The effect of back muscle fatigue on EMG and kinematics based estimation of low-back loads and active moments during manual lifting tasks

This study investigated the effects of back muscle fatigue on the estimation of low-back loads and active low-back moments during lifting, using an EMG and kinematics based model calibrated with data from an unfatigued state. Fourteen participants performed lifting tasks in unfatigued and fatigued states. Fatigue was induced through semi-static forward bending. EMG, kinematics, and ground reaction forces were measured, and low-back loads were estimated using inverse dynamics and EMG-driven muscle model. A regression model was developed using data from a set of calibration lifts, and its accuracy was evaluated for unfatigued and fatigued lifts. During the fatigue-inducing task, the EMG amplitude increased by 2.8 %MVC, representing a 38% increase relative to the initial value. However, during the fatigued lifts, the peak EMG amplitude was found to be 1.6 %MVC higher than that observed during the unfatigued lifts, representing a mere 4% increase relative to the baseline unfatigued peak EMG amplitude. Kinematics and low-back load estimates remained unaffected. Regression model estimation errors remained unaffected for 5 kg lifts, but increased by no more than 5% of the peak active low-back moment for 15 kg lifts. We conclude that the regression-based estimation quality of active low-back moments can be maintained during periods of muscle fatigue, although errors may slightly increase for heavier loads.

Effect of perturbation timing on recovering whole-body angular momentum during very slow walking

Humans prioritize regulation of the whole-body angular momentum (WBAM) during walking. When perturbed, modulations of the moment arm of the ground reaction force (GRF) with respect to the centre of mass (CoM) assist in recovering WBAM. For sagittal-plane perturbations of the WBAM given at toe off right (TOR), horizontal GRF modulations and not centre of pressure (COP) modulations were mainly responsible for these moment arm modulations. In this study, we aimed to find whether the instant of perturbations affects the contributions of the GRF and/or CoP modulations to the moment arm changes, in balance recovery during very slow walking. Perturbations of the WBAM were applied at three different instants of the gait cycle, namely at TOR, mid-swing (MS), and heel strike right (HSR). Forces equal to 16% of the participant's body weight were applied simultaneously to the pelvis and upper body in opposite directions for a duration of 150 ms. The results showed that the perturbation onset did not significantly affect the GRF moment arm modulation. However, the contribution of both the CoP and GRF modulation to the moment arm changes did change depending on the perturbation instant. After perturbations resulting in a forward pitch of the trunk a larger contribution was present from the CoP modulation when perturbations were given at MS or HSR, compared to perturbations at TOR. After backward pitch perturbations given at MS and HSR the CoP modulation counteracted the moment arm required for WBAM recovery. Therefore a larger contribution from the horizontal GRF was needed to direct the GRF posterior to the CoM and recover WBAM. In conclusion, the onset of WBAM perturbations does not affect the moment arm modulation needed for WBAM recovery, while it does affect the way CoP and GRF modulation contribute to that recovery.

Subject-Specific and COM Acceleration-Enhanced Reflex Neuromuscular Model to Predict Ankle Responses in Perturbed Gait

Subject-specific musculoskeletal models generate more accurate joint torque estimates from electromyography (EMG) inputs in relation to experimentally obtained torques. Similarly, reflex Neuromuscular Models (NMMs) that employ COM states in addition to musculotendon information generate muscle activations to musculoskeletal models that better predict ankle torques during perturbed gait. In this study, the reflex NMM of locomotion of one subject is identified by employing an EMG-calibrated musculoskeletal model in unperturbed and perturbed gait. A COM acceleration-enhanced reflex NMM is identified. Subject-specific musculoskeletal models improve torque tracking of the ankle joint in unperturbed and perturbed conditions. COM acceleration-enhanced reflex NMM improves ankle torque tracking especially in early stance and during backward perturbation. Results found herein can guide the implementation of reflex controllers in active prosthetic and orthotic devices.

Real-time lumbosacral joint loading estimation in exoskeleton-assisted lifting conditions via electromyography-driven musculoskeletal models

Lumbar joint compression forces have been linked to the development of chronic low back pain, which is specially present in occupational environments. Offline methodologies for lumbosacral joint compression force estimation are not commonly integrated in occupational or medical applications due to the highly time-consuming and complex post-processing procedures. Hence, applications such as real-time adjustment of assistive devices (i.e., back-support exoskeletons) for optimal modulation of compression forces remains unfeasible. Here, we present a real-time electromyography (EMG)-driven musculoskeletal model, capable of estimating accurate lumbosacral joint moments and plausible compression forces. Ten participants performed box-lifting tasks (5 and 15 kg) with and without the Laevo Flex back-support exoskeleton using squat and stoop lifting techniques. Lumbosacral kinematics and EMGs from abdominal and thoracolumbar muscles were used to drive, in real-time, subject-specific EMG-driven models, and estimate lumbosacral joint moments and compression forces. Real-time EMG-model derived moments showed high correlations (R2 = 0.76 - 0.83) and estimation errors below 30% with respect to reference inverse dynamic moments. Compared to unassisted lifting conditions, exoskeleton liftings showed mean lumbosacral joint moments and compression forces reductions of 11.9 - 18.7 Nm (6 - 12% of peak moment) and 300 - 450 N (5 - 10%), respectively. Our modelling framework was capable of estimating in real-time, valid lumbosacral joint moments and compression forces in line with in vivo experimental data, as well as detecting the biomechanical effects of a passive back-support exoskeleton. Our presented technology may lead to a new class of bio-protective robots in which personalized assistance profiles are provided based on subject-specific musculoskeletal variables.

Assisting walking balance using a bio-inspired exoskeleton controller

BACKGROUND

Balance control is important for mobility, yet exoskeleton research has mainly focused on improving metabolic energy efficiency. Here we present a biomimetic exoskeleton controller that supports walking balance and reduces muscle activity.

METHODS

Humans restore balance after a perturbation by adjusting activity of the muscles actuating the ankle in proportion to deviations from steady-state center of mass kinematics. We designed a controller that mimics the neural control of steady-state walking and the balance recovery responses to perturbations. This controller uses both feedback from ankle kinematics in accordance with an existing model and feedback from the center of mass velocity. Control parameters were estimated by fitting the experimental relation between kinematics and ankle moments observed in humans that were walking while being perturbed by push and pull perturbations. This identified model was implemented on a bilateral ankle exoskeleton.

RESULTS

Across twelve subjects, exoskeleton support reduced calf muscle activity in steady-state walking by 19% with respect to a minimal impedance controller (p < 0.001). Proportional feedback of the center of mass velocity improved balance support after perturbation. Muscle activity is reduced in response to push and pull perturbations by 10% (p = 0.006) and 16% (p < 0.001) and center of mass deviations by 9% (p = 0.026) and 18% (p = 0.002) with respect to the same controller without center of mass feedback.

CONCLUSION

Our control approach implemented on bilateral ankle exoskeletons can thus effectively support steady-state walking and balance control and therefore has the potential to improve mobility in balance-impaired individuals.

Sagittal-plane balance perturbations during very slow walking: Strategies for recovering linear and angular momentum

Spatiotemporal gait characteristics change during very slow walking, a relevant speed considering individuals with movement disorders or using assistive devices. However, we lack insights in how very slow walking affects human balance control. Therefore, we aimed to identify how healthy individuals use balance strategies while walking very slow. Ten healthy participants walked on a treadmill at an average speed of 0.43ms^-1, while being perturbed at toe off right by either perturbations of the whole-body linear momentum (WBLM) or angular momentum (WBAM). WBLM perturbations were given by a perturbation on the pelvis in forward or backward direction. The WBAM was perturbed by two simultaneous perturbations in opposite directions on the pelvis and upper body. The given perturbations had magnitudes of 4, 8, 12 and 16% of the participant's body weight, and lasted for 150ms. After perturbations of the WBLM the centre of pressure placement was modulated using the ankle joint, while keeping the moment arm of the ground reaction force (GRF) with respect to the centre of mass (CoM) small. After the perturbations of the WBAM a quick recovery was initiated, using the hip joint and adjusting the horizontal GRF to create a moment arm with respect to the CoM. These findings suggest no fundamental differences in the use of balance strategies at very slow walking compared to normal speeds. Still as the gait phases last longer, this time was exploited to counteract perturbations in the ongoing gait phase.

Whole Body Center of Mass Feedback in a Reflex-Based Neuromuscular Model Predicts Ankle Strategy During Perturbed Walking

Active prosthetic and orthotic devices have the potential to increase quality of life for individuals with impaired mobility. However, more research into human-like control methods is needed to create seamless interaction between device and user. In forward simulations the reflex-based neuromuscular model (RNM) by Song and Geyer shows promising similarities with real human gait in unperturbed conditions. The goal of this work was to validate and, if needed, extend the RNM to reproduce human kinematics and kinetics during walking in unperturbed and perturbed conditions. The RNM was optimized to reproduce joint torque, calculated with inverse dynamics, from kinematic and force data of unperturbed and perturbed treadmill walking of able-bodied human subjects. Torques generated by the RNM matched closely with torques found from inverse dynamics analysis on human data for unperturbed walking. However, for perturbed walking the modulation of the ankle torque in the RNM was opposite to the modulation observed in humans. Therefore, the RNM was extended with a control module that activates and inhibits muscles around the ankle of the stance leg, based on changes in whole body center of mass velocity. The added module improves the ability of the RNM to replicate human ankle torque response in response to perturbations. This reflex-based neuromuscular model with whole body center of mass velocity feedback can reproduce gait kinetics of unperturbed and perturbed gait, and as such holds promise as a basis for advanced controllers of prosthetic and orthotic devices.

Centre of pressure modulations in double support effectively counteract anteroposterior perturbations during gait

Centre of mass (CoM) motion during human balance recovery is largely influenced by the ground reaction force (GRF) and the centre of pressure (CoP). During gait, foot placement creates a region of possible CoP locations in the following double support (DS). This study aims to increase insight into how humans modulate the CoP during DS, and which CoP modulations are theoretically possible to maintain balance in the sagittal plane. Three variables sufficient to describe the shape, length and duration of the DS CoP trajectory of the total GRF, were assessed in perturbed human walking. To counteract the forward perturbations, braking was required and all CoP variables showed modulations correlated to the observed change in CoM velocity over the DS phase. These correlations were absent after backward perturbations, when only little propulsion was needed to counteract the perturbation. Using a linearized inverted pendulum model we could explore how the observed parameter modulations are effective in controlling the CoM. The distance the CoP travels forward and the instant the leading leg was loaded largely affected the CoM velocity, while the duration mainly affected the CoM position. The simulations also showed that various combinations of CoP parameters can reach a desired CoM position and velocity at the end of DS, and that even a full recovery in the sagittal plane within DS would theoretically have been possible. However, the human subjects did not exploit the therefore required CoP modulations. Overall, modulating the CoP trajectory in DS does effectively contributes to balance recovery.

Symbitron Exoskeleton: Design, Control, and Evaluation of a Modular Exoskeleton for Incomplete and Complete Spinal Cord Injured Individuals

In this paper, we present the design, control, and preliminary evaluation of the Symbitron exoskeleton, a lower limb modular exoskeleton developed for people with a spinal cord injury. The mechanical and electrical configuration and the controller can be personalized to accommodate differences in impairments among individuals with spinal cord injuries (SCI). In hardware, this personalization is accomplished by a modular approach that allows the reconfiguration of a lower-limb exoskeleton with ultimately eight powered series actuated (SEA) joints and high fidelity torque control. For SCI individuals with an incomplete lesion and sufficient hip control, we applied a trajectory-free neuromuscular control (NMC) strategy and used the exoskeleton in the ankle-knee configuration. For complete SCI individuals, we used a combination of a NMC and an impedance based trajectory tracking strategy with the exoskeleton in the ankle-knee-hip configuration. Results of a preliminary evaluation of the developed hardware and software showed that SCI individuals with an incomplete lesion could naturally vary their walking speed and step length and walked faster compared to walking without the device. SCI individuals with a complete lesion, who could not walk without support, were able to walk with the device and with the support of crutches that included a push-button for step initiation Our results demonstrate that an exoskeleton with modular hardware and control allows SCI individuals with limited or no lower limb function to receive tailored support and regain mobility.

Gait training with Achilles ankle exoskeleton in chronic incomplete spinal cord injury subjects

Powered exoskeletons (EXOs) have emerged as potential devices for Spinal Cord Injury (SCI) to support the intervention of physical therapists during therapy (rehabilitation EXOs) as well as to assist lower limb motion during the daily life (assistive EXOs). Although the ankle is considered a key joint for gait restoration after SCI, very few ankle exoskeletons were developed and tested in incomplete SCI (iSCI) population. Among those, the Achilles ankle exoskeleton is the only one embedding a Controller inspired by the neuromuscular system (NeuroMuscular Controller, NMC). In a previous study we demonstrated that a period dedicated to train iSCI subjects in using the Achilles EXO as an assistive aid, improved robot-aided walking speed and surprisingly also generated a positive trend in free walking speed on long and short distances thus suggesting a possible unexpected rehabilitation effect. To further investigate this result, a case-control longitudinal study was conducted in the present work. The aim of this study was to test the hypothesis that Achilles-aided training could improve performance of free walking of chronic iSCI people more than conventional intensity-matched gait rehabilitation. Before and after conventional and robot-aided rehabilitation a number of variables were analyzed, including spatiotemporal parameters, joint kinematics, ground reaction forces, muscle force, spasticity and its related symptoms, balance and personal experience about the training. Results showed that only the NMC-controlled Achilles training allowed participants to significantly walk faster, with a longer step length and a reduced gait cycle time. A slight force and spasticity improvements were also experienced. In terms of subjects' personal experience, Achilles training was perceived more interesting and less physically demanding than conventional rehabilitation.

Can Momentum-Based Control Predict Human Balance Recovery Strategies?

Human-like balance controllers are desired for wearable exoskeletons in order to enhance human-robot interaction. Momentum-based controllers (MBC) have been successfully applied in bipeds, however, it is unknown to what degree they are able to mimic human balance responses. In this paper, we investigated the ability of an MBC to generate human-like balance recovery strategies during stance, and compared the results to those obtained with a linear full-state feedback (FSF) law. We used experimental data consisting of balance recovery responses of nine healthy subjects to anteroposterior platform translations of three different amplitudes. The MBC was not able to mimic the combination of trunk, thigh and shank angle trajectories that humans generated to recover from a perturbation. Compared to the FSF, the MBC was better at tracking thigh angles and worse at tracking trunk angles, whereas both controllers performed similarly in tracking shank angles. Although the MBC predicted stable balance responses, the human-likeness of the simulated responses generally decreased with an increased perturbation magnitude. Specifically, the shifts from ankle to hip strategy generated by the MBC were not similar to the ones observed in the human data. Although the MBC was not superior to the FSF in predicting human-like balance, we consider the MBC to be more suitable for implementation in exoskeletons, because of its ability to handle constraints (e.g. ankle torque limits). Additionally, more research into the control of angular momentum and the implementation of constraints could eventually result in the generation of more human-like balance recovery strategies by the MBC.

Estimating ankle torque and dynamics of the stabilizing mechanism: No need for horizontal ground reaction forces

Changes in human balance control can objectively be assessed using system identification techniques in combination with support surface translations. However, large, expensive and complex motion platforms are required, which are not suitable for the clinic. A treadmill could be a simple alternative to apply support surface translations. In this paper we first validated the estimation of the joint stiffness of an inverted pendulum using system identification methods in combination with support surface translations, by comparison with the joint stiffness calculated using a linear regression method. Second, we used the system identification method to investigate the effect of horizontal ground reaction forces on the estimation of the ankle torque and the dynamics of the stabilizing mechanism of 12 healthy participants. Ankle torque and resulting frequency response functions, which describes the dynamics of the stabilizing mechanism, were calculated by both including and excluding horizontal ground reaction forces. Results showed that the joint stiffness of an inverted pendulum estimated using system identification is comparable to the joint stiffness estimated by a regression method. Secondly, within the induced body sway angles, the ankle torque and frequency response function of the joint dynamics calculated by both including and excluding horizontal ground reaction forces are similar. Therefore, the horizontal ground reaction forces play a minor role in calculating the ankle torque and frequency response function of the dynamics of the stabilizing mechanism and can thus be omitted.

Neuromuscular Controller Embedded in a Powered Ankle Exoskeleton: Effects on Gait, Clinical Features and Subjective Perspective of Incomplete Spinal Cord Injured Subjects

Powered exoskeletons are among the emerging technologies claiming to assist functional ambulation. The potential to adapt robotic assistance based on specific motor abilities of incomplete spinal cord injury (iSCI) subjects, is crucial to optimize Human-Robot Interaction (HRI). Achilles, an autonomous wearable robot able to assist ankle during walking, was developed for iSCI subjects and utilizes a NeuroMuscular Controller (NMC). NMC can be used to adapt robotic assistance based on specific residual functional abilities of subjects. The main aim of this pilot study was to analyze the effects of the NMC-controlled Achilles, used as an assistive device, on chronic iSCI participants' performance, by assessing gait speed during 10-session training of robot-aided walking. Secondary aims were to assess training impact on participants' motion, clinical and functional features and to evaluate subjective perspective in terms of attitude towards technology, workload, usability and satisfaction. Results showed that 5 training sessions were necessary to significantly improve robot-aided gait speed on short paths and consequently to optimize HRI. Moreover, the training allowed participants who initially were not able to walk for 6 minutes, to improve gait endurance during Achilles-aided walking and to reduce perceived fatigue. Improvements were obtained also in gait speed during free walking, thus suggesting a potential rehabilitative impact, even if Achilles-aided walking was not faster than free walking. Participants' subjective evaluations indicated a positive experience.

Reduced center of pressure modulation elicits foot placement adjustments, but no additional trunk motion during anteroposterior-perturbed walking

Understanding balance during human gait is complicated by the abundance of recovery options. Among all possible recovery options, three main strategies are often considered for human balance control, being the ankle, hip, and foot placement strategies. All can be addressed when balance is threatened during walking, but their relative importance remains uncertain. We have previously shown that healthy human subjects did not significantly adjust their foot placement relative to the body's center of mass (COM) in the first recovery step following anteroposterior pelvis perturbations, as compared to unperturbed walking. An ankle strategy could have contributed to the recovery instead. Here the goal is to further elucidate balance strategy preferences by investigating the stepping and hip strategies following these anteroposterior perturbations, but with an ankle strategy made ineffective. This was achieved by physically blocking each ankle and minimizing the support area of each foot through a pair of modified ankle-foot orthoses. These "pin-shoes" enabled stilt-like walking and ensured that foot placement adjustment was the only way to modulate the center of pressure location, comparable to "footless" inverted pendulum models of walking. Despite the pin-shoes, subjects did not additionally address a hip strategy compared to normal walking, but relied on foot placement adjustments instead. The observed foot placement adjustments were furthermore in line with concepts derived from a linear inverted pendulum model of walking. These results suggest low hip strategy priority when a foot placement strategy is available, while the latter can be predicted with concepts derived from a simple walking model.

Assessment of the underlying systems involved in standing balance: the additional value of electromyography in system identification and parameter estimation

Background

Closed loop system identification (CLSIT) is a method to disentangle the contribution of underlying systems in standing balance. We investigated whether taking into account lower leg muscle activation in CLSIT could improve the reliability and accuracy of estimated parameters identifying the underlying systems.

Methods

Standing balance behaviour of 20 healthy young participants was measured using continuous rotations of the support surface (SS). The dynamic balance behaviour obtained with CLSIT was expressed by sensitivity functions of the ankle torque, body sway and muscle activation of the lower legs to the SS rotation. Balance control models, 1) without activation dynamics, 2) with activation dynamics and 3) with activation dynamics and acceleration feedback, were fitted on the data of all possible combinations of the 3 sensitivity functions. The reliability of the estimated model parameters was represented by the mean relative standard errors of the mean (mSEM) of the estimated parameters, expressed for the basic parameters, the activation dynamics parameters and the acceleration feedback parameter. To investigate the accuracy, a model validation study was performed using simulated data obtained with a comprehensive balance control model. The accuracy of the estimated model parameters was described by the mean relative difference (mDIFF) between the estimated parameters and original parameters.

Results

The experimental data showed a low mSEM of the basic parameters, activation dynamics parameters and acceleration feedback parameter by adding muscle activation in combination with activation dynamics and acceleration feedback to the fitted model. From the simulated data, the mDIFF of the basic parameters varied from 22.2–22.4% when estimated using the torque and body sway sensitivity functions. Adding the activation dynamics, acceleration feedback and muscle activation improved mDIFF to 13.1–15.1%.

Conclusions

Adding the muscle activation in combination with the activation dynamics and acceleration feedback to CLSIT improves the accuracy and reliability of the estimated parameters and gives the possibility to separate the neural time delay, electromechanical delay and the intrinsic and reflexive dynamics. To diagnose impaired balance more specifically, it is recommended to add electromyography (EMG) to body sway (with or without torque) measurements in the assessment of the underlying systems.

Electronic supplementary material

The online version of this article (10.1186/s12984-017-0299-x) contains supplementary material, which is available to authorized users.

A Series Elastic- and Bowden-Cable-Based Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots

Within the context of impedance controlled exoskeletons, common actuators have important drawbacks. Either the actuators are heavy, have a complex structure or are poor torque sources, due to gearing or heavy nonlinearity. Considering our application, an impedance controlled gait rehabilitation robot for treadmill-training, we designed an actuation system that might avoid these drawbacks. It combines a lightweight joint and a simple structure with adequate torque source quality. It consists of a servomotor, a flexible Bowden cable transmission, and a force feedback loop based on a series elastic element. A basic model was developed that is shown to describe the basic dynamics of the actuator well enough for design purpose.

Further measurements show that performance is sufficient for use in a gait rehabilitation robot. The demanded force tracking bandwidths were met: 11 Hz bandwidth for the full force range (demanded 4 Hz) and 20 Hz bandwidth for smaller force range (demanded 12 Hz). The mechanical output impedance of the actuator could be reduced to hardly perceptible level. Maxima of about 0.7 Nm peaks for 4 Hz imposed motions appeared, corresponding to less than 2.5% of the maximal force output. These peaks were caused by the stick friction in the Bowden cables.

Spring stiffness variation showed that both a too stiff and a too compliant spring can worsen performance. A stiff spring reduces the maximum allowable controller gain. The relatively low control gain then causes a larger effect of stick in the force output, resulting in a less smooth output in general. Low spring stiffness, on the other side, decreases the performance of the system, because saturation will occur sooner.

Center of mass velocity-based predictions in balance recovery following pelvis perturbations during human walking

In many simple walking models, foot placement dictates the center of pressure location and ground reaction force components, whereas humans can modulate these aspects after foot contact. Because of the differences, it is unclear to what extent predictions made by models are valid for human walking. Yet, both model simulations and human experimental data have previously indicated that the center of mass (COM) velocity plays an important role in regulating stable walking. Here, perturbed human walking was studied to determine the relationship of the horizontal COM velocity at heel strike and toe-off with the foot placement location relative to the COM, the forthcoming center of pressure location relative to the COM, and the ground reaction forces. Ten healthy subjects received mediolateral and anteroposterior pelvis perturbations of various magnitudes at toe-off, during 0.63 and 1.25 m s(-1) treadmill walking. At heel strike after the perturbation, recovery from mediolateral perturbations involved mediolateral foot placement adjustments proportional to the mediolateral COM velocity. In contrast, for anteroposterior perturbations, no significant anteroposterior foot placement adjustment occurred at this heel strike. However, in both directions the COM velocity at heel strike related linearly to the center of pressure location at the subsequent toe-off. This relationship was affected by the walking speed and was, for the slow speed, in line with a COM velocity-based control strategy previously applied by others in a linear inverted pendulum model. Finally, changes in gait phase durations suggest that the timing of actions could play an important role during the perturbation recovery.

Adaptation of multijoint coordination during standing balance in healthy young and healthy old individuals

Standing balance requires multijoint coordination between the ankles and hips. We investigated how humans adapt their multijoint coordination to adjust to various conditions and whether the adaptation differed between healthy young participants and healthy elderly. Balance was disturbed by push/pull rods, applying two continuous and independent force disturbances at the level of the hip and between the shoulder blades. In addition, external force fields were applied, represented by an external stiffness at the hip, either stabilizing or destabilizing the participants' balance. Multivariate closed-loop system-identification techniques were used to describe the neuromuscular control mechanisms by quantifying the corrective joint torques as a response to body sway, represented by frequency response functions (FRFs). Model fits on the FRFs resulted in an estimation of time delays, intrinsic stiffness, reflexive stiffness, and reflexive damping of both the ankle and hip joint. The elderly generated similar corrective joint torques but had reduced body sway compared with the young participants, corresponding to the increased FRF magnitude with age. When a stabilizing or destabilizing external force field was applied at the hip, both young and elderly participants adapted their multijoint coordination by lowering or respectively increasing their neuromuscular control actions around the ankles, expressed in a change of FRF magnitude. However, the elderly adapted less compared with the young participants. Model fits on the FRFs showed that elderly had higher intrinsic and reflexive stiffness of the ankle, together with higher time delays of the hip. Furthermore, the elderly adapted their reflexive stiffness around the ankle joint less compared with young participants. These results imply that elderly were stiffer and were less able to adapt to external force fields.

Changes in sensory reweighting of proprioceptive information during standing balance with age and disease

With sensory reweighting, reliable sensory information is selected over unreliable information during balance by dynamically combining this information. We used system identification techniques to show the weight and the adaptive process of weight change of proprioceptive information during standing balance with age and specific diseases. Ten healthy young subjects (aged between 20 and 30 yr) and 44 elderly subjects (aged above 65 yr) encompassing 10 healthy elderly, 10 with cataract, 10 with polyneuropathy, and 14 with impaired balance, participated in the study. During stance, proprioceptive information of the ankles was disturbed by rotation of the support surface with specific frequency content where disturbance amplitude increased over trials. Body sway and reactive ankle torque were measured to determine sensitivity functions of these responses to the disturbance amplitude. Model fits resulted in a proprioceptive weight (changing over trials), time delay, force feedback, reflexive stiffness, and damping. The proprioceptive weight was higher in healthy elderly compared with young subjects and higher in elderly subjects with cataract and with impaired balance compared with healthy elderly subjects. Proprioceptive weight decreased with increasing disturbance amplitude; decrease was similar in all groups. In all groups, the time delay was higher and the reflexive stiffness was lower compared with young or healthy elderly subjects. In conclusion, proprioceptive information is weighted more with age and in patients with cataract and impaired balance. With age and specific diseases the time delay was higher and reflexive stiffness was lower. These results illustrate the opportunity to detect the underlying cause of impaired balance in the elderly with system identification.

Parkinson's disease patients compensate for balance control asymmetry

In Parkinson's disease (PD) subtle balance abnormalities can already be detected in early-stage patients. One feature of impaired balance control in PD is asymmetry: one leg produces more corrective joint torque than the other. We hypothesize that in mild to moderately affected PD patients, the least impaired leg compensates for the more impaired leg. Twenty PD patients and eleven healthy matched control subjects participated. Clinical asymmetry was determined by the difference between the left and right body side scores on the Unified Parkinson's Disease Rating Scale. Balance was perturbed with two independent continuous multisine perturbations in the forward-backward direction. Subsequently, we applied closed-loop system identification, which determined the spectral estimate of the stabilizing mechanisms, for each leg. Balance control behavior was similar in PD patients and control subjects at the ankle, but at the hip stiffness was increased. Control subjects exhibited symmetric balance control, but in PD patients the balance contribution of the leg of the clinically least affected body side was higher whereas the leg of the clinically most affected body side contributed less. The ratio between the legs helped to preserve a normal motor output at the ankle. Our results suggest that PD patients compensate for balance control asymmetries by increasing the relative contribution of the leg of their least affected body side. This compensation appears to be successful at the ankle but is accompanied by an increased stiffness at the hip. We discuss the possible implications of these findings for postural stability and fall risk in PD patients.

Improving the transparency of a rehabilitation robot by exploiting the cyclic behaviour of walking

To promote active participation of neurological patients during robotic gait training, controllers, such as "assist as needed" or "cooperative control", are suggested. Apart from providing support, these controllers also require that the robot should be capable of resembling natural, unsupported, walking. This means that they should have a transparent mode, where the interaction forces between the human and the robot are minimal. Traditional feedback-control algorithms do not exploit the cyclic nature of walking to improve the transparency of the robot. The purpose of this study was to improve the transparent mode of robotic devices, by developing two controllers that use the rhythmic behavior of gait. Both controllers use adaptive frequency oscillators and kernel-based non-linear filters. Kernelbased non-linear filters can be used to estimate signals and their time derivatives, as a function of the gait phase. The first controller learns the motor angle, associated with a certain joint angle pattern, and acts as a feed-forward controller to improve the torque tracking (including the zero-torque mode). The second controller learns the state of the mechanical system and compensates for the dynamical effects (e.g. the acceleration of robot masses). Both controllers have been tested separately and in combination on a small subject population. Using the feedforward controller resulted in an improved torque tracking of at least 52 percent at the hip joint, and 61 percent at the knee joint. When both controllers were active simultaneously, the interaction power between the robot and the human leg was reduced by at least 40 percent at the thigh, and 43 percent at the shank. These results indicate that: if a robotic task is cyclic, the torque tracking and transparency can be improved by exploiting the predictions of adaptive frequency oscillator and kernel-based nonlinear filters.

Speed-dependent reference joint trajectory generation for robotic gait support

For the control of actuated orthoses, or gait rehabilitation robotics, kinematic reference trajectories are often required. These trajectories, consisting of joint angles, angular velocities and accelerations, are highly dependent on walking-speed. We present and evaluate a novel method to reconstruct body-height and speed-dependent joint trajectories. First, we collected gait kinematics in fifteen healthy (middle) aged subjects (47-68), at a wide range of walking-speeds (0.5-5 kph). For each joint trajectory multiple key-events were selected (among which its extremes). Second, we derived regression-models that predict the timing, angle, angular velocity and acceleration for each key-event, based on walking-speed and the subject׳s body-height. Finally, quintic splines were fitted between the predicted key-events to reconstruct a full gait cycle. Regression-models were obtained for hip ab-/adduction, hip flexion/extension, knee flexion/extension and ankle plantar-/dorsiflexion. Results showed that the majority of the key-events were dependent on walking-speed, both in terms of timing and amplitude, whereas the body-height had less effect. The reconstructed trajectories matched the measured trajectories very well, in terms of angle, angular velocity and acceleration. For the angles the RMSE between the reconstructed and measured trajectories was 2.6°. The mean correlation coefficient between the reconstructed and measured angular trajectories was 0.91. The method and the data presented in this paper can be used to generate speed-dependent gait patterns. These patterns can be used for the control of several robotic gait applications. Alternatively they can assist the assessment of pathological gait, where they can serve as a reference for "normal" gait.

Lateral balance control for robotic gait training

For the rehabilitation of neurological patients robot-aided gait training is increasingly being used. Lack of balance training in these robotic gait trainers might contribute to the fact that they do not live up to the expectations. Therefore, in this study we developed and evaluated an algorithm to support lateral balance during walking, through controlling pelvis motions. This algorithm assists the pelvis, according to a natural pelvic sway pattern, rather than attracting it to the middle of the treadmill. The support algorithm was tested on six healthy young subjects who walked on a treadmill, while different support gains were introduced. Using a higher support gain resulted in a closer approximation of the pelvic sway towards the reference pattern. Step width and step width variability reduced when the external stabilization was provided, and the stability margin increased. This indicates that artificial stabilization reduces the need for active lateral balance control. The presented algorithm to support lateral balance provides a way to assist balance in a more physiological way, compared to attracting the subject to the centre of the treadmill. Here the user is attracted/assisted towards a more natural weight shift pattern. This also facilitates a more natural input of the load receptors, which are largely involved in the regulation of muscle activation patterns and the transitions between the different gait phases.

An explorative study into changes in circle drawing after gravity compensation training in chronic stroke patients

A way to reduce the influence of abnormal synergies on range of motion after stroke directly is to support the arm by a robotic gravity compensation device. However, it is not known whether a period of training with arm support improves independent, unsupported circle drawing, and what the role of abnormal synergies is. Seven chronic stroke patients received three 30 minute robotic gravity compensation training sessions per week for a period of six weeks. During baseline and evaluation measurements, Fugl-Meyer (FM) scores and circle drawing performance (area and roundness) were determined. After training, FM had improved in some subjects. Circle area increased significantly across subjects, whereas roundness did not. This indicates an improved unsupported active range of motion, but the influence of (reduced) abnormal synergies on this change remains unclear. Despite the small number of subjects, the present explorative study suggests that robotic gravity compensation training has the potential to increase the work area of the affected arm of chronic stroke patients. Further research into the impact of robotic gravity compensation training is warranted, to enhance insight into underlying mechanisms and optimal applications in clinical practice.

Rendering potential wearable robot designs with the LOPES gait trainer

In recent years, wearable robots (WRs) for rehabilitation, personal assistance, or human augmentation are gaining increasing interest. To make these devices more energy efficient, radical changes to the mechanical structure of the device are being considered. However, it remains very difficult to predict how people will respond to, and interact with, WRs that differ in terms of mechanical design. Users may adjust their gait pattern in response to the mechanical restrictions or properties of the device. The goal of this pilot study is to show the feasibility of rendering the mechanical properties of different potential WR designs using the robotic gait training device LOPES. This paper describes a new method that selectively cancels the dynamics of LOPES itself and adds the dynamics of the rendered WR using two parallel inverse models. Adaptive frequency oscillators were used to get estimates of the joint position, velocity, and acceleration. Using the inverse models, different WR designs can be evaluated, eliminating the need to build several prototypes. As a proof of principle, we simulated the effect of a very simple WR that consisted of a mass attached to the ankles. Preliminary results show that we are partially able to cancel the dynamics of LOPES. Additionally, the simulation of the mass showed an increase in muscle activity but not in the same level as during the control, where subjects actually carried the mass. In conclusion, the results in this paper suggest that LOPES can be used to render different WRs. In addition, it is very likely that the results can be further optimized when more effort is put in retrieving proper estimations for the velocity and acceleration, which are required for the inverse models.

Velocity-dependent reference trajectory generation for the LOPES gait training robot

The aim of this study is to assess the feasibility of an approach for generating velocity-dependent trajectories to train neurologically injured patients. The reference trajectories are constructed based on the gait patterns of subjects walking on a treadmill. By extracting key events (parameters) from these trajectories, the velocity dependency of the parameters is determined by regression analysis. Then, splines are fitted through these points to obtain gait patterns (position, velocity and acceleration) for specific walking velocities. Considering the severely injured patients, a feedforward controller is used in addition to the impedance controller. The approach is implemented on the LOPES gait rehabilitation robot and evaluated on healthy subjects. Results indicate that the subjects can walk naturally in the robot with the constructed reference trajectories. Further improvements to the technical design and additional testing of healthy and impaired subjects are required to show whether this approach can be transferred to clinical domain.

Soft artificial tactile sensors for the measurement of human-robot interaction in the rehabilitation of the lower limb

A new and alternative method to measure the interaction force between the user and a lower-limb gait rehabilitation exoskeleton is presented. Instead of using a load cell to measure the resulting interaction force, we propose a distributed measure of the normal interaction pressure over the whole contact area between the user and the machine. To obtain this measurement, a soft silicone tactile sensor is inserted between the limb and commonly used connection cuffs. The advantage of this approach is that it allows for a distributed measure of the interaction pressure, which could be useful for rehabilitation therapy assessment purposes, or for control. Moreover, the proposed solution does not change the comfort of the interaction; can be applied to connection cuffs of different shapes and sizes; and can be manufactured at a low cost. Preliminary results during gait assistance tasks show that this approach can precisely detect changes in the pressure distribution during a gait cycle.

Locomotor adaptation and retention to gradual and sudden dynamic perturbations

Motor learning principles are increasingly used in robotic rehabilitation. These principles are mainly derived from reaching studies and it is currently unknown whether the same principles apply in locomotion. The aim of this study was to determine the effect of a gradually and suddenly introduced dynamic perturbation on locomotor adaptation and on recalling the adaptation when being re-exposed to the same perturbation. Subjects walked on a treadmill and adapted to a viscous force field that was applied during the swing phase. In one group the strength of the force field was gradually increased over different steps whereas in a second group it was introduced suddenly at full strength. The gradual group showed less adaptation and a faster decay of the adaptation during the washout than the sudden group. Strikingly, when both groups were being re-exposed to the perturbation at full strength, the gradual group showed no adaptation whereas the sudden group showed a faster occurring adaptation than during the first exposure. In conclusion, in contrast to the reported beneficial effects of a gradual introduction of a perturbation on adaptation in reaching, it seems to have a detrimental effect on locomotor adaptation. These results indicate that caution should be taken when applying motor learning principles solely derived from reaching studies to improve robotic gait rehabilitation.

In vivo measurement of human knee and hip dynamics using MIMO system identification

This study presents a new method for the estimation of the dynamic impedance of multi-joint leg movements. The method is based on Multi Input Multi Output (MIMO) system identification techniques and is designed for continuous torque perturbations at the hip and knee joint. Preliminary results from this study indicate that MIMO system identification can successfully be used to estimate the hip and knee impedance and the interaction dynamics between both joints. It is also concluded that, in order to create a good model representation of the leg impedance, the effect of biarticular muscles needs to be taken into account. The obtained measures for joint impedance might be used for clinical assessment and follow up of patients, as well as for the development of supportive devices.

Influence of Gravity Compensation on Muscle Activation Patterns During Different Temporal Phases of Arm Movements of Stroke Patients

Background. Arm support to help compensate for the effects of gravity may improve functional use of the shoulder and elbow during therapy after stroke, but gravity compensation may alter motor control. Objective. To obtain quantitative information on how gravity compensation influences muscle activation patterns during functional, 3-dimensional reaching movements. Methods. Eight patients with mild hemiparesis performed 2 sets of repeated reach and retrieval movements, with and without unloading the arm, using a device that acted at the elbow and forearm to compensate for gravity. Electromyographic (EMG) patterns of 6 upper extremity muscles were compared during elbow and shoulder joint excursions with and without gravity compensation. Results. Movement performance was similar with and without gravity compensation. Smooth rectified EMG (SRE) values were decreased from 25% to 50% during movements with gravity compensation in 5 out of 6 muscles. The variation of SRE values across movement phases did not differ across conditions. Conclusions. Gravity compensation did not affect general patterns of muscle activation in this sample of stroke patients, probably since they had adequate function to complete the task without arm support. Gravity compensation did facilitate active arm movement excursions without impairing motor control. Gravity compensation may be a valuable modality in conventional or robot-aided therapy to increase the intensity of training for mildly impaired patients.

Preliminary results of training with gravity compensation of the arm in chronic stroke survivors

After stroke, arm function can be limited by a reduction in the selectivity of movements, due to involuntary coupling of shoulder abduction and elbow flexion, limiting the ability to reach. Gravity compensation of the arm reduces the required active shoulder abduction torques, which results in a larger range of motion instantaneously. Integration of a motivating rehabilitation game in the training program stimulates motor relearning processes during training. During 6 weeks, 8 chronic stroke survivors received 3 sessions of 30 minutes gravity compensated reach training per week using a rehabilitation game, which was evaluated by assessing motor status and a circle drawing task before and after training. After gravity compensation training, Fugl Meyer scores and the range of motion obtained from the circle drawing task had improved in a seven of the eight chronic stroke survivors. The present findings indicate that gravity compensation in combination with rehab games can be a valuable training modality for stroke rehabilitation.

Influence of gravity compensation on muscle activity during reach and retrieval in healthy elderly

INTRODUCTION

Arm support like gravity compensation may improve arm movements during stroke rehabilitation. It is unknown how gravity compensation affects muscle activation patterns during reach and retrieval movements. Since muscle activity during reach is represented by a component varying with movement velocity and a component supposedly counteracting gravity, we hypothesized that gravity compensation decreases the amplitude of muscle activity, but does not affect the pattern. To examine this, we compared muscle activity during well defined movements with and without gravity compensation in healthy elderly.

METHODS

Ten subjects performed reach and retrieval movements with and without gravity compensation. Muscle activity of biceps, triceps, anterior, middle and posterior parts of deltoid and upper trapezius was compared between the two conditions.

RESULTS

The level of muscle activity was lower with gravity compensation in all muscles, reaching significance in biceps, anterior deltoid and trapezius (p < or = 0.026). The muscle activation pattern did not differ between movements with and without gravity compensation (p > or = 0.662).

DISCUSSION

Gravity compensation only influenced the level of muscle activity but not the muscle activation pattern in terms of timing. Future studies should examine if the influence of gravity compensation is comparable for stroke patients. This may stimulate early and intensive training during rehabilitation.

Detecting asymmetries in balance control with system identification: first experimental results from Parkinson patients

Cognitive processes can influence balance in various ways, but not all changes in postural performance can easily be identified with the naked clinical eye. Various studies have shown that dynamic posturography is able to detect more subtle changes in balance control. For patients with Parkinson's disease (which is typically an asymmetric disease), changes in the symmetry of balance control might provide a sensitive measure of cognitive influences on balance. Here, we describe a new posturography technique that combines dynamic platform perturbations with system identification techniques to detect such asymmetries in balance control of two patients with Parkinson's disease. Results were compared to those of six healthy controls. Our pilot data show clear asymmetries in dynamic balance control, even though patients themselves were not aware of this and had no subjective problems with stability or standing. We also found asymmetries in weight bearing, but the asymmetries in dynamic balance contribution were larger. Finally, asymmetries in weight bearing and dynamic balance in patients were not tightly coupled as in healthy controls. Future studies could incorporate this approach when examining the influence of mental decline on postural regulation.

An adaptive model of sensory integration in a dynamic environment applied to human stance control

An adaptive estimator model of human spatial orientation is presented. The adaptive model dynamically weights sensory error signals. More specific, the model weights the difference between expected and actual sensory signals as a function of environmental conditions. The model does not require any changes in model parameters. Differences with existing models of spatial orientation are that: (1) environmental conditions are not specified but estimated, (2) the sensor noise characteristics are the only parameters supplied by the model designer, (3) history-dependent effects and mental resources can be modelled, and (4) vestibular thresholds are not included in the model; instead vestibular-related threshold effects are predicted by the model. The model was applied to human stance control and evaluated with results of a visually induced sway experiment. From these experiments it is known that the amplitude of visually induced sway reaches a saturation level as the stimulus level increases. This saturation level is higher when the support base is sway referenced. For subjects experiencing vestibular loss, these saturation effects do not occur. Unknown sensory noise characteristics were found by matching model predictions with these experimental results. Using only five model parameters, far more than five data points were successfully predicted. Model predictions showed that both the saturation levels are vestibular related since removal of the vestibular organs in the model removed the saturation effects, as was also shown in the e xperiments. It seems that the nature of these vestibular-related threshold effects is not physical, since in the model no threshold is included. The model results suggest that vestibular-related thresholds are the result of the processing of noisy sensory and motor output signals. Model analysis suggests that, especially for slow and small movements, the environment postural orientation can not be estimated optimally, which causes sensory illusions. The model also confirms the experimental finding that postural orientation is history dependent and can be shaped by instruction or mental knowledge. In addition the model predicts that: (1) vestibular-loss patients cannot handle sensory conflicting situations and will fall down, (2) during sinusoidal support-base translations vestibular function is needed to prevent falling, (3) loss of somatosensory information from the feet results in larger postural sway for sinusoidal support-base translations, and (4) loss of vestibular function results in falling for large support-base rotations with the eyes closed. These predictions are in agreement with experimental results.

A multisensory integration model of human stance control

A model is presented to study and quantify the contribution of all available sensory information to human standing based on optimal estimation theory. In the model, delayed sensory information is integrated in such a way that a best estimate of body orientation is obtained. The model approach agrees with the present theory of the goal of human balance control. The model is not based on purely inverted pendulum body dynamics, but rather on a three-link segment model of a standing human on a movable support base. In addition, the model is non-linear and explicitly addresses the problem of multisensory integration and neural time delays. A predictive element is included in the controller to compensate for time delays, necessary to maintain erect body orientation. Model results of sensory perturbations on total body sway closely resemble experimental results. Despite internal and external perturbations, the controller is able to stabilise the model of an inherently unstable standing human with neural time delays of 100 ms. It is concluded, that the model is capable of studying and quantifying multisensory integration in human stance control. We aim to apply the model in (1) the design and development of prostheses and orthoses and (2) the diagnosis of neurological balance disorders.