Approximately 50% of survivors of stroke experience lasting balance impairments that persist and that are often managed through compensatory, but suboptimal, strategies. Identifying neuromechanical control changes after stroke could enable more targeted and effective rehabilitation strategies. Computational modeling has begun to uncover balance control strategies in unimpaired adults, but efforts have been limited post-stroke. Here we show one of the first instances of a model of quiet stance that reveals distinct control strategies in post-stroke individuals compared to similarly-aged unimpaired participants. Quiet standing was modeled using a double-inverted pendulum with full-state feedback control. The controller parameters were fit to foot-ground force data collected from 12 post-stroke and 22 similarly-aged unimpaired participants. The best-fit models revealed a joint-torque-coordination pattern in the paretic limb of post-stroke participants that differed substantially from that of the unimpaired participants. The post-stroke participants’ non-paretic limb also showed increased reliance on neural feedback, which may quantify compensatory effort for the altered coordination in the paretic limb. The results demonstrate that model-based analysis of foot-ground force behavior could eventually reveal clinically meaningful insights that are not captured by traditional assessments.

Maintaining upright posture in quiet standing is an important skill that is often disrupted by stroke. Despite extensive study of human standing, current understanding is incomplete regarding the muscle coordination strategies that produce the ground-on-foot force (F) that regulates translational and rotational accelerations of the body. Even less is understood about how stroke disrupts that coordination. Humans produce sagittal plane variations in the location (center of pressure, xCP) and orientation (Fx/Fz) of F that, along with the force of gravity, produce sagittal plane body motions. As F changes during quiet standing there is a strong correlation between the xCP and Fx/Fz time-varying signals within narrow frequency bands. The slope of the correlation varies systematically with frequency in non-disabled populations, is sensitive to changes in both environmental and neuromuscular control factors, and emerges from the interaction of body mechanics and neural control. This study characterized the xCP versus Fx/Fz relationship as frequency-dependent Intersection Point (IP) heights for the paretic and non-paretic legs of individuals with history of a stroke (n = 12) as well as in both legs of nondisabled controls (n = 22) to reveal distinguishing motor coordination patterns. No inter-leg difference of IP height was present in the control group. The paretic leg IP height was lower than the non-paretic, and differences from control legs were in opposite directions. These results quantify disrupted coordination that may characterize the paretic leg balance deficit and non-paretic leg compensatory behavior, providing a means of monitoring balance impairment and a target for therapeutic interventions.

Aging can cause the decline of balance ability, which can lead to increased falls and decreased mobility. This work aimed to discern differences in balance control between healthy older and younger adults. Foot force data of 38 older and 65 younger participants (older and younger than 60 yr, respectively) were analyzed. To first determine whether the two groups exhibited any differences, this study incorporated the orientation of the foot-ground interaction force in addition to its point of application. Specifically, the frequency dependence of the "intersection point" of the lines of actions of the foot-ground interaction forces was evaluated. Results demonstrated that, like the mean center-of-pressure speed, a traditionally employed measure, the intersection-point analysis could distinguish between the two participant groups. Then, to further explore age-specific control strategies, simulations of standing balance were conducted. An optimal controller stabilized a double-inverted-pendulum model with torque-actuated ankle and hip joints corrupted with white noise. The experimental data were compared with the simulation results to identify the controller parameters that best described the human data. Older participants showed significantly more use of the ankle than hip compared with younger participants. Best-fit controller gains suggested increased preference for asymmetric inter-joint neural feedback, possibly to compensate for the effects of aging such as sarcopenia. These results underscore the advantages of the intersection-point analysis to quantify possible shifts in inter-joint control with age, thus highlighting its potential to be used as a balance assessment tool in research and clinical settings.NEW & NOTEWORTHY Age groups were distinguished by analyzing foot-ground force data during quiet standing in older and younger adults to calculate the foot-force vector intersection point that emerges across frequency bands. Modeling balance and comparing the simulations' outcomes to experimental results suggested that older adults increased reliance on neural feedback, possibly compensating for muscle strength deficiency. This novel analysis also quantified the apparent balance controller for each participant, highlighting its potential as a balance assessment tool.

Background: This study evaluated psychometric properties of the Intersection Point Height, derived from ground-on-feet force characteristics, as a tool for assessing balance control. We compare this metric with traditional center of pressure (CP) measurements. Methods: Data from a public dataset of 146 participants, divided into younger (<60 years old) and older (>= 60 years old) adults, were analyzed. Clinical tests included the Short Falls Efficacy ScaleInternational, International Physical Activity Questionnaire-Short Form, Trail Making Tests A and B, and the Mini-Balance Evaluation Systems Test. Reliability and validity were assessed through the intra-class correlation coefficient (ICC[3,1]) for mean Intersection Point Height in each test condition and Spearman's rho between summative Intersection Point Height (the sum of intra-condition mean values across all test conditions within one subject) and other variables of interest, respectively. Findings: Mean Intersection Point Height showed good to excellent reliability (ICC = 0.712-0.901), similar to that of CP velocity (ICC = 0.733-0.922) and greater than that of variance CPx (0.475-0.768). Summative Intersection Point Height exhibited strong convergent validity with Trail Making Tests A and B (rho = 0.49, p < 0.001) and the Mini-Balance Evaluation Systems Test (rho = -0.47, p < 0.001). At most, a weak to moderate association (rho = 0.39-0.49, p < 0.001) was found between intra-condition mean Intersection Point Height with CP metrics. Intra-condition mean Intersection Point Height demonstrated weak to moderate convergent validity with several clinical measures (rho = 0.32-0.52, p < 0.001). In contrast, at most, a weak to moderate association (rho = 0.39-0.49, p < 0.001) was found between intra-condition mean Intersection Point Height with CP metrics. Interpretation: The Intersection Point Height is a reliable and valid balance measure. Further, we believe that it is a more comprehensive evaluation than CP metrics.

A recent line of work suggests that the net behavior of the foot-ground interaction force provides insight into quiet-standing-balance dynamics and control. Through human-subject experiments, Boehm et al. found that the relative variations of the center of pressure and force orientation emerge as a distinct pattern in the frequency domain, termed the "intersection-point (IP) height." Subsequent empirical and simulation-based studies showed that different control strategies are reflected in the distribution of intersection-point height across frequency. To facilitate understanding of the strengths and limitations of the intersection-point height in describing the dynamics and control of standing, the present work establishes a spectral-based method that also enables derivation of a closed-form estimate of the intersection-point height from any linear model of quiet stance. This new method explained observations from prior work, including how the measure captures aspects of control and physiological noise. The analysis presented herein highlights the utility of the frequency-dependent foot-force dynamics in probing the balance controller and provides a tool for model development and validation to further our understanding of the neuromotor control of natural upright posture in humans.NEW & NOTEWORTHY The present work details a closed-form analytical result that reveals a direct link between frequency-domain patterns in the foot-force line-of-action and the closed-loop frequency response function of human upright stance. The analytical method developed herein dramatically simplifies the identification of the intersection-point-height measure of standing balance, and further demonstrates that the net behavior of the foot-ground interaction force quantifies essential characteristics of the underlying neuromotor control of human quiet standing.

Impedance based kinesthetic haptic devices have been a focus of study for many years. Factors such as delay and the dynamics of the device itself affect the stable rendering range of traditional active kinesthetic devices. A parallel hybrid actuation approach, which combines active energy supplying actuators and passive energy absorbing actuators into a single actuator, has recently been experimentally shown to increase the range of stable virtual stiffness a haptic device can achieve when compared to the active component of the actuator alone. This work presents both a stability and rendering range analysis that aims to identify the mechanisms and limitations by which parallel hybrid actuation increases the stable rendering range of virtual stiffness. Increases in actuator stability are analytically and experimentally shown to be linked to the stiffness of the passive actuator.

The multi-joint coordination responsible for maintaining upright posture in the standing human manifests in the pattern of variation of the support-surface force (F). Assessment of both the translational and rotational kinematics in the sagittal-plane requires understanding the critical relationship between the direction and location of F. Prior work demonstrated that band-pass filtered F direction and center-of-pressure (CoP) covary in time such that the F vector lines-of-action pass near a fixed point called an intersection point (IP). The height of that IP (IPz) varies systematically with the frequency of the pass band. From F measurements in able-bodied humans (n = 17) standing on various pitched surfaces, the present study also found the emergent property of an IP, with IPz located above the center of mass (CoM) at frequencies <1.75 Hz and below the CoM for higher frequencies. This property aids in maintaining upright posture for various perturbation modes within a single control structure. From purely mechanical effects, standing on a pitched surface should not change IPz, however these measurements of F show that IPz is generally closer to CoM height. This characterization of quiet standing provides simple means of assessing the complex multi-joint coordination of standing and relates directly to the physical demands of controlling the translational and rotational aspects of body posture.

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.

Cooperative robots or “cobots” promise to allow humans and robots to work together more closely while maintaining safety. However, to date the capabilities of cobots are greatly diminished compared to industrial robots in terms of the force and power they are able to safely produce. This is in part due to the actuation choices of cobots. Low impedance robotic actuators aim to solve this problem by attempting to provide an actuator with a combination of low output impedance and a large bandwidth of force control. In short the ideal actuator has a large dynamic range. Existing actuators success and performance has been limited. We propose a high force and high power balanced hybrid active-passive actuator which aims to increase the actuation capability of low impedance actuators and to safely enable high performance larger force and workspace robots. Our balanced hybrid actuator does so, by combining and controlling a series elastic actuator, a small DC motor, and a particle brake in parallel. The actuator provides low and high frequency power producing active torques, along with power absorbing passive torques. Control challenges and advantages of hybrid actuators are discussed and overcome through the use of trajectory optimization, and the safety of the new actuator is evaluated.

Quiet standing is a mechanically unstable postural objective that humans typically perform with ease. Control of upright posture requires stabilization of both translational and rotational degrees-offreedom that is accomplished by neuro-muscular coordination. This coordination produces a force at the ground-foot interface (F) that is quantified by magnitude, direction (theta(F)), and point of application (center-of-pressure, CP). Previous research has shown that the nervous system controls muscle activation such that CP motion occurs at both slow and fast time scales. However, it is unknown how OF varies with respect to CP and how that relationship varies across time scales. We present a novel method for assessing the frequency-dependent relative variation in theta(F) and CP. The center-of-pressure (CP) and direction of the ground-on-foot force (F) in the sagittal-plane during quiet standing were decomposed into 0.2 Hz-width frequency bands within 0.4-8.0 Hz. The relation between the direction and CP was approximately linear with a slope positively related to frequency. These frequency-dependent features of F have critical implications for understanding balance strategy because the translational and rotational acceleration effects of F were coupled, but with opposite phasing at high versus low frequencies. Such results suggest a system tuned for one stability mode at low frequencies and another mode at higher frequencies. This frequency-wise approach to examining the translational and rotational effects of humans' preferred F may be useful for establishing balance rehabilitation metrics, directing study of the underlying neural mechanisms responsible for the observed coordination, and for setting a biometric standard to inform biomimetic prosthetics and robotics. (C) 2018 The Authors. Published by Elsevier Ltd.

Background: Postural stability difficulties are commonly reported in people on the autism spectrum. However, it is unclear whether unsteady surfaces may exacerbate postural stability difficulties in children and adolescents with autism spectrum disorder (ASD). Understanding balance on unsteady surfaces is important because uneven surfaces are commonly encountered in daily life. Methods: Twenty-one youth on the autism spectrum and 16 youth with typical development (ages 6-16 years, IQ >= 79) stood on both a fixed and unsteady (tiltable) platform, and center of pressure was measured. Results: The group with ASD exhibited differentially more postural sway on the unsteady surface compared to the group with typical development. However, there was substantial variability within the ASD group. Follow-up analyses suggested that much of the variability in postural sway in the ASD group was accounted for by IQ. Conclusions: Clinically, these findings suggest that not all individuals with ASD struggle more with postural stability on unsteady surfaces. Instead children and adolescents with ASD and below-average IQ may have particular difficulty on unsteady surfaces and may require accommodations. Further, these findings lay the groundwork for future research to investigate the underlying mechanisms of poorer balance across the autism spectrum.

INTRODUCTION:The objective of this article is to introduce the robotic platform KIINCE and its emphasis on the potential of kinetic objectives for studying and training human walking and standing. The device is motivated by the need to characterize and train lower limb muscle coordination to address balance deficits in impaired walking and standing.METHODS:The device measures the forces between the user and his or her environment, particularly the force of the ground on the feet (F) that reflects lower limb joint torque coordination. In an environment that allows for exploration of the user's capabilities, various forms of real-time feedback guide neural training to produce F appropriate for remaining upright. Control of the foot plate motion is configurable and may be user driven or prescribed. Design choices are motivated from theory of motor control and learning as well as empirical observations of F during walking and standing.RESULTS:Preliminary studies of impaired individuals demonstrate the feasibility and potential utility of patient interaction with kinetic immersive interface for neuromuscular coordination enhancement.CONCLUSION:Applications include study and rehabilitation of standing and walking after injury, amputation, and neurological insult, with an initial focus on stroke discussed here.

Recovery of walking after stroke requires an understanding of how motor control deficits lead to gait impairment. Traditional therapy focuses on removing specific observable gait behaviors that deviate from unimpaired walking; however, those behaviors may be effective compensations for underlying problematic motor control deficits rather than direct effects of the stroke. Neurological deficits caused by stroke are not well understood, and thus, efficient interventions for gait rehabilitation likely remain unrealized. Our laboratory has previously characterized a post-stroke control deficit that yields a specific difference in direction of the ground reaction force ( F , limb endpoint force) exerted with the hemiplegic limb of study participants pushing on both stationary and moving pedals while seated. That task was not dependent on F to retain upright posture, and thus, the task did not constrain F direction. Rather, the F direction was the product of neural preference. It is not known if this specific muscle coordination deficit causes the observed walking deviations, but if present during walking, the deficit would prevent upright posture unless counteracted by compensatory behaviors. Compensations are presented that mechanically counteract the F misdirection to allow upright posture. Those compensations are similar to behaviors observed in stroke patients. Based on that alignment between predictions of this theory and clinical observations, we theorize that post-stroke gait results from the attempt to compensate for the underlying F misdirection deficit. Limb endpoint force direction has been shown to be trainable in the paretic upper limb, making it a feasible goal in the lower limb. If this F misdirection theory is valid, these ideas have tremendous promise for advancing the field of post-stroke gait rehabilitation.

A principle objective of human walking is controlling angular motion of the body as a whole to remain upright. The force of the ground on each foot (F) reflects that control, and recent studies show that in the sagittal plane F exhibits a specific coordination between F direction and center-of-pressure (CP) that is conducive to remaining upright. Typical walking involves the CP shifting relative to the body due to two factors: posterior motion of the foot with respect to the hip (stepping) and motion of the CP relative to the foot (foot roll-over). Recent research has also shown how adjusting ankle torque alone to shift CP relative to the foot systematically alters the direction of F, and thus, could play a key role in upright posture and the F measured during walking. This study explores how the CP shifts due to stepping and foot roll-over contribute to the observed F and its role in maintaining upright posture. Experimental walking kinetics and kinematics were combined with a mechanical model of the human to show that variation in F that was not attributable to foot roll-over had systematic correlation between direction and CP that could be described by an intersection point located near the center-of-mass. The findings characterize a component of walking motor control, describe how typical foot roll-over contributes to postural control, and provide a rationale for the increased fall risk observed in individuals with atypical ankle muscle function.

This study examined the prospective control of the swing phase in young healthy adults while walking at preferred speed over unobstructed ground and during obstacle clearance. Three aspects of swing were examined: (1) the relation of the body Center of Mass (CoM) to the stability boundaries at the base of support; (2) a dynamic time-to-contact analysis of the CoM and swing foot to these boundaries; and (3) the role of head movements in the prospective control of gait and field of view assessment. The time-to-contact analysis of CoM and swing foot showed less stable swing dynamics in the trail foot compared to the lead foot in the approach to the unstable equilibrium, with the CoM leading the swing foot and crossing the anterior stability boundary before the swing foot. Compensations in temporal coupling occurred in the trail limb during the late swing phase. Time-to-contact analysis of head movement showed stronger prospective control of the lead foot, while fixation of the field of view occurred earlier in swing and was closer to the body in the obstacle condition compared to unobstructed walking. The dynamic time-to-contact analysis offers a new approach to assessing the unstable swing phase of walking in different populations.

The neural control and mechanics of human bipedalism are inadequately understood. The variable at the interface of neural control and body mechanics that is key to upright posture during human walking is the force of the ground on the foot (ground reaction force, F). We present a model that predicts sagittal plane F direction as passing through a divergent point (DP) fixed in a reference frame attached to the person. Four reference frames were tested to identify which provided the simplest and most accurate description of F direction. For all reference frames, the DP model predicted nearly all the observed variation in F direction and whole body angular momentum during single leg stance. The reference frame with vertical orientation and with origin on the pelvis provided the best combination of accuracy and simplicity. The DP was located higher than the CM and the predicted F produced a pattern of torque about the CM that caused body pitch oscillations that disrupted upright posture. Despite those oscillations, that torque was evidence of a stability mechanism that may be a critical component enabling humans to remain upright while walking and performing other tasks.

Humans maintain upright bipedal posture by producing appropriate force against the environment through the interaction of neural controlled muscle force with the mechanics of the skeletal system. Characterizing these mechanics facilitates understanding of the neural control. We used a mechanical model of an upright human to analyze how the mechanical linkage aspects of the human body affect the force between the feet and the ground (F). Key parameters of F that directly regulate upright body posture are the direction of F (θF) and its point of application (xCP, anterior–posterior position of the center of pressure). Instantaneous analysis of the equations of motion demonstrated that θF varied systematically with xCP such that the F vectors intersected at a point called the Posture-specific force Intersection point or PI (Π). The Π was located above the center of mass when the hip and knee joints were modeled as rigid and was located near the knee when the hip and knee torques were held constant. Limb posture and the knee torque affected the location of Π. This Π behavior quantifies the purely mechanical effect of anterior–posterior center of pressure shifts on the direction of F, which has consequences for the control of whole body posture.

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb ( F t ) and index finger ( F i ) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant’s maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F t = F i was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F t = F i coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.