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Results for “Postural control”

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PubMed1992

Visual destabilisation of posture in normal subjects.

A new and simple method of assessing reliance on vision for postural control was evaluated in 41 normal subjects. Left-right reversal of peripheral vision induced by a head-mounted mirror device caused an initial dramatic instability in approximately half the subjects, when standing on foam to reduce the value of proprioception. Lateral reversal of central vision by means of a prism device evoked similar responses. Sensitivity to vision reversal was significantly correlated with motion sickness susceptibility. Despite some rapid habituation (partially retained over several weeks) sway remained as great as with eye closure in the anterior-posterior as well as lateral direction, indicating complete suppression of the visual input. Balancing with vision reversal caused a selective decrement in performance of a visuo-spatial memory task, suggesting that coping with misleading visual input may place continuous demands on cortical spatial processing.

Adult↗

PubMed2005

Slow dynamics of postural sway are in the feedback loop.

Postural sway is considered to have two fundamental stochastic components, a slow nonoscillatory component and a faster damped-oscillatory component. The slow component has been shown to account for the majority of sway variance during quiet stance. Postural control is generally viewed as a feedback loop in which sway is detected by sensory systems and appropriate motor commands are generated to stabilize the body's orientation. Whereas the mechanistic source for the damped-oscillatory sway component is most likely feedback control of an inverted pendulum, the underlying basis for the slow component is less clear. We investigated whether the slow process was inside or outside the feedback loop by providing standing subjects with sum-of-sines visual motion. Linear stochastic models were fit to the experimental sway trajectories to determine the stochastic structure of sway as well as the transfer function from visual motion to sway. The results supported a fifth-order stochastic model, consisting of a slow process and two damped-oscillatory components. Importantly, the slow process was determined to be inside the feedback loop. This supports the hypothesis that the slow component is due to errors in state estimation because state estimation is inside the feedback loop rather than a moving reference point or an exploratory process outside the feedback loop.

Adult↗

PubMed1986

Axial deviations of the spine in blind children.

The authors screened blind children from a special school to determine whether there is any correlation between idiopathic scoliosis and changes in the postural control system induced by blindness. Children with disabilities in addition to blindness, and children under the age of 8 years were excluded. They selected a homogeneous sample of 46 children of whom 39 were congenitally blind and 7 had become blind after birth. The results in the congenitally blind group were highly significant both quantitatively (59% exhibited structural deviations of the spine) and qualitatively. Apart from 2 cases they all exhibited multiple short scoliotic curves with unique characteristics rarely seen in sighted subjects. Despite this, the authors feel unable to make any categorical assertions regarding the possible role played by functional changes in the exterosensory and proprioceptive organs in the pathogenesis of idiopathic scoliosis. They suggest that the development of such multiple small curves in congenital blindness is simply the response of a congenitally deprived mechanism of body development to achieve stable equilibrium in the spine.

Adolescent↗

PubMed1984

Visual stabilization of posture. Physiological stimulus characteristics and clinical aspects.

Visual stabilization of posture is critically dependent on stimulus characteristics as well as on the performance of the visual system. The purpose of the present investigation was to obtain quantitative data in man by means of posturography of fore-aft and lateral body sway in relation to different visual stimulus characteristics. Visual acuity, when decreased logarithmically, causes a linearly increasing postural instability, twice as prominent for fore-aft than for lateral sway. Any measurable visual contribution for fore-aft sway ceases with an acuity lower than 0.03 and for lateral sway with an acuity lower than 0.01. The central area of the visual field as compared with the peripheral retina dominates postural control. The foveal region exhibits a powerful contribution, in particular for lateral sway. A partial but significant visual stabilization is preserved with a visual input rate between 1 to 4 Hz flicker frequency. As soon as continuous motion perception becomes involved with frequencies higher than 4 Hz, visual stabilization gradually improves with a saturation at frequencies higher than 16 Hz. Lateral body sway activity and eye-object distance are linearly related: body sway decreases with increasing distance corresponding to the linear decrease of net retinal displacement with increasing eye-object distance. Aspects of 'afferent' and 'efferent' visual motion perception, which involve fore-aft and lateral body sway differently, are evaluated. The clinical relevance is demonstrated in patients with oculomotor disturbances. The results are discussed with respect to the variety of related clinical disorders, which involve reduction in visual acuity, field defects, accommodation disturbances and ocular oscillations.

Accommodation, Ocular↗

PubMed1997

The role of vision and spatial orientation in the maintenance of posture.

This article reviews and analyzes the role of vision and spatial orientation in maintaining posture and balance. The key issues that relate to the development of postural control across the life span are discussed. Use of vision as a critical source of information that specifies spatial orientation in the environment is considered. We argue that the visual system functions as part of the perception-action cycle as promoted in ecological psychology by James Gibson. We compare and contrast theory and evidence of both standard and ecological accounts of how the visual system perceives the information and the findings relative to the role of the retinal vision in processing and acting on information related to motion. Changes in the ambient optical array (optical flow) as a non-force field are compared with gravity-based perturbations relative to the possible influence of the non-force field to changes in the motor system. Finally, a summary of some of our own work is presented, with comments about implications for further research and possible applications to clinical practice.

Humans↗

PubMed2003

Variability in constraints and functional competence in adults with multiple sclerosis.

OBJECTIVE: Examine intraindividual change in physical and psychological impairments associated with multiple sclerosis and assess the relationship between changes in specific deficits and functional competence in activities of daily living. DESIGN: A multivariate, replicated, single-subject, repeated-measures design was used to examine variability patterns across subjects. Five adults with multiple sclerosis were assessed on leg strength, upright postural control, mood, fatigue, stress, and self-efficacy for 4 mo. Functional competence in three activities of daily living was also evaluated. P-technique factor analyses were performed to examine which variables covaried with time. RESULTS: Analyses revealed covariation among physical and psychological variables for four of the five participants. Across all participants, coefficients of variation revealed greater variability in stress (32.3%-53.4%) and fatigue (23.4%-5.9%) than in any of the physical variables (<20%), and variability in all impairments was greater than variability in the functional tasks (0%-12.6%). CONCLUSIONS: The greater stability in functional performance compared with both physical and psychological constraints provides important insight into symptom management and clinical intervention. The magnitude of variability found in the psychological variables across individuals also has implications concerning psychological adjustment to the disease.

Activities of Daily Living↗

PubMed1990

Postural dynamics of walking in humans.

The dynamics of postural control in human biped locomotion were studied using (1) a model, and (2) experimentally applied impulsive force disturbances. The model was planar, and contained five rigid segments, articulating at frictionless pin joints. The model was used to identify joint torque combinations which would successfully correct for an impulsive force disturbance applied at different points in the walking cycle. The simulation results suggested that (1) early responses (within 80 ms) can be effective in compensating for impulsive disturbances, (2) the same strategies which successfully counteract similar disturbances during quiet standing are also effective in certain phases of the walking cycle, (3) modifications in the response strategies are needed to accommodate differences in the dynamics over the stride cycle, and (4) the swing leg is ineffective in compensating for disturbances in the short term. These model predictions were tested experimentally. Subject responses to an impulsive force disturbance applied during walking were studied. The electromyographic results generally support the model predictions.

Computer Simulation↗

PubMed1997

Stability limits for visual feedback posturography in vestibular rehabilitation.

Vestibular rehabilitation therapy (VRT) is used to enhance compensation after peripheral vestibular lesion. In visual feedback posturography (VFP) a patient can practice postural control actively on a force platform and progress in the VRT can be objectively measured. The aim of the study was to determine normative values for stability limits (SL) in the VFP. Eleven healthy subjects were measured with force platform in visual feedback condition. Subjects were instructed to move their center of gravity (COG) marker to given targets, which were displayed on a computer screen in front of them. Target distance from the center of the platform was increased step by step to determine the SL in 8 directions. Hit delays (HD) to the targets and hold percentages (HP) within the targets were measured to quantify the difficulty of the individual targets as a function of subject's shoe size. With fixed feet position the SL in anterior direction increased 1.2 times the absolute increase in shoe length. An almost similar increase was found in anterolateral direction. The knowledge of the mechanical SL is important, because targets near SL should be avoided for safety reasons.

Adult↗

PubMed2005

Variability of motoneuron activation and the modulation of force production in a postural reflex of the hermit crab abdomen.

The tri-phasic reflex in hermit crab (Pagurus pollicarus) abdomen is triggered by local mechanoreceptors and is essential for postural control. The reflex consists of three stereotypical phases: a brief, high-frequency burst, a transient cessation of firing, and a late-discharge that is much lower in frequency than the initial burst. To better understand the reflex generation of force, variability of motoneuron discharge in each of five parameters of reflex activation was assessed. An intracellular current injection routine was used to correlate each of these parameters with force production. Phase 3 motoneuron firing frequency showed the greatest correlation with force production. Phase 3 spike rate increased as a function of phase 2 duration, but the relationship between phase 2 duration and force produced by the reflex was weak. Junction potential amplitude decreased as phase 2 duration increased, and we hypothesize that this trend counteracts the increased phase 3 frequency, explaining the weak relationship of phase 2 duration and force production. Surprisingly, when phase 3 frequency was held constant and phase 2 was increased in duration, the concurrent decrease in junction potential amplitude did not reduce force production.

Abdomen↗

PubMed2006

Evidence for positive force feedback during involuntary aftercontractions.

We examined the involuntary isotonic muscle aftercontraction which follows a 1-min voluntary contraction at 60% of maximum voluntary force. Aftercontractions provide a unique paradigm for investigating the automatic postural control of the upper limb in man. Previous work has shown that the strength of these contractions in m. deltoid is linearly dependent on the inclination of the subject's body in the gravitational field. The current experiments were designed to test if this was due to the gravitational loading on the arm as this also changes with body inclination. The subject sat with the head stationary to prevent changes in vestibular orientation. Aftercontractions in m. deltoid were evoked with the normal weight of the arm present and also with arm loadings equal to 0, 25, 50 and 75% of this value. These reductions were produced by a counter lever apparatus. The results showed that the EMG amplitude of this involuntary contraction depended linearly on the loading of the arm. This is interpreted as being the result of positive load feedback. Aftercontraction EMG amplitude at 100% load was 2.7 times greater than at 0% load. The results are discussed in the context of recent work on positive load feedback in man and animals.

Adult↗

PubMed2006

Multisensory reweighting of vision and touch is intact in healthy and fall-prone older adults.

Unexplained falls in older adults are thought to arise from subtle deficits in multiple components of the postural control system, including peripheral sensory loss and central sensory processing. One commonly proposed central sensory processing deficit is a decline in the adaptive use of changing or conflicting sensory inputs for estimating body dynamics, i.e., multisensory reweighting. We examined the assumption of impaired multisensory reweighting in healthy and fall-prone older adults using quantitative methods that have previously demonstrated reweighting in young adults. Standing subjects were exposed to simultaneous medio-lateral oscillatory visual and fingertip touch inputs at varying relative amplitudes. No group differences in overall levels of vision and touch gain were found. Both healthy and fall-prone older adults demonstrated the same pattern of adaptive gain change as healthy young adults. Like the young adults, both elderly groups displayed clear evidence of intra- and inter-sensory reweighting to both vision and touch motion stimuli. These data suggest that, for small amplitude vision and touch stimuli, the central sensory reweighting adaptation process remains intact in healthy and fall-prone older adults with sufficiently intact peripheral sensation.

Accidental Falls↗

PubMed2003

Movement systems as dynamical systems: the functional role of variability and its implications for sports medicine.

In recent years, concepts and tools from dynamical systems theory have been successfully applied to the study of movement systems, contradicting traditional views of variability as noise or error. From this perspective, it is apparent that variability in movement systems is omnipresent and unavoidable due to the distinct constraints that shape each individual's behaviour. In this position paper, it is argued that trial-to-trial movement variations within individuals and performance differences observed between individuals may be best interpreted as attempts to exploit the variability that is inherent within and between biological systems. That is, variability in movement systems helps individuals adapt to the unique constraints (personal, task and environmental) impinging on them across different timescales. We examine the implications of these ideas for sports medicine, by: (i) focusing on intra-individual variability in postural control to exemplify within-individual real-time adaptations to changing informational constraints in the performance environment; and (ii) interpreting recent evidence on the role of the angiotensin-converting enzyme gene as a genetic (developmental) constraint on individual differences in physical performance. The implementation of a dynamical systems theoretical interpretation of variability in movement systems signals a need to re-evaluate the ubiquitous influence of the traditional 'medical model' in interpreting motor behaviour and performance constrained by disease or injury to the movement system. Accordingly, there is a need to develop new tools for providing individualised plots of motor behaviour and performance as a function of key constraints. Coordination profiling is proposed as one such alternative approach for interpreting the variability and stability demonstrated by individuals as they attempt to construct functional, goal-directed patterns of motor behaviour during each unique performance. Finally, the relative contribution of genes and training to between-individual performance variation is highlighted, with the conclusion that dynamical systems theory provides an appropriate multidisciplinary theoretical framework to explain their interaction in supporting physical performance.

Adaptation, Physiological↗

PubMed2007

Critical illness VR rehabilitation device (X-VR-D): evaluation of the potential use for early clinical rehabilitation.

We present a new critical illness VR rehabilitation device (X-VR-D) that enables diversified self-training and is applicable early in the rehabilitation of severely injured or ill patients. The X-VR-D consists of a VR program delivering a virtual scene on a flat screen and simultaneously processing commands to a moving chair mounted on a motion system. Sitting in the moving chair and exposed to a virtual reality environment the device evokes anticipatory and reactive muscle contractions in trunk and extremities for postural control. In this study we tested the device in 10 healthy subjects to evaluate whether the enforced perturbations indeed evoke sufficient and reproducible EMG muscle activations. We found that particular fast roll and pitch movements evoke adequate trunk and leg muscle activity. Higher angular velocities and higher angles of inclination elicited broader EMG bursts and larger amplitudes. The muscle activation pattern was highly consistent between different subjects and although we found some habituation of EMG responses in consecutive training sessions, the general pattern was maintained and was predictable for specific movements. The habituation was characterized by more efficient muscle contractions and better muscle relaxation during the rest positions of the device. Furthermore we found that the addition of a virtual environment to the training session evoked more preparatory and anticipatory muscle activation than sessions without a virtual environment. We conclude that the X-VR-D is safe and effective to elicit consistent and reproducible muscle activity in trunk and leg muscles in healthy subjects and thus can be used as a training method.

Abdominal Muscles↗

PubMed2002

Muscle response while holding an unstable load.

OBJECTIVE: Basic biomechanical measurement was applied to explore the physical risk associated with holding an unstable load. DESIGN: We investigated muscle response, before and after the initiation of the balance-recovery responses from perturbation when handling unstable loads. BACKGROUND: Most studies of muscle response and postural control during load handling focused on stable (fixed) loads. Investigation of muscle response while handling an unstable load deserves similar attention. METHODS: Twelve male subjects experienced time-variant loads (18 kg), with additional sudden 12 kgm/s impact momentum, under three different load-shift conditions (stable, posterior-to-anterior rolling, or anterior-to-posterior rolling load), and in three foot placements (straddle, wide, and narrow stance). Normalized electromyography (expressed as percentage of maximum voluntary contraction) for the brachioradialis, biceps, erector spinae, and hamstrings were measured. RESULTS: The maximal exertions for brachioradialis (91.0%) and biceps (42.5%) were observed approximately 0.1 s after posterior-to-anterior impact, with maximal measured contractions for erector spinae (57.2%) and hamstrings (59.9%) noted approximately 0.2 s after impact. CONCLUSIONS; The central nervous system detects and responds to the need to stabilize the joints close to the location of load-shift perturbation, with these responses determined according to the anatomical and functional limitation of each biomechanical link. RELEVANCE: Handling shifting loads, impact loading and sudden-unexpected loading may be more damaging to the musculoskeletal system than handling stable loads.

Adult↗

PubMed2006

Generalizability of center of pressure measures of quiet standing.

Center of pressure (COP) measures are commonly used as indicators of balance and postural control. At present, there are no universally accepted standards in research investigating fluctuations in the COP with regard to the number of trials or the length of a given trial. The purpose of this study was to use the tools of Generalizability Theory (G-Theory) to investigate the reliability of COP measures of quiet standing and to establish an optimal measurement protocol. G-Theory provides a tool that allows us to break down the sources of variation, or facets, in a measurement procedure and ultimately design a protocol that provides optimal reliability. Fifteen participants completed 10 90-s trials with eyes open and closed. COP measures of anterior-posterior standard deviation (SD(AP)), medial-lateral SD (SD(ML)), average velocity (Vel), and 95% confidence ellipse area (Area) were calculated using the first 30, 60, and 90s of each trial. A G-study and follow-up D-studies were performed to estimate reliability coefficients (G-coefficients). The results of the G-Theory analysis suggest that these COP measures reached acceptable levels of reliability (G>or=0.70) with at least five 60s trials.

Adult↗

PubMed1992

Characteristics of dynamic postural reactions in the locust hindleg.

The use of the locust (Schistocerca americana) hindleg in postural control was examined in animals that stood on a repeatedly swayed vertical substrate. Myograms were recorded from leg muscles and the angle of the femoro-tibial joint was monitored photographically. Two discrete strategies were observed; in compensatory reactions the hindleg was held in place, while in "flexion" reactions, the leg was moved, most often to complete flexion of the femoro-tibial joint. Tightly coupled, rhythmic bursting occurred in the flexor and levator muscles of the leg during compensatory reactions. Bursting was initiated repeatedly when the substrate was being pulled away from the animal. Bursting was correlated with subsequent decreases in the rate of change of the femoro-tibial joint angle. Compensatory and "flexion" reactions occurred preferentially in different ranges of joint angles: most often, compensatory reactions occurred in the mid-range, while "flexion" reactions were elicited in the extremes of joint angle. These differences may be due to the mechanical advantages of the tibial muscles and the leg may be moved to full flexion because of a locking mechanism of the flexor muscle tendon. These reactions are compared with known reflexes of hindleg proprioceptors and contrasted with similar responses of vertebrates.

Animals↗

PubMed2006

The risk of being fearful or fearless of falls in older people: an empirical validation.

PURPOSE: To investigate the risk of being fearful or fearless of falls in older people. METHODS: Using a force plate, postural control in different sensory and rhythmic conditions was measured in 263 community-dwelling older people. Other assessments included fear of falling, and handgrip strength. Fall incidence was assessed at baseline and during a one-year follow-up period. RESULTS: Logistic regression analysis revealed that increased lateral sway in near-tandem stance with eyes open (OR = 5.33; p < 0.01) and a worse performance on anteroposterior rhythmic weight shifts (OR = 0.65; p < 0.05) were related to falls. Univariate analyses revealed that older people with inappropriate high fear of falling according to their fall incidence had worse balance capacities on the rhythmic weight shifts (p < 0.05) but had similar static balance and physical capacities. Older people with inappropriate low fear of falling had a better hand grip (p < 0.05) but equally worse balance capacities than the comparison group. CONCLUSIONS: The results indicate the importance of lateral stability in relation to falls. They also suggest a substantial impact of inappropriate fear of falling on physical performance. Inappropriate high fear of falling may result in worse performance during dynamic balance tests, whereas older people with inappropriate low fear seem to overrate their capacities because of higher strength.

Accidental Falls↗

PubMed1997

Regional variability of stretch reflex amplitude in the cat medial gastrocnemius muscle during a postural task.

The relationship between local fibre stretch velocity (mechanical input) and the corresponding local reflex electromyographic (EMG) amplitude (a measure of the neural output) was assessed to determine the contribution of muscle spindle feedback in postural control. We hypothesized that traditionally measured input variables (e.g., the velocity of an external movement or whole muscle velocity) may not accurately represent the mechanical input to the muscle spindles, especially when the background forces are small. Three cats were trained to stand on pedestals while ankle rotations were applied to the left hindlimb. EMG and fiber movement in both proximal and distal regions of the muscle were recorded in addition to muscle length and tendon force. We found that local muscle velocity was correlated poorly with whole muscle velocity, demonstrating that internal and external muscle movements are often dissimilar, particularly during tasks that involve modest levels of muscle activation. Local EMG reflex amplitudes were correlated well with the corresponding local fiber stretch velocities (R values ranging from 0.5 to 0.8) but not with muscle stretch velocity. The lack of crossed correlations between fiber stretch velocities and reflex EMG amplitudes measured in proximal versus distal regions of the muscle suggests the presence of a local reflex component. It is concluded that changes in local muscle fiber length represent the mechanical input to spindles better than changes in the total muscle length. Additionally, spindles have a specific role in the reflex activation of nearby muscle fibers.

Animals↗