Measurement of passive ankle stiffness in subjects with chronic hemiparesis using a novel ankle robot

Reliability and minimal detectable change of stiffness and other mechanical properties of the ankle joint in standing and walking

BACKGROUND

Ankle joint stiffness and viscosity are fundamental mechanical descriptions that govern the movement of the body and impact an individual's walking ability. Hence, these internal properties of a joint have been increasingly used to evaluate the effects of pathology (e.g., stroke) and in the design and control of robotic and prosthetic devices. However, the reliability of these measurements is currently unclear, which is important for translation to clinical use.

RESEARCH QUESTION

Can we reliably measure the mechanical impedance parameters of the ankle while standing and walking?

METHODS

Eighteen able-bodied individuals volunteered to be tested on two different days separated by at least 24 h. Participants received several small random ankle dorsiflexion perturbations while standing and during the stance phase of walking using a custom-designed robotic platform. Three-dimensional motion capture cameras and a 6-component force plate were used to quantify ankle joint motions and torque responses during normal and perturbed conditions. Ankle mechanical impedance was quantified by computing participant-specific ensemble averages of changes in ankle angle and torque due to perturbation and fitting a second-order parametric model consisting of stiffness, viscosity, and inertia. The test-retest reliability of each parameter was assessed using intraclass correlation coefficients (ICCs). We also computed the minimal detectable change (MDC) for each impedance parameter to establish the smallest amount of change that falls outside the measurement error of the instrument.

RESULTS

In standing, the reliability of stiffness, viscosity, and inertia was good to excellent (ICCs=0.67-0.91). During walking, the reliability of stiffness and viscosity was good to excellent (ICCs=0.74-0.84) while that of inertia was fair to good (ICCs=0.47-0.68). The MDC for a single subject ranged from 20%- 65% of the measurement mean but was higher (>100%) for inertia during walking.

SIGNIFICANCE

Results indicate that dynamic measures of ankle joint impedance were generally reliable and could serve as an adjunct clinical tool for evaluating gait impairments.

Use of Lower Limb Exoskeletons as an Assessment Tool for Human Motor Performance: A Systematic Review

Exoskeletons are a promising tool to support individuals with a decreased level of motor performance. Due to their built-in sensors, exoskeletons offer the possibility of continuously recording and assessing user data, for example, related to motor performance. The aim of this article is to provide an overview of studies that rely on using exoskeletons to measure motor performance. Therefore, we conducted a systematic literature review, following the PRISMA Statement guidelines. A total of 49 studies using lower limb exoskeletons for the assessment of human motor performance were included. Of these, 19 studies were validity studies, and six were reliability studies. We found 33 different exoskeletons; seven can be considered stationary, and 26 were mobile exoskeletons. The majority of the studies measured parameters such as range of motion, muscle strength, gait parameters, spasticity, and proprioception. We conclude that exoskeletons can be used to measure a wide range of motor performance parameters through built-in sensors, and seem to be more objective and specific than manual test procedures. However, since these parameters are usually estimated from built-in sensor data, the quality and specificity of an exoskeleton to assess certain motor performance parameters must be examined before an exoskeleton can be used, for example, in a research or clinical setting.

Neuromechanical control of impact absorption during induced lower limb loading in individuals post-stroke

Decreased loading of the paretic lower limb and impaired weight transfer between limbs negatively impact balance control and forward progression during gait in individuals post-stroke. However, the biomechanical and neuromuscular control mechanisms underlying such impaired limb loading remain unclear, partly due to their tendency of avoiding bearing weight on the paretic limb during voluntary movement. Thus, an approach that forces individuals to more fully and rapidly load the paretic limb has been developed. The primary purpose of this study was to compare the neuromechanical responses at the ankle and knee during externally induced limb loading in people with chronic stroke versus able-bodied controls, and determine whether energy absorption capacity, measured during induced limb loading of the paretic limb, was associated with walking characteristics in individuals post-stroke. Results revealed reduced rate of energy absorption and dorsiflexion velocity at the ankle joint during induced limb loading in both the paretic and non-paretic side in individuals post-stroke compared to healthy controls. The co-contraction index was higher in the paretic ankle and knee joints compared to the non-paretic side. In addition, the rate of energy absorption at the paretic ankle joint during the induced limb loading was positively correlated with maximum walking speed and negatively correlated with double limb support duration. These findings demonstrated that deficits in ankle dorsiflexion velocity may limit the mechanical energy absorption capacity of the joint and thereby affect the lower limb loading process during gait following stroke.

Identification of Hip and Knee Joint Impedance During the Swing Phase of Walking

Knowledge on joint impedance during walking in various conditions is relevant for clinical decision-making and the development of robotic gait trainers, leg prostheses, leg orthotics and wearable exoskeletons. Whereas ankle impedance during walking has been experimentally assessed, knee and hip joint impedance during walking have not been identified yet. Here we developed and evaluated a lower limb perturbator to identify hip, knee and ankle joint impedance during treadmill walking. The lower limb perturbator (LOPER) consists of an actuator connected to the thigh via rods. The LOPER allows to apply force perturbations to a free-hanging leg, while standing on the contralateral leg, with a bandwidth of up to 39 Hz. While walking in minimal impedance mode, the interaction forces between LOPER and the thigh were low (<5 N) and the effect on the walking pattern was smaller than the within-subject variability during normal walking. Using a non-linear multibody dynamical model of swing leg dynamics, the hip, knee and ankle joint impedance were estimated at three time points during the swing phase for nine subjects walking at a speed of 0.5 m/s. The identified model was well able to predict the experimental responses for the hip and knee, since the mean variance accounted (VAF) for was 99% and 96%, respectively. The ankle lacked a consistent response and the mean VAF of the model fit was only 77%, and therefore the estimated ankle impedance was not reliable. The averaged across-subjects stiffness varied between the three time points within 34-66 and 0-3.5 Nm/rad Nm/rad for the hip and knee joint respectively. The damping varied between 1.9-4.6 and 0.02-0.14 Nms/rad Nms/rad for hip and knee respectively. The developed LOPER has a negligible effect on the unperturbed walking pattern and allows to identify hip and knee impedance during the swing phase.

Scapular Dynamic Muscular Stiffness Assessed through Myotonometry: A Narrative Review

Several tools have been used to assess muscular stiffness. Myotonometry stands out as an accessible, handheld, and easy to use tool. The purpose of this review was to summarize the psychometric properties and methodological considerations of myotonometry and its applicability in assessing scapular muscles. Myotonometry seems to be a reliable method to assess several muscles stiffness, as trapezius. This method has been demonstrated fair to moderate correlation with passive stiffness measured by shear wave elastography for several muscles, as well as with level of muscle contraction, pinch and muscle strength, Action Research Arm Test score and muscle or subcutaneous thickness. Myotonometry can detect scapular muscles stiffness differences between pre- and post-intervention in painful conditions and, sometimes, between symptomatic and asymptomatic subjects.

Multivariable passive ankle impedance in stroke patients: A preliminary study

Multivariable ankle mechanical impedance was estimated in four stroke survivors, in coupled dorsi- plantarflexion and inversion-eversion. We applied external torque perturbation with an ankle robot and used multi-input, multi-output stochastic system identification methods to estimate impedance, in both paretic and nonparetic limbs. Subjects were instructed to remain at rest throughout the four trials performed on each leg. Impedance projected onto the directions of maximum and minimum stiffness was fit to a 2nd order linear model, including inertia, viscosity and stiffness. For most trials, stiffness and damping in dorsi-plantarflexion are increased on the paretic side. However, for two subjects, overall impedance is not increased in the absence of sustained involuntary tonic contraction, registering values comparable to the non-paretic side. Thus, we speculate that the intrinsic properties of the paretic ankle remained unaffected at the evaluated pose. Spasticity (hyperflexive stretch reflex) would have systematically increased stiffness and damping, even in the absence of involuntary contraction. Hence, we speculate that these two subjects did not exhibit spasticity, while the remaining two subjects did, since impedance was increased, with no involuntary tonic muscle contraction. Regarding inversion-eversion, impedance in this direction remained unaffected by stroke. We evaluated two volunteers before and after the application of botulinum toxin. Surprisingly, ankle stiffness was not reduced, but anisotropy was normalized.

Characterization and clinical implications of ankle impedance during walking in chronic stroke

Individuals post-stroke experience persisting gait deficits due to altered joint mechanics, known clinically as spasticity, hypertonia, and paresis. In engineering, these concepts are described as stiffness and damping, or collectively as joint mechanical impedance, when considered with limb inertia. Typical clinical assessments of these properties are obtained while the patient is at rest using qualitative measures, and the link between the assessments and functional outcomes and mobility is unclear. In this study we quantify ankle mechanical impedance dynamically during walking in individuals post-stroke and in age-speed matched control subjects, and examine the relationships between mechanical impedance and clinical measures of mobility and impairment. Perturbations were applied to the ankle joint during the stance phase of walking, and least-squares system identification techniques were used to estimate mechanical impedance. Stiffness of the paretic ankle was decreased during mid-stance when compared to the non-paretic side; a change independent of muscle activity. Inter-limb differences in ankle joint damping, but not joint stiffness or passive clinical assessments, strongly predicted walking speed and distance. This work provides the first insights into how stroke alters joint mechanical impedance during walking, as well as how these changes relate to existing outcome measures. Our results inform clinical care, suggesting a focus on correcting stance phase mechanics could potentially improve mobility of chronic stroke survivors.

Reliability of Bi-Axial Ankle Stiffness Measurement in Older Adults

This study involves measurements of bi-axial ankle stiffness in older adults, where the ankle joint is passively moved along the talocrural and subtalar joints using a custom ankle movement trainer. A total of 15 elderly individuals participated in test-retest reliability measurements of bi-axial ankle stiffness at exactly one-week intervals for validation of the angular displacement in the device. The ankle's range of motion was also compared, along with its stiffness. The kinematic measurements significantly corresponded to results from a marker-based motion capture system (dorsi-/plantar flexion: r = 0.996; inversion/eversion: r = 0.985). Bi-axial ankle stiffness measurements showed significant intra-class correlations (ICCs) between the two visits for all ankle movements at slower (2.14°/s, ICC = 0.712) and faster (9.77°/s, ICC = 0.879) speeds. Stiffness measurements along the talocrural joint were thus shown to have significant negative correlation with active ankle range of motion (r = -0.631, p = 0.012). The ankle movement trainer, based on anatomical characteristics, was thus used to demonstrate valid and reliable bi-axial ankle stiffness measurements for movements along the talocrural and subtalar joint axes. Reliable measurements of ankle stiffness may help clinicians and researchers when designing and fabricating ankle-foot orthosis for people with upper-motor neuron disorders, such as stroke.

Biomechanical control of paretic lower limb during imposed weight transfer in individuals post-stroke

Background

Stroke is a leading cause of disability with associated hemiparesis resulting in difficulty bearing and transferring weight on to the paretic limb. Difficulties in weight bearing and weight transfer may result in impaired mobility and balance, increased fall risk, and decreased community engagement. Despite considerable efforts aimed at improving weight transfer after stroke, impairments in its neuromotor and biomechanical control remain poorly understood. In the present study, a novel experimental paradigm was used to characterize differences in weight transfer biomechanics in individuals with chronic stroke versus able-bodied controls

Methods

Fifteen participants with stroke and fifteen age-matched able-bodied controls participated in the study. Participants stood with one foot on each of two custom built platforms. One of the platforms dropped 4.3 cm vertically to induce lateral weight transfer and weight bearing. Trials involving a drop of the platform beneath the paretic lower extremity (non-dominant limb for control) were included in the analyses. Paretic lower extremity joint kinematics, vertical ground reaction forces, and center of pressure velocity were measured. All participants completed the clinical Step Test and Four-Square Step Test.

Results

Reduced paretic ankle, knee, and hip joint angular displacement and velocity, delayed ankle and knee inter-joint timing, increased downward displacement of center of mass, and increased center of pressure (COP) velocity stabilization time were exhibited in the stroke group compared to the control group. In addition, paretic COP velocity stabilization time during induced weight transfer predicted Four-Square Step Test scores in individuals post-stroke.

Conclusions

The induced weight transfer approach identified stroke-related abnormalities in the control of weight transfer towards the paretic limb side compared to controls. Decreased joint flexion of the paretic ankle and knee, altered inter-joint timing, and increased COP stabilization times may reflect difficulties in neuromuscular control during weight transfer following stroke. Future work will investigate the potential of improving functional weight transfer through induced weight transfer training exercise.

Neural and non-neural contributions to ankle spasticity in children with cerebral palsy

AIM

To assess the neural and non-neural contributions to spasticity in the impaired ankle of children with cerebral palsy (CP).

METHOD

Instrumented tapping of the Achilles tendon was done isometrically to minimize non-neural contributions and elicit neural contributions. Robot-controlled ankle stretching was done at various velocities, including slow stretching, with minimized neural contributions. Spasticity was assessed as having neural (phasic and tonic stretch reflex torque, tendon reflex gain, contraction rate, and half relaxation rate) and non-neural origin (elastic stiffness and viscous damping) in 17 children with CP (six females and 11 males; mean age [SD] 10y 8mo [3y 11mo], range 4y-18y) and 17 typically developing children (six females and 11 males; mean age [SD] 12y 7mo [2y 9mo], range 7y-18y). All torques were normalized to weight×height.

RESULTS

Children with CP showed increased phasic and tonic stretch reflex torque (p=0.004 and p=0.001 respectively), tendon reflex gain (p=0.02), contraction rate (p=0.038), half relaxation rate (p=0.02), elastic stiffness (p=0.01), and viscous damping (p=0.01) compared to typically developing children.

INTERPRETATION

Controlled stretching and instrumented tendon tapping allow the systematic quantification of various neural and non-neural changes in CP, which can be used to guide impairment-specific treatment.

WHAT THIS PAPER ADDS

Ankle spasticity is associated with increased phasic and tonic stretch reflexes, tendon reflex gain, and contraction and half relaxation rates. Ankle spasticity is also associated with increased elastic stiffness and viscous damping.

Neuromotor Regulation of Ankle Stiffness is Comparable to Regulation of Joint Position and Torque at Moderate Levels

Joint mechanical impedance, which describes the instantaneous relationship between kinematic perturbations and the resulting torque response, plays an important role in the way humans ambulate, interact with the environment, and respond to disturbances. Recent studies have quantified how the stiffness component of mechanical impedance varies during walking. However, the extent to which humans can voluntarily regulate leg joint stiffness is not yet known. Our study sought to quantify the accuracy and precision of the neuromotor system to voluntarily regulate ankle joint stiffness while seated, and compare these data to the well-known abilities to regulate ankle joint torque and position. We tested individuals’ ability to to regulate these quantities at three different magnitudes: 20%, 40%, and 60% of a maximum value. Our results showed that subjects were able to voluntarily regulate ankle joint stiffness, and that the normalized accuracy and precision of stiffness regulation were not different than those of position or torque for targets at magnitudes of 20% of a maximum value. However, the accuracy and precision of stiffness regulation were statistically different than those of position and torque for targets at magnitudes of 40% of the maximum values. At moderate targets, the similarity of the ability to regulate ankle joint stiffness when compared to the abilities to regulate joint torque and position highlights the importance of a comprehensive description of lower-limb biomechanics that includes consideration of joint mechanical impedance, in addition to the common descriptions of joint torque and position.

In-Vivo Study of Passive Musculotendon Mechanics in Chronic Hemispheric Stroke Survivors

We characterized the passive mechanical properties of the affected and contralateral musculotendon units in 9 chronic stroke survivors as well as in 6 neurologically-intact controls. Using a position-controlled motor, we precisely indented the distal tendon of the biceps brachii to a 20 mm depth from skin, recording both its sagittal motion using ultrasound movies and the compression force at the tip of the indenter. Length changes of 8 equally-spaced features along the aponeurosis axis were quantified using a pixel-tracking protocol. We report that, on the aggregate and with respect to contralateral and control, respectively, the affected side initiates feature motion at a shorter indentation distance by 61% and 50%, travels further by 15% and 9%, at a lower rate of 28% and 15%, and is stiffer by 40% and 57%. In an extended analysis including the spatial location of the 8 designated features, we report that in contrast to the contralateral and control muscles, the affected musculotendon unit does not strain measurably within the imaging window. These results confirm that chronic stroke-induced spasticity changes musculotendon unit passive mechanics, causing it to not strain under stretch. The mechanisms responsible for altered passive mechanics may lie within extracellular matrix fibrosis.

Quantifying joint stiffness in clubfoot patients

BACKGROUND

In clinical practice, clubfeet feel stiffer compared to healthy feet. Furthermore, the clinical impression is that stiffer clubfeet have a higher tendency to relapse. Until now, no objective measure has been available to determine the stiffness of clubfeet. The goal of the current project was to objectively quantify ankle and subtalar joint stiffness in clubfeet patients and to compare this stiffness between clubfeet patients and healthy controls using a newly developed measurement device.

METHODS

The newly developed Torque-Displacement-Handpiece in combination with an adjusted Abduction Dorsiflexion Mechanism clubfoot-brace, made it possible to move a foot over two rotational axis, while continuously capturing the applied torque and the achieved angulation. Based on this information, stiffness of the ankle and subtalar joint were assessed for 11 clubfoot patients with 17 clubfeet and 11 healthy subjects with 22 healthy feet.

FINDINGS

With the Torque-Displacement-Handpiece measuring device it was possible to measure torque, angulation and stiffness in a reliable and precise manner. Clubfoot patients showed less angulation and a higher stiffness for measurements over the ADM subtalar axis compared to controls. After adjusting for shoe size, the stiffness for measurements over the ADM tibiotalar axis was also significantly higher in clubfeet than controls.

INTERPRETATION

Overall, these results indicate that clubfoot patients have a higher ankle and subtalar joint stiffness in the affected joint compared to healthy controls. In the future, the Torque-Displacement-Handpiece could be used to monitor stiffness of clubfeet during treatment, and as such, play a potential role in the early detection of relapsing clubfeet.

Passive Wrist Stiffness: The Influence of Handedness

OBJECTIVE

This paper reports on the quantification of passive wrist joint stiffness and investigates the potential influence of handedness and gender on stiffness estimates.

METHODS

We evaluated the torque-angle relationship during passive wrist movements in 2 degrees of freedom (into flexion-extension and radial-ulnar deviation) in 13 healthy subjects using a wrist robot. Experimental results determined intrasubject differences between dominant and nondominant wrist and intersubject differences between male and female participants.

RESULTS

We found differences in the magnitude of passive stiffness of left- and right-hand dominant males and right-hand dominant females suggesting that the dominant hand tends to be stiffer than the nondominant hand. Left-hand stiffness magnitude was found to be 37% higher than the right-hand stiffness magnitude in the left-handed male group and the right-hand stiffness magnitude was 11% and 40% higher in the right-handed male and female groups, respectively. Other joint stiffness features such as the orientation and the anisotropy of wrist stiffness followed the expected pattern from previous studies.

CONCLUSION

The observed difference in wrist stiffness between the dominant and nondominant limb is likely due to biomechanical adaptations to repetitive asymmetric activities (such as squash, tennis, basketball, or activities of daily living such as writing, teeth brushing, etc.).

SIGNIFICANCE

Understanding and quantifying handedness influence on stiffness may have critical implication for the optimization of surgical and rehabilitative interventions.

Passive material properties of stroke-impaired plantarflexor and dorsiflexor muscles

BACKGROUND

Following a stroke, intrinsic muscle properties such as stiffness may be altered, which is accompanied by increased spasticity and contractures. Previously, quantification of muscle stiffness has been based off of indirect measurements. Using shear wave ultrasound elastography, direct measurements of muscle material properties can be made.

METHODS

Our aim was to evaluate material properties, specifically passive stiffness, using shear wave ultrasound elastography across a range of muscle lengths, in the medial gastrocnemius and the tibialis anterior in chronic stroke survivors.

FINDINGS

Our main results show significant increases of 27.7% and 26.9% in shear wave velocity of stroke-impaired medial gastrocnemius compared to the unimpaired contralateral side at 90° ankle angle (P=0.033) and 15° plantarflexion (P=0.001), respectively. However, no significant difference was found in the tibialis anterior between the two sides. Relatively weak correlations were found between SW velocity in the medial gastrocnemius and joint stiffness for both the non-paretic (ρ=0.384, P=0.001), and paretic side (ρ=0.363, P=0.002). Additionally, muscle stiffness estimates of stroke-impaired tibialis anterior from joint torque and angle measurements were significantly greater by 23.1% (P=0.033) than the unimpaired contralateral side. However, no significant difference was found in the medial gastrocnemius.

INTERPRETATION

These results indicate that there are non-uniform changes in passive stiffness of stroke-impaired muscle. Therefore, muscles need to be evaluated individually to assess alterations. Additionally, interpretation of joint-based calculations of muscle stiffness should be made cautiously. Having the ability to non-invasively assess muscle stiffness adaptations in vivo would aid in prognosis, evaluation, and treatment following a stroke.

A Feasibility Study of SSVEP-Based Passive Training on an Ankle Rehabilitation Robot

Objective

This study aims to establish a steady-state visual evoked potential- (SSVEP-) based passive training protocol on an ankle rehabilitation robot and validate its feasibility.

Method

This paper combines SSVEP signals and the virtual reality circumstance through constructing information transmission loops between brains and ankle robots. The robot can judge motion intentions of subjects and trigger the training when subjects pay their attention on one of the four flickering circles. The virtual reality training circumstance provides real-time visual feedback of ankle rotation.

Result

All five subjects succeeded in conducting ankle training based on the SSVEP-triggered training strategy following their motion intentions. The lowest success rate is 80%, and the highest one is 100%. The lowest information transfer rate (ITR) is 11.5 bits/min when the biggest one of the robots for this proposed training is set as 24 bits/min.

Conclusion

The proposed training strategy is feasible and promising to be combined with a robot for ankle rehabilitation. Future work will focus on adopting more advanced data process techniques to improve the reliability of intention detection and investigating how patients respond to such a training strategy.

Measurement of the passive stiffness of ankle joint in 3 DOF using stewart platform type ankle foot device

This paper presents a method to measure the passive stiffness of an ankle joint in three degrees of freedom (DOF) under two motion speeds (1 Hz and 5 degree/s) using a developed Stewart platform-type device. The developed device can reproduce input motions of the foot in 6 DOF by controlling six pneumatic linear motion actuators. We used the device to measure the passive stiffness of an ankle joint undergoing three kinds of motion, namely dorsi-plantar flexion, inversion-eversion, and adduction-abduction. The measured values of the passive stiffness of the ankle joint in dorsiflexion that we obtained agreed well with that obtained in a previous study, indicating that the developed device is useful for measuring the passive stiffness of ankle joint. In addition, the developed device can be used to measure the stiffness in inversion-eversion and adduction-abduction motions as well, parameters that have never been measured. The results we obtained demonstrated certain interesting features as we varied both the direction and pace of motion (e.g., there were significant differences in the stiffness not only between adduction and abduction during the faster pace, but also between these and the other motions).

Robot-assisted gait training versus treadmill training in patients with Parkinson's disease: a kinematic evaluation with gait profile score

The purpose of this study was to quantitatively compare the effects, on walking performance, of end-effector robotic rehabilitation locomotor training versus intensive training with a treadmill in Parkinson's disease (PD). Fifty patients with PD were randomly divided into two groups: 25 were assigned to the robot-assisted therapy group (RG) and 25 to the intensive treadmill therapy group (IG). They were evaluated with clinical examination and 3D quantitative gait analysis [gait profile score (GPS) and its constituent gait variable scores (GVSs) were calculated from gait analysis data] at the beginning (T0) and at the end (T1) of the treatment. In the RG no differences were found in the GPS, but there were significant improvements in some GVSs (Pelvic Obl and Hip Ab-Add). The IG showed no statistically significant changes in either GPS or GVSs. The end-effector robotic rehabilitation locomotor training improved gait kinematics and seems to be effective for rehabilitation in patients with mild PD.

Summary of Human Ankle Mechanical Impedance During Walking

The human ankle joint plays a critical role during walking and understanding the biomechanical factors that govern ankle behavior and provides fundamental insight into normal and pathologically altered gait. Previous researchers have comprehensively studied ankle joint kinetics and kinematics during many biomechanical tasks, including locomotion; however, only recently have researchers been able to quantify how the mechanical impedance of the ankle varies during walking. The mechanical impedance describes the dynamic relationship between the joint position and the joint torque during perturbation, and is often represented in terms of stiffness, damping, and inertia. The purpose of this short communication is to unify the results of the first two studies measuring ankle mechanical impedance in the sagittal plane during walking, where each study investigated differing regions of the gait cycle. Rouse et al. measured ankle impedance from late loading response to terminal stance, where Lee et al. quantified ankle impedance from pre-swing to early loading response. While stiffness component of impedance increases significantly as the stance phase of walking progressed, the change in damping during the gait cycle is much less than the changes observed in stiffness. In addition, both stiffness and damping remained low during the swing phase of walking. Future work will focus on quantifying impedance during the “push off” region of stance phase, as well as measurement of these properties in the coronal plane.

Measurement of Resistive Plantar Flexion Torque of the Ankle during Passive Stretch in Healthy Subjects and Patients with Poststroke Hemiplegia

BACKGROUND

Quantification of increased muscle tone for patients with spasticity has been performed to date using various devices to replace the manual scales, such as the modified Ashworth scale or the Tardieu scale. We developed a device that could measure resistive plantar flexion (PF) torque of the ankle during passive dorsiflexion (DF) as an indicator of muscle tone of ankle plantar flexors.

METHODS

The primary objective was to explore the test-retest intrarater reliability of a custom-built device. Participants were 11 healthy subjects (7 men, 4 women; mean age 47.0 years) and 22 patients with poststroke hemiplegia (11 hemorrhagic, 11 ischemic; 14 men, 8 women; mean age 57.2 years). The device was affixed to the ankle. Subjects were seated with knees either flexed or extended. The ankle was passively dorsiflexed from 20° of PF to more than 10° of DF at 5°/second (slow stretch) or 90°/second (fast stretch). Angle and torque were measured twice during the stretches. The intraclass correlation coefficients (ICCs) of torque at 10° of DF (T10) in the 4 conditions-slow and fast stretches with knee flexed or extended-were calculated.

RESULTS

The T10 ICCs of the 4 conditions were .95-.99 in both groups. The healthy subjects showed significantly higher T10 of knee extension than of knee flexion during slow and fast stretches. The patients showed increased velocity-dependent torque during fast stretches.

CONCLUSIONS

Excellent reliability was observed. The device is suitable for measuring resistive PF torque during passive stretch in a flexed knee condition.

A new method to assess passive and active ankle stiffness during quiet upright stance

Both passive and active ankle torque contribute to postural stability during quiet upright stance, yet directly measuring their relative contributions is difficult. Here, a new method was developed to estimate passive and active ankle stiffness (ST) and damping (DA). In contrast to earlier approaches, the proposed method does not require external mechanical or sensory perturbations. Instead, the method is based on the assumption that upright stance is intermittently controlled, and that active ankle torque is in-phase coherent with ankle angular acceleration. Thus, identifying the local maxima of ankle angular accelerations facilitates the identification of time windows that include substantial active ankle torque. After identifying these local maxima and associated windows, estimates of passive and active ankle ST and DA were obtained using linear regression analyses. Consistent with earlier work, passive ankle torque was estimated to account for 94-97% of the total ankle torque, and to have linear relationships with ankle angle and angular velocity. Predicted values of passive and active ankle stiffness were also consistent with earlier reports. This new approach may be a useful tool to efficiently investigate passive and active joint stiffness during quiet upright stance.

Relationship between activation of ankle muscles and quasi-joint stiffness in early and middle stances during gait in patients with hemiparesis

It is unclear whether muscle contraction is necessary to increase quasi-joint stiffness (QJS) of the ankle joint during gait in patients with hemiparesis. The purpose of the present study was to investigate the relationship between QJS and muscle activation at the ankle joint in the stance phase during gait in patients with hemiparesis. Spatiotemporal and kinetic gait parameters and activation of the medial head of the gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) muscles were measured using a 3-dimensional motion analysis system and surface electromyography, in 21 patients with hemiparesis due to stroke and 10 healthy individuals. In the early stance, the QJS on the paretic side (PS) of patients was greater than that on the non-PS (p<0.05) and not significantly correlated with activation of the three muscles. In the middle stance, the QJS on the PS was lower than that on the non-PS (p<0.05) and that on the right side of controls (p<0.001), which was positively correlated with activation of the MG (r=0.51, p<0.05) and SOL (r=0.49, p<0.05). In the patients with hemiparesis, plantarflexor activation may not contribute to QJS in the early stance. On the other hand, QJS in the middle stance may be attributed to activation of the MG and SOL. Our findings suggest that activation of the MG and SOL in the middle stance on the PS may require to be enhanced to increase QJS during gait in patients with hemiparesis.

Multivariable dynamic ankle mechanical impedance with active muscles

Multivariable dynamic ankle mechanical impedance in two coupled degrees-of-freedom (DOFs) was quantified when muscles were active. Measurements were performed at five different target activation levels of tibialis anterior and soleus, from 10% to 30% of maximum voluntary contraction (MVC) with increments of 5% MVC. Interestingly, several ankle behaviors characterized in our previous study of the relaxed ankle were observed with muscles active: ankle mechanical impedance in joint coordinates showed responses largely consistent with a second-order system consisting of inertia, viscosity, and stiffness; stiffness was greater in the sagittal plane than in the frontal plane at all activation conditions for all subjects; and the coupling between dorsiflexion-plantarflexion and inversion-eversion was small-the two DOF measurements were well explained by a strictly diagonal impedance matrix. In general, ankle stiffness increased linearly with muscle activation in all directions in the 2-D space formed by the sagittal and frontal planes, but more in the sagittal than in the frontal plane, resulting in an accentuated "peanut shape." This characterization of young healthy subjects' ankle mechanical impedance with active muscles will serve as a baseline to investigate pathophysiological ankle behaviors of biomechanically and/or neurologically impaired patients.

Quantifying changes in material properties of stroke-impaired muscle

BACKGROUND

Material properties of muscles are clinically important parameters for evaluating altered muscle function. Stroke survivors display motor impairments almost immediately after the vascular event, and then gradually develop altered muscle properties. Little is known about the magnitude of these changes in muscle material properties, specifically stiffness. Previous measures of stiffness are limited to estimates of joint stiffness or groups of muscles. Thus, our aim was to determine changes in passive muscle stiffness and composition by measuring: (1) shear wave speed using shear wave ultrasound elastography and (2) echo intensity of the B-mode ultrasound images of the biceps brachii muscle in individuals who have had a stroke.

METHODS

Shear wave ultrasound elastography and B-mode ultrasound images of the biceps brachii muscle of the paretic and non-paretic limbs of sixteen stroke survivors were captured at rest.

FINDINGS

Our main results show that shear wave speed and echo intensity of the paretic side were on average 69.5% and 15.5% significantly greater than those of the non-paretic side, respectively. Differences in shear wave speed between the non-paretic and the paretic muscles were strongly correlated with differences in echo intensity, time since stroke, and with Fugl-Meyer scores.

INTERPRETATION

Muscle stiffness and muscle composition, as indicated by SW speed and echo intensity, may be altered in stroke-impaired muscle at rest. These findings highlight the potential for SW elastography as a tool for both investigating the fundamental mechanisms behind changes in stroke-impaired muscle, and for evaluation of muscle mechanical properties as part of clinical examination.

Estimation of human ankle impedance during the stance phase of walking

Human joint impedance is the dynamic relationship between the differential change in the position of a perturbed joint and the corresponding response torque; it is a fundamental property that governs how humans interact with their environments. It is critical to characterize ankle impedance during the stance phase of walking to elucidate how ankle impedance is regulated during locomotion, as well as provide the foundation for future development of natural, biomimetic powered prostheses and their control systems. In this study, ankle impedance was estimated using a model consisting of stiffness, damping and inertia. Ankle torque was well described by the model, accounting for 98 ±1.2% of the variance. When averaged across subjects, the stiffness component of impedance was found to increase linearly from 1.5 to 6.5 Nm/rad/kg between 20% and 70% of stance phase. The damping component was found to be statistically greater than zero only for the estimate at 70% of stance phase, with a value of 0.03 Nms/rad/kg. The slope of the ankle's torque-angle curve-known as the quasi-stiffness-was not statistically different from the ankle stiffness values, and showed remarkable similarity. Finally, using the estimated impedance, the specifications for a biomimetic powered ankle prosthesis were introduced that would accurately emulate human ankle impedance during locomotion.

Clinical application of a modular ankle robot for stroke rehabilitation

BACKGROUND

Advances in our understanding of neuroplasticity and motor learning post-stroke are now being leveraged with the use of robotics technology to enhance physical rehabilitation strategies. Major advances have been made with upper extremity robotics, which have been tested for efficacy in multi-site trials across the subacute and chronic phases of stroke. In contrast, use of lower extremity robotics to promote locomotor re-learning has been more recent and presents unique challenges by virtue of the complex multi-segmental mechanics of gait.

OBJECTIVES

Here we review a programmatic effort to develop and apply the concept of joint-specific modular robotics to the paretic ankle as a means to improve underlying impairments in distal motor control that may have a significant impact on gait biomechanics and balance.

METHODS

An impedance controlled ankle robot module (anklebot) is described as a platform to test the idea that a modular approach can be used to modify training and measure the time profile of treatment response.

RESULTS

Pilot studies using seated visuomotor anklebot training with chronic patients are reviewed, along with results from initial efforts to evaluate the anklebot's utility as a clinical tool for assessing intrinsic ankle stiffness. The review includes a brief discussion of future directions for using the seated anklebot training in the earliest phases of sub-acute therapy, and to incorporate neurophysiological measures of cerebro-cortical activity as a means to reveal underlying mechanistic processes of motor learning and brain plasticity associated with robotic training.

CONCLUSIONS

Finally we conclude with an initial control systems strategy for utilizing the anklebot as a gait training tool that includes integrating an Internal Model-based adaptive controller to both accommodate individual deficit severities and adapt to changes in patient performance.

A robotic object hitting task to quantify sensorimotor impairments in participants with stroke

Background

Existing clinical scores of upper limb function often use observer-based ordinal scales that are subjective and commonly have floor and ceiling effects. The purpose of the present study was to develop an upper limb motor task to assess objectively the ability of participants to select and engage motor actions with both hands.

Methods

A bilateral robotic system was used to quantify upper limb sensorimotor function of participants with stroke. Participants performed an object hit task that required them to hit virtual balls moving towards them in the workspace with virtual paddles attached to each hand. Task difficulty was initially low, but increased with time by increasing the speed and number of balls in the workspace. Data were collected from 262 control participants and 154 participants with recent stroke.

Results

Control participants hit ~60 to 90% of the 300 balls with relatively symmetric performance for the two arms. Participants with recent stroke performed the task with most participants hitting fewer balls than 95% of healthy controls (67% of right-affected and 87% of left-affected strokes). Additionally, nearly all participants (97%) identified with visuospatial neglect hit fewer balls than healthy controls. More detailed analyses demonstrated that most participants with stroke displayed asymmetric performance between their affected and non-affected limbs with regards to number of balls hit, workspace area covered by the limb and hand speed. Inter-rater reliability of task parameters was high with half of the correlations above 0.90. Significant correlations were observed between many of the task parameters and the Functional Independence Measure and/or the Behavioural Inattention Test.

Conclusions

As this object hit task requires just over two minutes to complete, it provides an objective and easy approach to quantify upper limb motor function and visuospatial skills following stroke.

Position-dependent characterization of passive wrist stiffness

Because the dynamics of wrist rotations are dominated by stiffness, understanding wrist rotations requires a thorough characterization of wrist stiffness in multiple degrees of freedom. The only prior measurement of multivariable wrist stiffness was confined to approximately one-seventh of the wrist range of motion (ROM). Here, we present a precise nonlinear characterization of passive wrist joint stiffness over a range three times greater, which covers approximately 70% of the functional ROM of the wrist. We measured the torque-displacement vector field in 24 directions and fit the data using thin-plate spline smoothing optimized with generalized cross validation. To assess anisotropy and nonlinearity, we subsequently derived several different approximations of the stiffness due to this multivariable vector field. The directional variation of stiffness was more pronounced than reported previously. A linear approximation (obtained by multiple linear regression over the entire field) was significantly more anisotropic (eigenvalue ratio of 2.69 ± 0.52 versus 1.58 ± 0.39; ) though less misaligned with the anatomical wrist axes (12.1 ± 4.6° versus 21.2 ± 9.2°; ). We also found that stiffness over this range exhibited considerable nonlinearity-the error associated with a linear approximation was 20-30%. The nonlinear characterization over this greater range confirmed significantly greater stiffness in radial deviation compared to ulnar deviation. This study provides a characterization of passive wrist stiffness better suited to investigations of natural wrist rotations, which cover much of the wrist's ROM. It also provides a baseline for the study of neurological and/or orthopedic disorders that result in abnormal wrist stiffness.

Dynamic primitives in the control of locomotion

Humans achieve locomotor dexterity that far exceeds the capability of modern robots, yet this is achieved despite slower actuators, imprecise sensors, and vastly slower communication. We propose that this spectacular performance arises from encoding motor commands in terms of dynamic primitives. We propose three primitives as a foundation for a comprehensive theoretical framework that can embrace a wide range of upper- and lower-limb behaviors. Building on previous work that suggested discrete and rhythmic movements as elementary dynamic behaviors, we define submovements and oscillations: as discrete movements cannot be combined with sufficient flexibility, we argue that suitably-defined submovements are primitives. As the term “rhythmic” may be ambiguous, we define oscillations as the corresponding class of primitives. We further propose mechanical impedances as a third class of dynamic primitives, necessary for interaction with the physical environment. Combination of these three classes of primitive requires care. One approach is through a generalized equivalent network: a virtual trajectory composed of simultaneous and/or sequential submovements and/or oscillations that interacts with mechanical impedances to produce observable forces and motions. Reliable experimental identification of these dynamic primitives presents challenges: identification of mechanical impedances is exquisitely sensitive to assumptions about their dynamic structure; identification of submovements and oscillations is sensitive to their assumed form and to details of the algorithm used to extract them. Some methods to address these challenges are presented. Some implications of this theoretical framework for locomotor rehabilitation are considered.

Robot-assisted walking training for individuals with Parkinson's disease: a pilot randomized controlled trial

BACKGROUND

Over the last years, the introduction of robotic technologies into Parkinson's disease rehabilitation settings has progressed from concept to reality. However, the benefit of robotic training remains elusive. This pilot randomized controlled observer trial is aimed at investigating the feasibility, the effectiveness and the efficacy of new end-effector robot training in people with mild Parkinson's disease.

METHODS

Design. Pilot randomized controlled trial.

RESULTS

Robot training was feasible, acceptable, safe, and the participants completed 100% of the prescribed training sessions. A statistically significant improvement in gait index was found in favour of the EG (T0 versus T1). In particular, the statistical analysis of primary outcome (gait speed) using the Friedman test showed statistically significant improvements for the EG (p = 0,0195). The statistical analysis performed by Friedman test of Step length left (p = 0,0195) and right (p = 0,0195) and Stride length left (p = 0,0078) and right (p = 0,0195) showed a significant statistical gain. No statistically significant improvements on the CG were found.

CONCLUSIONS

Robot training is a feasible and safe form of rehabilitative exercise for cognitively intact people with mild PD. This original approach can contribute to increase a short time lower limb motor recovery in idiopathic PD patients. The focus on the gait recovery is a further characteristic that makes this research relevant to clinical practice. On the whole, the simplicity of treatment, the lack of side effects, and the positive results from patients support the recommendation to extend the use of this treatment. Further investigation regarding the long-time effectiveness of robot training is warranted.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01668407.

The difference between stiffness and quasi-stiffness in the context of biomechanical modeling

The ankle contributes the majority of mechanical power during walking and is a frequently studied joint in biomechanics. Specifically, researchers have extensively investigated the torque-angle relationship for the ankle during dynamic tasks, such as walking and running. The slope of this relationship has been termed the "quasi-stiffness." However, over time, researchers have begun to interchange the concepts of quasi-stiffness and stiffness. This is an especially important distinction as researchers currently begin to investigate the appropriate control systems for recently developed powered prosthetic legs. The quasi-stiffness and stiffness are distinct concepts in the context of powered joints, and are equivalent in the context of passive joints. The purpose of this paper is to demonstrate the difference between the stiffness and quasi-stiffness using a simple impedance-controlled inverted pendulum model and a more sophisticated biped walking model, each with the ability to modify the trajectory of an impedance controller's equilibrium angle position. In both cases, stiffness values are specified by the controller and the quasi-stiffness are shown during a single step. Both models have widely varying quasi-stiffness but each have a single stiffness value. Therefore, from this simple modeling approach, the differences and similarities between these two concepts are elucidated.

Static ankle impedance in stroke and multiple sclerosis: a feasibility study

Quantitative characterization of ankle mechanical impedance is critical for understanding lower extremity function in persons with neurological disorders. In this paper, we examine the feasibility of employing an ankle robot and multivariable analysis to determine static ankle impedance in 4 patients: 1 with multiple sclerosis and 3 with stroke. We employed a scalar based vector field approximation method which was successful in identifying young healthy subjects' ankle impedance. It enabled clear interpretation of spatial ankle impedance structure and intermuscular feedback at the ankle for both affected and unaffected legs. Measured impedance of two patients was comparable to healthy young subjects, while the other two patients had significantly different static ankle impedance properties.

The passive stiffness of the wrist and forearm

Because wrist rotation dynamics are dominated by stiffness (Charles SK, Hogan N. J Biomech 44: 614-621, 2011), understanding how humans plan and execute coordinated wrist rotations requires knowledge of the stiffness characteristics of the wrist joint. In the past, the passive stiffness of the wrist joint has been measured in 1 degree of freedom (DOF). Although these 1-DOF measurements inform us of the dynamics the neuromuscular system must overcome to rotate the wrist in pure flexion-extension (FE) or pure radial-ulnar deviation (RUD), the wrist rarely rotates in pure FE or RUD. Instead, understanding natural wrist rotations requires knowledge of wrist stiffness in combinations of FE and RUD. The purpose of this report is to present measurements of passive wrist stiffness throughout the space spanned by FE and RUD. Using a rehabilitation robot designed for the wrist and forearm, we measured the passive stiffness of the wrist joint in 10 subjects in FE, RUD, and combinations. For comparison, we measured the passive stiffness of the forearm (in pronation-supination), as well. Our measurements in pure FE and RUD agreed well with previous 1-DOF measurements. We have linearized the 2-DOF stiffness measurements and present them in the form of stiffness ellipses and as stiffness matrices useful for modeling wrist rotation dynamics. We found that passive wrist stiffness was anisotropic, with greater stiffness in RUD than in FE. We also found that passive wrist stiffness did not align with the anatomical axes of the wrist; the major and minor axes of the stiffness ellipse were rotated with respect to the FE and RUD axes by ∼20°. The direction of least stiffness was between ulnar flexion and radial extension, a direction used in many natural movements (known as the "dart-thrower's motion"), suggesting that the nervous system may take advantage of the direction of least stiffness for common wrist rotations.