Ankle intrinsic stiffness changes with postural sway

Biomechanical variations in patients with flatfoot deformity: Impact of gender, age, and BMI on foot kinetics and kinematics

Background

Flatfoot is considered by the collapse of the foot arch, altered biomechanics and impacting functional abilities. The biomechanical gait alteration of foot kinematics and kinetics in individuals with flatfoot, based on gender, age and Body mass index (BMI) in each cohort is unclear. This study explores how gender, age, and body mass index (BMI) impact distinct foot biomechanical characteristics, including ankle joint angle (Jc°), Ground force reaction angle (GFR°), Achilles tendon force (T), Ankle joint force (Jc) and vertical ground reaction force (VGRF) during the gait stance phase, in flatfoot versus normal-foot individuals on Indian Population.

Method

A foot pressure test and sagittal plane motion analysis were performed on 142 individuals with normal-foot arches and 102 with flatfoot, stratified by gender, age, and BMI. Calculations of the magnitude and direction of forces in ankle joint equilibrants relied on inverse dynamic analysis, vertical ground force reaction and mapping motion data of the gait stance phases.

Result

In the midstance phase, females with high BMI (HBMI) in the middle and older age group (p = 0.029 and p = 0.014), and males with HBMI in the older age group (p = 0.039) demonstrate significantly higher V G R F . Females and males with HBMI in middle and older age groups, along with males with normal BMI in the older age cohort, show positive and negative ranges of GFR°, indicating gait instability. In the push-off phase, females with HBMI in a middle-aged group exhibit significantly lower T a n d J c (p = 0.023 and p = 0.026) respectively.

Conclusion

The biomechanical issues in individuals with flatfoot, while accounting for the influence of gender, age and BMI, are crucial for tailored interventions and precise solutions to biomechanical issues, thereby enhancing foot function and reducing discomfort.

Aging Does Not Alter Ankle, Muscle, and Tendon Stiffness at Low Loads Relevant to Stance

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, there has been limited investigation into age-dependent changes in muscle stiffness. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads relevant to standing and the stance phase of walking, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p > 0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p = 0.008), muscle (37%; p = 0.02), and tendon stiffness (22%; p = 0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p = 0.004). Together, these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.

Temporal adaptation of the postural control following a prolonged fin swimming

PURPOSE

Postural control deteriorates following a transition between two environments, highlighting a sensory conflict when returning to natural conditions. Aquatic immersion offers new perspectives for studying postural control adaptation in transitional situations. Our aim is to study immediate and post-task static postural control adaptation on land after a prolonged fin swimming exercise in total immersion.

METHODS

Standing static postural control was assessed in 14 professional or recreational SCUBA divers (11 men, 3 women; 33.21 ± 10.70 years), with eyes open and closed, before, immediately after, and in the following 20 min following a fully-immersed 45-min fin swimming exercise. Centre-of-pressure metrics (COP) including average position, amplitude, velocity, length and 95% ellipse were evaluated in medial-lateral (x-axis) and anterior-posterior (y-axis) directions with a force platform. The Romberg ratio was also assessed for each metric.

RESULTS

A two-way repeated measures analysis of variance revealed a significant effect of the measurement period on COPx vel (p = 0.01), COPy vel (p < 0.01) and Length (p < 0.01), and of the visual condition on COPy vel (p < 0.01) and Length (p < 0.01). Eyes closed measures were systematically higher than eyes open measures despite there being no significant difference in the Romberg ratio in all periods. Post-immersion, the velocity and total trajectory of the centre of pressure remained systematically lower than baseline values in both visual conditions.

CONCLUSION

Post-immersion, COP velocity and length significantly decreased, suggesting a sensory reweighting strategy potentially associated with ankle stiffening.

Effects of foot and ankle mobilisations combined with home stretches in people with diabetic peripheral neuropathy: a proof-of-concept RCT

INTRODUCTION

People with diabetic peripheral neuropathy (DPN) and limited joint mobility syndrome (LJMS) can experience increased forefoot peak plantar pressures (PPPs), a known risk factor for ulceration. The aim of this study was to investigate whether ankle and 1st metatarsophalangeal (MTP) joint mobilisations and home-based stretches in people with DPN improve joint range of motion (ROM) and reduce forefoot PPPs.

DESIGN AND METHODS

Sixty-one people with DPN (IWGDF risk 2), were randomly assigned to a 6-week programme of ankle and 1st MTP joint mobilisations (n = 31) and home-based stretches or standard care only (n = 30). At baseline (T0); 6-week post intervention (T1) and at 3 months follow-up (T2), a blinded assessor recorded dynamic ankle dorsiflexion range using 3D (Codamotion) motion analysis and the weight bearing lunge test, static 1st MTP joint dorsiflexion ROM, dynamic plantar pressure and balance.

RESULTS

At T1 and T2 there was no difference between both groups in ankle dorsiflexion in stance phase, plantar pressure and balance. Compared to the control group, the intervention group showed a statistically significant increase in static ankle dorsiflexion range (Left 1.52 cm and 2.9cms, Right 1.62 cm and 2.7 cm) at 6 (T1) and 18 weeks (T2) respectively p < 0.01). Between group differences were also seen in left hallux dorsiflexion (2.75°, p < 0.05) at T1 and in right hallux dorsiflexion ROM (4.9°, p < 0.01) at T2 follow up. Further, functional reach showed a significant increase in the intervention group (T1 = 3.13 cm p < 0.05 and T2 = 3.9 cm p < 0.01). Intervention adherence was high (80%).

CONCLUSIONS

Combining ankle and 1st MTP joint mobilisations with home-based stretches in a 6-week programme in people with DPN is effective in increasing static measures of range. This intervention may be useful for improving ankle, hallux joint mobility and anteroposterior stability limits in people with diabetes and neuropathy but not for reducing PPP or foot ulcer risk.

TRIAL REGISTRATION

https://classic.

CLINICALTRIALS

gov/ct2/show/NCT03195855 .

Multi-muscle Synergies of Postural Control in Self- and External-Triggered Force Release During Simulated Archery Shooting

We investigated postural stability during simulated archery shooting. The experiment consisted of two force release conditions: self-triggered (time-set in a feedforward fashion) and external cue-triggered (time-set by reacting to external cue) conditions while standing on the force platform. The electromyography of leg muscles and the center of pressure (COP) were recorded. The notions of muscle-modes (M-modes) and multi-muscle synergies were employed to quantify the postural stability, which described covariation patterns of the M-modes to stabilize the COP. The result showed relatively strong postural stability in a self-triggered condition associated with consistent shooting performance. The current findings suggested that initiating force release in a feedforward fashion would be a beneficial strategy to ensure the consistency in shooting performance.

Simultaneous Quantification of Ankle, Muscle, and Tendon Impedance in Humans

OBJECTIVE

Regulating the impedance of our joints is essential for the effective control of posture and movement. The impedance of a joint is governed mainly by the mechanical properties of the muscle-tendon units spanning it. Many studies have quantified the net impedance of joints but not the specific contributions from the muscles and tendons. The inability to quantify both muscle and tendon impedance limits the ability to determine the causes underlying altered movement control associated with aging, neuromuscular injury, and other conditions that have different effects on muscle and tendon properties. Therefore, we developed a technique to quantify joint, muscle, and tendon impedance simultaneously and evaluated this technique at the human ankle.

METHODS

We used a single degree of freedom actuator to deliver pseudorandom rotations to the ankle while measuring the corresponding torques. We simultaneously measured the displacement of the medial gastrocnemius muscle-tendon junction with B-mode ultrasound. From these experimental measurements, we were able to estimate ankle, muscle, and tendon impedance using non-parametric system identification.

RESULTS

We validated our estimates by comparing them to previously reported measurements of muscle and tendon stiffness, the position-dependent component of impedance, to demonstrate that our technique generates reliable estimates of these properties.

CONCLUSION

Our approach can be used to clarify the respective contributions from the muscle and tendon to the net mechanics of a joint.

SIGNIFICANCE

This is a critical step forward in the ultimate goal of understanding how muscles and tendons govern ankle impedance during posture and movement.

Frontal plane ankle stiffness increases with axial load independent of muscle activity

Ankle sprains are the most common musculoskeletal injury, typically resulting from excessive inversion of the ankle. One way to prevent excessive inversion and maintain ankle stability is to generate a stiffness that is sufficient to resist externally imposed rotations. Frontal-plane ankle stiffness increases as participants place more weight on their ankle, but whether this effect is due to muscle activation or axial loading of the ankle is unknown. Identifying whether and to what extent axial loading affects ankle stiffness is important in understanding what role the passive mechanics of the ankle joint play in maintaining its stability. The objective of this study was to determine the effect of passive axial load on frontal-plane ankle stiffness. We had subjects seated in a chair as an axial load was applied to the ankle ranging from 10% to 50% body weight. Small rotational perturbations were applied to the ankle in the frontal plane to estimate stiffness. We found a significant, linear, 3-fold increase in ankle stiffness with axial load from the range of 0% body weight to 50% body weight. This increase could not be due to muscle activity as we observed no significant axial-load-dependent change in any of the recorded muscle activations. These results demonstrate that axial loading is a significant contributor to maintaining frontal-plane ankle stability, and that disruptions to the mechanism mediating this sensitivity of stiffness to axial loading may result in pathological cases of ankle instability.

Vertical ground reaction force oscillation during standing on hard and compliant surfaces: The “postural rhythm”

When a person stands upright quietly, the position of the Centre of Mass (CoM), the vertical force acting on the ground and the geometrical configuration of body segments is accurately controlled around to the direction of gravity by multiple feedback mechanisms and by integrative brain centres that coordinate multi-joint movements. This is not always easy and the postural muscles continuously produce appropriate torques, recorded as ground reaction force by a force platform. We studied 23 young adults during a 90 s period, standing at ease on a hard (Solid) and on a compliant support (Foam) with eyes open (EO) and with eyes closed (EC), focusing on the vertical component of the ground reaction force (VGRF). Analysis of VGRF time series gave the amplitude of their rhythmic oscillations (the root mean square, RMS) and of their frequency spectrum. Sway Area and Path Length of the Centre of Pressure (CoP) were also calculated. VGRF RMS (as well as CoP sway measures) increased in the order EO Solid ≈ EC Solid < EO Foam < EC Foam. The VGRF frequency spectra featured prevailing frequencies around 4–5 Hz under all tested conditions, slightly higher on Solid than Foam support. Around that value, the VGRF frequencies varied in a larger range on hard than on compliant support. Sway Area and Path Length were inversely related to the prevailing VGRF frequency. Vision compared to no-vision decreased Sway Area and Path Length and VGRF RMS on Foam support. However, no significant effect of vision was found on VGRF mean frequency for either base of support condition. A description of the VGRF, at the interface between balance control mechanisms and sway of the CoP, can contribute information on how upright balance is maintained. Analysis of the frequency pattern of VGRF oscillations and its role in the maintenance of upright stance should complement the traditional measures of CoP excursions in the horizontal plane.

Incongruity of Geometric and Spectral Markers in the Assessment of Body Sway

Different measurements of body oscillations in the time or frequency domain are being employed as markers of gait and balance abnormalities. This study investigates basic relationships within and between geometric and spectral measures in a population of young adult subjects. Twenty healthy subjects stood with parallel feet on a force platform with and without a foam pad. Adaptation effects to prolonged stance were assessed by comparing the first and last of a series of eight successive trials. Centre of Foot Pressure (CoP) excursions were recorded with Eyes Closed (EC) and Open (EO) for 90s. Geometric measures (Sway Area, Path Length), standard deviation (SD) of the excursions, and spectral measure (mean power Spectrum Level and Median Frequency), along the medio-lateral (ML) and antero-posterior (AP) direction were computed. Sway Area was more strongly associated than Path Length with CoP SD and, consequently, with mean Spectrum Level for both ML and AP, and both visual and surface conditions. The squared-SD directly specified the mean power Spectrum Level of CoP excursions (ML and AP) in all conditions. Median Frequency was hardly related to Spectrum Level. Adaptation had a confounding effect, whereby equal values of Sway Area, Path Length, and Spectrum Level corresponded to different Median Frequency values. Mean Spectrum Level and SDs of the time series of CoP ML and AP excursions convey the same meaning and bear an acceptable correspondence with Sway Area values. Shifts in Median Frequency values represent important indications of neuromuscular control of stance and of the effects of vision, support conditions, and adaptation. The Romberg Quotient EC/EO for a given variable is contingent on the compliance of the base of support and adaptation, and different between Sway Area and Path Length, but similar between Sway Area and Spectrum Level (AP and ML). These measures must be taken with caution in clinical studies, and considered together in order to get a reliable indication of overall body sway, of modifications by sensory and standing condition, and of changes with ageing, medical conditions and rehabilitation treatment. However, distinct measures shed light on the discrete mechanisms and complex processes underpinning the maintenance of stance.

Increased 18F-FDG Uptake in the Axillary Lymph Nodes of the Vaccinated Side Associated with COVID-19 Vaccination

A 50-year-old female patient underwent (¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) following modified radical mastectomy for cancer of the left breast. Ten days before the PET/CT, the coronavirus disease-2019 (COVID-19) vaccine was injected intramuscularly into the right deltoid muscle. Increased (¹⁸F-FDG uptake of maximum standardized uptake value (11.0) was observed in the lymph nodes of the right axilla, which had not been observed in the previous PET/CT. The size of the oval-shaped lymph nodes was up to approximately 11×9 mm; however, it was larger than that observed on the previous PET/CT. We contemplate that the increased (¹⁸F-FDG uptake was a reactive change in the lymph nodes associated with the COVID-19 vaccine.

Acoustic Environmental Conditions (Do Not?) Affect the Static Posturography Diagnostic Accuracy: A Test-Retest Reliability Study

Static posturography assessed with force platforms is a procedure used to obtain objective estimates related to postural adjustments. However, controlling multiple intrinsic and extrinsic factors influencing the diagnostic accuracy is essential to obtain reliable measurements and recommend its use with clinical or research purposes. We aimed to analyze how different environmental acoustic conditions affect the test−retest reliability and to analyze the most appropriate number of trials to calculate a valid mean average score. A diagnostic accuracy study was conducted enrolling 27 healthy volunteers. All procedures were taken considering consistent device settings, posture, feet position, recording time, and illumination of the room. Three trials were recorded in a silent environment (35−40 dB) and three trials were recorded in a noisy environment (85−90 dB). Results showed comparable reliability estimates for both acoustic conditions (ICC = 0.453−0.962 and 0.621−0.952), but silent conditions demonstrated better sensitivity to changes (MDC = 13.6−76%). Mean average calculations from 2 and 3 trials showed no statistically significant differences (p > 0.05). Cross-sectional studies can be conducted under noisy or silent conditions as no significantly different scores were obtained (p > 0.05) and ICC were comparable (except oscillation area). However, longitudinal studies should consider silent conditions as they demonstrated better sensitivity to real changes not derived from measurement errors.

Balance strategy in hoverboard control

This study examines how people learn to perform lower limb control in a novel task with a hoverboard requiring to maintain dynamic balance. We designed an experiment to investigate the learning of hoverboard balance and two control strategies: A hip strategy, which mainly uses hip movements to change the angle of the foot, and an ankle strategy relying more on ankle motion to control the orientation of hoverboard plates controlling the motion. Motor learning was indicated by a significant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10\pm 4$$\end{document}10±4% decrease in the trial completion time (p < 0.001) and a significant 24 ± 11% decrease in total muscle activation (p < 0.001). Furthermore, the participants, who had no prior experience riding a hoverboard, learned an ankle strategy to maintain their balance and control the hoverboard. This is supported by significantly stronger cross-correlation, phase synchrony, lower dynamic time warping distance between the hoverboard plate orientation controlling hoverboard motion, and the ankle angle when compared to the hip angle. The adopted ankle strategy was found to be robust to the foot orientation despite salient changes in muscle group activation patterns. Comparison with results of an experienced hoverboard rider confirmed that the first-time riders adopted an ankle strategy.

Frontal plane ankle stiffness increases with weight-bearing

Ankle sprains are among the most common musculoskeletal injuries. They are not isolated innocuous injuries as 30-40% of people who sprain their ankles develop chronic ankle instability. Ankle instability is typically assessed under passive unloaded conditions, ignoring any potential contribution of joint loading or muscle activation to the maintenance of ankle stability. Thus, the relevance of unloaded ankle stability assessments to the evaluation of impairments in chronic ankle instability or the prediction of future ankle sprains is questionable. Ankle impedance, which quantifies the resistance to an imposed rotation, has often been used to quantify ankle stability. However, few studies have investigated impedance in the frontal plane where sprains occur, and none have systematically investigated the effect of weight-bearing on ankle impedance. The objective of this study was to determine whether weight-bearing affects frontal plane ankle impedance. We had subjects systematically alter the weight on the tested ankle, while imposed frontal plane rotations were applied to estimate the impedance. We found that ankle stiffness, the static component of impedance, increased proportionally with the weight on the ankle. This increase in stiffness was due to a combination of the increase loading on the joint and the increase in muscle activation that occurs during weight-bearing. Finally, we found that men had a greater stiffness than women over the majority of the weight-bearing range. These results highlight the importance of clinically assessing ankle stability during weight-bearing conditions to better determine the impairments in chronic ankle instability and identify those at risk for ankle sprains.

Identification of Central and Stretch Reflex Contributions to Human Postural Control

Human postural control requires continuous modulation of ankle torque to stabilize the upright stance. The torque is generated by two components: active contributions, due to central control and stretch reflex, and passive mechanisms, due to joint intrinsic stiffness. Identifying the contribution of each component is difficult, since their effects appear together, and standing is controlled in closed-loop. This article presents a novel multiple-input, single-output method to identify central and stretch reflex contributions to human postural control. The model uses ankle muscle EMGs as inputs and requires no kinematic data. Application of the method to data from nine subjects during standing while subjected to perturbations of ankle position demonstrated that active torque accounted for 84.0± 5.5% of the ankle torque. The ankle plantar-flexors collectively produced the largest portion of the active torque through central control, with large inter-subject variability in the relative contributions of the individual muscles. In addition, reflex contribution of the plantar-flexors was substantial in half of the subjects, showing its potentially important functional role; finally, intrinsic contributions, estimated as the residual of the model, contributed to 15% of the torque. This study introduces a new method to quantify the contributions of the central and stretch reflex pathways to postural control; the method also provides an estimate of noisy intrinsic torque with significantly increased signal to noise ratio, suitable for identification of intrinsic stiffness in standing. The method can be used in different experimental conditions and requires minimal a-priori assumption regarding the role of different pathways in postural control.

Untreated adolescent idiopathic scoliotic girls display altered balance modalities during self-paced voluntary body sways compared to able-bodied girls

INTRODUCTION

This study investigated self-paced voluntary oscillations of scoliotic and non-scoliotic girls. Temporal variables and frequency coherence were calculated for the overall, low and high frequency bandwidths of the center of pressure excursions and free-moment to identify which variables best describe sway balance modalities in both groups.

METHODS

Twenty-three girls with adolescent idiopathic moderate scoliosis (spinal curves to the right) formed the scoliotic group and 19 matched able-bodied girls formed the non-scoliotic group. Each girl performed self-paced voluntary medio-lateral and antero-posterior sways while standing on a force platform. Center of pressure displacements, out of plane deviation and free-moment were measured and their frequency content calculated. The magnitude of the coherence was calculated for each signal pairs for three frequency ranges.

RESULTS

In both sway conditions, the center of pressure excursion parameters were on average 28% higher for the scoliotic group. Factor analysis revealed that balance modalities were essentially based on frequency coherence pair interactions whereas temporal parameters play a secondary role. However, these balance modalities were altered in the scoliotic group. They relied essentially on 2 additional principal components and 3 additional variables reflecting a fine tuning of the control mechanism to maintain dynamic balance.

INTERPRETATION

Scoliotic girls appear to be performing a wide ellipsoidal trajectory when performing whole body oscillations. Superfluous variables could be related to the difficulty in preserving balance during body sway tasks and could parasitize the scoliotic dynamic control balance modalities. Self-paced voluntary sways could be an appropriate complementary balance test for untreated scoliotic girls.

Individual differences in intrinsic ankle stiffness and their relationship to body sway and ankle torque

When standing, intrinsic ankle stiffness is smaller when measured using large perturbations, when sway size is large, and when background torque is low. However, there is a large variation in individual intrinsic ankle stiffness. Here we determine if individual variation has consequences for postural control. We examined the relationship between ankle stiffness, ankle torque and body sway across different individuals. Ankle stiffness was estimated in 19 standing participants by measuring torque responses to small, brief perturbations. Perturbation sizes of 0.2 & 0.9 degrees (both lasting 140 ms) measured short- and long-range stiffness respectively, while participants either stood quietly on a fixed platform or were imperceptibly tilted to reduce stability (0.1 Hz sinusoid; 0.2 & 0.4 deg). The spontaneous body sway component (natural random relatively rapid postural adjustments) and background ankle torque were averaged from sections immediately before perturbations. The results show that, first, intrinsic ankle stiffness is positively associated with ankle torque, and that this relationship is stronger for long-range stiffness. Second, intrinsic ankle stiffness is negatively associated with body sway, but, in contrast to the relationship with torque, this relationship is stronger for short-range stiffness. We conclude that high short-range intrinsic ankle stiffness is associated with reduced spontaneous sway, although the causal relationship between these two parameters is unknown. These results suggest that, in normal quiet standing where sway is very small, the most important determinant of intrinsic ankle stiffness may be stillness. In less stable conditions, intrinsic ankle stiffness may be more dependent on ankle torque.

Quantitative Modeling of Spasticity for Clinical Assessment, Treatment and Rehabilitation

Spasticity, a common symptom in patients with upper motor neuron lesions, reduces the ability of a person to freely move their limbs by generating unwanted reflexes. Spasticity can interfere with rehabilitation programs and cause pain, muscle atrophy and musculoskeletal deformities. Despite its prevalence, it is not commonly understood. Widely used clinical scores are neither accurate nor reliable for spasticity assessment and follow up of treatments. Advancement of wearable sensors, signal processing and robotic platforms have enabled new developments and modeling approaches to better quantify spasticity. In this paper, we review quantitative modeling techniques that have been used for evaluating spasticity. These models generate objective measures to assess spasticity and use different approaches, such as purely mechanical modeling, musculoskeletal and neurological modeling, and threshold control-based modeling. We compare their advantages and limitations and discuss the recommendations for future studies. Finally, we discuss the focus on treatment and rehabilitation and the need for further investigation in those directions.

Multi-Directional Ankle Impedance During Standing Postures

In this study, we estimated the multi-directional ankle mechanical impedance in two degrees-of-freedom (DOF) during standing, and determined how the stiffness, damping, and inertia vary with ankle angle and ankle torque at different postures. Fifteen subjects stood on a vibrating instrumented platform in four stationary postures, while subjected to pulse train perturbations in both the sagittal and frontal planes of motion. The four stationary postures were selected to resemble stages within the stance phase of the gait cycle: including post-heel-strike during the loading response, mid-stance, post-mid-stance, and just before the heel rises from the ground in terminal-stance phase. In general, the ankle stiffness and damping increased in all directions as the foot COP moved forward, and more torque is generated in plantarflexion. Interestingly, the multi-directional ankle impedance during standing showed a similar shape and major tilt axes to the results of non-loaded scenarios. However, there were notable differences in the impedance amplitude when the ankle was not under bodyweight loading. Last, the stiffness during standing had similar amplitudes ranges to the time-varying ankle stiffness during the stance phase of dynamic walking estimated in previous studies. These results have implications on the design of new, less physically intense, biomechanics experiments aimed at people with neuromuscular disorders or other physical impairments who cannot complete a standard gait test.

A Clustering-Based Approach to Identify Joint Impedance During Walking

Mechanical impedance, which changes with posture and muscle activations, characterizes how the central nervous system regulates the interaction with the environment. Traditional approaches to impedance estimation, based on averaging of movement kinetics, requires a large number of trials and may introduce bias to the estimation due to the high variability in a repeated or periodic movement. Here, we introduce a data-driven modeling technique to estimate joint impedance considering the large gait variability. The proposed method can be used to estimate impedance in both the stance and swing phases of walking. A 2-pass clustering approach is used to extract groups of unperturbed gait data and estimate candidate baselines. Then patterns of perturbed data are matched with the most similar unperturbed baseline. The kinematic and torque deviations from the baselines are regressed locally to compute joint impedance at different gait phases. Simulations using the trajectory data of a subject's gait at different speeds demonstrate a more accurate estimation of ankle stiffness and damping with the proposed clustering-based method when compared with two methods: i) using average unperturbed baselines, and ii) matching shifted and scaled average unperturbed velocity baselines. Furthermore, the proposed method requires fewer trials than methods based on average unperturbed baselines. The experimental results on human hip impedance estimation show the feasibility of clustering-based technique and verifies that it reduces the estimation variability.

Design of Muscle Reflex Control for Upright Standing Push-Recovery Based on a Series Elastic Robot Ankle Joint

In physical human-robot interaction environment, ankle joint muscle reflex control remains significant and promising in human bipedal stance. The reflex control mechanism contains rich information of human joint dynamic behavior, which is valuable in the application of real-time decoding motion intention. Thus, investigating the human muscle reflex mechanism is not only meaningful in human physiology study but also useful for the robotic system design in the field of human-robot physical interaction. In this paper, a specialized ankle joint muscle reflex control algorithm for human upright standing push-recovery is proposed. The proposed control algorithm is composed of a proportional-derivative (PD)-like controller and a positive force controller, which are employed to mimic the human muscle stretch reflex and muscle tendon force reflex, respectively. Reflex gains are regulated by muscle activation levels of contralateral ankle muscles. The proposed method was implemented on a self-designed series elastic robot ankle joint (SERAJ), where the series elastic actuator (SEA) has the potential to mimic human muscle-tendon unit (MTU). During the push-recovery experimental study, the surface electromyography (sEMG), ankle torque, body sway angle, and velocity of each subject were recorded in the case where the SERAJ was unilaterally kneed on each subject. The experimental results indicate that the proposed muscle reflex control method can easily realize upright standing push-recovery behavior, which is analogous to the original human behavior.

Biomechanics of Vertical Posture and Control with Referent Joint Configurations

Our study compared the results of two methods of analysis of postural sway during human quiet standing, the rambling-trembling (Rm-Tr) decomposition and the analysis of the point of intersection of the ground reaction forces (zIP analysis). Young, healthy subjects were required to stand naturally and with an increased level of leg/trunk muscle co-activation under visual feedback on the magnitude of a combined index of muscle activation (muscle mode). The main findings included the shift of zIP toward higher frequencies and strong correlations between Tr and zIP when the subjects stood with increased muscle co-activation. We interpret the results within the idea of whole-body control with a set of primitives associated with referent coordinates in the joint configuration space.

Patterns of muscle activation and modulation of ankle intrinsic stiffness in different postural operating conditions

Intrinsic stiffness describes the dynamic relationship between imposed angular perturbations to a joint and the resulting torque response, due to intrinsic mechanical properties of muscles and joint, and inertia of the limbs. Recently, we showed that ankle intrinsic stiffness changes substantially with sway in normal standing. In the present study, we documented how ankle intrinsic stiffness changes with postural operating conditions. Subjects stood on an apparatus while subjected to ankle position perturbations in five conditions: normal standing, toe-up and toe-down standing, and backward and forward lean. In each condition, ankle intrinsic stiffness was estimated while its modulation with sway was accounted for. The results demonstrated that ankle intrinsic stiffness varies widely, from 0.08 to 0.75 of critical stiffness, across postural operating conditions; however, it is always smaller than the critical stiffness. Therefore, other contributions are necessary to ensure stable standing. The mean intrinsic stiffness was highest in forward lean and lowest in backward lean. Moreover, within each operating condition, the intrinsic stiffness changed with center-of-pressure position in one of three ways, each associated with a distinct muscle activation pattern; these include 1) monotonically increasing stiffness-center of pressure relation, associated with a progressive increase in triceps surae activation, 2) decreasing-increasing stiffness-center of pressure relation, associated with initial activation of tibialis anterior and later activation of triceps surae, and 3) monotonically decreasing stiffness-center of pressure relation, associated with decreasing activation of tibialis anterior. Thus intrinsic stiffness varies greatly within and across postural operating conditions, and a correct understanding of postural control requires accounting for such variations.NEW & NOTEWORTHY Ankle intrinsic stiffness changes with sway in normal standing. We quantified such changes in different postural operating conditions and demonstrated that the intrinsic stiffness changes in a manner associated with different activation patterns of ankle plantarflexors and dorsiflexors, emerging in different operating conditions. Large modulations of the intrinsic stiffness within and across postural operating conditions show that the stiffness importance and contribution change and must be accounted for in the study of postural control.

A Closed-Loop Method to Identify EMG-Torque Dynamics in Human Balance Control

Human balance control requires continuous modulation of ankle torque by central and spinal activation of the ankle muscles combined with the intrinsic mechanical stiffness of the joint. These components appear together and cannot be measured separately. This work presents a novel multiple-input, single-output, closed-loop identification method that decomposes the ankle torque in human balance control into its central, stretch reflex, and intrinsic components. The method models separate transfer functions for each EMG-torque relation for central and stretch reflex mechanisms and estimates the ankle intrinsic torque from the residuals. The method uses only EMG measurements, requires no kinematic data, and has few parameters, resulting in robust performance. Application of the method to perturbed standing data from two healthy subjects demonstrated that the central and stretch reflex torques accounted for 80-93% of the ankle torque variation, while the intrinsic stiffness was responsible for most of the remaining torque.