Human Upright Posture Research Articles

Frequency-domain patterns in foot-force line-of-action: an emergent property of standing balance control.

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 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.

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.

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.

Thinking about Animals in Thirteenth-Century Paris: Theologians on the Boundary between Humans and Animals

Thinking about Animals in Thirteenth-Century Paris: Theologians on the Boundary between Humans and Animals

Thinking about Animals in Thirteenth-Century Paris: Theologians on the Boundary between Humans and Animals

Thinking about Animals in Thirteenth-Century Paris: Theologians on the Boundary between Humans and Animals

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.

Covariation of fetal skull and maternal pelvis during the perinatal period in rhesus macaques and evolution of childbirth in primates

A large brain combined with an upright posture in humans has resulted in a high cephalopelvic proportion and frequently obstructed labor. Fischer and Mitteroecker [B. Fischer, P. Mitteroecker, Proc. Natl. Acad. Sci. U.S.A. 112, 5655-5660 (2015)] proposed that the morphological covariations between the skull and pelvis could have evolved to ameliorate obstructed labor in humans. The availability of quantitative data of such covariation, especially of the fetal skull and maternal pelvis, however, is still scarce. Here, we present direct evidence of morphological covariations between the skull and pelvis using actual mother-fetus dyads during the perinatal period of Macaca mulatta, a species that exhibits cephalopelvic proportions comparable to modern humans. We analyzed the covariation of the three-dimensional morphology of the fetal skull and maternal pelvis using computed tomography-based models. The covariation was mostly observed at the pelvic locations related to the birth canal, and the forms of the birth canal and fetal skull covary in such a way that reduces obstetric difficulties. Therefore, cephalopelvic covariation could have evolved not only in humans, but also in other primate taxa in parallel, or it could have evolved already in the early catarrhines.

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.

Ancient Views on the Upright Human Posture and the New Testament Concept of Resurrection

In ancient anthropological views, the ability to stand upright was often regarded as a distinctively human ability given by the gods. Sometimes it was connected not only to the uprightness of the human body but also to that of the human soul, and indicated the human ability to contemplate one’s immortality. In the ancient Graeco-Roman literature this feature of the human soul was also connected with the restoration of physical life as a process of waking up from the sleep of death and was often represented by the verbs ἀνίστημι and ἐγέιρω . In biblical and cognate texts, these verbs represent the concept of resurrection. The present article discusses the link between the idea of physical uprightness and that of resurrection in ancient sources, especially the New Testament. In doing so, it applies some ideas of cognitive linguistics that help us to establish this link by means of the image-scheme UP-DOWN, which is used for creating the metaphor of resurrection as waking up and getting up from the sleep of death.

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.

Special Issue: Nonlinear and Computational Dynamics in Biomedical Applications

Special Issue: Nonlinear and Computational Dynamics in Biomedical Applications

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.

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.

Effect of axial vertical vibration on degeneration of lumbar intervertebral discs in modified bipedal rats: An in-vivo study.

To assess the effects of axial vibrations on gene expression and lumbar intervertebral disc degeneration invivo. A modified bipedal rat model was established using a brachial plexus rhizotomy approach to imitate human upright posture. The experimental animals were randomly divided into three groups: control, vertical vibration, and whole-body vibration. Gene expression in degeneration of the intervertebral discs was assessed by reverse transcription-quantitative polymerase chain reaction. The expression of aggrecan, Col1α1, Col2α1, and decorin were shown to be up-regulated in 14-week-old rats in the vertical vibration and whole-body vibration groups, whereas biglycan and versican expression was down-regulated in 14-week-old rats of the two experimental groups. Furthermore, biglycan and versican expression levels were shown to be lower in the whole-body vibration group than in the vertical vibration group (P<0.05). This in-vivo study demonstrated that vibrations can influence the expression of anabolic genes. Furthermore, whole-body vibrations seem to have a greater effect in this regard than vertical vibrations. A new method is expected to relieve the low back pain of the patients through our research.

A novel approach to study human posture control: “Principal movements” obtained from a principal component analysis of kinematic marker data

Human upright posture is maintained by postural movements, which can be quantified by “principal movements” (PMs) obtained through a principal component analysis (PCA) of kinematic marker data. The current study expands the concept of “principal movements” in analogy to Newton׳s mechanics by defining “principal position” (PP), “principal velocity” (PV), and “principal acceleration” (PA) and demonstrates that a linear combination of PPs and PAs determines the center of pressure (COP) variance in upright standing. Twenty-one subjects equipped with 27-markers distributed over all body segments stood on a force plate while their postural movements were recorded using a standard motion tracking system. A PCA calculated on normalized and weighted posture vectors yielded the PPs and their time derivatives, the PVs and PAs. COP variance explained by the PPs and PAs was obtained through a regression analysis. The first 15 PMs quantified 99.3% of the postural variance and explained 99.60%±0.22% (mean±SD) of the anterior–posterior and 98.82±0.74% of the lateral COP variance in the 21 subjects. Calculation of the PMs thus provides a data-driven definition of variables that simultaneously quantify the state of the postural system (PPs and PVs) and the activity of the neuro-muscular controller (PAs). Since the definition of PPs and PAs is consistent with Newton׳s mechanics, these variables facilitate studying how mechanical variables, such as the COP motion, are governed by the postural control system.

Modeling of processes for vertical posture support

Study the mechanisms for regulation of human posture is one of the urgent tasks of physiology of movement. This is due to significance of the results of research for development of methods for diagnosis and treatment of various disorders of the central nervous system (CNS) and the musculoskeletal system (MSS) as well as to theoreti cal elaborations concerning the general principles of organization of control of person’s movement. Objective: To conceptually deter mine the role of statogram as a result of interaction of the CNS and the execution system (ES) in the evaluation of the MSS function. Results: Based on the analysis of existing physiological models for maintaining of equilibrium there was developed a conceptual model of interaction between the CNS and the MSS for the person in maintaining upright posture. The system approach to the dif ferential assessment of the MSS on the basis of statography was proposed. The research process can be divided into three stages: I — definition of anthropometric parameters of the patient (weight, height, length of segments, bearing surface of feet); II — registra tion of movement of the common center of mass in the patient with natural standing, and the correlation of these parameters with the anthropometric data; III — study with using disturbances that meet location of the lesion, its nature and extent. The conclusion about the state of the MSS is formed on the basis of a comparative analysis of the results obtained in the second and third stages of the study. Conclusion: There were developed a conceptual model of the process of maintaining upright human posture as a system of automated management and justified the general direction of statographic studies when assessing the MSS function. Applying of methods of disturbances at statographic studies that allow to activate the affected elements of the MSS or block compensatory mechanisms was substantiated.

Neuro-humoral control during orthostasis in health and disease.

Different from many other species, human beings spend nearly two-thirds of the day in an upright posture. Due to gravity, about 700 to 900 ml of blood shift into the lower body during quiet standing. As a result, central blood volume decreases and this leads to a series of neural-humoral adjustments that aim to maintain the mean arterial pressure during orthostasis. Both the sympathetic nervous system and the renin-angiotensin-aldosterone system contribute importantly to the arterial pressure maintenance in upright humans. There is a growing interest in the blood pressure regulation during orthostasis in healthy individuals and patient populations. This e-book provides some updates on sympathetic neural control and renal-adrenal function in health and disease. Sympathetic neural activity regulates blood pressure mainly through the baroreflex-mediated vasoconstriction during orthostasis. The baroreflex is a closed-loop feedback system, but requires open-loop experiments to identify its characteristics, which is impossible in human research. Kamiya et al. demonstrated that system identification based analysis and numerical simulation using baroreflex subsystem characteristics identified in animal models may contribute to our understanding of human sympathetic physiology under orthostasis (Kamiya et al., 2014). Neural and renal-adrenal control of blood pressure can be affected by not only age but also race and physical fitness. For example, it was found that young normotensive Caucasian women may rely on sympathetic neural activity more so than African-American women who had a tendency to rely on the renal-adrenal system to regulate blood pressure during orthostatic challenges (Jarvis et al., 2014). Conversely, Sugawara et al. reported that endurance-trained men had increased orthostatic tachycardia and enhanced cardiovagal baroreflex sensitivity, which may favorably mitigate accumulated risks for post-exercise orthostatic intolerance (Sugawara et al., 2014). Failure of the sympathetic nervous system during orthostasis may result in syncope (Iwase et al., 2014). However, orthostatic tolerance does not seem to be related to resting sympathetic neural activity or sympathetic baroreflex sensitivity (Hinojosa-Laborde et al., 2014). Studies in healthy young individuals indicate that in addition to sympathetic vasoconstriction, the heart rate response may also contribute importantly to orthostatic tolerance (Convertino, 2014). On the other hand, it is argued that cardiac effector mechanisms appear not to be important for the adjustment of arterial pressure to the upright posture in humans (Wieling et al., 2014). Indeed, investigations into the role of the sympathetic nervous system in orthostatic intolerance have yielded mixed results. The review article by Lambert and Lambert (2014) outlines the current knowledge of the function of the sympathetic nervous system in vasovagal syncope and the Postural Orthostatic Tachycardia Syndrome (POTS, also called Chronic Orthostatic Intolerance). There are multiple pathophysiological mechanisms that underlie POTS; some POTS patients have evidence of elevated sympathoneural tone, others have a hypovolemic state, and many patients have a perturbed renin-angiotensin-aldosterone system profile (Mar and Raj, 2014). Patients with POTS have acute changes in cognitive performance in response to orthostatic challenges, and these patients also have an increased prevalence of depression and higher levels of anxiety (Anderson et al., 2014). Impaired cerebral autoregulation may underlie upright cognitive dysfunction in POTS (Medow et al., 2014). Finally, patients with autonomic failure often have orthostatic hypotension and (pre)syncopal symptoms. Arnold et al. showed that ergotamine combined with caffeine could be used as an alternate treatment in carefully selected patients without comorbid coronary artery disease (Arnold et al., 2014). Taken together, this e-book highlights only a small portion of the recent advances in neural-humoral control during orthostasis in humans. More research is needed to better understand the role of the sympathetic nervous system and the renin-angiotensin-aldosterone system in the regulation of blood pressure in health and disease.

Maintenance of the Upright Posture in Humans upon Disturbance of Stability of the Visually Perceived Environment: Effect of an Instruction for Controlling Body Sway

We examined the maintenance of the upright posture under conditions of immersion of the tested subject in a 3D virtual visual environment, VVE. The latter consisted of two plans, the foreground looked like a window of the room, while the background was a fragment of the urban landscape. The software used allowed us to correlate body sway within the sagittal plane with shifts of the VVE foreground doing the latter mobile. The linkage in the course of testing could be either antiphase (APh) or synphase (SPh); the background always remained immobile. Each experiment was conventionally divided into two parts. In the first part, the subjects were not informed that body sways in some trials can be linked with shifts of the visual environment and were proposed to look at some object within the stable background. In the second part, the subjects were preliminarily informed of the existence of such a linkage and proposed to minimize body sways within the sagittal plane by tracking shifts of the foreground with respect to some, selected preliminarily, object within the background. Trials with a linkage between body sways and VVE shifts were altered by trials with standing of the subject facing an absolutely immobile visual pattern (IVP) and trials with the eyes closed (EC). The impact of visual control on the postural maintenance was estimated according to changes in the amplitude and frequency characteristics of two variables; i) shifts of the vertical projection of the center of gravity (CG) of the body and ii) difference between the position of the center of feet pressure (CFP) and CG projection. Changes in the latter were considered the main controlled variable in the postural maintenance, while the CFP-CG was interpreted as a variable containing information on changes in the resulting muscle/joint stiffness in the ankle joints related to activation of the shin muscles. The analysis of CG and CFP-CG oscillations demonstrated clear dependence of their spectra on the direction of linkage between body sways and oscillations of the foreground and on the instruction obtained by the subjects. In the first group of tests, RMSs of the spectra of oscillations of both variables at the SPh linkage were found to be within the same range as in standing with the EC, while at the APh linkage these oscillations were closer to the range typical of IVP conditions. In the second group of tests, the RMS of the spectra calculated for both variables were qualitatively different. In the case of APh relations between body sways and foreground shifts, these values were smaller than values in standing at IVP, while under SPh conditions they were smaller than those typical of standing with the EC. Introduction of the instruction of the second test group was also reflected in changes of the parameters of the CFP-CG variable. At the APh relation, the RMSs of the spectra for this variable decreased somewhat, and their median frequency increased significantly. The data obtained allow us to hypothesize that the improvement of stability standing after introduction of the instruction specifying visual control and concentrating the latter on one direction of body sway results from more active using of visual feedback under conditions of perception of the VVE unstable foreground. This should be considered additional proof in favor of effective involvement of vision in postural control upon disturbances in stability of the external environment.

Maintenance of the Upright Posture in Humans upon Manipulating the Direction and Delay of Visual Feedback

We examined the maintenance of the upright posture in subjects immersed in a 3D virtual visual environment, VVE. The latter consisted of two plans; the VVE foreground represented the image of a room window, while the background looked like an outside urban landscape. The position of the foreground (window) was linked, using the respective software, with body sways in the sagittal plane and shifted in accordance with body movements, while the background always remained immobile. By manipulating the direction and time delay of the relationship between the body sways and shifts of the VVE foreground, we tried to estimate the contribution of visual signals to postural control. For this purpose, we used the technique of frequency filtration of displacements of the center of feet pressure (CFP) and, in such a way, detected signals proportional to movements of the general center of gravity (CG) of the body (CG variable). After this, we calculated the difference between the CFP and CG shifts (CFP-CG-variable). The latter variable is known to be proportional to the horizontal acceleration; this is why it was used for estimation of the muscular torques correcting CG shifts. Analysis of changes in the above variables (CG and CFP-CG) demonstrated a clear dependence between the root mean square (RMS) of their amplitude spectra on the direction of feedback between body sways and shifts of the VVE foreground. With a synphase (SPh) pattern of the link between shifts of the body and those of the VVE foreground, the RMS values were closer to the range typical of standing with the eyes closed, while with an antiphase (APh) link, they were closer to the zone of values observed under conditions of normal vision. Introduction of time delays in feedback between sagittal body sways and shifts of the VVE foreground exerted nearly similar effects on the CG and CFP-CG variables. In both SPh and APh relations, 0.2- and 0.5-sec-long time delays resulted in increases in the RMS of the sway spectrum, while 0.8- and 1.0-sec-long delays led to RMS decreases. Thus, our findings show that the maintenance of upright stance under conditions of manipulations of the direction and time delay of visual feedback is performed, at least partly, with the use of an unstable VVE foreground as the reference. This can be considered proof of a special (frequently, dominating) role of the visual signals in postural control.

Functional Synergies Underlying Control of Upright Posture during Changes in Head Orientation

BackgroundStudies of human upright posture typically have stressed the need to control ankle and hip joints to achieve postural stability. Recent studies, however, suggest that postural stability involves multi degree-of-freedom (DOF) coordination, especially when performing supra-postural tasks. This study investigated kinematic synergies related to control of the body’s position in space (two, four and six DOF models) and changes in the head’s orientation (six DOF model).Methodology/Principal FindingsSubjects either tracked a vertically moving target with a head-mounted laser pointer or fixated a stationary point during 4-min trials. Uncontrolled manifold (UCM) analysis was performed across tracking cycles at each point in time to determine the structure of joint configuration variance related to postural stability or tracking consistency. The effect of simulated removal of covariance among joints on that structure was investigated to further determine the role of multijoint coordination. Results indicated that cervical joint motion was poorly coordinated with other joints to stabilize the position of the body center of mass (CM). However, cervical joints were coordinated in a flexible manner with more caudal joints to achieve consistent changes in head orientation.Conclusions/SignificanceAn understanding of multijoint coordination requires reference to the stability/control of important performance variables. The nature of that coordination differs depending on the reference variable. Stability of upright posture primarily involved multijoint coordination of lower extremity and lower trunk joints. Consistent changes in the orientation of the head, however, required flexible coordination of those joints with motion of the cervical spine. A two-segment model of postural control was unable to account for the observed stability of the CM position during the tracking task, further supporting the need to consider multijoint coordination to understand postural stability.