Postural dynamics in the standing human

The purpose of this study was to develop a mathematical model of the linkage dynamics in upright standing, and to use this model to study output principles for postural control. The standing human was modelled in the sagittal plane as a three-segment linkage. Mechanical disturbances were simulated as forces which could be applied at various points in this linkage. An iterative approach was used to find joint torque combinations which would restore balance within 80 ms of these mechanical disturbances. The model predicted that a specific proportional relationship was necessary between the hip, knee and ankle torques in order for balance to be restored. This proportional relationship was shown to be a function of the model structure, but independent of the location, direction and amplitude of the disturbance. These predictions were tested experimentally. A disturbance apparatus was designed to apply an impulsive force to the subjects. The joint torque responses of the subjects were in quantitative agreement with the predictions of the model. The results suggest that a fixed relationship between joint torques may be required to restore balance, and this fixed relationship may make the task of postural control simpler for the nervous system.     

Possible functional roles of phase resetting during walking

The walking rhythm is known to show phase shift or "reset" in response to external impulsive perturbations. We tried to elucidate functional roles of the phase reset possibly used for the neural control of locomotion. To this end, a system with a double pendulum as a simplified model of the locomotor control and a model of bipedal locomotion were employed and analyzed in detail. In these models, a movement corresponding to the normal steady-state walking was realized as a stable limit cycle solution of the system. Unexpected external perturbations applied to the system can push the state point of the system away from its limit cycle, either outside or inside the basin of attraction of the limit cycle. Our mathematical analyses of the models suggested functional roles of the phase reset during walking as follows. Function 1: an appropriate amount of the phase reset for a given perturbation can contribute to relocating the system's state point outside the basin of attraction of the limit cycle back to the inside. Function 2: it can also be useful to reduce the convergence time (the time necessary for the state point to return to the limit cycle). In experimental studies during walking of animals and humans, the reset of walking rhythm induced by perturbations was investigated using the phase transition curve (PTC) or the phase resetting curve (PRC) representing phase-dependent responses of the walking. We showed, for the simple double-pendulum model, the existence of the optimal phase control and the corresponding PTC that could optimally realize the aforementioned functions in response to impulsive force perturbations. Moreover, possible forms of PRC that can avoid falling against the force perturbations were predicted by the biped model, and they were compared with the experimentally observed PRC during human walking. Finally, physiological implications of the results were discussed.     

Tolerance of two invasive thistles to repeated disturbance

Many invasive species have short life cycles, high reproduction, and easily dispersed offspring that make them good ruderal species under disturbance. However, the tolerance of such ruderal species to disturbance is often overlooked. In a 2-year mowing study, we applied frequent intense disturbances to examine the tolerance of two congeneric invasive thistles, Carduus acanthoides and Carduus nutans, and potential differences in their responses. Our results show that both species can survive multiple mowing events, with C. acanthoides surviving repeated intense mowing through a whole season. Furthermore, C. acanthoides was found to adjust its growth form to the disturbance regime, and successfully overwintered and reproduced in the subsequent growing season if the disturbance was terminated. Our results support the idea that tolerance to disturbance should be considered when examining invasions by short-lived monocarpic species, since avoidance of disturbance via rapid life cycle completion and seed production, and tolerance of disturbance via regrowth can co-occur in these species. Consequently, management of short-lived invasives should take both life history strategies into account.     

A fitness function for optimal life history in relation to density effects,competition, predation and stability of the environment

A fitness function (function maximized under natural selection) is studied in a population model in which the growth of a population is suppressed by crowding, density-independent continuous mortality (by euryphagous predators) and periodic disturbances. The dynamics of the population density between occurrence of disturbance can be expressed as,dN/dt=(F(N/K)−D)N, whereN is the population density,K is the carrying capacity,D is the density-independent continuous mortality, andF is the growth regulation factor described as a function of crowding (N/K). The period of disturbance isS. The survival rate under disturbance isu. It is concluded that the fitness function is (approximately) a product of competitive ability (C), carrying capacity, and degree of saturation, and is given byCKF ⁻¹(D−(lnu)/S). The degree of saturation is the inverse function of regulation factor (F) at the death rate due to predators and disturbance. I assume a population in which density is regulated only through survival. In this case, a low survival rate at the critical age-group means a high value ofCKF ⁻¹(D−(lnu)/S). Therefore, the reciprocal of the density-dependent survival rate at critical age-group is a measure of the fitness function. Using this measure, I predict the optimal age (body size) at first reproduction of a species of salamander. I also found that fitness calculated from observed values ofl(x) andm(x) includes a tautology. When the concept of fitness function is compared with the ESS method, the latter is more flexible. However, there is a possibility that an ESS is at the minimum of fitness function.     

Disturbance regimes and life-history evolution

Disturbance regimes are ecologically important, but many of their evolutionary consequences are poorly understood. A model is developed here that combines the within- and among-season dynamics of disturbances with evolutionary life-history theory. "Disturbance regime" is defined in terms of disturbance timing, frequency, predictability, and severity. The model predicts the optimal body size and time at which organisms should abandon a disturbance-prone growth habitat by maturing and moving to a disturbance-free, nongrowth habitat. The effects of both coarse-grained (those affecting the entire population synchronously) and fine-grained disturbances (those occurring in a patch dynamics setting) are explored. Several predictions are congruent with previous theory. Infrequent or temporally unpredictable disturbances should have little effect on the evolution of life-history strategies, even though they may cause high mortality. Similar to seasonal time constraints on reproduction, disturbance regimes can synchronize metamorphosis within a population, resulting in a seasonal decline in body size at maturity. Other model predictions are novel. When disturbances cause high mortality, coarse-grained disturbances have a much stronger effect on life-history strategies than fine-grained disturbances, suggesting that population structure (relative to the scale of disturbance) plays a critical evolutionary role when disturbances are severe. When within-population variance in juvenile body size is high, two consecutive seasonal declines in body size at maturity can occur, the first associated with disturbance regime and the second associated with seasonal time constraints.     

Stabilisation of walking by intrinsic muscle properties revealed in a three-dimensional muscle-driven simulation

A fundamental question in movement science is how humans perform stable movements in the presence of disturbances such as contact with objects. It remains unclear how the nervous system, with delayed responses to disturbances, maintains the stability of complex movements. We hypothesised that intrinsic muscle properties (i.e. the force–length–velocity properties of muscle fibres and tendon elasticity) may help stabilise human walking by responding instantaneously to a disturbance and providing forces that help maintain the movement trajectory. To investigate this issue, we generated a 3D muscle-driven simulation of walking and analysed the changes in the simulation's motion when a disturbance was applied to models with and without intrinsic muscle properties. Removing the intrinsic properties reduced the stability; this was true when the disturbing force was applied at a variety of times and in different directions. Thus, intrinsic muscle properties play a unique role in stabilising walking, complementing the delayed response of the central nervous system.     

Are Large, Infrequent Disturbances Qualitatively Different from Small, Frequent Disturbances?

In this article, we develop a heuristic model of ecosystem-disturbance dynamics that illustrates a range of responses of disturbance impact to gradients of increasing disturbance extent, intensity, or duration. Three general kinds of response are identified and illustrated: (a) threshold response, (b) scale-independent response, and (c) continuous response. Threshold responses are those in which the response curve shows a discontinuity or a sudden change in slope along the axis of increasing disturbance extent, intensity, or duration. The response threshold occurs at a point where the force of the disturbance exceeds the capacity of internal mechanisms to resist disturbance, or where new mechanisms of recovery become involved. Within this conceptual framework, we find that some unusually large or intense disturbances, but not all, produce qualitatively different responses compared with similar disturbances of lesser magnitude. If disturbance impact does not increase with increasing disturbance extent, intensity, or duration, or if the response curve changes monotonically, then large disturbances are not qualitatively different from small ones. For example, jack pine tends to become reestablished after stand-replacing fire in boreal forests, regardless of fire size, because its serotinous cones provide an adequate seed source throughout the burned area. Thus, large fires are not qualitatively different from small fires in terms of jack pine reproduction. However, if disturbance impact does increase abruptly at some point with increasing disturbance extent, intensity, or duration, often because of thresholds in the capacity of internal mechanisms to resist or respond to disturbance impact, then large disturbances are qualitatively different from small ones, at least for some parameters of ecological response. For example, balsam fir and white cedar can recolonize a small burned patch of boreal forest in close proximity to surviving individuals of these species, but they will be eliminated from a large burn because of their susceptibility to fire-caused mortality and their inability to disperse their seeds over long distances. The conceptual framework presented here permits some new insights into the dynamics of natural systems and may provide a useful tool with which managers can assess the potential for catastrophic damages resulting from large, infrequent disturbances. Received 14 July 1998; accepted 29 September 1998     

Linking radial growth patterns and moderate‐severity disturbance dynamics in boreal old‐growth forests driven by recurrent insect outbreaks: A tale of opportunities,successes, and failures

In boreal landscapes, emphasis is currently placed on close‐to‐nature management strategies, which aim to maintain the biodiversity and ecosystem services related to old‐growth forests. The success of these strategies, however, depends on an accurate understanding of the dynamics within these forests. While moderate‐severity disturbances have recently been recognized as important drivers of boreal forests, little is known about their effects on stand structure and growth. This study therefore aimed to reconstruct the disturbance and postdisturbance dynamics in boreal old‐growth forests that are driven by recurrent moderate‐severity disturbances. We studied eight primary old‐growth forests in Québec, Canada, that have recorded recurrent and moderately severe spruce budworm (Choristoneura fumiferana [Clem.]) outbreaks over the 20th century. We applied an innovative dendrochronological approach based on the combined study of growth patterns and releases to reconstruct stand disturbance and postdisturbance dynamics. We identified nine growth patterns; they represented trees differing in age, size, and canopy layer. These patterns highlighted the ability of suppressed trees to rapidly fill gaps created by moderate‐severity disturbances through a single and significant increase in radial growth and height. Trees that are unable to attain the canopy following the disturbance tend to remain in the lower canopy layers, even if subsequent disturbances create new gaps. This combination of a low stand height typical of boreal forests, periodic disturbances, and rapid canopy closure often resulted in stands constituted mainly of dominant and codominant trees, similar to even‐aged forests. Overall, this study underscored the resistance of boreal old‐growth forests owing to their capacity to withstand repeated moderate‐severity disturbances. Moreover, the combined study of growth patterns and growth release demonstrated the efficacy of such an approach for improving the understanding of the fine‐scale dynamics of natural forests. The results of this research will thus help develop silvicultural practices that approximate the moderate‐severity disturbance dynamics observed in primary and old‐growth boreal forests.     

Combined disturbances and the role of their spatial and temporal properties in shaping community structure

Disturbances are characteristic for many ecosystems. However, we still lack generalizations concerning their role in shaping communities, particularly when disturbances co-occur. To study such effects, we used a novel modeling approach that is unrestricted by a priori tradeoffs among specific plant traits, except for those generated by allocation principles. Thus, trait combinations were emergent properties associated with biotic and abiotic constraints. Specifically, we asked which traits dominate under specific disturbance regimes, whether single and combined disturbance regimes promote similar trait tradeoffs and how complex disturbance regimes affect species richness and functional diversity. Overall, disturbances’ temporal properties governed the outcome of combined disturbances and were a stronger assortative force than spatial disturbance properties: low temporal predictability decreased seed-dispersability and dormancy, but increased competitive ability and disturbance tolerance. Evidence for tradeoffs between different colonization modes and between dormancy and disturbance tolerance were found, while surprisingly, the widely accepted colonization–competition tradeoff was not generated. Diversity was highest at intermediate disturbance intensity, but decreased monotonically with increasing unpredictability. In accordance with our results, future models should avoid restrictive assumptions about tradeoffs to generate robust and more general predictions about the role of disturbances for community dynamics.     

Dealing with Uncertainty in Spatially Explicit Population Models

It has been argued that spatially explicit population models (SEPMs) cannot provide reliable guidance for conservation biology because of the difficulty of obtaining direct estimates for their demographic and dispersal parameters and because of error propagation. We argue that appropriate model calibration procedures can access additional sources of information, compensating the lack of direct parameter estimates. Our objective is to show how model calibration using population-level data can facilitate the construction of SEPMs that produce reliable predictions for conservation even when direct parameter estimates are inadequate. We constructed a spatially explicit and individual-based population model for the dynamics of brown bears (Ursus arctos) after a reintroduction program in Austria. To calibrate the model we developed a procedure that compared the simulated population dynamics with distinct features of the known population dynamics (=patterns). This procedure detected model parameterizations that did not reproduce the known dynamics. Global sensitivity analysis of the uncalibrated model revealed high uncertainty in most model predictions due to large parameter uncertainties (coefficients of variation CV 0.8). However, the calibrated model yielded predictions with considerably reduced uncertainty (CV 0.2). A pattern or a combination of various patterns that embed information on the entire model dynamics can reduce the uncertainty in model predictions, and the application of different patterns with high information content yields the same model predictions. In contrast, a pattern that does not embed information on the entire population dynamics (e.g., bear observations taken from sub-areas of the study area) does not reduce uncertainty in model predictions. Because population-level data for defining (multiple) patterns are often available, our approach could be applied widely.     

New mechanics of traumatic brain injury

The prediction and prevention of traumatic brain injury is a very important aspect of preventive medical science. This paper proposes a new coupled loading-rate hypothesis for the traumatic brain injury (TBI), which states that the main cause of the TBI is an external Euclidean jolt, or SE(3)-jolt, an impulsive loading that strikes the head in several coupled degrees-of-freedom simultaneously. To show this, based on the previously defined covariant force law, we formulate the coupled Newton–Euler dynamics of brain’s micro-motions within the cerebrospinal fluid and derive from it the coupled SE(3)-jolt dynamics. The SE(3)-jolt is a cause of the TBI in two forms of brain’s rapid discontinuous deformations: translational dislocations and rotational disclinations. Brain’s dislocations and disclinations, caused by the SE(3)-jolt, are described using the Cosserat multipolar viscoelastic continuum brain model.

How does spatio-temporal disturbance influence species diversity in a hierarchical competitive system? Prospective order of species coexistence and extinction

To address how habitat destruction and hierarchical competition among species affect the spatio-temporal dynamics of a multi-species community, we present a compartment model in which multiple species undergo dispersal and competitive interactions in a patchy habitat arranged in a two-dimensional lattice. We assume that disturbances are periodically imposed on some parts of the lattice in a block, followed by a period free of disturbance. For convenience, species are ranked in order of competitive ability. We further assume that the intrinsic growth rate of species i, _i , and the dispersal ability, D _i , increase in decreasing order of rank. Our model can analytically determine the exact number of surviving species when disturbance is absent. In the presence of disturbance, we numerically examine how spatio-temporal changes in environmental heterogeneity affect species coexistence and extinction, for the case in which the value of _i /D _i monotonically increases or decreases with rank. The results demonstrate that (1) when the interspecific competition is smaller than the intraspecific competition, we can provide predictions on the prospective order of species to be driven extinct and the order of potential species to revive with increasing extents of disturbance; (2) when the interspecific competition is stronger than intraspecific competition, a small difference in the disturbance level can lead to drastic changes in the species composition, their densities and the order of species extinction. In addition, comparison with other similar models reveals that differences in species interaction in local population dynamics critically affect the disturbance-mediated species diversity.     

Trajectory formation of arm movement by cascade neural network model based on minimum torque-change criterion

We proposed that the trajectory followed by human subject arms tended to minimize the time integral of the square of the rate of change of torque (Uno et al. 1987). This minimum torque-change model predicted and reproduced human multi-joint movement data quite well (Uno et al. 1989). Here, we propose a neural network model for trajectory formation based on the minimum torque-change criterion. Basic ideas of information representation and algorithm are(i) spatial representation of time,(ii) learning of forward dynamics and kinetics model and(iii) relaxation computation based on the acquired model. The model can resolve ill-posed inverse kinematics and inverse dynamics problems for redundant controlled object as well as ill-posed trajectory formation problems. By computer simulation, we show that the model can produce a multi-joint arm trajectory while avoiding obstacles or passing through viapoints.     

Sexual conflicts, loss of flight, and fitness gains in locomotion of polymorphic water striders

In insect wing polymorphism, morphs with fully developed, intermediate, and without wings are recognized. The morphs are interpreted as a trade‐off between flight and flightlessness; the benefits of flight are counterbalanced by the costs of development and the maintenance of wings and flight muscles. Such a trade‐off has been widely shown for reproductive and developmental parameters, and wing reduction is associated with species of stable habitats. However, in this context, the role of water locomotion performance has not been well explored. We chose seven water striders (Heteroptera: Gerridae) as a model to study this trade‐off and its relation to sexual conflicts, namely, Aquarius elongatus (Uhler), Aquarius paludum (Fabr.), Gerris insularis (Motschulsky), Gerris nepalensis Distant, Gerris latiabdominis Miyamoto, Metrocoris histrio (White), and Rhagadotarsus kraepelini Breddin. We estimated the locomotion performance as the legs’ stroke force, measured on tethered specimens placed on water with a force transducer attached to their backs. By dividing force by body weight, we made performance comparisons. We found a positive relationship between weight and force, and a negative one between weight and the force‐to‐weight ratio among species. The trade‐off between water and flight locomotion was manifested as differences in performance in terms of the force/weight ratio. However, the bias toward winged or wing‐reduced morphs was species dependent, and presumably related to habitat preference. Water strider species favouring a permanent habitat (G. nepalensis) showed higher performance in the apterous morph, but in those favouring temporary habitats (A. paludum and R. kraepelini) morphs’ performance did not differ significantly. Males had higher performance than females in all but three species studied (namely, A. elongatus, G. nepalensis, and R. kraepelini); these three have a type II mating strategy with minimized mating struggle. We hypothesized that in type I mating system, in which males must struggle strongly to subdue the female, males should outperform females to copulate successfully. This was not necessarily true among males of species with type II mating.     

Stride lengths and frequencies of arboreal walking in seven species of didelphid marsupials

Didelphid marsupials differ in their use of the forest strata, with corresponding differences in morphology and arboreal walking performances. Similar performances may be reached by different combinations of stride length and frequency, but it has been suggested that arboreal walkers increase velocity by longer strides. Our objective was to determine how stride length and frequency contribute to the velocity in the arboreal walking of seven species of didelphid marsupials of the Atlantic Forest of Brazil. Animals were stimulated to cross five 3-m long horizontal supports of different diameters. The cycle of maximum velocity was chosen to measure relative stride length, frequency, and relative velocity. Except forCaluromys philander, the more arboreal species were faster than the terrestrial species, but maximum velocity of arboreal species was reached by two strategies, increasing stride frequency (Gracilinanus microtarsus, Micoureus demerarae, andDidelphis aurita), or reducing frequency and increasing stride length (Marmosops incanus andC. philander). Increasing velocity in arboreal walking by more frequent strides may reduce oscillations of the body, whereas longer strides may reduce branch swaying. Among the terrestrial species,Philander frenatus performed similarly to more arboreal species, suggesting a potential ability to use the canopy, undetected in field observations.     

Terrestrial flatworm (Platyhelminthes: Tricladida: Terricola) diversity versus man-induced disturbance in an ombrophilous forest in southern Brazil

Terrestrial flatworms, or Terricola, are sensitive to environmentalchanges and therefore might be excellent indicators of the conservation statusof natural habitats. The present study aimed to answer two main questions: (1)is terrestrial planarian diversity affected by human disturbances, and (2) isthere any species or group of species that indicates such disturbance? The studysite, National Forest of São Francisco de Paula, Brazil, was originallycovered by a mixed ombrophilous forest, but successive reforestations andselective logging have modified the original landscape. We studied Terricoladiversity in the four main habitats in the study area: mixed ombrophilous forest(NA), ombrophilous forest with selective Araucariaangustifolia logging (N), A. angustifoliareforestation (A), and reforestation of Pinus elliottii(P). According to an increasing degree of disturbance, the habitats might beordered as follows: (NA)<(N)<(A)<(P). We conducted 24 surveys in eachhabitat over a period of 1 year. Our results indicate that: (1)Terricola diversity is inversely related to the degree of habitat disturbance;(2) there are species (Geoplana franciscana,Geoplana sp. 5, and possibly Geoplanidae 3 andNotogynaphallia guaiana) that prefer habitats located onthe extreme right along the main axis of a detrended correspondence analysisordination and therefore can be considered as indicators of well preserved,natural habitats. On the other hand there are species(Xerapoa sp. 1, Choeradoplanaiheringi, G. marginata sensu Marcus andGeoplana sp. 2) preferring more disturbed habitats, whichmay form biological indicators of such disturbances.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Purposive behavior and cognitive mapping: a neural network model

This study presents a real-time, biologically plausible neural network approach to purposive behavior and cognitive mapping. The system is composed of (a) an action system, consisting of a goal-seeking neural mechanism controlled by a motivational system; and (b) a cognitive system, involving a neural cognitive map. The goal-seeking mechanism displays exploratory behavior until either (a) the goal is found or (b) an adequate prediction of the goal is generated. The cognitive map built by the network is a top logical map, i.e., it represents only the adjacency, but not distances or directions, between places. The network has recurrent and non-recurrent properties that allow the reading of the cognitive map without modifying it. Two types of predictions are introduced: fast-time and real-time predictions. Fast-time predictions are produced in advance of what occurs in real time, when the information stored in the cognitive map is used to predict the remote future. Real-time predictions are generated simultaneously with the occurrence of environmental events, when the information stored in the cognitive map is being updated. Computer simulations show that the network successfully describes latent learning and detour behavior in rats. In addition, simulations demonstrate that the network can be applied to problem-solving paradigms such as the Tower of Hanoi puzzle.     

Structure and regeneration ofFraxinus spaethiana-Pterocarya rhoifolia forests in unstable valleys in the Chichibu Mountains,central Japan

The structure of temperate deciduous forests dominated byFraxinus spaethiana andPterocarya rhoifolia in V-shaped valleys in the Chichibu Mountains, central Japan was studied to clarify their dynamics, with special reference to disturbance. Forest structure and annual ring analysis revealed that the forests have been subjected to mass movement (mud flow) in the past. The size and age structure, and diameter growth curves, indicated thatF. spaethiana is relatively shade tolerant, long living and slow-growing, whileP. rhoifolia is less shade tolerant, short living, but fast growing.Pterocarya rhoifolia appears to depend on large-scale but rare disturbances.Fraxinus spaethiana establishes its seedlings on relatively unstable substrate in V-shaped valleys, but once established, they survive a long time with high shade tolerance. The dominance and coexistence of the two species may primarily depend on the intensity and frequency of geomorphological processes.     

Habitat gradient and reproductive habits of the sevenStellaria species in Japan

The author studied the habitat preference, life form, life cycle and reproductive characteristics of the seven species of the genusStellaria along a valley in the northeastern part of Kyoto City. Along the valley 150 quadrats were set from the village up to the hills. The quadrats were sorted into 15 quadrat groups by their affinity in the environment. The quadrat groups were ordinated by the quotients of community and the Bray-Curtis method in order to reveal the complex gradient in the environment. The complex gradient could be best understood by the degree of human disturbance. The seven species showed a continuous distribution along the complex gradient. There existed three patterns in their habitat preference and reproductive characteristics: (1)S. media andS. neglecta grew as annuals in the habitat of severe human disturbance. (2)S. tomentella, S. sessiliflora, andS. diversiflora grew as perennials in the habitat of slight human disturbance. (3)S. aquatica andS. alsine grew as facultative annuals either in the habitat of severe or slight human disturbance.