Effects of stance width on postural movement pattern and anticipatory postural control associated with unilateral arm abduction

We investigated the effects of stance width on postural movement pattern and activation timing of postural muscles during unilateral arm abduction. Thirty-two healthy subjects abducted the right arm at their own timing. Stance width was 0, 9, 18 or 27 cm. Movement angles of leg lateral inclination and trunk lateral flexion to the leg in the frontal plane were analyzed. Based on movement angles at 0 cm width, subjects were classified into three groups: contralateral whole body leaning (CWBLg); ipsilateral trunk flexion (ITFg); and contralateral trunk flexion (CTFg). A high correlation between the movement angles was obtained at 0 cm width (r = 0.82). With increasing stance width, postural movement pattern in the ITFg shifted to patterns characterized by lateral flexion of the trunk toward the side opposite to arm movement, and movement angle of leg-inclination in ITFg and CWBLg decreased. At 0 cm width, left gluteus medius and tensor fascia latae were activated significantly about 40 ms ahead of the right middle deltoid in CWBLg and CTFg, but not in ITFg. However, preceding activation became prominent (about 20 ms) in ITFg for wide stances. Moreover, bilateral activation of the tensor fascia latae was observed in CTFg for all widths.     

A new method to analyze postural stability during a transition task from double-leg stance to single-leg stance

Time to stabilization (TTS) has been introduced as a method to analyze dynamic postural stability during jump and landing tasks, but has also been applied during the transition task from double-leg stance (DLS) to single-leg stance (SLS). However, the application of the originally described TTS technique during the latter task has some important limitations. The first goal of this study was to present an adapted version of the TTS technique to provide an effective alternative method to better analyze postural stability during the transition from DLS to SLS. The second goal was to study the influence of pathology and different speeds on postural stability outcomes. Fifteen healthy control subjects and 15 subjects with chronic ankle instability (CAI) performed the transition task on their preferred speed and as fast as possible, with eyes open and with eyes closed. Subjects with CAI performed the transition significantly slower when moving at their preferred speed with eyes closed. The time subjects needed to reach a new stability point was not discriminative between groups and largely dependent on movement speed. However, the amount of sway after this new stability point was significantly increased in the CAI group and when eyes were closed. The results of this study suggest that subjects with CAI have a decreased ability to overcome the postural perturbation created by the voluntary movement from DLS to SLS. Focusing only on TTS during the transition from DLS to SLS may lead at least in some cases to misinterpretations when assessing postural stability.     

How area restricted search of a pelagic seabird changes while performing a dual foraging strategy

Movements and foraging strategies of marine predators should cope with the hierarchical spatial distribution of resources. Therefore, in order to predict the at‐sea distribution of aerial predators, it is crucial to understand the factors governing trajectory decisions at different scales. Using first passage time (FPT) analysis on precision tracking information (GPS‐loggers data) we were able to examine the foraging strategy of Cory's shearwaters Calonectris diomedea and to detect the adoption of area‐restricted search (ARS), measuring the scale and duration of this behaviour. Data were collected from three different populations foraging in different oceanographic conditions. During long excursions birds only commuted between their colony and prey patches, while on their short movements birds increased the amount of looping movements. On short trips, birds addopted ARS behaviour at an average scale of 18 km and at a second nested scale of around 2 km. When engaging in long trips, first scale of ARS occurred on average at about 67 km of radii and than a second nested scale at a radii of 24 km. Overall, the different populations showed foraging patterns matching the habitats exploited: a) at smaller scales of ARS, sea‐surface temperature, chlorophyll‐a concentration and depth influenced the time of residence (i.e. FPT) of birds (with variations at a population level); b) at larger scales of ARS, FPT increased within regions of higher gradients of sea‐surface temperature, chlorophyl‐a concentration and depth. This study demonstrates that Cory's shearwaters adopt scale‐dependent adjustments of movement in relation to the hierarchical distribution of the environment they exploit, matching the scale and duration of ARS with the hierarchical distribution of the environmental features.     

Effect of a dual task on postural control in dyslexic children

Several studies have examined postural control in dyslexic children; however, their results were inconclusive. This study investigated the effect of a dual task on postural stability in dyslexic children. Eighteen dyslexic children (mean age 10.3±1.2 years) were compared with eighteen non-dyslexic children of similar age. Postural stability was recorded with a platform (TechnoConcept®) while the child, in separate sessions, made reflex horizontal and vertical saccades of 10° of amplitude, and read a text silently. We measured the surface and the mean speed of the center of pressure (CoP). Reading performance was assessed by counting the number of words read during postural measures. Both groups of children were more stable while performing saccades than while reading a text. Furthermore, dyslexic children were significantly more unstable than non-dyslexic children, especially during the reading task. Finally, the number of words read by dyslexic children was significantly lower than that of non-dyslexic children and, in contrast to the non-dyslexic children. In line with the U-shaped non-linear interaction model, we suggest that the attention consumed by the reading task could be responsible for the loss of postural control in both groups of children. The postural instability observed in dyslexic children supports the hypothesis that such children have a lack of integration of multiple sensorimotor inputs.     

An adaptive model of sensory integration in a dynamic environment applied to human stance control

An adaptive estimator model of human spatial orientation is presented. The adaptive model dynamically weights sensory error signals. More specific, the model weights the difference between expected and actual sensory signals as a function of environmental conditions. The model does not require any changes in model parameters. Differences with existing models of spatial orientation are that: (1) environmental conditions are not specified but estimated, (2) the sensor noise characteristics are the only parameters supplied by the model designer, (3) history-dependent effects and mental resources can be modelled, and (4) vestibular thresholds are not included in the model; instead vestibular-related threshold effects are predicted by the model. The model was applied to human stance control and evaluated with results of a visually induced sway experiment. From these experiments it is known that the amplitude of visually induced sway reaches a saturation level as the stimulus level increases. This saturation level is higher when the support base is sway referenced. For subjects experiencing vestibular loss, these saturation effects do not occur. Unknown sensory noise characteristics were found by matching model predictions with these experimental results. Using only five model parameters, far more than five data points were successfully predicted. Model predictions showed that both the saturation levels are vestibular related since removal of the vestibular organs in the model removed the saturation effects, as was also shown in the experiments. It seems that the nature of these vestibular-related threshold effects is not physical, since in the model no threshold is included. The model results suggest that vestibular-related thresholds are the result of the processing of noisy sensory and motor output signals. Model analysis suggests that, especially for slow and small movements, the environment postural orientation can not be estimated optimally, which causes sensory illusions. The model also confirms the experimental finding that postural orientation is history dependent and can be shaped by instruction or mental knowledge. In addition the model predicts that: (1) vestibular-loss patients cannot handle sensory conflicting situations and will fall down, (2) during sinusoidal support-base translations vestibular function is needed to prevent falling, (3) loss of somatosensory information from the feet results in larger postural sway for sinusoidal support-base translations, and (4) loss of vestibular function results in falling for large support-base rotations with the eyes closed. These predictions are in agreement with experimental results. Received: 12 November 1999 / Accepted in revised form: 30 June 2000     

Dominance by a canopy forming seaweed modifies resource and consumer control of bloom-forming macroalgae

Degradation of ecological resources by large-scale disturbances highlights the need to demonstrate biological properties that increase resistance to change and promote the resilience of ecosystem regimes. Coastal eutrophication is a global-scale disturbance that drives ecosystem change by increasing primary production and favouring ephemeral and bloom-forming life-forms. Recent synthesis indicates that consumption processes increase the resistance of coastal communities to nutrient loading by controlling the responses of ephemeral macroalgae. Here we suggest a similar ecological function for canopy cover by demonstrating that the presence of a canopy species modifies both resource and consumer control of bloom-forming algae associated with nutrient enrichment. We tested effects of canopy presence on the interaction between consumer and resource control, by field-manipulations of a dominant canopy forming seaweed ( Fucus vesiculosus ), grazer presence (dominated by the gastropod Littorina littorea ) and nutrient enrichment (common agricultural NPK fertilizer). Canopy cover and grazers jointly controlled strong increases of ephemeral bloom-forming algae (dominated by Ulva spp) from nutrient enrichment; nutrients increased ephemeral recruitment almost 10-fold, but only in the absence of both grazers and canopy cover. Recruitment success of the canopy-forming seaweed itself decreased additively with 56.1, 71.3 and 50.5% from independent effects of canopy cover, grazers and nutrient enrichment, respectively. A meta-analysis of nine nutrient enrichment experiments including seaweed, seagrass and stream communities, showed that in the presence of canopies average nutrient effects were reduced by more than 90% compared to without canopies. This corroborates the generality of our finding that dominating canopy species are important for aquatic ecosystems by increasing community resistance to the propagation of nutrient effects.     

Analysis of the multi-segmental postural movement strategies utilized in bipedal,tandem and one-leg stance as quantified by a principal component decomposition of marker coordinates

Postural control research describes ankle-, hip-, or multi-joint strategies as mechanisms to control upright posture. The objectives of this study were, first, development of an analysis technique facilitating a direct comparison of the structure of such multi-segment postural movement patterns between subjects; second, comparison of the complexity of postural movements between three stances of different difficulty levels; and third, investigation of between-subject differences in the structure of postural movements and of factors that may contribute to these differences.     

Evaluation of a generalized model of human postural dynamics and control in the sagittal plane

A two-step identification method is used to evaluate a generalized model of human postural control in the sagittal plane. Postural dynamics are represented as a planar open-chain linkage system supported by a triangular foot. The control mechanism is modeled as a state feedback element in which the torque acting at a given link is an arbitrary function of the state variables — angles and angular velocities. To validate the approach, six normal subjects underwent two series of experiments. The first series were used to determine an appropriate model of the system dynamics. The second series were used to estimate the parameters of the feedback model. A computer simulation of the complete system shows that the model predictions closely match the observed responses. These results suggest that the proposed model provides a useful framework for analysis of postural control mechanisms.This work was supported by the National Institutes of Health under Grant NS 21363     

How a RING finger protein and a steroid hormone control autophagy

Previous work in our laboratory has indicated that the steroid hormone ecdysone triggers programmed autophagy in the fat body of Drosophila larvae by downregulating the class I phosphoinositide 3-kinase (PI3K) pathway. We recently found evidence that Deep orange (Dor), a Drosophila RING finger protein implicated in late-endosomal trafficking, controls ecdysone signaling as well as autolysosome fusion, thus exerting a dual regulation of autophagy. We found that dor mutants are defective in programmed autophagy. The mutant larvae showed impaired upregulation of ecdysone signaling during development, accompanied by a failure to downregulate the PI3K pathway. Downregulation of the PI3K pathway could be restored by feeding the dor mutants with ecdysone. Even though ecdysone signaling and autophagy were impaired in the dor mutants, we detected an accumulation of autophagosomes in dor mutant fat bodies. This could probably be attributed to the failure of autophagosomes to fuse with lysosomes. In this Addendum we review these findings and provide some speculations about how Dor may control both ecdysone signalling and autolysosomal fusion.     

Functional postural responses after perturbations in multiple directions in a standing man: a principle of decoupled control

The objective of this study was to assess functional postural responses by analyzing the net joint torques (NJT) in the ankles and the hips resulting from perturbations delivered in multiple directions to subjects standing quietly. A total of eight subjects were standing on two force platforms while an apparatus randomly delivered controlled perturbations at the level of the pelvis in eight directions: anterio-posterior (AP), medio-lateral (ML), and four combinations of these principal directions. Perturbations were repeated five times in each direction for six conditions (i.e., three different perturbation strengths and three different feet orientations). The comparison of the averaged ankle sum NJT (AP) responses showed that the time courses of the responses elicited by a perturbation acting only in the AP direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. In contrast the observed averaged ankle NJT (ML) responses did not follow the same similarity. The comparison of the averaged ankle and hip sum NJT (ML) responses revealed that the time courses of the responses elicited by a perturbation acting only in the ML direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. These findings were invariable of the experimental conditions and were consistent among all the eight subjects. Thereby, we conclude that the ankle sum NJT (AP) and the ankle and hip sum NJT (ML) are the global variables being controlled. This shows that CNS controls the recovery from the multiple direction perturbations of moderate strength by decoupling the AP-ML postural space into two orthogonal directions (AP and ML).     

Rapid evolution of a heteroplasmic repetitive sequence in the mitochondrial DNA control region of carnivores

We describe a repetitive DNA region at the 3 end of the mitochondrial DNA (mtDNA) control region and compare it in 21 carnivore species representing eight carnivore families. The sequence and organization of the repetitive motifs can differ extensively between arrays; however, all motifs appear to be derived from the core motif ACGT. Sequence data and Southern blot analysis demonstrate extensive heteroplasmy. The general form of the array is similar between heteroplasmic variants within an individual and between individuals within a species (varying primarily in the length of the array, though two clones from the northern elephant seal are exceptional). Within certain families, notably ursids, the array structure is also similar between species. Similarity between species was not apparent in other carnivore families, such as the mustelids, suggesting rapid changes in the organization and sequence of some arrays. The pattern of change seen within and between species suggests that a dominant mechanism involved in the evolution of these arrays is DNA slippage. A comparative analysis shows that the motifs that are being reiterated or deleted vary within and between arrays, suggesting a varying rate of DNA turnover. We discuss the evolutionary implications of the observed patterns of variation and extreme levels of heteroplasmy.By acceptance of this article, the publisher acknowledges the right of the US Government to retain non-exclusive, royalty-free license in and to any copyright covering the article. Correspondence to: A.R. Hoetzel     

Resonance in a mathematical model of baroreflex control: arterial blood pressure waves accompanying postural stress

A mathematical model of the arterial baroreflex was developed and used to assess the stability of the reflex and its potential role in producing the low-frequency arterial blood pressure oscillations called Mayer waves that are commonly seen in humans and animals in response to decreased central blood volume. The model consists of an arrangement of discrete-time filters derived from published physiological studies, which is reduced to a numerical expression for the baroreflex open-loop frequency response. Model stability was assessed for two states: normal and decreased central blood volume. The state of decreased central blood volume was simulated by decreasing baroreflex parasympathetic heart rate gain and by increasing baroreflex sympathetic vaso/venomotor gains as occurs with the unloading of cardiopulmonary baroreceptors. For the normal state, the feedback system was stable by the Nyquist criterion (gain margin = 0.6), but in the hypovolemic state, the gain margin was small (0.07), and the closed-loop frequency response exhibited a sharp peak (gain of 11) at 0.07 Hz, the same frequency as that observed for arterial pressure fluctuations in a group of healthy standing subjects. These findings support the theory that stresses affecting central blood volume, including upright posture, can reduce the stability of the normally stable arterial baroreflex feedback, leading to resonance and low-frequency blood pressure waves.     

Comparative analysis of the dynamics of human postural control during fixation and pursuit of a visual target

The properties of the system maintaining the upright posture were compared in different states of the oculomotor system: during target fixation and horizontal fast and slow pursuit (0.1 and 0.01 Hz), recording the trajectories of the center of pressure in the frontal and the sagittal planes. Methods of nonlinear analysis were applied to assess the similarity in pairwise comparisons. The overall similarity of the frontal plane dynamics proved to be higher than that of the sagittal plane dynamics. However, differences were revealed in fast pursuit versus slow pursuit or fixation in the frontal but not in the sagittal plane. Such differences may reflect the different inertia of the oculomotor and the balance control systems. In general, the results are consistent with the current notions on the two orthogonal subsystems of postural control.     

Neural control of a cyclic postural behavior in the crayfish,Procambarus clarkii: the pattern-initiating interneurons

As part of its repertoire of defensive behaviors, the crayfish, Procambarus clarkii, may respond to mildly threatening tactile or visual stimuli from the front of its body by walking backwards. During this behavior, the abdomen undergoes complex cyclical movements involving flexion and extension of the postural musculature which cause the tail to alternately contact and withdraw from the substrate. Intracellular neuropil recordings and dye injections were used to search for the interneurons responsible for initiating this postural motor pattern in the crayfish abdomen. Several diverse morphological types of interganglionic pattern-initiating (PI) interneurons were found. Each interneuron, when driven intracellularly, was capable of eliciting the same motor program, in its entirety, throughout the abdominal nerve cord. During pattern generation, PI interneurons exhibited a burst of spikes preceding the motor output. Silencing single PI interneurons with hyperpolarizing current during pattern generation failed to affect the motor program, indicating a redundancy of pattern-initiating function. The observations of extensive dye-coupling with other parallel axons, consistent dye-coupling with other identified cells in the pattern-initiating system, and the presence of multiple spike amplitudes in the bursts suggested electrotonic coupling among the PI interneurons. An additional group of interganglionic interneurons, the partial pattern-initiating (PPI) interneurons, were found to comprise a significant subset of the pattern-initiating system. As with the PI cells, the PPI interneurons exhibited a complex burst of spikes just preceding the patterned motor program. However, the PPI interneurons were only capable of eliciting an incomplete, though recognizable, postural motor pattern. Silencing any PPI interneuron during pattern generation caused a deficit in the motor pattern, indicating either an absence or lesser degree of functional redundancy within the PPI interneuron population compared to that occurring within the PI interneuron group. We conclude that a large number of PI interneurons are presynaptic to a relatively small group of PPI interneurons which, in turn, conduct pattern-initiating signals to the ganglionic oscillators. Our results indicate that pattern-initiation is accomplished through a command system involving multiple command elements organized in a coordinated interganglionic network.     

Non-linear stimulus-response behavior of the human stance control system is predicted by optimization of a system with sensory and motor noise

We developed a theory of human stance control that predicted (1) how subjects re-weight their utilization of proprioceptive and graviceptive orientation information in experiments where eyes closed stance was perturbed by surface-tilt stimuli with different amplitudes, (2) the experimentally observed increase in body sway variability (i.e. the “remnant” body sway that could not be attributed to the stimulus) with increasing surface-tilt amplitude, (3) neural controller feedback gains that determine the amount of corrective torque generated in relation to sensory cues signaling body orientation, and (4) the magnitude and structure of spontaneous body sway. Responses to surface-tilt perturbations with different amplitudes were interpreted using a feedback control model to determine control parameters and changes in these parameters with stimulus amplitude. Different combinations of internal sensory and/or motor noise sources were added to the model to identify the properties of noise sources that were able to account for the experimental remnant sway characteristics. Various behavioral criteria were investigated to determine if optimization of these criteria could predict the identified model parameters and amplitude-dependent parameter changes. Robust findings were that remnant sway characteristics were best predicted by models that included both sensory and motor noise, the graviceptive noise magnitude was about ten times larger than the proprioceptive noise, and noise sources with signal-dependent properties provided better explanations of remnant sway. Overall results indicate that humans dynamically weight sensory system contributions to stance control and tune their corrective responses to minimize the energetic effects of sensory noise and external stimuli.     

Frequency dependence of the action-perception cycle for postural control in a moving visual environment: relative phase dynamics

When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion. Received: 7 September 1993/Accepted in revised form: 2 May 1994