified instance of a previouslyvalidated control scheme that has demonstrated an ability to produce a metabolic benefit in unimpaired and impaired individuals and across different terrains and walking conditions [22], [23]. The control scheme included a high-level controller that prescribed a torque profile in real time using the footplate FSR as in 6: ...
... where T was the instantaneous desired torque setpoint, t set was the desired peak exoskeleton torque during stance phase (e.g., 15 Nm), f inst was the instantaneous FSR signal, f cal was the average peak FSR signal established across three strides for each user and walking speed, and C limit was the motor current that was allowed to be sourced during each phase (Fig. 1D) [23]. A low-level PD closed-loop controller was used to closely track the desired torque signal (Fig. 1D, E). ...
The widespread adoption of powered lower-limb exoskeletons for augmenting mobility requires energy-efficient actuation to provide meaningful assistance over relevant walking durations. We designed, modeled, and validated an ankle exoskeleton design with a parallel elastic element in the form of a carbon fiber leaf spring that stored and returned energy in parallel to a cable-drive ankle joint during stance phase. We assessed the impact of the parallel elastic element on the performance of the untethered robotic ankle exoskeleton at walking speeds of 0.75-1.25 m/s using a previously validated-controller, and a controller designed specifically to maximize the spring's benefit. The spring selected for our adult cohort had a stiffness of 97.2 Nm/rad, engaged best at 0-degrees ankle dorsiflexion, and produced 10-15 Nm peak assistive torque at all walking speeds. When tracking the previously validated exoskeleton controller, peak motor current was reduced by 14-20% and integrated current was reduced by 16-19% for parallel-elastic design vs. without spring engagement; this translated to 15-26% more assisted steps for the same battery capacity. When utilizing the controller designed to take advantage of the parallel spring torque, the number of assisted steps for the same battery capacity increased 46-76% compared to the no spring configuration depending on walking speed. Seeking to facilitate real-world adoption, this powered parallel elastic ankle exoskeleton design holds potential to significantly extend powered walking duration, and improve battery and motor life of operation by reducing peak and mean motor current requirements.
... T HE improvement and restoration of the gait performance while ambulating in daily living conditions, which involves activities of ascending/descending stairs and overground walking, has been an important aspect of the robotic rehabilitation community [1]- [3]. Walking in a controlled laboratory setting does not represent a realistic scenario of the real world where users involve multiple walking modes [1], [2]. Walking in multimodal conditions with the trivial interventions of aerobic exercises, strength training, and treadmill walking, stroke, older adults (aged 71 years and above) and Parkinson's Disease patients have shown generalizability and transference effects of gait training into unsupervised walking [4], [5]. ...
... In Kang et al. [1], different walking mode-specific hip-joint moment trajectories were implemented to design multimodal gait interventions with the controller manually switching its intervention type. Anklejoint moment trajectory being different in each walking mode, a ground reaction-based model for the ankle moment control strategy was designed for multimodal gait intervention [2]. For effective and successful implementation of multimodal interventions in activities of daily living, there is a need to automatically switch from one walking mode to another for specific gait intervention. ...
... The classification algorithms based on support vector machine [19], [21], [22], neural networks [21], [23], fuzzy logic [24], [25], k-nearest neighbors [18], [20], decision tree [21], Bayesian networks [26], [27], and Gaussian Mixture Model [28] have been developed providing accuracies from 77.3% to 97.6%. These methods rely on a large dataset for training the model offline, post-processing algorithms, or utilizing multiple sensorial inputs for walking mode classification, which have been identified to add computation load and processing speed constraints [2], [27]. Further, such approaches have been projected as black box models [27], where the biomechanical significance of different walking modes remains unknown. ...
The human ankle joint interacts with the environment during ambulation to provide mobility and maintain stability. This association changes depending on the different gait patterns of day-to-day life. In this study, we investigated this interaction and extracted kinematic information to classify human walking mode into upstairs, downstairs, treadmill, overground and stationary in real-time using a single-DoF IMU axis. The proposed algorithm's uniqueness is twofold - it encompasses components of the ankle's biomechanics and subject-specificity through the extraction of inherent walking attributes and user calibration. The performance analysis with forty healthy participants (mean age: 26.8 ± 5.6 years yielded an accuracy of 89.57% and 87.55% in the left and right sensors, respectively. The study, also, portrays the implementation of heuristics to combine predictions from sensors at both feet to yield a single conclusive decision with better performance measures. The simplicity yet reliability of the algorithm in healthy participants and the observation of inherent multimodal walking features, similar to young adults, in elderly participants through a case study, demonstrate our proposed algorithm's potential as a high-level automatic switching framework in robotic gait interventions for multimodal walking.
... However, because it is impractical to estimate the foot anthropometric parameters for each individual, this method uses approximated normative data, which affects its accuracy. In the model developed by Bishe et al. [13] for an ankle exoskeleton, the ankle plantarflexion moment was obtained as the product of the normal GRF under the forefoot measured by a customized force-sensitive resistor (FSR) and the average tangential distance between the forefoot and the ankle joint, which was assumed to be a fixed parameter independent of gait phase, walking speed, or foot length. The estimator proposed by Gasparri et al. [10] for the same exoskeleton predicts the normalized plantarflexion moment by feeding the normalized sum of two forefoot FSR signals as input to a best-fit quadratic polynomial. ...
... Because some authors reported the NRMSE normalized over the entire gait cycle (NRMSE GC ), as opposed to the stance phase (NRMSE ST ), we computed both metrics for our bestperforming ensemble GPR models. These GPR models yielded smaller NRMSE ST than the models in [12] and [13], despite not relying on knowledge of the foot anthropometric parameters. This confirms the superior explanatory capability of learning-based models compared with simplistic biomechanical models. ...
... A master-slave robotics system, underpinned by reinforcement learning, was presented in [16] for the purpose of mirror therapy for patients with limb impairments. In another study, an adaptive ankle exoskeleton control was validated, demonstrating its potential to enhance the walking capabilities of patients [17]. Deep Reinforcement Learning (DRL) approaches have demonstrated their capacity to tackle high-dimensionality problems, a longstanding limitation of traditional RL techniques. ...
... Another challenge in the current state of adaptive robot imitation [17], [22], [18], [25] is the absence of generalization and standardization across different settings. Given that these studies employ diverse training strategies and network architectures, they may yield varying results, even for similar robot-assisted physiotherapy routines. ...
Robot-assisted physiotherapy offers a promising
avenue for easing the burden on healthcare professionals and
providing treatment in the comfort of one’s home. Typically,
physiotherapy requires the repetitive movements until a certain
efficiency metric is achieved. In the field of robot-assisted
physiotherapy challenges include accurately determining the
quality of imitation between robot and human movements, and
tailoring the robot behavior to match the subject’s abilities. This
paper presents an innovative modular framework for Adaptive
Motion Imitation (AMI) in the context of multi-joint robotassisted physiotherapy. The proposed framework utilizes a deep
Gated Recurrent Unit (GRU) Neural Network and Segment
Online Dynamic Time Wrapping (SODTW). The SODTW cost
is employed as a measure to determine the closeness between
the movements of the robot and the subject. The GRU, which
uses the range of motions and the fundamental frequency
components of joint trajectories as inputs, forecasts dynamic
and periodic reference trajectories for the robot joints. By modifying the input frequency coefficients according to the subject’s
SODTW cost, the output of the GRU is adapted to adapt
the robot’s motion with the subject’s imitation capabilities.
The division of the prediction and adaptation elements of our
framework greatly streamlines testing and coding, and boosts
the scalability of the algorithm. The efficacy of the proposed
AMI framework was experimentally assessed with a group of
15 participants and the social robot Zeno in our lab. The results
demonstrate the validity of the proposed framework in adapting
the behavior of the robot according to the subject’s imitation
abilities.
... Both exoskeleton configurations employed proportional joint moment control, providing plantarflexion assistance proportional to the reaction force perpendicular to the forefoot, and constant dorsiflexion assistance during the swing phase [37] [38]. Custom torque sensors at the ankle were used for torque-feedback closed-loop PD control. ...
Motor-powered ankle exoskeletons have been shown to improve walking and rehabilitation outcomes in individuals with and without gait impairments (e.g., cerebral palsy (CP)). To date, ankle exoskeleton designs have either placed the motors on the shanks (direct or quasi-direct drive) or around the waist with Bowden cable transmissions. The former offers better transmission efficiency, while the latter reduces added mass biomechanical penalty. The biomechanical effects of motor placement may be magnified for individuals with CP due to weakened lower limb strength. To date, no study has compared how motor placement alters the biomechanical responses and user perceptions of individuals with or without gait impairment (e.g., CP). In this study involving 7 individuals with CP and 9 unimpaired individuals, we compared their metabolic cost of transport, lower limb muscle activities, and user perceptions when using ankle exoskeletons with either waist-mounted motors (and Bowden cables) or shank-mounted motors that were otherwise identical. Despite changes in lower leg muscle recruitment, results showed no statistical differences in the metabolic cost of transport. Shank-mounted motors were preferred by more participants in both cohorts (e.g., 6/7 in CP). These results help inform the ergonomics and mechanical designs of ankle exoskeletons and how they may be perceived.
... Assistance patterns significantly affect exoskeletonassisted human gait performance [13] [14]. Assistance patterns have previously been derived from simulation [15], real-time biomechanical measurements [16], and manually adjustment based on users' responses [17]. Physiological, biomechanical, and neurological differences between individuals can cause divergent responses to the same assistance pattern, which means one participant's optimal pattern may perform poorly on another. ...
Exoskeletons have been shown be able to reduce metabolic cost of human walking. Determining the suitable assistance is challenging due to individual variability in response and the need for tailored assistance across different gait conditions. Human-in-the-loop (HIL) optimization has been proposed to address this issue, but current implementations often suffer from prolonged optimization cycles. In this study, we establish a model capturing the relationship between optimized assistance parameters and gait conditions (speed and slope) through a series of HIL optimization experiments spanning various gait conditions. The validation results showed that the desired assistance torque calculated by the model closely aligns with the assistance torque obtained through HIL optimization, and the calculated assistance reduced metabolic cost by 11.95% (p<0.001) and RMS soleus activity by 22.28% (p=0.049) compared to the case without assistance. The optimized assistance using model values for initialization after two generations significantly reduced metabolic cost by 12.1% (p<0.001) and RMS soleus activity by 24.8% (p=0.033), and produced larger benefits than using the empirical values after four generations, with a 50% increase in efficiency. Results suggested that the relationship model can help to improve multi-gait exoskeleton assistance customization efficiency and effectiveness both by optimization parameter initialization and direct parameter assertion. These advancements expand the applicability of HIL optimization and improve the effectiveness of exoskeleton assistance.
... Actuators 2024, 13, 428 2 of 18 healthy people, leading to an increased metabolic cost [15,16]. Consequently, a lower limb exoskeleton has been developed to assist with gait in stair environments [17][18][19]. Lower limb exoskeletons enhance user functionality and assist in daily activities by transmitting assistive forces to the lower limb joints during locomotion. Yinsheng Xu et al. proposed a soft exosuit that assists hip flexion during stair climbing by utilizing angular information of the hip joint, which effectively reduces the metabolic consumption and muscle fatigue in users [20]. ...
Lower limb exoskeletons have been developed to improve functionality and assist with daily activities in various environments. Although these systems utilize sensors for gait phase detection, they lack anticipatory information about environmental changes, which limits their adaptability. This paper presents a vision-based intelligent gait environment detection algorithm for a lightweight ankle exosuit designed to enhance gait stability and safety for stroke patients, particularly during stair negotiation. The proposed system employs YOLOv8 for real-time environment classification, combined with a long short-term memory (LSTM) network for spatio-temporal feature extraction, enabling the precise detection of environmental transitions. An experimental study evaluated the classification algorithm and soft ankle exosuit performance through three conditions using kinematic analysis and muscle activation measurements. The algorithm achieved an overall accuracy of over 95% per class, which significantly enhanced the exosuit’s ability to detect environmental changes, and thereby improved its responsiveness to various conditions. Notably, the exosuit increased the ankle dorsiflexion angles and reduced the muscle activation during the stair ascent, which enhanced the foot clearance. The results of this study indicate that advanced spatio-temporal feature analysis and environment classification improve the exoskeleton’s gait assistance, improving adaptability in complex environments for stroke patients.
... However, it remains a significant challenge to estimate hip joint moment based on measurable data from the human body. Although some researchers have obtained joint moments by establishing approximate dynamic models [14], [15], these methods rely on complex sensor equipment such as plantar pressure sensors, which may decrease the stability of the exoskeleton system and reduce user comfort. ...
The hip joint moment during walking is a crucial basis for hip exoskeleton control. Compared to generating assistive torque profiles based on gait estimation, estimating hip joint moment directly using hip joint angles offers advantages such as simplified sensing and adaptability to variable walking speeds. Existing methods that directly estimate moment from hip joint angles are mainly used for offline biomechanical estimation. However, they suffer from long computation time and lack of personalization, rendering them unsuitable for personalized control of hip exoskeletons. To address these challenges, this paper proposes a fast hip joint moment estimation method based on generalized moment features (GMF). The method first employs a GMF generator to learn a feature representation of joint moment, namely the proposed GMF, which is independent of individual differences. Subsequently, a GRU-based neural network with fast computational performance is trained to learn the mapping from the joint kinematics to the GMF. Finally, the predicted GMF is decoded into the joint moment with a GMF decoder. The joint estimation model is trained and tested on a dataset comprising 20 subjects under 28 walking speed conditions. Results show that the proposed method achieves a root mean square error of 0.1180 0.0021 Nm/kg for subjects in test dataset, and the computation time per estimation using the employed GRU-based estimator is 1.3420 0.0031 ms, significantly faster than mainstream neural network architectures, while maintaining comparable network accuracy. These promising results demonstrate that the proposed method enhances the accuracy and computational speed of joint moment estimation neural networks, with potential for guiding exoskeleton control.
... Another study by the same group [18] exploited a reinforcement learning-based master-slave robotics system for mirror therapy in patients with limb impairments. Bishe et al. [19] proposed and tested a novel adaptive ankle exoskeleton control that showed promise in improving the walking ability of patients. The use of Deep Reinforcement Learning (DRL) in place of a standard RL has been useful in solving complex problems that traditional reinforcement learning struggles with. ...
This work proposes a modular learning framework (MIRA) for rehabilitation robots based on a new deep recurrent neural network (RNN) that achieves adaptive multi-joint motion imitation. The RNN is fed with the fundamental frequencies as well as the ranges of the joint trajectories, in order to predict the future joint trajectories of the robot. The proposed framework also uses a Segment Online Dynamic Time Warping (SODTW) algorithm to quantify the closeness between the robot and patient motion. The SODTW cost decides the amount of modification needed in the inputs to our deep RNN network, which in turn adapts the robot movements. By keeping the prediction mechanism (RNN) and adaptation mechanism (SODTW) separate, the framework achieves modularity, flexibility, and scalability. We tried both Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU) RNN architectures within our proposed framework. Experiments involved a group of 15 human subjects performing a range of motion tasks in conjunction with our social robot, Zeno. Comparative analysis of the results demonstrated the superior performance of the LSTM RNN across multiple task variations, highlighting its enhanced capability for adaptive motion imitation.
... In another study, stair descending results were the lowest with a mean accuracy of 83.5% for ankle torque among other locomotion tasks including level walking and slope walking for an ankle exoskeleton controller. Additionally, stair ascending was the second lowest one[61]. Gupta et al. developed a neural network for ankle angle estimation using sEMG signals from 6 lower limb muscles and knee angle as input. ...
... Gasparri et al. [5] developed a proportional joint-moment control strategy for an ankle exoskeleton that can adjust assistance to meet the changing biomechanical demand during variable gaits. This strategy was validated under various walking conditions [7]. Kikuchi et al. [8] used an accelerometer and a rotary potentiometer to estimate gait and achieved optimal control of the initial drop speed of the foot by developing an intelligent and controllable ankle-foot orthosis system. ...
As the population ages, the number of elderly people suffering from systemic diseases such as stroke increases. To address this problem, various wearable walking assistive robots have been developed to promote physical exercise for stroke prevention. Wearable assistive robots have shown the ability to improve human mobility. However, most of these robots are heavy, bulky, and impractical. In this study, we developed a compact ankle assistive robot for elderly users to promote walking exercise. By informing the user of correct motion and timing, the robot can guide the user to achieve a healthy gait by only assisting their ankle joint. The robot provides faster-than-ankle motion to allow the user to feel supported while walking. Users can adjust the robot’s assistance parameters through a graphical user interface (GUI) according to their needs. Furthermore, we proposed a gait-adaptive method for ankle assistive robots to adapt to the user’s changing gait. Hence, the robot can automatically adjust the parameters to provide more accurate walking assistance. Finally, the results of an evaluation experiment demonstrated the feasibility of human gait adaptation. The proposed methods have the advantages of low cost and easy implementation.
... Using previous research for this case of study, ankle assistance is provided to an individual at different stages during the walking cycle [47][48][49]52]. Initially, at the stance phase, the foot rests with an angle of 0 • respect to the ground and the assisting torque needed is zero (figure 9, stage 1). ...
Soft actuators show much promise for use in bioinspired and biomimetic robotics as they share many similarities with actuation systems found in nature. Twisted and coiled actuators are soft actuators that have been shown to outperform many metrics of biological muscles, leading researchers to derive actuation models for future control and implementation. Although models have been proposed for twisted and coiled carbon nanotubes and polymer fibers, cavatappi artificial muscles—a novel twisted and coiled fluidic soft actuator—have not been modeled yet. This work establishes a framework for modeling cavatappi using the thick-wall pressure vessel stress analysis and the spring theory. The presented model uses the mechanical properties of the precursor drawn material used for fabrication, initial twist (internal fiber angle), muscle geometry, and internal pressure to predict the artificial muscles contraction under different external loads. The model predictions agree with the experimental results for cavatappi of different internal fiber angles and load conditions. Given their potential implementation in bioinspired applications, our model can help better design, optimize, and control the actuation response of cavatappi.
... By using fore-foot pressure sensors, the control signal adapted to the instantaneous demand on each user's ankle plantar flexor muscles, automatically adjusting to the type of disability (e.g., hemiplegia vs diplegia) and walking pattern (e.g., crouch gait), and the terrain (e.g., stairs). The controller was previously validated for incline walking and stair ascent in healthy individuals (adaptive accuracy: 88% for incline walking and 83% for stair ascent) [24]. Nominal stance phase torque was set to ~0.30 Nm/kg during stance phase. ...
Graded terrains, like slopes and stairs, are particularly challenging for people with neurological disorders like cerebral palsy (CP) due to increased selective muscle control and muscle strength requirements. Lower-limb exoskeletons may be able to assist individuals with CP when navigating graded terrains. This study sought to determine the effects of untethered ankle exoskeleton assistance on lower-limb joint angles, moments, and muscle activity during up-incline walking and up-stair stepping in CP (n=7). We hypothesized that powered assistance would result in improved ankle mechanics (i.e., increased total ankle moments) across both terrains. During incline walking, we found that peak ankle dorsiflexion angle increased by (p=0.006) during walking with ankle assistance compared to walking without the device (Shod). Compared to without the device, the peak total ankle plantarflexor moment increased by 8% (p=0.022) while peak biological ankle plantarflexor moment decreased by 17% (p< 0.001). Incline walking with ankle assistance reduced stance phase muscle activity of the soleus (20%, p=0.010) and vastus lateralis (18%, p=0.004), and swing phase tibialis activity (19%, p=0.028) compared to Shod. During stair ascent with the device, the peak total ankle plantarflexor moment increased by 17% (p=0.011) and the peak knee extensor moment increased by 40% (p=0.018) compared to Shod. These findings provide insight into the biomechanical benefits of ankle exoskeleton assistance during incline and stair walking. This work aims to advance the use of robotic assistive technology to improve mobility for people with CP.
... Proportional myoelectric control can provide users adaptive control over robotic assistance from lower limb exoskeletons and prostheses [9], [16]- [19]. Proportional myoelectric control transforms direct input of muscle activity amplitude into torque output of the exoskeleton, allowing for feedforward stride-to-stride variability in walking that is controlled by the human wearer [20]- [23]. Most bouts of daily walking in humans are less than 20 consecutive steps [24]. ...
Proportional myoelectric control of robotic lower limb exoskeletons can increase the variability and adaptability of biomechanical behaviors for assisting human movement compared to traditional state-based control. Previous exoskeletons using proportional myoelectric control have relied on pneumatic actuators and been limited to laboratory use. We applied proportional myoelectric control to a robotic ankle exoskeleton using a brushless DC motor (Dephy) and enabled it to work in community settings. Benchtop testing verified electromechanical responses similar to biological values (electromechanical delay of 22 ms and time to peak activation of 123 ms). Four healthy participants trained for thirty minutes each using bilateral ankle exoskeletons. From minute one of powered walking to minute 30 of powered walking, peak soleus EMG reduced by 17.9% as they learned to walk with exoskeleton assistance. Our future work will extend the powered walking period, measure metabolic cost, and measure gait variability between participants using proportional myoelectric control on fully portable, electromechanical ankle exoskeletons.
... The related research results showed that the use of the new evaluation function to optimize the power assist curve in the loop control of the human body could further reduce the degree of muscle activation during walking. Zachary's team designed a realtime adaptive ankle exoskeleton controller capable of accurately assisting in a variety of walking conditions without the need for walking condition classification or real-time assessment of muscle activity, which provided the foundation for the application of AAFO in free-living situation [60]. However, the muscle coordination mode of the human body during walking can be changed to a certain extent affected by AFO, and then result in the compensatory phenomenon of some muscle groups. ...
The ankle joint is one of the important joints of the human body to maintain the ability to walk. Diseases such as stroke and ankle osteoarthritis could weaken the body’s ability to control joints, causing people’s gait to be out of balance. Ankle–foot orthoses can assist users with neuro/muscular or ankle injuries to restore their natural gait. Currently, passive ankle–foot orthoses are mostly designed to fix the ankle joint and provide support for walking. With the development of materials, sensing, and control science, semi-active orthoses that release mechanical energy to assist walking when needed and can store the energy generated by body movement in elastic units, as well as active ankle–foot orthoses that use external energy to transmit enhanced torque to the ankle, have received increasing attention. This article reviews the development process of ankle–foot orthoses and proposes that the integration of new ankle–foot orthoses with rehabilitation technologies such as monitoring or myoelectric stimulation will play an important role in reducing the walking energy consumption of patients in the study of human-in-the-loop models and promoting neuro/muscular rehabilitation.
... The control method of ankle exoskeletons standing balance is normally hierarchical, with a high-level controller estimating the behavior-related desired joint torques and the lower-level controller performing the torque control [9]. The high-level controller plays the most important role in the ankle exoskeleton's work, and the majority of the research on the control of exoskeletons is usually based on time [10,11], joint angle [12], and electromyographic measurements [13,14]. ...
The exoskeleton is often regarded as a tool for rehabilitation and assistance of human movement. The control schemes were conventionally implemented by developing accurate physical and kinematic models, which often lack robustness to external variational disturbing forces. This paper presents a virtual neuromuscular control for robotic ankle exoskeleton standing balance. The robustness of the proposed method was improved by applying a specific virtual neuromuscular model to estimate the desired ankle torques for ankle exoskeleton standing balance control. In specialty, the proposed control method has two key components, including musculoskeletal mechanics and neural control. A simple version of the ankle exoskeleton was designed, and three sets of comparative experiments were carried out. The experimentation results demonstrated that the proposed virtual neuromuscular control could effectively reduce the wearer’s lower limb muscle activation, and improve the robustness of the different external disturbances.
... The torque profiles, generated through calibration and measurement of the fore-foot pressure sensors, inherently matched each individual's walking pattern and walking speed, contributing to a seamless interaction between the device and its user. The adaptive controller was previously validated and achieved >90% accuracy in estimating biological ankle moment during level walking when providing assistance (Gasparri et al., 2019;Bishe et al., 2021) and resistance . Under assistance mode, we provided bilateral stance-phase plantar flexor assistance that reached a peak of 0.3-0.35 ...
Background
Age-related deficits in plantar flexor muscle function during the push-off phase of walking likely contribute to the decline in mobility that affects many older adults. New mobility aids and/or functional training interventions may help slow or prevent ambulatory decline in the elderly.
Objective
The overarching objective of this study was to explore the feasibility of using an untethered, dual-mode ankle exoskeleton as a mobility aid to reduce energy consumption, and as a resistive gait training tool to facilitate functional recruitment of the plantar flexor muscles.
Methods
We recruited six older adults (68–83 years old) to evaluate acute metabolic and neuromuscular adaption to ankle exoskeleton assistance and to evaluate the potential for ankle resistance with biofeedback to facilitate utilization of the ankle plantar flexors. We also conducted a 12-session ankle resistance training protocol with one pilot participant.
Results
Participants reached the lowest net metabolic power and soleus integrated electromyography (iEMG) at 6.6 ± 1.6 and 5.8 ± 4.9 min, respectively, during the 30-min exoskeleton assistance adaptation trial. Four of five participants exhibited a reduction (up to 19%) in metabolic power during walking with assistance. Resistance increased stance-phase soleus iEMG by 18–186% and stance-phase average positive ankle power by 9–88%. Following ankle resistance gait training, the participant exhibited increased walking speed, endurance, and strength.
Conclusions
Our results suggest that dual-mode ankle exoskeletons appear highly applicable to treating plantar flexor dysfunction in the elderly, with assistance holding potential as a mobility aid and resistance holding potential as a functional gait training tool.
... We implemented a real-time exoskeleton control strategy that had been previously validated to provide assistance proportional to the biological ankle joint moment (estimation model accuracy of 90% in unimpaired level walking) [23]. Custom embedded forefoot pressure sensors (A502, Tekscan, South Boston, MA)) were used to estimate real-time biological ankle moment of the user and to identify the stance and swing phase of a gait cycle. ...
People with lower-limb hemiparesis have impaired function on one side of the body that affects their walking ability. Wearable robotic assistance has been investigated to treat hemiparetic gait by applying assistance to the paretic limb. In this exploratory case series, we sought to compare the effects of bilateral vs. paretic-limb-only ankle exoskeleton assistance on walking performance in a case series of three heterogeneous presentations of lower-limb hemiparesis. A secondary goal was to validate the use of a real-time ankle-moment-adaptive exoskeleton control system for effectively assisting hemiparetic gait; the ankle moment controller accuracy ranged from 72 90% across all conditions and participants. Compared to walking without the device, both paretic-limb-only and bilateral assistance resulted in greater average total ankle power (up to 72%), improved treadmill walking efficiency (up to 28%), and increased over-ground walking distance (up to 41%). All participants achieved a more symmetrical, efficient gait pattern with bilateral assistance, indicating that assisting both limbs may be more beneficial than assisting only the paretic side in people with hemiparetic gait. The results of this case series are intended to inform future clinical studies and exoskeleton designs in a wide range of patient populations.
... Each participant was outfitted with a custom ankle exoskeleton device (Fig. 2B). Briefly, this untethered, battery-powered device consisted of a motor assembly worn at the waist, and bilateral ankle assemblies that provided stance-phase plantar flexor assistance proportional to each user's real-time, estimated ankle moment, seamlessly adjusting to turns and variable walking speed [11]; the peak nominal stance-phase torque setpoint was between 0.26 -0.34 Nm/kg. As such, the stance-phase torque profile (shape and timing) was unique to each participant because it was governed by the ankle moment estimation scheme. ...
Objective:
To determine the effects of providing battery-powered ankle dorsiflexor and plantar flexor exoskeleton assistance on six-minute walk test performance and efficiency in children and young adults with cerebral palsy by comparing distance walked under exoskeleton assisted (Assisted) and no device (Shod) walking conditions, and explore the acclimation rate to maximal walking with ankle exoskeleton assistance.
Results:
Six-minute walk test performance significantly improved under the final Assisted condition test compared to the Shod condition (42 ± 27 m, p = 0.02), surpassing the minimum clinically important difference range for children and young adults with CP. There was no difference in walking efficiency (-0.06 ± 0.1, p = 0.3). Participants had an average acclimation rate of 19.6 m per session.
Conclusions:
Powered ankle assistance can significantly improve six-minute walk test performance in individuals with mild-to-moderate gait impairment from CP, supporting the use of this intervention to improve functional mobility and walking capacity in this patient population.
Introduction
Walking is essential for daily life but poses a significant challenge for many individuals with neurological conditions like cerebral palsy (CP), which is the leading cause of childhood walking disability. Although lower-limb exoskeletons show promise in improving walking ability in laboratory and controlled overground settings, it remains unknown whether these benefits translate to real-world environments, where they could have the greatest impact.
Methods
This feasibility study evaluated whether an untethered ankle exoskeleton with an adaptable controller can improve spatiotemporal outcomes in eight individuals with CP after low-frequency exoskeleton-assisted gait training on real-world terrain.
Results
Comparing post- and pre-assessment, assisted walking speed increased by 11% and cadence by 7% ( p = 0.003; p = 0.006), while unassisted walking speed increased by 8% and cadence by 5% ( p = 0.009; p = 0.012). In the post-assessment, assisted walking speed increased by 9% and stride length by 8% relative to unassisted walking ( p < 0.001; p < 0.001). Improvements in walking speed were more strongly associated with longer strides than higher cadence ( R ² = 0.92; R ² = 0.68). Muscle activity outcomes, including co-contraction of the soleus and tibialis anterior, did not significantly change after training.
Discussion
These findings highlight the spatiotemporal benefits of an adaptive ankle exoskeleton for individuals with CP in real-world settings after short-term training. This work paves the way for future randomized controlled trials (RCTs) to evaluate the isolated effects of adaptive ankle exoskeletons on gait performance and neuromuscular outcomes in individuals with CP in real-world environments
Lower-limb exoskeletons have the potential to transform the way we move1–14, but current state-of-the-art controllers cannot accommodate the rich set of possible human behaviours that range from cyclic and predictable to transitory and unstructured. We introduce a task-agnostic controller that assists the user on the basis of instantaneous estimates of lower-limb biological joint moments from a deep neural network. By estimating both hip and knee moments in-the-loop, our approach provided multi-joint, coordinated assistance through our autonomous, clothing-integrated exoskeleton. When deployed during 28 activities, spanning cyclic locomotion to unstructured tasks (for example, passive meandering and high-speed lateral cutting), the network accurately estimated hip and knee moments with an average R² of 0.83 relative to ground truth. Further, our approach significantly outperformed a best-case task classifier-based method constructed from splines and impedance parameters. When tested on ten activities (including level walking, running, lifting a 25 lb (roughly 11 kg) weight and lunging), our controller significantly reduced user energetics (metabolic cost or lower-limb biological joint work depending on the task) relative to the zero torque condition, ranging from 5.3 to 19.7%, without any manual controller modifications among activities. Thus, this task-agnostic controller can enable exoskeletons to aid users across a broad spectrum of human activities, a necessity for real-world viability.
Robotic exoskeletons are increasingly being used for gait rehabilitation in individuals with neuromuscular disorders, such as cerebral palsy (CP). A primary rehabilitation goal for those with CP is to improve ankle push-off power, which is crucial for enhancing gait function. Previous research suggests that interleaving assistance and resistance within the same training session may improve certain aspects of gait, such as joint trajectories and torque profiles. This feasibility study sought to investigate the efficacy of priming the plantar flexor muscles with ankle exoskeleton plantar flexor assistance to facilitate increased ankle push-off power during subsequent resisted gait training bouts in individuals with CP. Specifically, we hypothesized that providing plantar-flexor assistance immediately prior to walking with resistance would increase peak biological ankle power and muscle activity compared to walking with resistance alone. We found that peak biological ankle power increased by 25% (p = 0.021) during assistance-primed resisted walking compared to the baseline resisted walking trail. While ankle angular velocity also increased alongside power, there was no significant difference in plantar flexor muscle activity, suggesting more efficient recruitment. These results contribute to our overarching goal of optimizing robotic exoskeleton interventions, potentially leading to the future design of more effective gait rehabilitation strategies
: This pilot study investigated the feasibility and efficacy of using autonomous ankle exoskeletons in community settings among individuals with cerebral palsy (CP). Five participants completed two structured community walking protocols: a week-long ankle exoskeleton acclimation and training intervention, and a dose-matched Sham intervention of unassisted walking. Results: Results demonstrated significant improvements in acclimatized walking performance with the ankle exoskeleton, including increased speed and stride length. Participants also reported increased enjoyment and perceived benefits of using the exoskeleton. While ankle exoskeleton training did not lead to significant improvements in unassisted walking, this study demonstrates the feasibility of using ankle exoskeletons in the real world by people with CP. Conclusions: This study highlights the potential of wearable exoskeletons to augment community walking performance in CP, laying a foundation for further exploration in real-world environments
Individuals with diminished walking performance caused by neuromuscular impairments often lack plantar flexion muscle activity. Robotic devices have been developed to address these issues and increase walking performance. While these devices have shown promise in their ability to increase musculature engagement of the lower limbs when used on a treadmill, most have not been developed or validated for overground walking and community use. Overground walking may limit the effectiveness of robotic devices due to differences in gait characteristics between walking terrains and reduced user engagement. The purpose of this study was to validate our multimodal robotic gait training system for overground walking in individuals with neuromuscular gait impairments. This untethered wearable robotic device can provide an ankle resistive torque proportional to the users’ biological ankle torque. The device can also provide audio biofeedback based on users’ plantar pressure intending to increase ankle power and muscle activity of the plantar flexors. Seven individuals with cerebral palsy participated. Participants walked overground and on a treadmill with our robotic gait training system in a single testing session. Results showed all seven participants to increase peak plantar flexor muscle activity, 10.3% on average, when walking with the gait trainer overground compared to treadmill. When compared to typical baseline overground walking, overground gait trainer use caused individuals to have slightly less knee joint excursion (3°) and moderately more ankle joint excursion (7°). This work supports our vision of using the wearable robotic device as a gait aid and rehabilitation tool in the home and community settings.
Robotic lower-limb exoskeletons can augment human mobility, but current systems require extensive, context-specific considerations, limiting their real-world viability. Here, we present a unified exoskeleton control framework that autonomously adapts assistance on the basis of instantaneous user joint moment estimates from a temporal convolutional network (TCN). When deployed on our hip exoskeleton, the TCN achieved an average root mean square error of 0.142 newton-meters per kilogram across 35 ambulatory conditions without any user-specific calibration. Further, the unified controller significantly reduced user metabolic cost and lower-limb positive work during level-ground and incline walking compared with walking without wearing the exoskeleton. This advancement bridges the gap between in-lab exoskeleton technology and real-world human ambulation, making exoskeleton control technology viable for a broad community.
Lower limb robotic exoskeletons are often studied in the context of steady-state treadmill walking in laboratory environments. However, the end goal of these devices is often adoption into our everyday lives. To move outside of the laboratory, there is a need to study exoskeletons in real world, complex environments. One way to study the human-machine interaction is to look at how the exoskeleton affects the user’s gait. In this study we assessed changes in gait spatiotemporal variability when using a robotic ankle exoskeleton under proportional myoelectric control both inside on a treadmill and outside overground. We hypothesized that walking with the exoskeletons would not lead to significant changes in variability inside on a treadmill or outside compared to not using the exoskeletons. In addition, we hypothesized that walking outside would lead to higher variability both with and without the exoskeletons compared to treadmill walking. In support of our hypothesis, we found significantly higher coefficients of variation of stride length, stance time, and swing time when walking outside both with and without the exoskeleton. We found a significantly higher variability when using the exoskeletons inside on the treadmill, but we did not see significantly higher variability when walking outside overground. The value of this study to the literature is that it emphasizes the importance of studying exoskeletons in the environment in which they are meant to be used. By looking at only indoor gait spatiotemporal measures, we may have assumed that the exoskeletons led to higher variability which may be unsafe for certain target populations. In the context of the literature, we show that variability due to robotic ankle exoskeletons under proportional myoelectric control does not elicit different changes in stride time variability than previously found in other daily living tasks (uneven terrain, load carriage, or cognitive tasks).
Despite medical treatment focused on addressing walking disability, many millions of people with neurological conditions, like cerebral palsy (CP), struggle to maintain independent mobility. Lower limb exoskeletons and exosuits may hold potential for augmenting walking ability. However, it remains unknown whether these wearable robots are safe and beneficial for use outside of highly controlled laboratory environments, the demonstration of which is necessary for clinical translation. Here, we show that a lightweight, portable, ankle exoskeleton with an adaptable one-size-works-for-all assistance controller can improve energy efficiency and walking speed for individuals with CP spanning a wide spectrum of lower limb impairment in a multi-terrain real-world environment. Tested on an outdoor walking route with level, sloped, and stair terrain, robotic assistance resulted in a 15–18% (p
0.013–0.026) reduction in estimated energy cost and a 7–8% (p
0.001–0.004) increase in average walking speed across “shorter” 6-minute and “longer” 20-minute walking durations relative to unassisted walking. This study provides evidence that wearable robots may soon improve mobility in neighborhood, school, and community settings for individuals with CP.
The current research on exoskeleton torque control is relatively extensive, but the performance is suboptimal. Exoskeletons driven by series elastic actuator (SEA) can achieve high force control accuracy through simple position control. However, motor placement at the distal end increases the mass of the body terminal device, which not only increases the moment of inertia of the lower limb but also interferes with the normal movement of the patient, limiting their applicability in assisting the wearer. In this study, to prevent foot drop, we designed a lightweight, unilateral ankle exoskeleton that provides assistance for dorsiflexion via a single motor. By placing the motor and other heavier electronic components at the waist, the inertial load on the human leg is reduced achieving better assistance performance. Torque control assistance for the ankle joint is implemented using modified cascade PI control. When a person walks at a speed of 0.4 m/s, the desired maximum torque is set to 1.5 Nm, and the actual maximum torque is 1.5±0.187 Nm, which is close to the expected value. This demonstrates that the exoskeleton exhibits satisfactory force-tracking performance and can precisely assist individuals with foot drop in their daily walking activities.
Slowing the decline in walking mobility in the elderly is critical for maintaining the quality of life. Wearable assistive devices may 1 day facilitate mobility in older adults; however, we need to ensure that such devices do not impair stability in this population that is predisposed to fall-related injuries. This study sought to quantify the effects of untethered ankle exoskeleton assistance on measures of stability, whole-body dynamics, and strategies to maintain balance during normal and perturbed walking in older adults. Eight healthy participants (69–84 years) completed a treadmill-based walking protocol that included perturbations from unexpected belt accelerations while participants walked with and without ankle exoskeleton assistance. Exoskeleton assistance increased frontal plane range of angular momentum (8–14%, p ≤ 0.007), step width (18–34%, p ≤ 0.006), and ankle co-contraction (21–29%, p ≤ 0.039), and decreased biological ankle moment (16–27%, p ≤ 0.001) during unperturbed and perturbed walking; it did not affect the anteroposterior margin-of-stability, step length, trunk variability, or soleus activity during unperturbed and perturbed walking. Our finding that ankle exoskeleton assistance did not affect the anteroposterior margin-of-stability supports additional investigation of assistive exoskeletons for walking assistance in the elderly.
When applying exoskeletons for walking assistance, one important consideration is to ensure that the users retain full control over the exoskeleton-provided assistance, which is quite limited in existing exoskeletons due to the absence of a suitable control system. In this article, a time-independent exoskeleton control system is developed based on a novel assistance profile generation method and an iterative force control method to enable continuous assistance adjustment. The assistance profile is formulated as a Gaussian function with a human state variable and can be updated online to adapt to different users. The proposed profile continuously self-adjusts along the movement of the user's leg, especially when users change their walking patterns. The proposed control system iteratively compensates for the force control lag and amplitude attenuation to enable precise tracking of the assistance profile during natural human walking. Experiments have been conducted using a soft exoskeleton on subjects with and without prior experience using an exoskeleton. The experimental results have shown the effectiveness of the proposed control system compared with a common time-dependent control system.
This paper presents a survey on recent advances of wearable actuated ankle-foot orthoses (AAFOs). First of all, their medical functions are investigated. From the short-term aspect, they lead to rectification of pathological gaits, reduction of metabolic cost, and improvement of gait performance. After AAFO-based walking training with sufficient time, free walking performance can be enhanced. Then, key design factors are studied. First, primary design parameters are investigated. Second, common actuators are analysed. Third, human-robot interaction (HRI), ergonomics, safety, and application places, are considered. In the following section, control technologies are reviewed from the aspects of rehabilitation stages, gait feature quantities, and controller characteristics. Finally, existing problems are discussed; development trends are prospected.
Individuals with cerebral palsy (CP) display motor control patterns that suggest decreased supraspinal input, but it remains unknown if they are able to modulate lower-limb reflexes in response to more complex tasks, or whether global motor control patterns relate to reflex modulation capacity in this population. Eight ambulatory individuals with CP (12–18 years old) were recruited to complete a task complexity protocol, where soleus H-reflex excitability was compared between bilateral (baseline) and unilateral (complex) standing. We also investigated the relationship between each participant’s ability to modulate soleus H-reflex excitability and the complexity of their walking neural control pattern determined from muscle synergy analysis. Finally, six of the eight participants completed an exoskeleton walking protocol, where soleus H-reflexes were collected during the stance phase of walking with and without stance-phase plantar flexor resistance. Participants displayed a significant reduction in soleus H-reflex excitability (− 26 ± 25%, p = 0.04) with unilateral standing, and a strong positive relationship was observed between more refined neural control during walking and an increased ability to modulate reflex excitability (R = 0.79, p = 0.04). There was no difference in neuromuscular outcome measures with and without the ankle exoskeleton (p values all > 0.05), with variable reflex responses to walking with ankle exoskeleton resistance. These findings provide evidence that ambulatory individuals with CP retain some capacity to modulate lower-limb reflexes in response to increased task complexity, and that less refined neural control during walking appears to be related to deficits in reflex modulation.
Background
Ankle exoskeletons can improve walking mechanics and energetics, but few untethered devices have demonstrated improved performance and usability across a wide range of users and terrains. Our goal was to design and validate a lightweight untethered ankle exoskeleton that was effective across moderate-to-high intensity ambulation in children through adults with and without walking impairment.
Methods
Following benchtop validation of custom hardware, we assessed the group-level improvements in walking economy while wearing the device in a diverse unimpaired cohort (n = 6, body mass = 42–92 kg). We also conducted a maximal exertion experiment on a stair stepping machine in a small cohort of individuals with cerebral palsy (CP, n = 5, age = 11–33 years, GMFCS I-III, body mass = 40–71 kg). Device usability metrics (device don and setup times and System Usability Score) were assessed in both cohorts.
Results
There was a 9.9 ± 2.6% (p = 0.012, range = 0–18%) reduction in metabolic power during exoskeleton-assisted inclined walking compared to no device in the unimpaired cohort. The cohort with CP was able to ascend 38.4 ± 23.6% (p = 0.013, range = 3–132%) more floors compared to no device without increasing metabolic power (p = 0.49) or perceived exertion (p = 0.50). Users with CP had mean device don and setup times of 3.5 ± 0.7 min and 28 ± 6 s, respectively. Unimpaired users had a mean don time of 1.5 ± 0.2 min and setup time of 14 ± 1 s. The average exoskeleton score on the System Usability Scale was 81.8 ± 8.4 (“excellent”).
Conclusions
Our battery-powered ankle exoskeleton was easy to use for our participants, with initial evidence supporting effectiveness across different terrains for unimpaired adults, and children and adults with CP.
Trial registration Prospectively registered at ClinicalTrials.gov (NCT04119063) on October 8, 2019.
Individuals with neuromuscular impairment from conditions like cerebral palsy face reduced quality of life due to diminishing mobility and independence. Lower-limb exoskeletons have potential to aid mobility, yet few studies have investigated their use during over-ground walking – an exercise that may contribute to our understanding of potential benefit in free-living settings. The goal of this study was to determine the potential for adaptive plantar-flexor assistance from an untethered ankle exoskeleton to improve over-ground walking economy and speed. Six individuals with cerebral palsy completed three consecutive daily over-ground training sessions to acclimate to, and tune, assistance. During a final assessment visit, metabolic cost, walking speed, and soleus electromyography were collected for baseline, unpowered, low, training-tuned, and high assistance conditions. Compared to each participant’s baseline condition, we observed a 3.9 ± 1.9% (p = 0.050) increase in walking speed and a 22.0 ± 4.5% (p = 0.002) reduction in soleus activity with training-tuned assistance; metabolic cost of transport was unchanged (p = 0.130). High assistance resulted in an 8.5 ± 4.0% (p = 0.042) reduction in metabolic cost of transport, a 6.3 ± 2.6% (p = 0.029) increase in walking speed, and a 25.0 ± 4.0% (p < 0.001) reduction in soleus activity. Improvement in exoskeleton-assisted walking economy was related to pre-training baseline walking speed (
, p = 0.001); the slower and more impaired participants improved the most. Energy cost and preferred walking speed remained generally unchanged for the faster and less impaired participants. These findings demonstrate that powered ankle exoskeletons have the potential to improve mobility-related outcomes for some people with cerebral palsy.
Background
Ankle exoskeletons offer a promising opportunity to offset mechanical deficits after stroke by applying the needed torque at the paretic ankle. Because joint torque is related to gait speed, it is important to consider the user’s gait speed when determining the magnitude of assistive joint torque. We developed and tested a novel exoskeleton controller for delivering propulsive assistance which modulates exoskeleton torque magnitude based on both soleus muscle activity and walking speed. The purpose of this research is to assess the impact of the resulting exoskeleton assistance on post-stroke walking performance across a range of walking speeds.
Methods
Six participants with stroke walked with and without assistance applied to a powered ankle exoskeleton on the paretic limb. Walking speed started at 60% of their comfortable overground speed and was increased each minute (n00, n01, n02, etc.). We measured lower limb joint and limb powers, metabolic cost of transport, paretic and non-paretic limb propulsion, and trailing limb angle.
Results
Exoskeleton assistance increased with walking speed, verifying the speed-adaptive nature of the controller. Both paretic ankle joint power and total limb power increased significantly with exoskeleton assistance at six walking speeds (n00, n01, n02, n03, n04, n05). Despite these joint- and limb-level benefits associated with exoskeleton assistance, no subject averaged metabolic benefits were evident when compared to the unassisted condition. Both paretic trailing limb angle and integrated anterior paretic ground reaction forces were reduced with assistance applied as compared to no assistance at four speeds (n00, n01, n02, n03).
Conclusions
Our results suggest that despite appropriate scaling of ankle assistance by the exoskeleton controller, suboptimal limb posture limited the conversion of exoskeleton assistance into forward propulsion. Future studies could include biofeedback or verbal cues to guide users into limb configurations that encourage the conversion of mechanical power at the ankle to forward propulsion.
Trial registration
N/A.
Electronic supplementary material
The online version of this article (10.1186/s12984-019-0523-y) contains supplementary material, which is available to authorized users.
Exoskeletons can influence human gait. A healthy gait is characterized by a certain amount of variability compared to a non-healthy gait that has more inherent variability; however which exoskeleton assistance parameters are necessary to avoid increasing gait variability or to potentially lower gait variability below that of unassisted walking are unknown. This study investigated the interaction effects of exoskeleton timing and power on gait variability. Ten healthy participants walked on a treadmill with bilateral ankle-foot exoskeletons under ten conditions with different timing (varied from 36% to 54% of the stride) and power (varied from 0.2 to 0.5 W∙kg⁻¹) combinations. We used the largest Lyapunov exponent (LyE) and maximum Floquet multiplier (FM) to evaluate the stride-to-stride fluctuations of the kinematic time series. We found the lowest LyE at the ankle and a significant reduction versus powered-off with exoskeleton power (summed for both legs) of 0.45 W∙kg⁻¹ and actuation timing at 48% of the stride cycle. At the knee, a significant positive effect of power and a negative interaction effect of power and timing were found for LyE. We found significant positive interaction effects of the square of timing and power for LyE at the knee and hip joints. In contrast, the FM at the ankle increased with increasing power and later timing. We found a significant negative effect of power and a positive interaction effect of power and timing for FM at the knee and no significant effects of any of the exoskeleton parameters for FM at the hip. The ability of the exoskeleton to reduce the LyE at the ankle joint offers new possibilities in terms of altering gait variability, which could have applications for using exoskeletons as rehabilitation devices. Further efforts could examine if it is possible to simultaneously reduce the LyE and FM at one or more lower limb joints.
The knee joint of the human body involves both rotation and translation, while the magnitude of anterior-posterior translation during flexion/extension movement of the knee joint is very small compared with the length of the human lower limb. It is therefore desirable for an exoskeleton leg to have two degrees of freedom to accommodate the motion of the human knee joint, and for there to be a precision measuring method to obtain its trajectory. This paper presents a novel parallel mechanism which can be used as a precise measuring device to realize closed-loop control for a biologically inspired 3-degree of freedom (DOF) lower limb exoskeleton (BLLE-3) for human gait rehabilitation. In this work, mechanical design and kinematics of the exoskeleton are described. Errors of exoskeleton motion are modelled and analyzed. Closed-loop control law is implemented to enable accurate trajectory following the motions of the exoskeleton. Simulations and experimental results are included to show the effectiveness of the new measuring and control method.
Background
Soft exosuits are a recent approach for assisting human locomotion, which apply assistive torques to the wearer through functional apparel. Over the past few years, there has been growing recognition of the importance of control individualization for such gait assistive devices to maximize benefit to the wearer. In this paper, we present an updated version of autonomous multi-joint soft exosuit, including an online parameter tuning method that customizes control parameters for each individual based on positive ankle augmentation power. Methods
The soft exosuit is designed to assist with plantarflexion, hip flexion, and hip extension while walking. A mobile actuation system is mounted on a military rucksack, and forces generated by the actuation system are transmitted via Bowden cables to the exosuit. The controller performs an iterative force-based position control of the Bowden cables on a step-by-step basis, delivering multi-articular (plantarflexion and hip flexion) assistance during push-off and hip extension assistance in early stance. To individualize the multi-articular assistance, an online parameter tuning method was developed that customizes two control parameters to maximize the positive augmentation power delivered to the ankle. To investigate the metabolic efficacy of the exosuit with wearer-specific parameters, human subject testing was conducted involving walking on a treadmill at 1.50 m s− 1 carrying a 6.8-kg loaded rucksack. Seven participants underwent the tuning process, and the metabolic cost of loaded walking was measured with and without wearing the exosuit using the individualized control parameters. ResultsThe online parameter tuning method was capable of customizing the control parameters, creating a positive ankle augmentation power map for each individual. The subject-specific control parameters and resultant assistance profile shapes varied across the study participants. The exosuit with the wearer-specific parameters significantly reduced the metabolic cost of load carriage by 14.88 ± 1.09% (P = 5 × 10− 5) compared to walking without wearing the device and by 22.03 ± 2.23% (P = 2 × 10− 5) compared to walking with the device unpowered. Conclusion
The autonomous multi-joint soft exosuit with subject-specific control parameters tuned based on positive ankle augmentation power demonstrated the ability to improve human walking economy. Future studies will further investigate the effect of the augmentation-power-based control parameter tuning on wearer biomechanics and energetics.
Background
Controllers for assistive robotic devices can be divided into two main categories: controllers using neural signals and controllers using mechanically intrinsic signals. Both approaches are prevalent in research devices, but a direct comparison between the two could provide insight into their relative advantages and disadvantages. We studied subjects walking with robotic ankle exoskeletons using two different control modes: dynamic gain proportional myoelectric control based on soleus muscle activity (neural signal), and timing-based mechanically intrinsic control based on gait events (mechanically intrinsic signal). We hypothesized that subjects would have different measures of metabolic work rate between the two controllers as we predicted subjects would use each controller in a unique manner due to one being dependent on muscle recruitment and the other not.
Methods
The two controllers had the same average actuation signal as we used the control signals from walking with the myoelectric controller to shape the mechanically intrinsic control signal. The difference being the myoelectric controller allowed step-to-step variation in the actuation signals controlled by the user’s soleus muscle recruitment while the timing-based controller had the same actuation signal with each step regardless of muscle recruitment.
Results
We observed no statistically significant difference in metabolic work rate between the two controllers. Subjects walked with 11% less soleus activity during mid and late stance and significantly less peak soleus recruitment when using the timing-based controller than when using the myoelectric controller. While walking with the myoelectric controller, subjects walked with significantly higher average positive and negative total ankle power compared to walking with the timing-based controller.
Conclusions
We interpret the reduced ankle power and muscle activity with the timing-based controller relative to the myoelectric controller to result from greater slacking effects. Subjects were able to be less engaged on a muscle level when using a controller driven by mechanically intrinsic signals than when using a controller driven by neural signals, but this had no affect on their metabolic work rate. These results suggest that the type of controller (neural vs. mechanical) is likely to affect how individuals use robotic exoskeletons for therapeutic rehabilitation or human performance augmentation.
Electronic supplementary material
The online version of this article (10.1186/s12984-018-0379-6) contains supplementary material, which is available to authorized users.
Control systems for powered prosthetic legs typically divide the gait cycle into several periods with distinct controllers, resulting in dozens of control parameters that must be tuned across users and activities. To address this challenge, this paper presents a control approach that unifies the gait cycle of a powered knee–ankle prosthesis using a continuous, user-synchronized sense of phase. Virtual constraints characterize the desired periodic joint trajectories as functions of a phase variable across the entire stride. The phase variable is computed from residual thigh motion, giving the amputee control over the timing of the prosthetic joint patterns. This continuous sense of phase enabled three transfemoral amputee subjects to walk at speeds from 0.67 to 1.21 m/s and slopes from to +9.0 . Virtual constraints based on task-specific kinematics facilitated normative adjustments in joint work across walking speeds. A fixed set of control gains generalized across these activities and users, which minimized the configuration time of the prosthesis.
Exoskeletons are a promising technology that enables individuals with mobility limitations to walk again. As the 2016 Cybathlon illustrated, however, the community has a considerable way to go before exoskeletons have the necessary capabilities to be incorporated into daily life. While most exoskeletons power only hip and knee flexion, Team Institute for Human and Machine Cognition (IHMC) presents a new exoskeleton, Mina v2, which includes a powered ankle dorsi/plantar flexion (Figure 1). As our entry to the 2016 Cybathlon Powered Exoskeleton Competition, Mina v2's performance allowed us to explore the effectiveness of its powered ankle compared to other powered exoskeletons for pilots with paraplegia. We designed our gaits to incorporate powered ankle plantar flexion to help improve mobility, which allowed our pilot to navigate the given Cybathlon tasks quickly, including those that required ascending movements, and reliably achieve average, conservative walking speeds of 1.04 km/h (0.29 m/s). This enabled our team to place second overall in the Powered Exoskeleton Competition in the 2016 Cybathlon.
The evolution to bipedalism forced humans to develop suitable strategies for dynamically controlling their balance, ensuring stability, and preventing falling. The natural aging process and traumatic events such as lower-limb loss can alter the human ability to control stability significantly increasing the risk of fall and reducing the overall autonomy. Accordingly, there is an urgent need, from both end-users and society, for novel solutions that can counteract the lack of balance, thus preventing falls among older and fragile citizens. In this study, we show a novel ecological approach relying on a wearable robotic device (the Active Pelvis Orthosis, APO) aimed at facilitating balance recovery after unexpected slippages. Specifically, if the APO detects signs of balance loss, then it supplies counteracting torques at the hips to assist balance recovery. Experimental tests conducted on eight elderly persons and two transfemoral amputees revealed that stability against falls improved due to the “assisting when needed” behavior of the APO. Interestingly, our approach required a very limited personalization for each subject, and this makes it promising for real-life applications. Our findings demonstrate the potential of closed-loop controlled wearable robots to assist elderly and disabled subjects and to improve their quality of life.
This paper presents a review on design issues and solutions found in active lower limb prostheses. This review is based on a systematic literature search with a methodical search strategy. The search was carried out across four major technical databases and the retrieved records were screened for their relevance. A total of 21 different active prostheses, including 8 above-knee, 9 below-knee and 4 combined knee-ankle prostheses were identified. While an active prosthesis may help to restore the functional performance of an amputee, the requirements regarding the actuation unit as well as for the control system are high and the development becomes a challenging task. Regarding mechanical design and the actuation unit high force/torque delivery, high efficiency, low size and low weight are conflicting goals. The actuation principle and variable impedance actuators are discussed. The control system is paramount for a “natural functioning” of the prosthesis. The control system has to enable locomotion and should react to the amputee’s intent. For this, multi-level control approaches are reviewed.
Background
Carrying load alters normal walking, imposes additional stress to the musculoskeletal system, and results in an increase in energy consumption and a consequent earlier onset of fatigue. This phenomenon is largely due to increased work requirements in lower extremity joints, in turn requiring higher muscle activation. The aim of this work was to assess the biomechanical and physiological effects of a multi-joint soft exosuit that applies assistive torques to the biological hip and ankle joints during loaded walking. Methods
The exosuit was evaluated under three conditions: powered (EXO_ON), unpowered (EXO_OFF) and unpowered removing the equivalent mass of the device (EXO_OFF_EMR). Seven participants walked on an instrumented split-belt treadmill and carried a load equivalent to 30 % their body mass. We assessed their metabolic cost of walking, kinetics, kinematics, and lower limb muscle activation using a portable gas analysis system, motion capture system, and surface electromyography. ResultsOur results showed that the exosuit could deliver controlled forces to a wearer. Net metabolic power in the EXO_ON condition (7.5 ± 0.6 W kg−1) was 7.3 ± 5.0 % and 14.2 ± 6.1 % lower than in the EXO_OFF_EMR condition (7.9 ± 0.8 W kg−1; p = 0.027) and in the EXO_OFF condition (8.5 ± 0.9 W kg−1; p = 0.005), respectively. The exosuit also reduced the total joint positive biological work (sum of hip, knee and ankle) when comparing the EXO_ON condition (1.06 ± 0.16 J kg−1) with respect to the EXO_OFF condition (1.28 ± 0.26 J kg−1; p = 0.020) and to the EXO_OFF_EMR condition (1.22 ± 0.21 J kg−1; p = 0.007). Conclusions
The results of the present work demonstrate for the first time that a soft wearable robot can improve walking economy. These findings pave the way for future assistive devices that may enhance or restore gait in other applications.
Background
Ankle exoskeletons can now reduce the metabolic cost of walking in humans without leg disability, but the biomechanical mechanisms that underlie this augmentation are not fully understood. In this study, we analyze the energetics and lower limb mechanics of human study participants walking with and without an active autonomous ankle exoskeleton previously shown to reduce the metabolic cost of walking.
Methods
We measured the metabolic, kinetic and kinematic effects of wearing a battery powered bilateral ankle exoskeleton. Six participants walked on a level treadmill at 1.4 m/s under three conditions: exoskeleton not worn, exoskeleton worn in a powered-on state, and exoskeleton worn in a powered-off state. Metabolic rates were measured with a portable pulmonary gas exchange unit, body marker positions with a motion capture system, and ground reaction forces with a force-plate instrumented treadmill. Inverse dynamics were then used to estimate ankle, knee and hip torques and mechanical powers.
Results
The active ankle exoskeleton provided a mean positive power of 0.105 ± 0.008 W/kg per leg during the push-off region of stance phase. The net metabolic cost of walking with the active exoskeleton (3.28 ± 0.10 W/kg) was an 11 ± 4 % (p = 0.019) reduction compared to the cost of walking without the exoskeleton (3.71 ± 0.14 W/kg). Wearing the ankle exoskeleton significantly reduced the mean positive power of the ankle joint by 0.033 ± 0.006 W/kg (p = 0.007), the knee joint by 0.042 ± 0.015 W/kg (p = 0.020), and the hip joint by 0.034 ± 0.009 W/kg (p = 0.006).
Conclusions
This study shows that the ankle exoskeleton does not exclusively reduce positive mechanical power at the ankle joint, but also mitigates positive power at the knee and hip. Furthermore, the active ankle exoskeleton did not simply replace biological ankle function in walking, but rather augmented the total (biological + exoskeletal) ankle moment and power. This study underscores the need for comprehensive models of human-exoskeleton interaction and global optimization methods for the discovery of new control strategies that optimize the physiological impact of leg exoskeletons.
Technological advancements have led to the development of numerous wearable robotic devices for the physical assistance and restoration of human locomotion. While many challenges remain with respect to the mechanical design of such devices, it is at least equally challenging and important to develop strategies to control them in concert with the intentions of the user.
This work reviews the state-of-the-art techniques for controlling portable active lower limb prosthetic and orthotic (P/O) devices in the context of locomotive activities of daily living (ADL), and considers how these can be interfaced with the user’s sensory-motor control system. This review underscores the practical challenges and opportunities associated with P/O control, which can be used to accelerate future developments in this field. Furthermore, this work provides a classification scheme for the comparison of the various control strategies.
As a novel contribution, a general framework for the control of portable gait-assistance devices is proposed. This framework accounts for the physical and informatic interactions between the controller, the user, the environment, and the mechanical device itself. Such a treatment of P/Os – not as independent devices, but as actors within an ecosystem – is suggested to be necessary to structure the next generation of intelligent and multifunctional controllers.
Each element of the proposed framework is discussed with respect to the role that it plays in the assistance of locomotion, along with how its states can be sensed as inputs to the controller. The reviewed controllers are shown to fit within different levels of a hierarchical scheme, which loosely resembles the structure and functionality of the nominal human central nervous system (CNS). Active and passive safety mechanisms are considered to be central aspects underlying all of P/O design and control, and are shown to be critical for regulatory approval of such devices for real-world use.
The works discussed herein provide evidence that, while we are getting ever closer, significant challenges still exist for the development of controllers for portable powered P/O devices that can seamlessly integrate with the user’s neuromusculoskeletal system and are practical for use in locomotive ADL.
Lower extremity exoskeletons are wearable robots that integrate human intelligence with the strength of legged robots. Recently, lower extremity exoskeletons have been specifically developed for transportation of disabled individuals. This paper summarizes the anthropomorphic design of a lower extremity exoskeleton named "walking supporting exoskeleton (WSE)." WSE has been developed to support some fundamental motions (walking, sitting, standing, etc.) of disabled individuals who lost leg muscular activities completely or partially. WSE has two degrees of freedom per leg which are powered by electrical actuators. This paper discusses critical design criteria considered in mechanical design and actuator selection of WSE.
Robotic lower limb exoskeletons have been built for augmenting human performance, assisting with disabilities, studying human physiology, and re-training motor deficiencies. At the University of Michigan Human Neuromechanics Laboratory, we have built pneumatically-powered lower limb exoskeletons for the last two purposes. Most of our prior research has focused on ankle joint exoskeletons because of the large contribution from plantar flexors to the mechanical work performed during gait. One way we control the exoskeletons is with proportional myoelectric control, effectively increasing the strength of the wearer with a physiological mode of control. Healthy human subjects quickly adapt to walking with the robotic ankle exoskeletons, reducing their overall energy expenditure. Individuals with incomplete spinal cord injury have demonstrated rapid modification of muscle recruitment patterns with practice walking with the ankle exoskeletons. Evidence suggests that proportional myoelectric control may have distinct advantages over other types of control for robotic exoskeletons in basic science and rehabilitation.
To investigate plantar pressure differences between obese and non-obese adults during standing and walking protocols using a pressure distribution platform.
Thirty-five males (age 42.4+/-10.8 y; 67-179 kg) and 35 females (age 40.0+/-12.6 y; 46-150 kg) divided into obese (body mass index (BMI) 38.75+/-5.97 kg/m2) and non-obese (BMI 24.28+/-3.00 kg/m2) sub-groups, respectively.
Data collection was performed with a capacitive pressure distribution platform with a resolution of 2 sensors/cm2 (Emed F01, Novel GmbH, München). The measurement protocol included half and full body weight standing on the left, right and both feet, respectively, and walking across the platform, striking with the right foot. Pressures were evaluated for eight anatomical sites under the feet.
For both men and women, the mean pressure values of the obese were higher under all anatomical landmarks during half body weight standing. Significant increases in pressure were found under the heel, mid-foot and metatarsal heads II and IV for men and III and IV for women. Foot width during standing was also significantly increased in obese subjects. For walking, significantly higher peak pressures were also found in both obese males and females.
Compared to a non-obese group, obese subjects showed increased forefoot width and higher plantar pressures during standing and walking. The greatest effect of body weight on higher peak pressures in the obese was found under the longitudinal arch of the foot and under the metatarsal heads. The higher pressures for obese women compared to obese men during static weight bearing (standing) may be the result of reduced strength of the ligaments of the foot.
Robotic lower limb exoskeletons that can alter joint mechanical power output are novel tools for studying the relationship between the mechanics and energetics of human locomotion. We built pneumatically powered ankle exoskeletons controlled by the user's own soleus electromyography (i.e. proportional myoelectric control) to determine whether mechanical assistance at the ankle joint could reduce the metabolic cost of level, steady-speed human walking. We hypothesized that subjects would reduce their net metabolic power in proportion to the average positive mechanical power delivered by the bilateral ankle exoskeletons. Nine healthy individuals completed three 30 min sessions walking at 1.25 m s(-1) while wearing the exoskeletons. Over the three sessions, subjects' net metabolic energy expenditure during powered walking progressed from +7% to -10% of that during unpowered walking. With practice, subjects significantly reduced soleus muscle activity (by approximately 28% root mean square EMG, P<0.0001) and negative exoskeleton mechanical power (-0.09 W kg(-1) at the beginning of session 1 and -0.03 W kg(-1) at the end of session 3; P=0.005). Ankle joint kinematics returned to similar patterns to those observed during unpowered walking. At the end of the third session, the powered exoskeletons delivered approximately 63% of the average ankle joint positive mechanical power and approximately 22% of the total positive mechanical power generated by all of the joints summed (ankle, knee and hip) during unpowered walking. Decreases in total joint positive mechanical power due to powered ankle assistance ( approximately 22%) were not proportional to reductions in net metabolic power ( approximately 10%). The ;apparent efficiency' of the ankle joint muscle-tendon system during human walking ( approximately 0.61) was much greater than reported values of the ;muscular efficiency' of positive mechanical work for human muscle ( approximately 0.10-0.34). High ankle joint ;apparent efficiency' suggests that recoiling Achilles' tendon contributes a significant amount of ankle joint positive power during the push-off phase of walking in humans.
Dynamic simulations of movement allow one to study neuromuscular coordination, analyze athletic performance, and estimate internal loading of the musculoskeletal system. Simulations can also be used to identify the sources of pathological movement and establish a scientific basis for treatment planning. We have developed a freely available, open-source software system (OpenSim) that lets users develop models of musculoskeletal structures and create dynamic simulations of a wide variety of movements. We are using this system to simulate the dynamics of individuals with pathological gait and to explore the biomechanical effects of treatments. OpenSim provides a platform on which the biomechanics community can build a library of simulations that can be exchanged, tested, analyzed, and improved through a multi-institutional collaboration. Developing software that enables a concerted effort from many investigators poses technical and sociological challenges. Meeting those challenges will accelerate the discovery of principles that govern movement control and improve treatments for individuals with movement pathologies.
The VT-Lowe's exoskeleton is a novel passive lift-assistive device designed to offload the back muscles during repetitive lifting. In this study, the effect of the exoskeleton on electromyographic (EMG) signals was investigated in four different lifting types (stoop, squat, freestyle and asymmetric) and two box weights (0% and 20% of body weight). Twelve young healthy adults ages 18–31 years (mean = 22.75, SD = 4.35) were participants. The EMG signals for twelve muscles (iliocostalis erector spinae (IL), longissimus erector spinae (LT), multifidus (MF), bicep femoris (BF), vastus lateralis (VL) and abdominal external oblique (AEO) muscles) were measured. The exoskeleton significantly decreased the peak and mean activity of back muscles (IL and LT) by 31.5% and 29.3%, respectively, for symmetric lifts and by 28.2% and 29.5%, respectively, for asymmetric lifts. The peak and mean EMG of leg muscles were significantly reduced by 19.1% and 14.1% during symmetric lifts, and 17.4% and 14.6% during asymmetric lifts. Although the exoskeleton reduced the activation of back and leg muscles, it slightly increased the activity of external oblique muscles, although this was not statistically significant. In conclusion, the exoskeleton is promising as a lift-assist device for manual material handlers and workers performing repetitive lifting.
Lower-limb exoskeletons used to improve free-living mobility for individuals with neuromuscular impairment must be controlled to prescribe assistance that adapts to the diverse locomotor conditions encountered during daily life, including walking at different speeds and across varied terrain. The goal of this study was to design and establish clinical feasibility of an ankle exoskeleton control strategy that instantly and appropriately adjusts assistance to the changing biomechanical demand during variable walking. To accomplish this goal, we developed a proportional joint-moment control strategy that prescribes assistance as a function of the instantaneous estimate of the ankle joint moment and conducted a laboratory-based feasibility study. Four individuals with neuromotor impairment and one unimpaired individual completed exoskeleton-assisted slow and fast gait transition tasks that involved gait initiation and changing walking speed. We found that the controller was effective in instantaneously prescribing exoskeleton assistance that was proportional to the ankle moment with less than 14% root-mean-square error, on average. We also performed a three-subject pilot investigation to determine the ability of the proportional joint-moment controller to improve walking economy. Evaluated in two individuals with cerebral palsy and one unimpaired individual, metabolic cost of transport improved 17-27% during treadmill and over-ground walking with proportional control compared to wearing the exoskeleton unassisted. These preliminary findings support the continued investigation of proportional joint-moment control for assisting individuals with neuromuscular disabilities during walking in real-world settings.
Neuromuscular impairment associated with cerebral palsy (CP) often leads to life-long walking deficits. Our goal was to evaluate the ability of a novel untethered wearable ankle exoskeleton to reduce the severity of gait pathology from CP. In this clinical feasibility study of five individuals with CP, we used instrumented gait analysis to quantify how powered plantar-flexor assistance affected gait mechanics following multi-visit acclimation. Compared to how each participant walked normally, walking with untethered exoskeleton assistance resulted in improved ankle plantar-flexion and knee extension; residual flexion deformity across the lower-extremity improved by a clinically significant 14.4° (p = 0.022). Powered plantar-flexor assistance increased average total positive ankle power by 44% (p = 0.037), and resulted in a 30% reduction in average negative biological ankle power (p = 0.004) and a 29% reduction in average positive hip power (p = 0.009). These findings suggest that powered ankle assistance augmented, rather than simply replaced, biological function to produce a more efficient gait pattern, which was corroborated by a 19% improvement in metabolic cost of transport (p = 0.011). This study provides evidence in support of the continued investigation of ankle assistance in mobility and rehabilitation interventions for this patient population.
The high energy cost of walking in individuals with cerebral palsy (CP) contributes significantly to reduced mobility and quality of life. The purpose of this study was to develop and clinically evaluate an untethered ankle exoskeleton with the ability to reduce the metabolic cost of walking in children and young adults with gait pathology from CP. We designed a battery-powered device consisting of an actuator-and-control module worn above the waist with a Bowden cable transmission used to provide torque to pulleys aligned with the ankle. Special consideration was made to minimize adding mass to the body, particularly distal portions of the lower-extremity. The exoskeleton provided plantar-flexor assistance during the mid-to-late stance phase, controlled using a real-time control algorithm and embedded sensors. We conducted a device feasibility and a pilot clinical evaluation study with five individuals with CP ages five through thirty years old. Participants completed an average of 130 minutes of exoskeleton-assisted walking practice. We observed a 19 ± 5% improvement in the metabolic cost of transport (p = 0.011) during walking with untethered exoskeleton assistance compared to how participants walked normally. These preliminary findings support the future investigation of powered ankle assistance for improving mobility in this patient population.
This paper introduces TWIICE, a lower-limb exoskeleton that enables people suffering from complete paraplegia to stand up and walk again. TWIICE provides complete mobilization of the lower-limbs, which is a first step toward enabling the user to regain independence in activities of the daily living. The tasks it can perform include level and inclined walking (up to 20° slope), stairs ascent and descent, sitting on a seat, and standing up. Participation in the world's first Cybathlon (Zurich, 2016) demonstrated good performance at these demanding tasks. In this paper, we describe the implementation details of the device and comment on preliminary results from a single user case study.
The longitudinal arch between the heel and the forefoot and the transverse arch between the first and fifth metatarsal heads, absorb shock, energy and force. A device to measure plantar pressure was used in 66 normal healthy subjects and in 294 patients with various types of foot disorder. Only 22 (3%) of a total of 720 feet, had a dynamic metatarsal arch during the stance phase of walking, and all had known abnormality.
Our findings show that there is no distal transverse metatarsal arch during the stance phase. This is important for the classification and description of disorders of the foot.
The ability to walk contributes considerably to physical health and overall well-being, particularly in children with motor disability, and is therefore prioritized as a rehabilitation goal. However, half of ambulatory children with cerebral palsy (CP), the most prevalent childhood movement disorder, cease to walk in adulthood. Robotic gait trainers have shown positive outcomes in initial studies, but these clinic-based systems are limited to short-term programs of insufficient length to maintain improved function in a lifelong disability such as CP. Sophisticated wearable exoskeletons are now available, but their utility in treating childhood movement disorders remains unknown. We evaluated an exoskeleton for the treatment of crouch (or flexed-knee) gait, one of the most debilitating pathologies in CP. We show that the exoskeleton reduced crouch in a cohort of ambulatory children with CP during overground walking. The exoskeleton was safe and well tolerated, and all children were able to walk independently with the device. Rather than guiding the lower limbs, the exoskeleton dynamically changed the posture by introducing bursts of knee extension assistance during discrete portions of the walking cycle, a perturbation that resulted in maintained or increased knee extensor muscle activity during exoskeleton use. Six of seven participants exhibited postural improvements equivalent to outcomes reported from invasive orthopedic surgery. We also demonstrate that improvements in crouch increased over the course of our multiweek exploratory trial. Together, these results provide evidence supporting the use of wearable exoskeletons as a treatment strategy to improve walking in children with CP.
Optimum human input
Exoskeletons can be used to augment human abilities—for example, to lift very heavy loads or to provide greater endurance. For each user, though, a device will need to be adjusted for optimum effect, which can be time-consuming. Zhang et al. show that the human can be included in the optimization process, with real-time adaptation of an ankle exoskeleton (see the Perspective by Malcolm et al. ). By using indirect calorimetry to measure metabolic rates, the authors were able to adjust the torque provided by the device while users were walking, running, and carrying a load.
Science , this issue p. 1280 ; see also p. 1230
For a physical human-robot interaction (pHRI) system such as an exoskeleton, it has been an important issue to control the interaction force between the user and the actuators, because it directly determines the accuracy of the realized impedance. However, the accurate force control of the exoskeleton system has not been fully achieved due to the difficulties caused by uncertainties from pHRI. To overcome this problem, in this study, the series elastic actuator (SEA) was operated by a time delay control (TDC) which directly compensated the uncertainties without an accurate model of them, realizing the designed model dynamics in the actuator. However, in spite of successful application of TDC, the delays between the desired and model dynamics severely degraded the force control performance of the SEA. Thus, a new strategy named model-inverse time delay control (MiTDC) was proposed by introducing a virtual reference predicted by the inverse of model dynamics in TDC. The proposed method significantly improved performance of the force control, and it was verified by demonstrating zero impedance control with pHRI, free oscillation of a pendulum, and force control in the exoskeleton.
The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.
Individuals with cerebral palsy frequently exhibit crouch gait, a pathological walking pattern characterized by excessive knee flexion. Knowledge of the knee joint moment during crouch gait is necessary for the design and control of assistive devices used for treatment. Our goal was to 1) develop statistical models to estimate knee joint moment extrema and dynamic stiffness during crouch gait, and 2) use the models to estimate the instantaneous joint moment during weight-Acceptance. We retrospectively computed knee moments from 10 children with crouch gait and used stepwise linear regression to develop statistical models describing the knee moment features. The models explained at least 90% of the response value variability: peak moment in early (99%) and late (90%) stance, and dynamic stiffness of weight-Acceptance flexion (94%) and extension (98%). We estimated knee extensor moment profiles from the predicted dynamic stiffness and instantaneous knee angle. This approach captured the timing and shape of the computed moment (root-mean-squared error: 2.64 Nm); including the predicted early-stance peak moment as a correction factor improved model performance (root-mean-squared error: 1.37 Nm). Our strategy provides a practical, accurate method to estimate the knee moment during crouch gait, and could be used for real-Time, adaptive control of robotic orthoses.
The human ankle produces a large burst of "push-off" mechanical power late in the stance phase of walking, reduction of which leads to considerably poorer energy economy. It is, however, uncertain whether the energetic penalty results from poorer efficiency when the other leg joints substitute for the ankle's push-off work, or from a higher overall demand for work due to some fundamental feature of push-off. Here we show that greater metabolic energy expenditure is indeed explained by a greater demand for work. This is predicted by a simple model of walking on pendulum-like legs, because proper push-off reduces collision losses from the leading leg. We tested this by experimentally restricting ankle push-off bilaterally in healthy adults (N=8) walking on a treadmill at 1.4 m⋅s(-1), using ankle-foot orthoses with steel cables limiting motion. These produced up to about 50% reduction in ankle push-off power and work, resulting in up to about 50% greater net metabolic power expenditure to walk at the same speed. For each 1 J reduction in ankle work, we observed about 0.6 J more dissipative collision work by the other leg, 1.3 J more positive work from the leg joints overall, and 3.94 J more metabolic energy expended. Loss of ankle push-off required more positive work elsewhere to maintain walking speed. That additional work was performed by the knee, apparently at reasonably high efficiency. Ankle push-off may contribute to walking economy by reducing dissipative collision losses and thus overall work demand.
With efficiencies derived from evolution, growth and learning, humans are very well-tuned for locomotion. Metabolic energy used during walking can be partly replaced by power input from an exoskeleton, but is it possible to reduce metabolic rate without providing an additional energy source? This would require an improvement in the efficiency of the human-machine system as a whole, and would be remarkable given the apparent optimality of human gait. Here we show that the metabolic rate of human walking can be reduced by an unpowered ankle exoskeleton. We built a lightweight elastic device that acts in parallel with the user's calf muscles, off-loading muscle force and thereby reducing the metabolic energy consumed in contractions. The device uses a mechanical clutch to hold a spring as it is stretched and relaxed by ankle movements when the foot is on the ground, helping to fulfil one function of the calf muscles and Achilles tendon. Unlike muscles, however, the clutch sustains force passively. The exoskeleton consumes no chemical or electrical energy and delivers no net positive mechanical work, yet reduces the metabolic cost of walking by 7.2 ± 2.6% for healthy human users under natural conditions, comparable to savings with powered devices. Improving upon walking economy in this way is analogous to altering the structure of the body such that it is more energy-effective at walking. While strong natural pressures have already shaped human locomotion, improvements in efficiency are still possible. Much remains to be learned about this seemingly simple behaviour.
Starting from the early research in the 1960s, especially in the last two decades, orthoses and exoskeletons have been significantly developed. They are designed in different architectures to assist their users’ movements. The research literature has been more prolific on lower-limb devices: a main reason is that they address a basic but fundamental motion task, walking. Leg exoskeletons are simpler to design, compared to upper-limb counterparts, but still have particular cognitive and physical requirements from the emerging human–robot interaction systems. In the state of the art, different control strategies and approaches can be easily found: it is still a challenge to develop an assistive strategy which makes the exoskeleton supply efficient and natural assistance. So, this paper aims to provide a systematic overview of the assistive strategies utilized by active locomotion–augmentation orthoses and exoskeletons. Based on the literature collected from Web of Science and Scopus, we have studied the main robotic devices with a focus on the way they are controlled to deliver assistance; the relevant validations are as well investigated, in particular experimentations with human in the loop. Finally current trends and major challenges in the development of an assistive strategy are concluded and discussed.
The study was designed to assess the co-contractions of tibialis anterior (TA) and gastrocnemius lateralis (GL) in healthy young adults during gait at self-selected speed and cadence, in terms of variability of onset-offset muscular activation and occurrence frequency. Statistical gait analysis (SGA), a recent methodology performing a statistical characterization of gait by averaging spatio-temporal and EMG-based parameters over numerous strides, was performed in twenty-four healthy young adults. Co-contractions were assessed as the period of overlap between activation intervals of TA and GL. Results showed that GL and TA act as pure agonist/antagonists for ankle plantar/dorsiflexion (no co-contractions) in only 21.3±8.2% of strides. In the remaining strides, statistically significant (p<0.05) co-contractions appear in early stance (29.2±1.7%), mid-stance (32.1±18.3%) and swing (62.2±2.0%). This significantly increased complexity in muscle recruitment strategy beyond the activation as pure ankle plantar/dorsiflexors, suggests that co-contractions are likely functional to further physiological tasks as foot inversion, balance improvement, control of ankle stability and knee flexion. This study represents the first attempt for the development in healthy young adults of a "normality" reference frame for GL/TA co-contractions, able to include the physiological variability of the phenomenon and eliminate the confounding effect of age.
Copyright © 2014 Elsevier Ltd. All rights reserved.
Correlation is a statistical method used to assess a possible linear association between two continuous variables. It is simple both to calculate and to interpret. However, misuse of correlation is so common among researchers that some statisticians have wished that the method had never been devised at all. The aim of this article is to provide a guide to appropriate use of correlation in medical research and to highlight some misuse. Examples of the applications of the correlation coefficient have been provided using data from statistical simulations as well as real data. Rule of thumb for interpreting size of a correlation coefficient has been provided.
This study investigated the use of surface electromyography (EMG) combined with pattern recognition (PR) to identify user locomotion modes. Due to the nonstationary characteristics of leg EMG signals during locomotion, a new phase-dependent EMG PR strategy was proposed for classifying the user's locomotion modes. The variables of the system were studied for accurate classification and timely system response. The developed PR system was tested on EMG data collected from eight able-bodied subjects and two subjects with long transfemoral (TF) amputations while they were walking on different terrains or paths. The results showed reliable classification for the seven tested modes. For eight able-bodied subjects, the average classification errors in the four defined phases using ten electrodes located over the muscles above the knee (simulating EMG from the residual limb of a TF amputee) were 12.4% +/- 5.0%, 6.0% +/- 4.7%, 7.5% +/- 5.1%, and 5.2% +/- 3.7%, respectively. Comparable results were also observed in our pilot study on the subjects with TF amputations. The outcome of this investigation could promote the future design of neural-controlled artificial legs.
The architectural features of the major knee extensors and flexors and ankle plantar flexors and dorsiflexors were determined in three human cadavers. There was marked uniformity of fiber length throughout a given muscle and a trend toward similar fiber lengths within muscles of a synergistic group. Muscle length/fiber length ratios were remarkably similar for all three limbs. Angles of fiber pinnation were relatively small (0 degree-15 degrees) and generally consistent throughout the muscle. From these architectural data, the performance of a muscle was studied with respect to its tension production and velocity of shortening potentials. The tension is a function of the number of sarcomeres in parallel, and the velocity of shortening is a function of the number of sarcomeres in series. Muscles were grouped according to whether they showed a predilection for tension or velocity of shortening.
To examine the relative magnitude of the knee abduction-adduction moments during stair climbing, ten normal subjects (average weight 660 N, leg length 0.962 m, height 1.74 m) were studied during repeated trials of stair ascent and descent. Data were collected using a four-camera video system and two forces plates incorporated within a flight of three stairs. The inverse dynamics approach was used to calculate internal moments at the knee, and these moments were normalized in magnitude (to percent body weight and leg length) and time (percent stance). The primary findings were: (1) knee joint moments were similar in shape and magnitude for the first and second steps during both stair ascent and descent; (2) the abduction knee moments, although comparable in magnitude (25-45 Nm), were statistically smaller than the extension moments (60-85 Nm) for stair ascent and descent; and (3) the moment patterns were exclusively abductor throughout stance, indicating that the ground reaction vector always passed medial to the knee joint center. Although the knee abduction-adduction moment is not in the primary plane of motion, its magnitude should not be ignored when trying to understand the stability and function of the knee during stair climbing.
The study was designed to determine the strategies used by diplegic subjects to change walking speed. Two groups, limited community ambulators and community ambulators, were compared with controls to determine if ability to increase speed would decrease as a function of motor impairment. Compared with matched controls, diplegic subjects were slower and relied more on cadence to increase speed. The ability to change velocity and stride length was significantly less in the diplegic groups than in controls and accounted for the wider difference in their fast walking velocity. Velocity and stride length decreased, whereas stance time increased as a function of motor involvement. In the limited community ambulators, pelvic excursion was increased, whereas hip and knee excursion was reduced. By assessing fast speed, differences between controls and diplegic groups became more apparent.
The longitudinal arch between the heel and the forefoot and the transverse arch between the first and fifth metatarsal heads, absorb shock, energy and force. A device to measure plantar pressure was used in 66 normal healthy subjects and in 294 patients with various types of foot disorder. Only 22 (3%) of a total of 720 feet, had a dynamic metatarsal arch during the stance phase of walking, and all had known abnormality. Our findings show that there is no distal transverse metatarsal arch during the stance phase. This is important for the classification and description of disorders of the foot.
The ReWalk powered exoskeleton to restore ambulatory function to individuals with thoracic-level motor-complete spinal cord injury
- M F Abel
M. F. Abel et al., "The ReWalk powered exoskeleton to restore ambulatory function to individuals with thoracic-level motor-complete spinal
cord injury," J. Pediatr. Orthop., vol. 6, no. 11, pp. 333-340, 2012.