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1.
Motivated by experimental studies of insects, we propose a model for legged locomotion in the horizontal plane. A three-degree-of
freedom, energetically conservative, rigid-body model with a pair of compliant virtual legs in intermittent contact with the
ground allows us to study how dynamics depends on parameters such as mass, moment of inertia, leg stiffness, and length. We
find periodic gaits, and show that mechanics alone can confer asymptotic stability of relative heading and body angular velocity.
We discuss the relevance of our idealized models to experiments and simulations on insect running, showing that their gait
and force characteristics match observations reasonably well. We perform parameter studies and suggest that our model is relevant
to the understanding of locomotion dynamics across species.
Received: 17 April 2001 / Accepted in revised form: 20 November 2001 相似文献
2.
We develop a model for anguilliform (eel-like) swimming as an elastic rod actuated via time-dependent intrinsic curvature and subject to hydrodynamic drag forces, the latter as proposed by Taylor (in Proc Roy Proc Lond A 214:158–183, 1952). We employ a eometrically exact theory and discretize the resulting nonlinear partial differential evolution both to perform numerical simulations, and to compare with previous models consisting of chains of rigid links or masses connected by springs, dampers, and prescribed force generators representing muscles. We show that muscle activations driven by motoneuronal spike trains via calcium dynamics produce intrinsic curvatures corresponding to near-sinusoidal body shapes in longitudinally-uniform rods, but that passive elasticity causes Taylor’s assumption of prescribed shape to fail, leading to time-periodic motions and lower speeds than those predicted Taylor (in Proc Roy Proc Lond A 214:158–183, 1952). We investigate the effects of bending stiffness, body geometry, and activation patterns on swimming speed, turning behavior, and acceleration to steady swimming. We show that laterally-uniform activation yields stable straight swimming and laterally differential activation levels lead to stable turns, and we argue that tapered bodies with reduced caudal (tail-end) activation (to produce uniform intrinsic curvature) swim faster than ones with uniform activation. 相似文献
3.
We develop a simple model for insect locomotion in the horizontal (ground) plane. As in earlier work by Seipel et al. (Biol
Cybern 91(0):76–90, 2004) we employ six actuated legs that also contain passive springs, but the legs, with “hip” and ‘knee’
joints, better represent insect morphology. Actuation is provided via preferred angle inputs at each joint, corresponding
to zero torques in the hip and knee springs. The inputs are determined from estimates of foot forces in the cockroach Blaberus discoidalis via an inverse problem. The head–thorax–body is modeled as a single rigid body, and leg masses, inertia and joint dissipation
are ignored. The resulting three degree-of-freedom dynamical system, subject to feedforward joint inputs, exhibits stable
periodic gaits that compare well with observations over the insect’s typical speed range. The model’s response to impulsive
perturbations also matches that of freely-running cockroaches (Jindrich and Full, J Exp Biol 205:2803–2823, 2002), and stability
is maintained in the face of random foot touchdowns representative of real insects. We believe that this model will allow
incorporation of realistic muscle models driven by a central pattern generator in place of the joint actuators, and that it
will ultimately permit the study of proprioceptive feedback pathways involving leg force and joint angle sensing. 相似文献
4.
Daniel Krüger Sandro Wartzack 《Computer methods in biomechanics and biomedical engineering》2017,20(15):1589-1598
Musculoskeletal multibody models are increasingly used to analyze and optimize physical interactions between humans and technical artifacts. Since interaction is conveyed by contact between the human body and the artifact, a computationally robust modeling approach for frictional contact forces is a crucial aspect. In this contribution, we propose a parametric contact model and formulate an associated force optimization problem to simultaneously estimate unknown muscle and contact forces in an inverse dynamic manner from a prescribed motion trajectory. Unlike existing work, we consider both the static and the kinetic regime of Coulomb’s friction law. The approach is applied to the analysis of a leg extension training machine with the objective to reduce the stress on the tibiofemoral joint. The uncertainty of the simulation results due to a tunable parameter of the contact model is of particular interest. 相似文献
5.
We extend the analysis of simple, energy-conserving models for the dynamics of insect locomotion in the horizontal plane
developed in Schmitt and Holmes (2000a,b, 2001), where gaits characteristic of steady cockroach running and turning were evoked.
In this paper, we include dissipation and energy inputs via active “muscles” in three forms: via prescribed torques at the
“hip” pivot, via an active spring element of variable length, and via a pair of Hill-type muscle models representing an extensor/flexor
system. Due to mechanical feedback of passive elastic forces, the stable gaits of the conservative models are preserved, and
now energy input and absorption balances to additionally stabilize a preferred speed, with only modest neural sensing and
feedback being required. However, these bipedal models still cannot simultaneously match observed moment-yaw magnitudes and
fore-aft dynamics.
Received: 17 September 2001 / Accepted: 20 February 2003 /
Published online: 20 May 2003
Correspondence to: P. Holmes (e-mail: pholmes@math.Princeton.EDU)
Acknowledgements. This work was supported by DARPA/ONR: N00014-98-1-0747 and DoE: DE-FG02-95ER25238. John Schmitt was partially supported by
a DoD Graduate Fellowship, a Wu Fellowship of the School of Engineering and Applied Science, and a George Van Ness Lothrop
Honorific Fellowship of the Graduate School at Princeton University. We thank Kenneth Meijer for allowing us to use his muscle
model in Sect. 4 and Bob Full and Dan Koditschek for numerous helpful suggestions. 相似文献
6.
BACKGROUND: Keratocytes are specialised, rapidly moving cells that generate substantial contractile force perpendicular to their direction of locomotion. Potential roles for contractile force in cell motility include cell-body transport, regulation of adhesion, and retraction of the cell's trailing edge. RESULTS: To investigate contact dynamics, we used simultaneous confocal fluorescence and interference reflection microscopy to image keratocytes injected with fluorescent vinculin. We found that contacts formed behind the leading edge and grew beneath both the lamellipodium and the cell body. Contacts in the middle of the cell remained stationary relative to the substrate and began to disassemble as the cell body passed over them. In contrast, contacts in the lobes of the cell grew continuously and more rapidly, incorporated more vinculin, and slid inwards towards the sides of the cell body. Contact sliding often led to merging of contacts before their removal from the substrate. CONCLUSIONS: We suggest a synthesis of two existing, apparently conflicting models for keratocyte motility, in which network contraction progressively reorients actin filaments using the contacts as pivots, forming bundles that then generate lateral tension by a sliding-filament mechanism. Contact dynamics vary between the middle of the cell and the lobes. We propose that laterally opposed contractile forces first enhance contact growth and stability, but escalating force eventually pulls contacts from the substrate at the back of the cell, without interfering with the cell's forward progress. 相似文献
7.
The theory of optimal foraging predicts abrupt changes in consumer behavior which lead to discontinuities in the functional
response. Therefore population dynamical models with optimal foraging behavior can be appropriately described by differential
equations with discontinuous right-hand sides. In this paper we analyze the behavior of three different Lotka–Volterra predator–prey
systems with optimal foraging behavior. We examine a predator–prey model with alternative food, a two-patch model with mobile
predators and resident prey, and a two-patch model with both predators and prey mobile. We show that in the studied examples,
optimal foraging behavior changes the neutral stability intrinsic to Lotka–Volterra systems to the existence of a bounded
global attractor. The analysis is based on the construction and use of appropriate Lyapunov functions for models described
by discontinuous differential equations.
Received: 23 March 1999 相似文献
8.
Body size and food web structure: testing the equiprobability assumption of the cascade model 总被引:2,自引:0,他引:2
M. G. Neubert S. C. Blumenshine D. E. Duplisea T. Jonsson B. Rashleigh 《Oecologia》2000,123(2):241-251
The cascade model successfuly predicts many patterns in reported food webs. A key assumption of this model is the existence
of a predetermined trophic hierarchy; prey are always lower in the hierarchy than their predators. At least three studies
have suggested that, in animal food webs, this hierarchy can be explained to a large extent by body size relationships. A
second assumption of the standard cascade model is that trophic links not prohibited by the hierarchy occur with equal probability.
Using nonparametric contingency table analyses, we tested this ”equiprobability hypothesis” in 16 published animal food webs
for which the adult body masses of the species had been estimated. We found that when the hierarchy was based on body size,
the equiprobability hypothesis was rejected in favor of an alternative, ”predator-dominance” hypothesis wherein the probability
of a trophic link varies with the identity of the predator. Another alternative to equiprobabilty is that the probability
of a trophic link depends upon the ratio of the body sizes of the two species. Using nonparametric regression and liklihood
ratio tests, we show that a size-ratio based model represents a significant improvement over the cascade model. These results
suggest that models with heterogeneous predation probabilities will fit food web data better than the homogeneous cascade
model. They also suggest a new way to bridge the gap between static and dynamic food web models.
Received: 3 February 1999 / Accepted: 26 October 1999 相似文献
9.
Firing-rate models describing neural-network activity can be formulated in terms of differential equations for the synaptic
drive from neurons. Such models are typically derived from more general models based on Volterra integral equations assuming
exponentially decaying temporal coupling kernels describing the coupling of pre- and postsynaptic activities. Here we study
models with other choices of temporal coupling kernels. In particular, we investigate the stability properties of constant
solutions of two-population Volterra models by studying the equilibrium solutions of the corresponding autonomous dynamical
systems, derived using the linear chain trick, by means of the Routh–Hurwitz criterion. In the four investigated synaptic-drive
models with identical equilibrium points we find that the choice of temporal coupling kernels significantly affects the equilibrium-point
stability properties. A model with an α-function replacing the standard exponentially decaying function in the inhibitory coupling kernel is in most of our examples
found to be most prone to instability, while the opposite situation with an α-function describing the excitatory kernel is found to be least prone to instability. The standard model with exponentially
decaying coupling kernels is typically found to be an intermediate case. We further find that stability is promoted by increasing
the weight of self-inhibition or shortening the time constant of the inhibition. 相似文献
10.
Anthropologists and psychiatrists traditionally have used the salience of a mind–body dichotomy to distinguish Western from
non-Western ethnopsychologies. However, despite claims of mind–body holism in non-Western cultures, mind–body divisions are
prominent in non-Western groups. In this article, we discuss three issues: the ethnopsychology of mind–body dichotomies in
Nepal, the relationship between mind–body dichotomies and the hierarchy of resort in a medical pluralistic context, and, finally,
the role of mind–body dichotomies in public health interventions (biomedical and psychosocial) aimed toward decreasing the
stigmatization of mental illness. We assert that, by understanding mind–body relations in non-Western settings, their implications,
and ways in which to reconstitute these relations in a less stigmatizing manner, medical anthropologists and mental health
workers can contribute to the reduction of stigma in global mental health care.
相似文献
Brandon A. KohrtEmail: |
11.
Predator-prey models with delay and prey harvesting 总被引:1,自引:0,他引:1
It is known that predator-prey systems with constant rate harvesting exhibit very rich dynamics. On the other hand, incorporating
time delays into predator-prey models could induce instability and bifurcation. In this paper we are interested in studying
the combined effects of the harvesting rate and the time delay on the dynamics of the generalized Gause-type predator-prey
models and the Wangersky-Cunningham model. It is shown that in these models the time delay can cause a stable equilibrium
to become unstable and even a switching of stabilities, while the harvesting rate has a stabilizing effect on the equilibrium
if it is under the critical harvesting level. In particular, one of these models loses stability when the delay varies and
then regains its stability when the harvesting rate is increased. Computer simulations are carried to explain the mathematical
conclusions.
Received: 1 March 2000 / Revised version: 7 September 2000 /?Published online: 21 August 2001 相似文献
12.
Under normal conditions human walking or running consists of stable cyclic movements. Minor perturbances such as a stone
or a pothole do not disrupt the cycle, and the system returns to its prescribed trajectory. We investigated whether a pair
of antagonistic muscles is able to stabilize the movement without neuronal feedback. The human is represented by a model consisting
of a massless two-segment linkage system (leg) topped by a point mass. Both the extensor and flexor muscles are described
by a Hill-type muscle model. Conditions for stability are calculated analytically based on the Ljapunov Theory and the results
are illustrated by numerical examples. The activation functions of both the extensor and flexor muscles can be calculated
for a prescribed trajectory to maintain the self-stabilizing ability of such a system. Experimental evidence supports the
prediction.
Our investigation shows that a moving center of rotation of the kneejoint, a biarticular flexor muscle group, the force-velocity
relation, and the ascending limb of the force-length relation improves the self-stabilizing ability of human movement.
Received: 24 March 1999 / Accepted: 20 February 2003 /
Published online: 22 May 2003
Correspondence to: H. Wagner (e-mail: heiko.wagner@uni-jena.de, Tel.:+49-3641-945706)
Acknowledgements. The authors wish to express their gratitude to Arnd Friedrichs and Lars Wendrock for their contribution to data acquisition
and analysis. Veit Wank is acknowledged for making available for our use the x-ray photos of several knee joints. We would
like to thank Anna N. Ahn for her very detailed and helpful comments. With support from DFG (Innovationskolleg “Motion Systems”). 相似文献
13.
A. M. de Roos O. Diekmann P. Getto M. A. Kirkilionis 《Bulletin of mathematical biology》2010,72(2):259-297
In this paper, we present methods for a numerical equilibrium and stability analysis for models of a size structured population
competing for an unstructured resource. We concentrate on cases where two model parameters are free, and thus existence boundaries
for equilibria and stability boundaries can be defined in the (two-parameter) plane. We numerically trace these implicitly
defined curves using alternatingly tangent prediction and Newton correction. Evaluation of the maps defining the curves involves
integration over individual size and individual survival probability (and their derivatives) as functions of individual age.
Such ingredients are often defined as solutions of ODE, i.e., in general only implicitly. In our case, the right-hand sides
of these ODE feature discontinuities that are caused by an abrupt change of behavior at the size where juveniles are assumed
to turn adult. So, we combine the numerical solution of these ODE with curve tracing methods. We have implemented the algorithms
for “Daphnia consuming algae” models in C-code. The results obtained by way of this implementation are shown in the form of graphs. 相似文献
14.
Stability of discrete one-dimensional population models 总被引:3,自引:0,他引:3
Paul Cull 《Bulletin of mathematical biology》1988,50(1):67-75
We give conditions for local and global stability of discrete one-dimensional population models. We give a new test for local
stability when the derivative is −1. We give several sufficient conditions for global stability. We use these conditions to
show that local and global stability coincide for the usual models from the literature and even for slightly more complicated
models. We give population models, which are in some sense the simplest models, for which local and global stability do not
coincide. 相似文献
15.
During wound healing, both chemotaxis and contact guidance can contribute to the migration of blood and tissue cells to the
wound. In order to understand the wound healing process, we must thus understand how cells respond to both these simultaneous
directional cues, which are not necessarily coaligned. Although chemotaxis and contact guidance have been studied individually,
the interaction between them has not been addressed. We extend a stochastic cell movement model, developed by Dickinson and
Tranquillo (1995) [6] for individual cues, for simultaneous chemotaxis and contact guidance by a two-parameter perturbation
analysis in terms of the two associated cues, a chemotactic factor gradient and aligned tissue fibers. We present results
from analysis of the first-order perturbation, which includes the cell flux expression heuristically proposed by others, but
reveals paradoxical results for other indices of cell movement, such as the mean-squared displacement. We then present second-order
perturbation results that resolve these paradoxical results. Finally, we relate these results to a continuum mechanical model
developed by Barocas and Tranquillo (1997) [3] that predicts fiber alignment due to cell traction induced tissue contraction.
Received: 30 April 1999 / Revised version: 30 October 1999 / Published online: 14 September 2000 相似文献
16.
Kazunori Sato 《Population Ecology》2009,51(3):411-418
In this paper, we investigate a spatially explicit metapopulation model with Allee effects. We refer to the patch occupancy
model introduced by Levins (Bull Entomol Soc Am 15:237–240, 1969) as a spatially implicit metapopulation model, i.e., each local patch is either occupied or vacant and a vacant patch can
be recolonized by a randomly chosen occupied patch from anywhere in the metapopulation. When we transform the model into a
spatially explicit one by using a lattice model, the obtained model becomes theoretically equivalent to a “lattice logistic
model” or a “basic contact process”. One of the most popular or standard metapopulation models with Allee effects, developed
by Amarasekare (Am Nat 152:298–302, 1998), supposes that those effects are introduced formally by means of a logistic equation. However, it is easier to understand
the ecological meaning of associating Allee effects with this model if we suppose that only the logistic colonization term
directly suffers from Allee effects. The resulting model is also well defined, and therefore we can naturally examine it by
Monte Carlo simulation and by doublet and triplet decoupling approximation. We then obtain the following specific features
of one-dimensional lattice space: (1) the metapopulation as a whole does not have an Allee threshold for initial population
size even when each local population follows the Allee effects; and (2) a metapopulation goes extinct when the extinction
rate of a local population is lower than that in the spatially implicit model. The real ecological metapopulation lies between
two extremes: completely mixing interactions between patches on the one hand and, on the other, nearest neighboring interactions
with only two nearest neighbors. Thus, it is important to identify the metapopulation structure when we consider the problems
of invasion species such as establishment or the speed of expansion. 相似文献
17.
David Sternberg Stephen R. Balcombe Jonathan C. Marshall Jaye S. Lobegeiger Angela H. Arthington 《Environmental Biology of Fishes》2012,93(1):95-104
The ecology of dryland rivers is driven by their highly variable hydrology, particularly flooding regimes, whereby intermittent
floods typically generate ‘booms’ of primary and secondary productivity, including massive fish production. We tested these
concepts in the Moonie River, Australia, using the percichthyid, Macquaria ambigua, a dryland river species known to display pronounced ‘boom and bust’ abundance patterns in response to floodplain inundation
followed by extended periods of low to no channel flow. We expected that body condition (as measured by whole body lipid content)
and biomass of M. ambigua would be related to prey biomass, and that these factors would all ‘spike’ following widespread flooding. Instead we found
more subtle responses. There were ‘booms’ in biomass of Macrobrachium and zooplankton, two important food items, whereas M. ambigua maintained relatively low but sustained lipid and biomass levels following flooding. It appears that instead of a ‘boom’
in fish biomass, abundant invertebrate food resources and sustained lipid levels contributed to high survivorship of this
species during the ‘bust’ period over cool dry months. 相似文献
18.
Rivalan P Prévot-Julliard AC Choquet R Pradel R Jacquemin B Girondot M 《Oecologia》2005,145(4):564-574
The trade-off between current and future reproduction plays an important role in demographic analyses. This can be revealed
by the relationship between the number of years without reproduction and reproductive investment within a reproductive year.
However, estimating both the duration between two successive breeding season and reproductive effort is often limited by variable
recapture or resighting effort. Moreover, a supplementary difficulty is raised when nonbreeder individuals are not present
sampling breeding grounds, and are therefore unobservable. We used capture–recapture (CR) models to investigate intermittent
breeding and reproductive effort to test a putative physiological trade-off in a long-lived species with intermittent breeding,
the leatherback sea turtle. We used CR data collected on breeding females on Awa:la-Ya:lima:po beach (French Guiana, South
America) from 1995 to 2002. By adding specific constraints in multistate (MS) CR models incorporating several nonobservable
states, we modelled the breeding cycle in leatherbacks and then estimated the reproductive effort according to the number
of years elapsed since the last nesting season. Using this MS CR framework, the mean survival rate was estimated to 0.91 and
the average resighting probability to 0.58 (ranged from 0.30 to 0.99). The breeding cycle was found to be limited to 3 years.
These results therefore suggested that animals whose observed breeding intervals are greater than 3 years were most likely
animals that escaped detection during their previous nesting season(s). CR data collected in 2001 and 2002 allowed us to compare
the individual reproductive effort between females that skipped one breeding season and females that skipped two breeding
seasons. These inferences led us to conclude that a trade-off between current and future reproduction exists in leatherbacks
nesting in French Guiana, likely linked to the resource provisioning required to invest in reproduction. 相似文献
19.
Daphnia revisited: local stability and bifurcation theory for physiologically structured population models explained by way of an example 总被引:1,自引:0,他引:1
Odo Diekmann Mats Gyllenberg J. A. J. Metz Shinji Nakaoka Andre M. de Roos 《Journal of mathematical biology》2010,61(2):277-318
We consider the interaction between a general size-structured consumer population and an unstructured resource. We show that
stability properties and bifurcation phenomena can be understood in terms of solutions of a system of two delay equations
(a renewal equation for the consumer population birth rate coupled to a delay differential equation for the resource concentration).
As many results for such systems are available (Diekmann et al. in SIAM J Math Anal 39:1023–1069, 2007), we can draw rigorous
conclusions concerning dynamical behaviour from an analysis of a characteristic equation. We derive the characteristic equation
for a fairly general class of population models, including those based on the Kooijman–Metz Daphnia model (Kooijman and Metz in Ecotox Env Saf 8:254–274, 1984; de Roos et al. in J Math Biol 28:609–643, 1990) and a model introduced
by Gurney–Nisbet (Theor Popul Biol 28:150–180, 1985) and Jones et al. (J Math Anal Appl 135:354–368, 1988), and next obtain
various ecological insights by analytical or numerical studies of special cases. 相似文献
20.
We study a system of differential equations that models the population dynamics of an SIR vector transmitted disease with two pathogen strains. This model arose from our study of the population dynamics of dengue
fever. The dengue virus presents four serotypes each induces host immunity but only certain degree of cross-immunity to heterologous
serotypes. Our model has been constructed to study both the epidemiological trends of the disease and conditions that permit
coexistence in competing strains. Dengue is in the Americas an epidemic disease and our model reproduces this kind of dynamics.
We consider two viral strains and temporary cross-immunity. Our analysis shows the existence of an unstable endemic state
(‘saddle’ point) that produces a long transient behavior where both dengue serotypes cocirculate. Conditions for asymptotic
stability of equilibria are discussed supported by numerical simulations. We argue that the existence of competitive exclusion
in this system is product of the interplay between the host superinfection process and frequency-dependent (vector to host)
contact rates.
Received 4 December 1995; received in revised form 5 March 1996 相似文献