Lateral Perturbations Research Articles

A robotic device for understanding neuromechanical interactions during standing balance control

Postural stability in standing balance results from the mechanics of body dynamics as well as active neural feedback control processes. Even when an animal or human has multiple legs on the ground, active neural regulation of balance is required. When the postural configuration, or stance, changes, such as when the feet are placed further apart, the mechanical stability of the organism changes, but the degree to which this alters the demands on neural feedback control for postural stability is unknown. We developed a robotic system that mimics the neuromechanical postural control system of a cat in response to lateral perturbations. This simple robotic system allows us to study the interactions between various parameters that contribute to postural stability and cannot be independently varied in biological systems. The robot is a ‘planar’, two-legged device that maintains compliant balance control in a variety of stance widths when subject to perturbations of the support surface, and in this sense reveals principles of lateral balance control that are also applicable to bipeds. Here we demonstrate that independent variations in either stance width or delayed neural feedback gains can have profound and often surprisingly detrimental effects on the postural stability of the system. Moreover, we show through experimentation and analysis that changing stance width alters fundamental mechanical relationships important in standing balance control and requires a coordinated adjustment of delayed feedback control to maintain postural stability.

Using postglacial sea level, crustal velocities and gravity-rate-of-change to constrain the influence of thermal effects on mantle lateral heterogeneities

Lateral heterogeneities in the mantle can be caused by thermal, chemical and non-isotropic pre-stress effects. Here, we investigate the possibility of using observations of the glacial isostatic adjustment (GIA) process to constrain the thermal contribution to lateral variations in mantle viscosity. In particular, global historic relative sea level, GPS in Laurentide and Fennoscandia, altimetry together with tide-gauge data in the Great Lakes area, and GRACE data in Laurentide are used. The lateral viscosity perturbations are inferred from the seismic tomography model S20A by inserting the scaling factor β to determine the contribution of thermal effects versus compositional heterogeneity and non-isotropic pre-stress effects on lateral heterogeneity in mantle viscosity. When β = 1, lateral velocity variations are caused by thermal effects alone. With β < 1, the contribution of thermal effect decreases, so that for β = 0, there is no lateral viscosity variation and the Earth is laterally homogeneous. These lateral viscosity variations are superposed on four different reference models which differ significantly in the lower mantle viscosity. The Coupled Laplace Finite Element method is used to predict the GIA response on a spherical, self-gravitating, compressible, viscoelastic Earth with self-gravitating oceans, induced by the ICE-4G deglaciation model. Results show that the effect of β on uplift rates and gravity rate-of-change is not simple and involves the trade-off between the contribution of lateral viscosity variations in the transition zone and in the lower mantle. Models with small viscosity contrast in the lower mantle cannot explain the observed uplift rates in Laurentide and Fennoscandia. However, the RF3S20 model with a reference viscosity profile simplified from Peltier's VM2 with the value of β around 0.2–0.4 is found to explain most of the global RSL data, the uplift rates in Laurentide and Fennoscandia and the BIFROST horizontal velocity data. In addition, the changes in GIA signals caused by changes in the value of β are large enough to be detected by the data, although uncertainty in other parameters in the GIA models still exists. This may encourage us to further utilize GIA observations to constrain the thermal effect on mantle lateral heterogeneity as geodetic and satellite gravity measurements are improved.

Thinning of continental backarc lithosphere by flow-induced gravitational instability

Many continental backarcs have thin (∼ 60 km) lithosphere for 100s of km behind the volcanic arc, even where there has been no significant extension. One mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction-related mantle flow. The stability of backarc mantle lithosphere is examined using thermal–mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average Phanerozoic continental lithosphere. Subduction-induced mantle flow shears the base of the backarc lithosphere, producing lateral density perturbations. Owing to the non-Newtonian lithosphere rheology, shearing also reduces the effective viscosity of the lowermost lithosphere, enabling the density perturbations to become gravitationally unstable. The perturbations develop into rapidly-growing downwellings which result in removal of lower backarc lithosphere on timescales of 5–10 Ma. Conductive heating and shearing of the remaining lithosphere results in a second, more muted, phase of gravitational instability and thinning. Lower lithosphere instability is enhanced by higher subduction rates, weaker intrinsic rheology, higher compositional density, and hotter initial thermal structure. The numerical model results are in good agreement with a buoyancy stability analysis, which includes the vertical gradients in temperature and strain rate through the lithosphere. As both rheology and density depend on lithosphere composition, significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water. To produce a final thickness comparable to that observed without lithospheric contraction or extension, it is necessary to have a weak lithosphere rheology, an initial thermal structure hotter than average Phanerozoic continental geotherms, or a combination of the two. The region of thin lithosphere at the northern Cascadia backarc coincides with terranes that were accreted to the North American craton. The known fertile composition of the backarc lithosphere may allow it to be thinned, while the drier, more refractory craton lithosphere is resistant to thinning.

Effects of lateral perturbations and changing stance conditions on anticipatory postural adjustment

The study investigates the role of lateral muscles and changing stance conditions in anticipatory postural adjustments (APAs). Subjects stood laterally to an aluminum pendulum released by an experimenter and were required to stop it with their right or left hand. Stance conditions were manipulated by having the subjects stand in the following positions: on a single limb (SS), with feet together (narrow base of support, NB), and with feet shoulder width apart (regular base of support, RB). Bilateral EMG activity of dorsal, ventral, and lateral trunk and leg muscles and ground reaction forces were recorded and quantified within the time intervals typical of APAs. Anticipatory postural adjustments were seen in all experimental conditions, and their magnitudes depended on the stance and the side of perturbation. Accordingly, APAs in lateral muscles increased on the side of perturbation in SS condition, while simultaneous activation of dorsal muscles occurred on the contralateral side. Smaller APAs were seen in lateral muscles in conditions with a wider base of support (NB, RB) and APAs in dorsal muscles were smaller in NB – in comparison to RB – stance. The results of the present study provide new data on the role of lateral, ventral, and dorsal muscles in anticipatory postural control when dealing with lateral perturbations in conditions of postural instability.

Coordination of Upper and Lower Body during Balance Recovery following a Support Translation

Strategies for recovery of posture were studied after lateral mechanical perturbations. 11 participants standing in tandem stance were unexpectedly submitted to lateral support translations with the eyes open or closed at two translation amplitudes. The trajectories of the center of mass of the upper and lower body and muscle activities allowed identification of three strategies, involving either the ankle or the hip only, or both. Hip use increased with vision and with amplitude of perturbation. Short-to-medium latency electromyographic activities were observed in leg and trunk muscles, and long-latency responses in the back leg muscles. Vision increased the activity of both leg and trunk muscles but did not influence the onset of the muscular responses. These data suggest a hierarchy in the selection of these different strategies: the hip is mobilized when the perturbation is more destabilizing but this strategy has a cost and needs specific sensory information supplied by vision.

Unsteady locomotion: integrating muscle function with whole body dynamics and neuromuscular control

By integrating studies of muscle function with analysis of whole body and limb dynamics, broader appreciation of neuromuscular function can be achieved. Ultimately, such studies need to address non-steady locomotor behaviors relevant to animals in their natural environments. When animals move slowly they likely rely on voluntary coordination of movement involving higher brain centers. However, when moving fast, their movements depend more strongly on responses controlled at more local levels. Our focus here is on control of fast-running locomotion. A key observation emerging from studies of steady level locomotion is that simple spring-mass dynamics, which help to economize energy expenditure, also apply to stabilization of unsteady running. Spring-mass dynamics apply to conditions that involve lateral impulsive perturbations, sudden changes in terrain height, and sudden changes in substrate stiffness or damping. Experimental investigation of unsteady locomotion is challenging, however, due to the variability inherent in such behaviors. Another emerging principle is that initial conditions associated with postural changes following a perturbation define different context-dependent stabilization responses. Distinct stabilization modes following a perturbation likely result from proximo-distal differences in limb muscle architecture, function and control strategy. Proximal muscles may be less sensitive to sudden perturbations and appear to operate, in such circumstances, under feed-forward control. In contrast, multiarticular distal muscles operate, via their tendons, to distribute energy among limb joints in a manner that also depends on the initial conditions of limb contact with the ground. Intrinsic properties of these distal muscle-tendon elements, in combination with limb and body dynamics, appear to provide rapid initial stabilizing mechanisms that are often consistent with spring-mass dynamics. These intrinsic mechanisms likely help to simplify the neural control task, in addition to compensating for delays inherent to subsequent force- and length-dependent neural feedback. Future work will benefit from integrative biomechanical approaches that employ a combination of modeling and experimental techniques to understand how the elegant interplay of intrinsic muscle properties, body dynamics and neural control allows animals to achieve stability and agility over a variety of conditions.

AFM-tip-induced crystallization of poly(ethylene oxide) melt droplets

The AFM-tip-induced crystallization of poly(ethylene oxide) (PEO) melt droplets was studied. The melt droplets with a height of 50–100 nm and a lateral size of 2–3 μm were obtained by melting the PEO ultra-thin films on a mica surface. For the PEO samples with average molecular weights (Mn) ranging from 1.0 × 103 g/mol to 1.0 × 104 g/mol, the lateral perturbation from the AFM tip in the hard-tapping or nanoscratch modes could not induce the growth of the flat-on lamellae. In contrast, under AFM nanoindentation mode, the tip-induced crystallization occurred when a sufficiently high vertical tip force was applied to the melt droplets of PEO with Mn ⩾ 1.0 × 104 g/mol. Moreover, the experimental results indicated that the AFM-tip-induced crystallization of PEO in the nanoindentation process had molecular weight dependence.

Feature Evaluation and Pattern Recognition of Lower Limb Muscle EMG during Postural Balance Control

We recognized EMG signal patterns of lower limb muscles by using neural networks and performed feature evaluation during the recovery of postural balance of human body. Surface electrodes were attached to lower limb and EMG signals were collected during the balance recovery process from a perturbation without permitting compensatory stepping. A waist pulling system was used to apply transient perturbations in five horizontal directions. The EMG signals of fifty repetitions of five motions were analyzed for ten subjects. Twenty features were extracted from EMG signals of one event. Feature evaluation was also performed by using DB (Davies-Bouldin) index. By using neural networks, EMG signals were classified into five categories, such as forward perturbation, backward perturbation, lateral perturbation and two oblique perturbations. As results, motions were recognized with mean success rates of 75 percent. With the neural networks classifier of this study, the EMG patterns of lower limb muscles during the recovery of postural balance can be classified with high accuracy of recognition.

Deciphering the Effects of Climate on Animal Populations: Diagnostic Analysis Provides New Interpretation of Soay Sheep Dynamics

Soay sheep on the island of Hirta exhibit periodic population collapses that have been proposed to result from nonlinear interactions between weather, population density, and age structure. Here we employ a diagnostic approach to reanalyze the data from 1985 to 2004 and find that climate mainly affects the equilibrium population size, thus acting as a lateral perturbation. From this, we derive a simple energetic model for a population interacting with its food supply in the presence of variable winter weather. This model explains the strong nonlinearity in the Soay sheep population regulation function and provides a framework for evaluating climatic perturbations. We examined two integrative climatic indexes, one representing effects on forage production and the other representing the severity of winter weather. Results suggest that the latter has the main effect on Soay sheep population dynamics. Models incorporating this variable provided fairly accurate predictions of Soay sheep population fluctuations. The diagnostic approach offers an objective way to develop simple, nonstructured population models that are useful for understanding the causes of population fluctuations and predicting population changes, provided they are based on a careful consideration of the underlying biological and/or ecological mechanisms.

Role of background viscosity in the investigation of postglacial rebound induced crustal motion in a laterally heterogeneous mantle

The effect of laterally homogeneous background viscosity profile on postglacial rebound induced surface motion on a laterally heterogeneous earth is investigated. The lateral viscosity perturbations are inferred from seismic tomography model S20A. Three different background viscosity models are investigated—they differ in lithospheric thickness and the viscosity jump across the 670 km discontinuity. The coupled Laplace-finite-element method is used to calculate present-day crustal motion on a spherical, self-gravitating, incompressible, viscoelastic earth with self-gravitating oceans, induced by the ICE-4G deglaciation model. Results show that background viscosity significantly affects the magnitude of the uplift rate, and both direction and magnitude of the tangential velocity. Thus a good handle on the background viscosity is very important in the study and interpretation of glacial isostatic data.

AFM-TIP-INDUCED CRYSTALLIZATION OF POLY(ETHYLENE OXIDE) MELT DROPLETS

The AFM-tip-induced crystallization of poly(ethylene oxide) (PEO) melt droplets was studied. The melt droplets with the height of 50～100 nm and the lateral size of 2～3 μm were obtained by melting the PEO ultrathin films on the mica surface. For the PEO samples with the number average molecular weights (M_n) ranged from 1.0×10~3 to 1.0×10~4 g/mol,the lateral perturbation from the AFM tip with hard-tapping or nanoscratch mode could not induce the growth of the flat-on lamellae. By contrast,under AFM nanoindentation mode,the tip-induced crystallization occurred when a sufficiently high vertical tip force was applied to the melt droplets of PEO with M_n≥1.0×10~4 g/mol. Moreover,the experimental results indicated that AFM-tip-induced crystallization with nanoindentation method had PEO molecular weight (viscosity) dependence.

Lithospheric structure of Tasmania from a novel form of teleseismic tomography

In 2001 and 2002, a temporary array of 72 seismic recorders was deployed across northern Tasmania (SE Australia), with the aim of imaging the underlying crust and upper mantle using three‐dimensional (3‐D) teleseismic tomography. Using a recently developed adaptive stacking technique, 6520 relative P wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period. A novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method, a grid‐based eikonal solver, is used to map the residual patterns as P wave velocity anomalies. The new scheme proves to be both fast and robust, making it well suited to large data sets and the reconstruction of complex anomalies. The resultant tomographic images of Tasmania exhibit significant lateral perturbations in P wave velocity structure (≤5%) from a 1‐D reference model. A marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern Tasmania is underlain by dense rocks with an oceanic crustal affinity, contrasting with the continentally derived siliciclastic core of western Tasmania. Significantly, the Tamar Fracture System does not overlie the narrow transition from relatively fast to slow velocities, which suggests that it may be a near‐surface feature rather than a manifestation of deeper crustal‐scale suturing as previously thought. Farther west, an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group and Arthur Lineament may be related to remnant subduction of oceanic lithosphere associated with the mid‐Cambrian Delamerian Orogeny.

Effect of lateral perturbations on psychophysical acceleration detection thresholds

BackgroundIn understanding how the human body perceives and responds to small slip-like motions, information on how one senses the slip is essential. The effect of aging and plantar sensory loss on detection of a slip can also be studied. Using psychophysical procedures, acceleration detection thresholds of small lateral whole-body perturbations were measured for healthy young adults (HYA), healthy older adults (HOA) and older adults with diabetic neuropathy (DOA). It was hypothesized that young adults would require smaller accelerations than HOA's and DOA's to detect perturbations at a given displacement.MethodsAcceleration detection thresholds to whole-body lateral perturbations of 1, 2, 4, 8, and 16 mm were measured for HYAs, HOAs, and DOAs using psychophysical procedures including a two-alternative forced choice protocol. Based on the subject's detection of the previous trial, the acceleration magnitude of the subsequent trial was increased or decreased according to the parameter estimation by sequential testing methodology. This stair-stepping procedure allowed acceleration thresholds to be measured for each displacement.ResultsResults indicate that for lateral displacements of 1 and 2 mm, HOAs and DOAs have significantly higher acceleration detection thresholds than young adults. At displacements of 8 and 16 mm, no differences in threshold were found among groups or between the two perturbation distances. The relationship between the acceleration threshold and perturbation displacement is of particular interest. Peak acceleration thresholds of approximately 10 mm/s2 were found at displacements of 2, 4, 8, and 16 mm for HYAs; at displacements of 4, 8, and 16 mm for HOAs; and at displacements of 8 and 16 mm for DOAs. Thus, 2, 4, and 8 mm appear to be critical breakpoints for HYAs, HOAs, and DOAs respectively, where the psychometric curve deviated from a negative power law relationship. These critical breakpoints likely indicate a change in the physiology of the system as it responds to the stimuli.ConclusionAs a function of age, the displacement at which the group deviates from a negative power law relationship increases from 2 mm to 4 mm. Additionally, the displacement at which subjects with peripheral sensory deficits deviate from the negative power law relations increases to 8 mm. These increases as a function of age and peripheral sensory loss may help explain the mechanism of falls in the elderly and diabetic populations.

Deciphering the Effects of Climate on Animal Populations: Diagnostic Analysis Provides New Interpretation of Soay Sheep Dynamics

Soay sheep on the island of Hirta exhibit periodic population collapses that have been proposed to result from nonlinear interactions between weather, population density, and age structure. Here we employ a diagnostic approach to reanalyze the data from 1985 to 2004 and find that climate mainly affects the equilibrium population size, thus acting as a lateral perturbation. From this, we derive a simple energetic model for a population interacting with its food supply in the presence of variable winter weather. This model explains the strong nonlinearity in the Soay sheep population regulation function and provides a framework for evaluating climatic perturbations. We examined two integrative climatic indexes, one representing effects on forage production and the other representing the severity of winter weather. Results suggest that the latter has the main effect on Soay sheep population dynamics. Models incorporating this variable provided fairly accurate predictions of Soay sheep population fluctuations. The diagnostic approach offers an objective way to develop simple, nonstructured population models that are useful for understanding the causes of population fluctuations and predicting population changes, provided they are based on a careful consideration of the underlying biological and/or ecological mechanisms.

Predicting nonlinear and non-additive effects of climate: the Alpine ibex revisited

ABSTRACT: Climate can have complex effects on demographic rates and the endogenous feedbackstructure regulating mammal populations, and this can create problems for predictive modelling. Innorthern and Alpine environments, weather appears to influence ungulate population growth ratesmainly during years of high population density, suggesting the possibility of nonlinear interactions be-tween the 2 variables. Threshold models have been employed to account for these nonlinearities. Forexample, in the case of Alpine ibex Capra ibex in the Gran Paradiso National Park of Italy, strongerdensity dependence is assumed to occur after snowfall exceeds 1.54 m. In this paper we use more ob-jective nonparametric methods to evaluate the form of the functional relationships governing thedynamics of this ibex population. No evidence was found for a threshold effect in the data. Instead weuncovered a non-additive and nonlinear interaction between climate and population density. The re-sulting models predict ibex numbers as well or better than previous threshold models despite requir-ing fewer parameters, and also conform well to traditional ecological concepts. We conclude with sev-eral lessons for those who wish to predict the effects of climate change on animal population dynamics.KEY WORDS: Ibex · Population dynamics · Lateral perturbations · Theoretical models

Climate and small rodent dynamics in semi-arid Chile: the role of lateral and vertical perturbations and intra-specific processes

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 30:125-132 (2006) - doi:10.3354/cr030125 Climate and small rodent dynamics in semi-arid Chile: the role of lateral and vertical perturbations and intra-specific processes Mauricio Lima1,*, M. Andrea Previtali2, Peter L. Meserve2 1Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Católica de Chile, 193 Correo 22, Santiago CP 6513677, Chile 2Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60115, USA *Email: mlima@genes.bio.puc.cl ABSTRACT: The study of the relative importance of the feedback structure (intrinsic processes) and exogenous (climatic or environmental) factors in determining population dynamics, in particular the interaction between density-dependence and climate, is a major question in population ecology. We sought to explain the numerical fluctuations of 2 sympatric rodent species at one well-studied site in semi-arid Chile using simple theoretically based population dynamics models and Royama’s theoretical framework for analyzing the dynamics of populations influenced by exogenous climatic forces. We found that rainfall effects appear to operate in a different manner on the 2 rodent species. For one species (Phyllotis darwini), rainfall appeared to influence the carrying capacity of the environment, whereas for a second (Akodon olivaceus) the rainfall effect had a primarily additive influence on the maximum per capita growth rates. KEY WORDS: Small rodents · Population dynamics models · ENSO · Lateral perturbations Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 30, No. 2. Online publication date: February 23, 2006 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2006 Inter-Research.

Cervical Muscle Response to Head Rotation in Whiplash-Type Right Lateral Impacts

To determine the electromyogram (EMG) response of the cervical muscles to a right lateral impact whiplash-type perturbation when the head is rotated. Twenty healthy volunteers were subjected to right lateral impacts of 4.2, 8.1, 10.3, and 12.5 m/s2 and were looking either left or right. Bilateral EMGs of the sternocleidomastoid, trapezius, and splenius capitis muscles were recorded. Triaxial accelerometers recorded the acceleration of the chair, torso at the shoulder level, and head of the participant. In a right lateral impact, muscle responses were of low magnitude with the head rotated to either the left or the right. At the highest acceleration of 12.5 m/s2, all generated less than 39% of their maximal voluntary contraction EMG. The sternocleidomastoid muscle showed a greater EMG response than its counterpart and the muscles contralateral to the direction of impact had higher EMG responses. The time to onset of the EMG for the splenii capitis and trapezii generally decreased with increasing levels of acceleration. As anticipated, an increase in applied acceleration resulted in an increase in accompanying head accelerations (P < .05), and when the head acceleration increased, so too did the force equivalent exertions by the various muscles. Overall, a right lateral impact with head rotation to either right or left appears to reduce the activity and thus the risk of muscle injury, perhaps because of "bracing" by muscles actively producing rotation or because of greater spinal stability from other structures when the head is in the rotated position.

20.20 A randomised controlled trial to evaluate task-related-exerciseclasses for older people with movement difficulties

When balance is disturbed, location of the center of pressure (COP) contributes to a person’s ability to recover from a perturbation. This study investigated COP control prior to first step lift-off (FSLO) during lateral perturbations in older non-fallers and fallers. 38 non-fallers and 16 fallers received lateral waist-pulls at 5 different intensities. Crossover stepping responses at the intensity level where the largest number of subjects responded with crossover steps were analyzed. Whole-body center of mass (COM) and COP positions in the medio-lateral (ML) direction with respect to the base of support (BOS), and COP velocity were calculated. An inverted pendulum model was used to define the BOS stability boundary at FSLO, which was also adjusted using the COP position at FSLO (functional boundary). No significant differences were found in the COP velocities between fallers and non-fallers (p>.093). However, the COP positions for fallers were located significantly more medial at FSLO (p≤.01), resulting in a significantly reduced functional boundary. Although the stability margins, measures of stability based on the BOS, were significantly larger than zero for fallers (p≤.004), they were not significantly different from zero for the functional boundary, i.e., reaching the functional stability limit. Fallers had reduced functional limits of stability in the ML direction, which would predispose them to more precarious stability conditions than non-fallers. This could be a cause for taking more steps than non-fallers for balance recovery as we observed. The functional boundary estimation may be a more sensitive marker of balance instability than the BOS boundary.

22.26 Postural instability in the frontal plane induced by lateral perturbations during treadmill walking

The purpose of this investigation was to determine the effect of unexpected forward perturbations (FP) during gait on lower extremity joint mechanics and muscle Electromyographic (EMG) patterns in healthy adults. The muscles surrounding the hip were found to be most important in maintaining control of the trunk and preventing collapse in response to the FP. Distinct lower extremity joint moment and power patterns were observed in response to the FP but an overall positive moment of support (Ms) was maintained. Therefore, reactive balance control was a synchronized effort of the lower extremity joints to prevent collapse during the FP.

6.15 Postural sway during lateral perturbation of tandem romberg test

6.15 Postural sway during lateral perturbation of tandem romberg test