Human Upright Research Articles

Editorial: Human upright walking from past to present

Editorial: Human upright walking from past to present

Impact of professional dance training on characteristics of postural sway.

The stability of human upright posture determines the range and dynamics of movements performed. Consequently, the repertoire and quality of the movements performed by a dancer are mainly determined by the efficiency of postural control. This is of particular importance in professional dance training that should focus on shaping optimal movement‑posture interaction. To get a deeper insight into this problem, the impact of the training on postural sway characteristics during quiet stance was analyzed in 16 female students in the seventh grade of a ballet school and compared with the size‑ and age‑matched group of secondary school students. Center of pressure trajectories were recorded for 25.6 s while standing quiet with eyes open (EO) and then with eyes closed (EC). The assessment of postural control was based on novel normalized sway parameters including sway vector (SV), sway anteroposterior (AP) and mediolateral (ML) directional indices (DIAP and DIML), and sway ratios (SRAP and SRML). The results document a significant contribution of vision to postural stability control in ballet students, which seems to compensate for training‑related changes in joint mobility and altered activity ranges of the legs' muscles. In the control group standing with EC, SV amplitude increased only by 18% whereas in the ballet students tested in the same conditions, the increase exceeded 72%. Under full control of standing posture (EO test), the training‑related increase of leg muscle forces allows dancers to maintain balance with lesser effort as documented here by 21% reduced SRAP. Additionally, the dancers while tested with their EC exhibited a 12% increase in the anteroposterior sway with a concomitant reduction of the mediolateral sway. The resulting changes in the postural control asymmetry were documented by both DIAP/DIML and SV azimuth. In conclusion, our novel analysis of postural sway seems a useful tool in monitoring the effects of training as well as the proper course of postural control development in children and adolescents.

Bioinformatics analysis of paravertebral muscles atrophy in adult degenerative scoliosis

Paravertebral muscles (PVM) act as one of the major dynamic factors to maintain human upright activities and play a remarkable role in maintaining the balance of the trunk. Adult degenerative scoliosis (ADS) has become one of the important causes of disability in the elderly population owing to the changes in spinal biomechanics, atrophy and degeneration of PVM, and imbalance of the spine. Previously, many studies focused on the physical evaluation of PVM degeneration. However, the molecular biological changes are still not completely known. In this study, we established a rat model of scoliosis and performed the proteomic analysis of the PVM of ADS. The results showed that the degree of atrophy, muscle fat deposition, and fibrosis of the PVM of rats positively correlated with the angle of scoliosis. The proteomic results showed that 177 differentially expressed proteins were present in the ADS group, which included 105 upregulated proteins and 72 downregulated proteins compared with the PVM in individuals without spinal deformities. Through the construction of a protein–protein interaction network, 18 core differentially expressed proteins were obtained, which included fibrinogen beta chain, apolipoprotein E, fibrinogen gamma chain, thrombospondin-1, integrin alpha-6, fibronectin-1, platelet factor 4, coagulation factor XIII A chain, ras-related protein Rap-1b, platelet endothelial cell adhesion molecule 1, complement C1q subcomponent subunit A, cathepsin G, myeloperoxidase, von Willebrand factor, integrin beta-1, integrin alpha-1, leukocyte surface antigen CD47, and complement C1q subcomponent subunit B. Further analysis of the Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) and immunofluorescence showed that the neutrophil extracellular traps (NETs) formation signaling pathway plays a major role in the pathogenesis of PVM degeneration in ADS. The results of the present study preliminarily laid the molecular biological foundation of PVM atrophy in ADS, which will provide a new therapeutic target for alleviating PVM atrophy and decreasing the occurrence of scoliosis.

Control of structural redundancy from the head to trunk in the human upright standing revealed using a data-driven approach

The human being dynamically and highly controls the head–trunk with redundant mechanical structures to maintain a stable upright standing position that is inherently unstable. The posture control strategies are also affected by the differences in the conditions of sensory inputs. However, it is unclear how the head–trunk segmental properties are altered to respond to situations that require appropriate changes in standing posture control strategies. We used a data-driven approach to conduct a multipoint measurement of head–trunk sway control in a quiet standing position with differences in the conditions of sensory inputs. Healthy young subjects with 22 accelerometers attached to their backs were evaluated for head–trunk vibration during quiet standing under two conditions: one with open eyes and one with closed eyes. The synchronization of the acceleration and the instantaneous phase was then calculated. The results showed that the synchronization of acceleration and instantaneous phase varied depending on the visual condition, and there were some continuous coherent patterns in each condition. Findings were that the structural redundancy of the head–trunk, which is multi-segmental and has a high mass ratio in the whole body, must be adjusted adaptively according to the conditions to stabilize upright standing in human-specific bipeds.

Change in task conditions leads to changes in intermittency in intermittent feedback control employed by CNS in control of human stance.

Event-driven intermittent feedback control is a form of feedback control in which the corrective control action is only initiated intermittently when the variables of interest exceed certain threshold criteria. It has been reported in the literature that the CNS uses an event-driven intermittent control strategy to stabilize the human upright posture. However, whether the threshold criteria may change under different postural task conditions is not yet well understood. We employ a numerical study with inverted pendulum models and an experimental study with 51 young healthy individuals (13 females and 38 males; age: 27.8 ± 6.5 years) with stabilogram-diffusion, temporal and spectral analysis applied to COP (Center of Pressure) trajectories measured from these experiments to examine this aspect. The present study provides compelling evidence that inducing a natural arm swing during quiet stance appears to lead to higher sensory dead zone in neuronal control reflecting higher intermittency thresholds in active feedback control and a corresponding lower sensory dependence. Beyond the obvious scientific interest in understanding this aspect of how CNS controls the standing posture, an investigation of the said control strategy may subsequently help uncover insights about how control of quiet stance degrades with age and in diseased conditions. Additionally, such an understanding will also be of interest to the humanoid robotics community as it may lead to insights leading to improving control strategies for posture control in robots.

Neuromuscular Control Modelling of Human Perturbed Posture Through Piecewise Affine Autoregressive With Exogenous Input Models.

In this study, the neuromuscular control modeling of the perturbed human upright stance is assessed through piecewise affine autoregressive with exogenous input (PWARX) models. Ten healthy subjects underwent an experimental protocol where visual deprivation and cognitive load are applied to evaluate whether PWARX can be used for modeling the role of the central nervous system (CNS) in balance maintenance in different conditions. Balance maintenance is modeled as a single-link inverted pendulum; and kinematic, dynamic, and electromyography (EMG) data are used to fit the PWARX models of the CNS activity. Models are trained on 70% and tested on the 30% of unseen data belonging to the remaining dataset. The models are able to capture which factors the CNS is subjected to, showing a fitting accuracy higher than 90% for each experimental condition. The models present a switch between two different control dynamics, coherent with the physiological response to a sudden balance perturbation and mirrored by the data-driven lag selection for data time series. The outcomes of this study indicate that hybrid postural control policies, yet investigated for unperturbed stance, could be an appropriate motor control paradigm when balance maintenance undergoes external disruption.

Long term correlation and inhomogeneity of the inverted pendulum sway time-series under the intermittent control paradigm

In this study the extended detrended fluctuation analysis (EDFA) was applied to the sway data generated from an inverted pendulum (IP) model, intermittently controlled at the ankle. The time series taken into account was the center of pressure (COP), since it represents the widest used time series in posturography, and it constitutes a natural link between model and data-based analysis approaches for studying the dynamics of the human balance maintenance. COP time-series were obtained by varying the intermittent control parameters (ICP) in a uniform distribution range that ensures IP stability to quantify changes in the long-term correlation and inhomogeneity of the time-series. Globally, EDFA coefficients (α and β) showed to be sensitive to the variations of derivative control gain (D), whereas for proportional gain (P) and ρ parameters no significant trends were observed. However, relations between EDFA coefficients and ρ arose whether derivative gain is examined within a low and high regions of value. For low D gains, both α and β showed a significant correlation with ρ, which disappears when higher D values were considered. Thus EDFA coefficients can provide useful insights about the long-term correlation and local characteristics of COP timeseries, which are strictly related to the control policy adopted for maintaining balance. This supports the validity of the intermittent motor control paradigm for the human upright stance and suggests the use of EDFA in real posturography applications, in order to extract meaningful information regarding the properties of COP timeseries for different groups of patients.

Identifying human postural dynamics and control from unperturbed balance

BackgroundUpright standing requires control of an inherently unstable multi-joint human body within a small base of support, despite biological motor and / or sensory noise which challenge balance. Without applying perturbations, system identification methods have been regarded as inadequate, because the relevant internal biological noise processes are not accessible to direct measurement. As a result, unperturbed balance studies have been limited to investigation of behavioral patterns rather than possible underlying control strategies.MethodsIn this paper, we present a mathemathically rigorous system identification method that is applicable to study the dynamics and control of unperturbed balance. The method is derived from autocorrelation matrices with non-zero time lags and identifies the system matrix of a discrete-time dynamic system in the presence of unknown noise processes, without requiring any information about the strength of the noise.ResultsUnlike reasonable ‘least-squares’ approaches, the performance of the new method is consistent across a range of different combinations of internal and measurement noise strengths, even when measurement noise is substantial. We present a numerical example of a model that simulates human upright balancing and show that its dynamics can be identified accurately. With a biomechanically reasonable choice of state and input variables, a state feedback controller can also be identified.ConclusionsThis study provides a new method to correctly identify the dynamics of human standing without the need for known external perturbations. The method was numerically validated using simulation that included realistic features of human balance. This method avoids potential issues of adaptation or possible reflex responses evoked by external perturbations, and does not require expensive in-lab, high-precision measurement equipment. It may eventually enable diagnosis and treatment of individuals with impaired balance, and the development of safe and effective assistive and / or rehabilitative technologies.

Validation of an ankle-hip model of balance on a balance board via kinematic frequency-content

BackgroundThe ankle-strategy model, where the human body is modeled as a single inverted pendulum hinged at the ankle, has been used for decades to study the dynamics and the stability of the human upright posture (UP). However, the contribution of the hip joints is critical whenever postural disturbances are considered. To account for hip contribution, a double inverted pendulum (DIP) model rotating about the ankle and hip joints has been recently proposed in our previous work but experimental validation efforts are scarce. MethodIn the present study, it is investigated whether the DIP model is able to reproduce the experimentally observed frequency spectrum of the ankle and hip joint kinematic for young and elderly subjects balancing on a compliant surface. The DIP model based and experimental kinematics are compared via Fourier analysis to obtain their corresponding amplitude spectrum density (ASD) functions. Quantitative comparisons of the ASD functions are accomplished through Bland-Altman (B&A) analysis, and Pearson correlation coefficient (PCC). ResultsThe DIP model can reproduce part of the experimental frequency spectrum of the ankle and hip joint angle position and velocity, especially for frequencies larger than 0.35 Hz. Moreover, the model captures the decaying behavior of the experimental ASD functions as frequency increases. With respect to joint angle velocities, the highest PCC between model-based and experimental ASD functions is found for the hip joint of elderly subjects. The B&A analysis shows that the zero-difference between model-based and experimental ASD functions lies between the 95 % confidence interval, especially for the joint angle position results. These suggest that the DIP model reproduces part of the experimentally observed frequency spectrum, which validates the model to study the dynamics and stability of the human upright posture.

Contributions of Vision in Human Postural Control: A Virtual Reality-based Study.

During human standing, it has been previously observed that information about the position and frequency of visual surround motion improves balance by reducing sway responses to external disturbances. However, experimental limitations only allowed for independent investigation of such parameters while being incapable of providing a fully immersive experience of a real environment. The aim of this study is to investigate the effect of visual information on dynamic body sway in the human upright stance by presenting perturbations through a virtual reality (VR) system. Moreover, we designed a new perturbation signal based on trapezoidal velocity (TrapV) pulses enabling us to simultaneously examine the effects of amplitude and velocity on balance control. The experiments included four different peak-to-peak amplitudes (1-10 degrees), and three velocities (2-10 degree/sec). The body angle, ankle torques and shank angles were measured and analyzed in response to each perturbation. The results reveal that stimuli with higher amplitudes evoked larger responses, while they were initially increased and reached a peak, then decreased by increasing the motion velocity of visual surround.

Cosine tuning determines plantarflexors' activities during human upright standing and is affected by incomplete spinal cord injury.

Plantarflexors such as the soleus (SOL) and medial gastrocnemius (MG) play key roles in controlling bipedal stance; however, how the central nervous system controls the activation levels of these plantarflexors is not well understood. Here we investigated how the central nervous system controls the plantarflexors' activation level during quiet standing in a cosine tuning manner where the maximal activation is achieved in a preferred direction (PD). Furthermore, we investigated how spinal cord injury affects these plantarflexors' activations. Thirteen healthy adults (AB) and thirteen individuals with chronic, incomplete spinal cord injury (iSCI) performed quiet standing trials. Their body kinematics and kinetics as well as electromyography signals from the MG and SOL were recorded. In the AB group, we found that the plantarflexors followed the cosine tuning manner during quiet standing. That is, MG was most active when the ratio of plantarflexion torque to knee extension torque was ~2:-3, whereas SOL was most active when the ratio was ~2:1. This suggests that the SOL muscle, despite being a monoarticular muscle, is sensitive to both ankle plantarflexion and knee extension during quiet standing. The difference in the PDs accounts for the phasic activity of MG and for the tonic activity of SOL. Unlike the AB group, the MG's activity was similar to the SOL's activity in the iSCI group, and the SOL PDs were similar to those in the AB group. This result suggests that chronic iSCI affects the control strategy, i.e., cosine tuning, for MG, which may affect standing balance in individuals with iSCI.NEW & NOTEWORTHY Soleus muscle shows a tonic activity whereas medial gastrocnemius muscle shows a phasic activity during quiet standing. Cosine tuning and their preferred direction account for the different muscle activation patterns between these two muscles. In individuals with chronic incomplete spinal cord injury, the preferred direction of gastrocnemius medial head is affected, which may result in their deteriorated standing balance.

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.

Human postural ankle torque control model during standing posture with a series elastic muscle-tendon actuator

The main motivation behind the creation of a compliant actuation system is to provide safety, capability of storing energy, and improved performance levels in dynamic behavior of the mechanical human ankle. In this paper, a torsional flat spring (TFS) is proposed to provide high compliance and deformation values for human ankle series elastic muscle-tendon actuator system (HA-SEMTAS). The proposed torsional flat spring uses two torsional flat spiral spring in opposite directions, as the torsional flat spiral spring generates torque only in one direction. First, we present the characteristics of the TFS for an ankle joint, and the design and modeling of the HA-SEMTAS are developed. Modelling of the HA-SEMTAS simulates the responses of fluctuations in the center of mass and center of pressure and offers the possibility of measuring sensor activation, decomposition of reactive torque and participation of each set of muscle groups to balance posture. In order to control ankle angles and torque of Human system, torque controllers (PID and PD-feedforward) are generated for (HA-SEMTAS). The controller inputs are torsional flat spring position errors and error rates. Feedforward and integral action were applied to reduce the steady-state error of the system. It is seen that PID + ff of the system developed here is robust to step and ramp-type disturbances. Real-time controllers are embedded in GoogolTech GT-800 Industrial PC. Experimental results confirm that the step tracking of human upright posture behavior is satisfactory and overall system stability has improved using the proposed controller and compliant actuation system. It is also shown that PD + feedforward yields better performance than PID controller.

Free Energy Principle in Human Postural Control System: Skin Stretch Feedback Reduces the Entropy

Human upright standing involves an integration of multiple sensory inputs such as vision, vestibular and somatosensory systems. It has been known that sensory deficits worsen the standing balance. However, how the modulation of sensory information contributes to postural stabilization still remains an open question for researchers. The purpose of this work was to formulate the human standing postural control system in the framework of the free-energy principle, and to investigate the efficacy of the skin stretch feedback in enhancing the human standing balance. Previously, we have shown that sensory augmentation by skin stretch feedback at the fingertip could modulate the standing balance of the people with simulated sensory deficits. In this study, subjects underwent ten 30-second trials of quiet standing balance with and without skin stretch feedback. Visual and vestibular sensory deficits were simulated by having each subject close their eyes and tilt their head back. We found that sensory augmentation by velocity-based skin stretch feedback at the fingertip reduced the entropy of the standing postural sway of the people with simulated sensory deficits. This result aligns with the framework of the free energy principle which states that a self-organizing biological system at its equilibrium state tries to minimize its free energy either by updating the internal state or by correcting body movement with appropriate actions. The velocity-based skin stretch feedback at the fingertip may increase the signal-to-noise ratio of the sensory signals, which in turn enhances the accuracy of the internal states in the central nervous system. With more accurate internal states, the human postural control system can further adjust the standing posture to minimize the entropy, and thus the free energy.

Experimental Methods to Study Human Postural Control.

Many components of the nervous and musculoskeletal systems act in concert to achieve the stable, upright human posture. Controlled experiments accompanied by appropriate mathematical methods are needed to understand the role of the different sub-systems involved in human postural control. This article describes a protocol for performing perturbed standing experiments, acquiring experimental data, and carrying out the subsequent mathematical analysis, with the aim of understanding the role of musculoskeletal system and central control in human upright posture. The results generated by these methods are important, because they provide insight into the healthy balance control, form the basis for understanding the etiology of impaired balance in patients and the elderly, and aid in the design of interventions to improve postural control and stability. These methods can be used to study the role of somatosensory system, intrinsic stiffness of ankle joint, and visual system in postural control, and may also be extended to investigate the role of vestibular system. The methods are to be used in the case of an ankle strategy, where the body moves primarily about the ankle joint and is considered a single-link inverted pendulum.

Numerical Nonlinear Analysis for Dynamic Stability of an Ankle-Hip Model of Balance on a Balance Board

Abstract Study of human upright posture (UP) stability is of great relevance to fall prevention and rehabilitation, especially for those with balance deficits for whom a balance board (BB) is a widely used mechanism to improve balance. The stability of the human-BB system has been widely investigated from a dynamical system point of view. However, most studies assume small disturbances, which allow to linearize the nonlinear human-BB dynamical system, neglecting the effect of the nonlinear terms on the stability. Such assumption has been useful to simplify the system and use bifurcation analyses to determine local dynamic stability properties. However, dynamic stability analysis results through such linearization of the system have not been verified. Moreover, bifurcation analyses cannot provide insight on dynamical behaviors for different points within the stable and unstable regions. In this study, we numerically solve the nonlinear delay differential equation that describes the human-BB dynamics for a range of selected parameters (proprioceptive feedback and time-delays). The resulting solutions in time domain are used to verify the stability properties given by the bifurcation analyses and to compare different dynamical behaviors within the regions. Results show that the selected bifurcation parameters have significant impacts not only on UP stability but also on the amplitude, frequency, and increasing or decaying rate of the resulting trajectory solutions.

Special Issue: Nonlinear and Computational Dynamics in Biomedical Applications

Special Issue: Nonlinear and Computational Dynamics in Biomedical Applications

Toe flexor strength is not related to postural stability during static upright standing in healthy young individuals

BackgroundThe human foot has adapted specifically to support body weight when standing upright. At the base of the postural control system, the unique arch structure of the foot still has an uncertain role in human upright standing. Because the toe flexor muscles help to support the foot arches, they might be an important contributor to postural stability. However, no research has identified the influence of the toe flexor strength or the foot arch height on postural stability in static upright standing.Research question: The aim of this study was to examine whether the toe flexor strength and the foot arch height were related to postural stability in static standing in healthy young individuals. MethodsFifty healthy young individuals were recruited into this cross-sectional study. Toe flexor strength was measured using a toe grip dynamometer, and it was normalised by body mass (rTFS). Foot arch height was assessed as the distance between the navicular tuberosity of the foot and the floor, and it was normalised by height (rFAH). Postural stability was evaluated using the path of the centre of pressure (COP) during double-leg standing with eyes open and single-leg standing with eyes open. ResultsrTFS and rFAH were 2.6 ± 0.8 N/kg and 2.8 ± 0.4%, respectively, and they were not significantly correlated (r = 0.094), indicating that they were independent variables. The results of Pearson’s correlation analysis revealed that any body size related variables (height, body mass, BMI) were not significantly correlated with COP variables under either double-leg or single-leg standing, rTFS was not significantly correlated with COP variables under either double-leg or single-leg standing and rFAH was not significantly correlated with COP variables under double-leg standing. SignificanceToe flexor strength has no significant role in maintaining postural stability during static upright standing.

Effect of sensory-motor latencies and active muscular stiffness on stability for an ankle-hip model of balance on a balance board

To achieve human upright posture (UP) and avoid falls, the central nervous system processes visual, vestibular, and proprioceptive information to activate the appropriate muscles to accelerate or decelerate the body’s center of mass. In this process, sensory-motor (SM) latencies and muscular deficits, even in healthy older adults, may cause falls. This condition is worse for people with chronic neuromuscular deficits (stroke survivors, patients with multiple sclerosis or Parkinson’s disease). One therapeutic approach is to recover or improve quiet UP by utilizing a balance board (BB) (a rotating surface with a tunable stiffness and time delay), where a patient attempts to maintain UP while task difficulty is manipulated. While BBs are commonly used, it is unclear how UP is maintained or how changes in system parameters such as SM latencies and BB time delay affect UP stability. To understand these questions, it is important that mathematical models be developed with enough degrees-of-freedom to capture the many responses evoked during the maintenance of UP on a BB. This paper presents an ankle-hip model of balance on a BB, which is used to study the combined effect of SM latencies and active muscular stiffness of the ankle and hip joints, and the BB stiffness and time delay on UP stability. The analysis predicts that people with proprioceptive, visual, vestibular loss, or increased SM latencies may show either leaning postures or larger body-sway. The results show that the BB time delay and the visual and vestibular feedback have the largest impact on UP stability.

Upright posture and the meaning of meronymy: A synthesis of metaphoric and analytic accounts

AbstractCross-linguistic strategies for mapping lexical and spatial relations from body partonym systems to external object meronymies (as in English ‘table leg,’ ‘mountain face’) have attracted substantial research and debate over the past three decades. Due to the systematic mappings, lexical productivity, and geometric complexities of body-based meronymies found in many Mesoamerican languages, the region has become focal for these discussions, prominently including contrastive accounts of the phenomenon in Zapotec and Tzeltal, leading researchers to question whether such systems should be explained as global metaphorical mappings from bodily source to target holonym or as vector mappings of shape and axis generated “algorithmically.” I propose a synthesis of these accounts in this paper by drawing on the species-specific cognitive affordances of human upright posture grounded in the reorganization of the anatomical planes, with a special emphasis on antisymmetrical relations that emerge between arm-leg and face-groin antinomies cross-culturally. Whereas Levinson argues that the internal geometry of objects “stripped of their bodily associations” (1994: 821) is sufficient to account for Tzeltal meronymy, making metaphorical explanations entirely unnecessary, I propose a more powerful, elegant explanation of Tzeltal meronymic mapping that affirms both the geometric-analytic and the global-metaphorical nature of Tzeltal meaning construal. I do this by demonstrating that the “algorithm” in question arises from the phenomenology of movement and correlative body memories — an experiential ground that generates a culturally selected pair of inverse contrastive paradigm sets with marked and unmarked membership emerging antithetically relative to the transverse anatomical plane. These relations are then selected diagrammatically for the classification of object orientations according to systematic geometric iconicities. Results not only serve to clarify the case in question but also point to the relatively untapped potential that upright posture holds for theorizing the emergence of human cognition, highlighting in the process the nature, origins and theoretical validity of markedness and double scope conceptual integration.