A physical model suggests that hip-localized balance sense in birds improves state estimation in perching: implications for bipedal robots

In addition to a vestibular system, birds uniquely have a balance-sensing organ within the pelvis, called the lumbosacral organ (LSO). The LSO is well developed in terrestrial birds, possibly to facilitate balance control in perching and terrestrial locomotion. No previous studies have quantified the functional benefits of the LSO for balance. We suggest two main benefits of hip-localized balance sense: reduced sensorimotor delay and improved estimation of foot-ground acceleration. We used system identification to test the hypothesis that hip-localized balance sense improves estimates of foot acceleration compared to a head-localized sense, due to closer proximity to the feet. We built a physical model of a standing guinea fowl perched on a platform, and used 3D accelerometers at the hip and head to replicate balance sense by the LSO and vestibular systems. The horizontal platform was attached to the end effector of a 6 DOF robotic arm, allowing us to apply perturbations to the platform analogous to motions of a compliant branch. We also compared state estimation between models with low and high neck stiffness. Cross-correlations revealed that foot-to-hip sensing delays were shorter than foot-to-head, as expected. We used multi-variable output error state-space (MOESP) system identification to estimate foot-ground acceleration as a function of hip- and head-localized sensing, individually and combined. Hip-localized sensors alone provided the best state estimates, which were not improved when fused with head-localized sensors. However, estimates from head-localized sensors improved with higher neck stiffness. Our findings support the hypothesis that hip-localized balance sense improves the speed and accuracy of foot state estimation compared to head-localized sense. The findings also suggest a role of neck muscles for active sensing for balance control: increased neck stiffness through muscle co-contraction can improve the utility of vestibular signals. Our engineering approach provides, to our knowledge, the first quantitative evidence for functional benefits of the LSO balance sense in birds. The findings support notions of control modularity in birds, with preferential vestibular sense for head stability and gaze, and LSO for body balance control,respectively. The findings also suggest advantages for distributed and active sensing for agile locomotion in compliant bipedal robots

Identifying Plant and Feedback in Human Posture Control

Human upright bipedal stance is a classic example of a control system consisting of a plant (i.e., the physical body and its actuators) and feedback (i.e., neural control) operating continuously in a closed loop. Determining the mechanistic basis of behavior in a closed loop control system is problematic because experimental manipulations or deficits due to trauma/injury influence all parts of the loop. Moreover, experimental techniques to open the loop (e.g., isolate the plant) are not viable because bipedal upright stance is not possible without feedback. The goal of the proposed study is to use a technique called closed loop system identification (CLSI) to investigate properties of the plant and feedback separately. Human upright stance has typically been approximated as a single-joint inverted pendulum, simplifying not only the control of a multi-linked body but also how sensory information is processed relative to body dynamics. However, a recent study showed that a single-joint approximation is inadequate. Trunk and leg segments are in-phase at frequencies below 1 Hz of body sway and simultaneously anti-phase at frequencies above 1 Hz during quiet stance. My dissertation studies have investigated the coordination between the leg and trunk segments and how sensory information is processed relative to that coordination. For example, additional sensory information provided through visual or light touch information led to a change of the in-phase pattern but not the anti-phase pattern, indicating that the anti-phase pattern may not be neurally controlled, but more a function of biomechanical properties of a two-segment body. In a subsequent study, I probed whether an internal model of the body processes visual information relative to a single or double-linked body. The results suggested a simple control strategy that processes sensory information relative to a single-joint internal model providing further evidence that the anti-phase pattern is biomechanically driven. These studies suggest potential mechanisms but cannot rule out alternative hypotheses because the source of behavioral changes can be attributed to properties of the plant and/or feedback. Here I adopt the CLSI approach using perturbations to probe separate processes within the postural control loop. Mechanical perturbations introduce sway as an input to the feedback, which in turn generates muscle activity as an output. Visual perturbations elicit muscle activity (a motor command) as an input to the plant, which then triggers body sway as an output. Mappings of muscle activity to body sway and body sway to muscle activity are used to identify properties of the plant and feedback, respectively. The results suggest that feedback compensates for the low-pass properties of the plant, except at higher frequencies. An optimal control model minimizing the amount of muscle activation suggests that the mechanism underlying this lack of compensation may be due to an uncompensated time delay. These techniques have the potential for more precise identification of the source of deficits in the postural control loop, leading to improved rehabilitation techniques and treatment of balance deficits, which currently contributes to 40% of nursing home admissions and costs the US health care system over $20B per year

Computational and Robotic Models of Human Postural Control

Currently, no bipedal robot exhibits fully human-like characteristics in terms of its postural control and movement. Current biped robots move more slowly than humans and are much less stable. Humans utilize a variety of sensory systems to maintain balance, primary among them being the visual, vestibular and proprioceptive systems. A key finding of human postural control experiments has been that the integration of sensory information appears to be dynamically regulated to adapt to changing environmental conditions and the available sensory information, a process referred to as "sensory re-weighting." In contrast, in robotics, the emphasis has been on controlling the location of the center of pressure based on proprioception, with little use of vestibular signals (inertial sensing) and no use of vision. Joint-level PD control with only proprioceptive feedback forms the core of robot standing balance control. More advanced schemes have been proposed but not yet implemented. The multiple sensory sources used by humans to maintain balance allow for more complex sensorimotor strategies not seen in biped robots, and arguably contribute to robust human balance function across a variety of environments and perturbations. Our goal is to replicate this robust human balance behavior in robots.In this work, we review results exploring sensory re-weighting in humans, through a series of experimental protocols, and describe implementations of sensory re-weighting in simulation and on a robot

Effect of Aging on Human Postural Control and the Interaction with Attention

The ability to stand upright and walk is generally taken for granted, yet control of balance utilizes many processes involving the neuromuscular and sensory systems. As we age, balance function begins to decline and can become problematic for many older adults. In particular, adults 65 years of age and older exhibit a higher incidence of falls than younger adults, and falls are a leading cause of injury in older adults, contributing to significant medical costs. Without better understanding of the impact of aging on balance and means to ameliorate those effects, this problem is expected to grow as life expectancy continues to increase.In addition to sensori-motor declines with age that impact balance, another factor known to affect balance, particularly in older adults, is attention, meaning the amount of cognitive resources utilized for a particular task. When two or more tasks vie for cognitive resources, performance in one or more tasks can be compromised (a common example today is driving while talking on a cell phone). Attention has been observed to be a critical factor in many falls reported by older adults. However, it is still not fully understood how aging and attentional demand affect balance and how they interact with each other.In this dissertation, we conducted dual-task experiments and model-based analyses to study upright standing and the interaction of the effects of age and attention on postural control. The effect of age was investigated by testing two age groups (young and older adults) with no evident balance and cognitive impairment and by comparing results of the two groups. The effect of attention and its interaction with age was studied by comparing body sway in the two age groups in response to a moving platform, while either concurrently performing a cognitive task (dual-task) or not (single-task). Our findings highlight postural control differences between young and older adults, as quantified by experimental measures of body motion as well as by model parameter values, such as stiffness, damping and processing delay

A Nonlinear Dynamic Approach for Evaluating Postural Control

Recent research suggests that traditional biomechanical models of postural stability do not fully characterise the nonlinear properties of postural control. In sports medicine, this limitation is manifest in the postural steadiness assessment approach, which may not be sufficient for detecting the presence of subtle physiological change after injury. The limitation is especially relevant given that return-to-play decisions are being made based on assessment results. This update first reviews the theoretical foundation and limitations of the traditional postural stability paradigm. It then offers, using the clinical example of athletes recovering from cerebral concussion, an alternative theoretical proposition for measuring changes in postural control by applying a nonlinear dynamic measure known as ‘approximate entropy’. Approximate entropy shows promise as a valuable means of detecting previously unrecognised, subtle physiological changes after concussion. It is recommended as an important supplemental assessment tool for determining an athlete’s readiness to resume competitive activity

The development of a novel pitch-side concussion balance assessment: a comparison between a virtual reality based balance tool and the modified balance error scoring system

Background: Balance deficits are a key measurable marker of concussion injuries. An objective pitch-side concussion balance assessment needs to replace current subjective, insensitive, unportable tests. A novel pitch-side dual-task VR test has been developed to evoke perturbations, and measure COP path length changes, via a WBB. Aims: To establish whether a VR WBB system is effectively able to assess postural stability, by evoking perturbations, and to measure subsequent changes in COP path length. To establish whether mBESS error scores, or objective mBESS COP path lengths correlate with changes in COP path length post-perturbation. Methods: 14 female University of Birmingham hockey players aged 18-21 performed both the mBESS and the VR WBB assessment at the pitch-side. Results: The mean COP path length post-perturbation was significantly greater than pre- perturbation, as the tilt induced a compensatory sway response. SL error scores significantly correlated with SL COP path length, and COP path length percentage change from pre to post-perturbation. Conclusion: The dual-task VR WBB system effectively assesses postural stability by measuring subsequent changes in COP path length. The objective nature and plethora of information provided by the VR WBB system, heightens its appeal over the mBESS, as an assessment of postural stability

Biomechanical and psychophysical underpinnings of balance dysfunction in individuals with traumatic brain injury

Falls are a major burden on healthcare infrastructure, especially in older adults and even more so in older individuals that are living in institutions. According to data from the Centers for Disease Control and Prevention (CDC), from 2010 to 2020, unintentional falls were the leading cause of nonfatal emergency department visits for all age groups except among individuals from 15-24 years of age, where unintentional falls ranked a very close second to being unintentionally struck by or against. Among older individuals living in the community, approximately 30-35% fall at least once in a given year, and around three times as many falls over the same time period among adults living in institutions. Sustaining a fall can double an older individual\u27s chances of sustaining a subsequent fall, and about one in five falls results in some kind of serious injury, such as a broken bone or head injury, with falls being the leading cause of traumatic brain injury (TBI). Based on the CDC\u27s 2014 data set, about 2 8 million people sustain a TBI in the United States every year, including over 837,000 children, contributing to over 56,000 deaths with over 2,500 of them being children. During this period, falls were the leading cause of TBI, accounting for approximately 48% of all TBI related emergency room (ER) visits, with older adults 65 and older and children ages 17 and below being most likely to suffer a TBI as a result of a fall, with falls being the cause in 49% and 81% of cases, respectively. Depending on the exact nature and severity of a particular TBI, the resulting impairments can be both acute and long lasting, range from mild to debilitating, and can include sensory and perceptual deficiencies, reduced or altered cognitive function, and various types of motor deficits including paralysis, spasticity, and weakness. Balance dysfunction is a significant disabling factor post TBI, as it can increase the risk of falls which can in turn lead to subsequent brain injuries. Postural control, the process of maintaining one\u27s balance, is an essential ability that plays a crucial role in many everyday activities and is accomplished through the integration and coordination of visual, vestibular, proprioceptive inputs, and motor control to create a feedback system that maintains the body\u27s center of mass (COM) above its\u27 base of support. A TBI can affect postural control in multiple different ways, including direct damage to sensory processing regions responsible for handling essential balance related internal and external stimuli, or damaging motor control and planning areas that are responsible for generating appropriate muscular response to the above-mentioned stimuli. While damage to either of these cortical regions can lead to balance dysfunction, the specific regions effected, and the extent of the damage can necessitate different rehabilitation approaches when attempting to restore lost functionality. A loss or reduction of a particular sensory input or the ability to process that sensory input may require training an individual to shift their focus to the remaining sensory inputs, while the loss or reduction of motor control to a limb or joint would require the training of an individual to develop and utilize compensatory motor strategies for other limbs and joints to safely maintain their balance. When standing quietly (or moving in an unperturbed fashion), an individual\u27s postural control system only needs to account for internal changes resulting from the body\u27s own movements, and the individual is more easily able to control the timing and intensity of required postural adjustments by limiting their movements to within the range of what their sensory and motor control systems can handle, i.e., if they have reduced muscle strength, perception, or reaction time, they can limit the speed or extent of their movements to make it easier for them to maintain their balance, rather than moving rapidly. However, when an individual\u27s base of support is perturbed unexpectedly, their body needs to be able to determine and perform the necessary postural adjustments based on the perturbation that occurred, which can pose an increasing challenge depending upon the extent of injuries. Since they are less in control of the dynamics of the situation, their ability to perceive the perturbation is critical to their ability to adapt to it properly. The slower or less accurately they perceive the disturbance the less likely they are to be able to make the minimum postural adjustments needed to maintain stability, and the more likely they become to require exaggerated or significant postural adjustments that may be beyond their ability to safely perform. Since falls are a major concerned, as mentioned above, and falls occur when the central nervous system fails to identify an impending loss of balance and or make the necessary adjustments to an individual\u27s COM or base of support in time, an individual\u27s ability to accurately perceive their environment during dynamic situations is critical to avoiding falls. Existing research that specifically quantifies impaired sensory integration in an objective manner post TBI is limited, and no prior work has investigated sensory acuity in individuals post TBI. While there are many well establish methods for testing postural function that have been validated in both healthy individuals and individuals with a variety of diagnoses, the evidence of these measures in association with balance interventions in individuals post TBI is sparse. To address this deficiency, this preliminary study employed a novel psychophysical approach to assess the perception of perturbation threshold of individuals post TBI and evaluate their kinematic and center of pressure (COP) behaviors during perturbed standing to attempt to detect differences in how individuals post TBI employ known balance control strategies

Mechanisms underlying muscle recruitment in response to postural perturbations

The neural and sensory mechanisms underlying appropriate muscle recruitment in response to balance challenges remains elusive. We asked whether the decerebrate cat preparation might be employed for further investigation of postural mechanisms. First, we evaluated the muscular activation patterns and three-dimensional whole limb forces generated by a modified premammillary decerebrated cat. We hypothesized that directionally appropriate muscle activation does not require the cerebral cortices. Furthermore, we hypothesized that the muscle responses would generate functionally appropriate and constrained force responses similar to those reported in the intact animal. Data confirmed both of our hypotheses and suggested important roles for the brainstem and spinal cord in mediating directionally appropriate muscular activation. Second, we investigated how individual muscle activation is translated to functional ground reaction forces. We hypothesized that muscles are selectively activated based upon their potential counteractive endpoint force. Data demonstrated that the endpoint force generated by each muscle through stimulation was directed oppositely to the principal direction of each muscle's EMG tuning curve. Further, muscles that have variable tuning curves were found to have variable endpoint forces in the XY plane. We further hypothesized that the biomechanical constraints of individual muscle actions generate the constrained ground reaction forces created in response to support surface perturbations. We found that there was a lack of muscles with strong medial-lateral actions in the XY plane. This was further exaggerated at long stance conditions, which corresponds to the increased force constraint present in the intact animal under the same conditions. Third, we investigated how loss of cutaneous feedback from the footpads affects the muscle recruitment in response to support surface perturbations. We utilized our decerebrate cat model as it allows 1) isolation of the proprioceptive system (cutaneous and muscle receptor) and 2) observation of the cutaneous loss before significant compensation by the animal. We hypothesized that muscle spindles drive directionally sensitive muscle activation during postural disturbances. Therefore, we expected that loss of cutaneous feedback from the foot soles would not alter the directional properties of muscle activation. While background activity was significantly diminished, the directionally sensitive muscular activation remained intact. Due to fixation of the head, the decerebrate cat additionally does not have access to vestibular or visual inputs. Therefore, this result strongly implicates muscle receptors as the primary source of directional feedback. Finally to confirm that muscle receptors, specifically muscle spindles, are capable of generating feedback to drive the directionally tuning, we investigated the response properties of muscle spindles to horizontal support surface perturbations in the anesthetized cat. As previously stated, we hypothesized that muscle spindles provide the feedback necessary for properly directed muscular responses. We further hypothesized that muscle spindles can relay feedback about the perturbation parameters such as velocity and the initial stance condtion. Results confirmed that muscle spindle generate activation patterns remarkably similar to muscular activation patterns generated in the intact cat. This information, along the knowledge that cutaneous feedback does not substantially eliminate directional tuning, strongly suggests that muscle spindles contribute the critical directional feedback to drive muscular activation in response to support surface perturbations.Ph.D.Committee Chair: T. Richard Nichols; Committee Member: Lena Ting; Committee Member: Shawn Hochman; Committee Member: Thomas Burkholder; Committee Member: Timothy Cop

Blood alcohol concentration at 0.06 and 0.10% causes a complex multifaceted deterioration of body movement control.

Alcohol-related falls are recognized as a major contributor to the occurrence of traumatic brain injury. The control of upright standing balance is complex and composes of contributions from several partly independent mechanisms such as appropriate information from multiple sensory systems and correct feedback and feed forward movement control. Analysis of multisegmented body movement offers a rarely used option for detecting the fine motor problems associated with alcohol intoxication. The study aims were to investigate whether (1) alcohol intoxication at 0.06 and 0.10% blood alcohol concentration (BAC) affected the body movements under unperturbed and perturbed standing; and (2) alcohol affected the ability for sensorimotor adaptation. Body movements were recorded in 25 participants (13 women and 12 men, mean age 25.1 years) at five locations (ankle, knee, hip, shoulder, and head) during quiet standing and during balance perturbations from pseudorandom pulses of calf muscle vibration over 200s with eyes closed or open. Tests were performed at 0.00, 0.06, and 0.10% BAC. The study revealed several significant findings: (1) an alcohol dose-specific effect; (2) a direction-specific stability decrease from alcohol intoxication; (3) a movement pattern change related to the level of alcohol intoxication during unperturbed standing and perturbed standing; (4) a sensorimotor adaptation deterioration with increased alcohol intoxication; and (5) that vision provided a weaker contribution to postural control during alcohol intoxication. Hence, alcohol intoxication at 0.06 and 0.10% BAC causes a complex multifaceted deterioration of human postural control