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The aim of the paper is to investigate the application of control schemes based on "internal models" to the stabilization of the standing posture. The computational complexities of the control problems are analyzed, showing that muscle stiffness alone is insufficient to carry out the task. The paper also re-visits the concept of the cerebellum as a Smith's predictor.
We analyzed the temporal evolution of the displacement of the center of pressure (COP) during prolonged unconstrained standing (30 min) in non-impaired human subjects. The COP represents the collective outcome of the postural control system and the force of gravity and is the main parameter used in studies on postural control. Our analysis showed that the COP displacement during human standing displays fractal properties that were quantified by the Hurst exponent obtained from the classical rescaled adjusted range analysis. The average fractal or Hurst exponent (H) was 0.35+/-0.06. The presence of long-range correlations from a few seconds to several minutes due to the fractal characteristics of the postural control system has several important implications for the analysis of human balance.
1. In addition to giant cells, originally described by Deiters, the lateral vestibular nucleus contains also medium- and small-size cells. The role that these neurons exert in the static control of posture has been investigated in precollicular decerebrate cats in which the resting discharge of spontaneously active vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (IVS neurons) has been related to the cell size inferred on the basis of the conduction velocity of their axons. 2. In control experiments, the IVS neurons with slower axonal conduction velocity and, by inference, having thinner axons and smaller cell bodies differed from those having faster conduction velocity by displaying a higher resting discharge rate and a relatively regular interspike interval distribution, i.e. a lower coefficient of variation (CV). 3. The resting discharge of the IVS neurons, which corresponded on the average to 24.5 +/- 15.7, S.D. imp./sec, in control experiments, increased significantly to 44.1 +/- 23.8, S.D. imp./sec after ablation of the cerebellar vermis and the fastigial nuclei, leading to a great increase in postural activity, while the proportion of regularly discharging units (with the lowest CV) increased. Moreover, the negative correlation between resting discharge of all the recorded IVS neurons and the conduction velocity of the corresponding axons, which was quite slight in the experiments with the cerebellum intact, greatly increased after partial cerebellectomy. This finding was due to a prominent increase in resting discharge of the small-size IVS neurons, while the discharge of the large-size IVS neurons was, on the average, comparable to that obtained in the controls. It appears, therefore, that the cerebellum exerts a prominent tonic inhibitory influence on the small-size IVS neurons, which are thus responsible for the great increase in decerebrate rigidity after cerebellectomy. 4. The resting discharge rate of the IVS neurons was not, on the average, greatly modified after ipsilateral acute (aVN) and chronic vestibular neurectomy (cVN) with respect to the controls. However, the proportion of regularly discharging units (with the lowest CV) decreased after aVN, but increased after cVN. The relation found in control experiments, i.e. the faster the conduction velocity of VS axon the lower was the unit discharge at rest, was lost after aVN, due to a decrease in resting discharge rate of the slow neurons. The mean discharge rate of these units, however, recovered after cVN, so that the negative correlation between resting discharge rate and axonal conduction velocity was reestablished.(ABSTRACT TRUNCATED AT 400 WORDS)
Although low back pain (LBP) is a widespread and disabling health problem, there is a lack of evidence based medicine with respect to its treatment and rehabilitation. A major reason for this is the poor understanding of the underlying mechanisms of the LBP syndromes. In an attempt to fill this gap, the present review article provides an overview of the sensory-motor control aspects of trunk stabilization and postural control of the trunk, and how they may relate to the evolution of LBP. In particular, the anatomy and physiology of the sensory-motor control mechanisms of the trunk muscles that contribute to general and segmental stability of the lumbar spine will be elucidated. Furthermore, a brief overview of current theories of postural control will be provided with respect to spinal stabilization. Finally, a concept of the pathophysiological changes within the sensory-motor control mechanisms of the lumbar spine in the presence of muscle injury and pain will be presented. The impact of pain and muscle injury on the muscular support for the lumbar motion segment will be discussed along with the deficits in neuromuscular control in LBP patients with decreased segmental lumbar stability.
If muscles are viewed as spring-like torque generators, then the integral of torque with respect to joint angle is the potential energy of that muscle. An energy function for the musculoskeletal system can be defined by summing the energy contribution of each muscle and the potential energy stored in the limb. Any local minimum in this energy landscape is a possible equilibrium position for the limb. The gradient of this function with respect to joint angles is a torque field, and the task of postural control is to find a set of muscle activations to produce a desired field. We consider one technique by which this approximation may be achieved: A postural module is defined as a synergy of muscles that produce a class of torque functions that converge at a constant equilibrium position, but whose stiffness at this position varies as a function of activation of the postural module. For a single-joint system, we show that through control of two such modules it is possible to produce any stiffness at any desired equilibrium position. To extend this scheme to a multijoint system, we initially derive the mechanical constraints on the shape of the restoring force field when a multijoint limb is displaced from equilibrium. Next, we consider voluntary control of the force field when the human arm is displaced from equilibrium: Mussa-Ivaldi, Hogan, and Bizzi (1985) have suggested that subjects are unable to voluntarily change the shape and orientation of the field, although they can readily scale it. This suggests existence of a limitation on the independent recruitment of arm muscles. We show, through simulation, that the inability to voluntarily control the shape and orientation of the restoring force field can be attributed to an organization of postural modules that act as local stiffness controllers. We propose that through coactivation, postural modules coarsely encode the work space and serve as an intermediate control system in the motor control hierarchy.
BACKGROUND: Ageing is associated with a reduction in balance, in particular through dysfunction of each level of postural control, which results in an increased risk of falling. Conversely, the practice of physical activities has been shown to modulate postural control in elderly people. OBJECTIVE: This study examined the potential positive effects of two types of regular physical and sporting activities on vestibular information and their relation to posture. METHODS: Gaze and postural stabilisation was evaluated by caloric and rotational vestibular tests on 18 healthy subjects over the age of 60 who regularly practised low-energy or bioenergetic physical activities and on 18 controls of a similar age who only walked on a regular basis. These subjects were also submitted to static and dynamic posturographic tests. RESULTS: The control group displayed less balance control, with a lower vestibular sensitivity and a relatively high dependency on vision compared to the group practising low-energy physical activities, which had better postural control with good vestibular sensitivity and less dependency on vision. The postural control and vestibular sensitivity of subjects practising bioenergetic activities was average, and required higher visual afferent contribution. CONCLUSION: Low-energy exercises, already shown to have the most positive impact on balance control by relying more on proprioception, also appear to develop or maintain a high level of vestibular sensitivity allowing elderly people practising such exercises to reduce the weight of vision.
Studies in both decerebrate, and intact cats have already established the presence of specific areas in the brainstem that subserve control of posture and locomotion. They are the subthalamic locomotor region (SLR) in the lateral hypothalamic area (LHA), the mesencephalic locomotor region (MLR) in the posterior midbrain, the dorsal tegmental field (DTF) and the ventral tegmental field (VTF) of caudal pons along its midline. These areas can be stimulates either electrically or chemically to induce site-specific changes in posture and locomotor synergies. Our observations indicate that the postural and locomotor synergies are structured in a hierarchy within rostro-caudal axis of the brainstem, and that the command routing through the brainstem relies on interactions with the SLR, the MLR, the DTF area and the VTF area. These results and our concepts for postural and locomotor control and their interactions are discussed with the concepts of command hierarchies for motor control.
Postural control is a common mechanism to compensate for unexpected displacements of the body. In the older population, a slip or fall due to a failure of postural control is a common cause of morbidity and mortality. The ability of postural control decreases with aging or neuropathy. In this study, 2 groups, diabetics and non-diabetics in the older population, were compared to determine how patterns of postural sway during quiet standing were related to the detection of perturbation. The SLIP-FALLS system was applied to the measurement of sway and detection of perturbation. In phase 1 of the development of the predictive model, neural network algorithms were applied to find determinant variables for perturbation detection. In phase 2, a fuzzy logic inference system was developed to investigate the relationship between sway and perturbation detection. Results of this study may be applied to the design of floor mats or shoe insoles for preventing fatigue in workplaces.
We tested the hypothesis that fall anxiety would differentially influence the regulation of upright standing among younger and among older adults. Fall anxiety was imposed by a manipulation of environmental context that increased the threat to postural control by introducing the potential for injurious consequences should a fall occur. Fifteen younger and 15 older adults participated in this study. Regardless of age, postural control was more conservative when fall anxiety increased, however, age did not affect how anxiety influenced the regulation of postural control. Our findings imply that the motor consequences of fall anxiety are no more pervasive for older adults than for younger adults, and that age does not alter the ability to accommodate to environmental demands that heighten fall anxiety.
Computer aided design (CAD) in conjunction with digitised anthropometric manikins can be used for analysis and control of stressful work postures, one of the most frequently cited occupational risk factors of upper extremity cumulative trauma disorders. This paper describes the use of macros for manipulating manikins and workstation components and for designing the workplace. AutoCAD, a popular computer aided design software package, was used to demonstrate the feasibility of these concepts. Specifically, macros are used for drawing work equipment using parametric designs, manipulating manikins and analysing jobs. In comparing the macros to the use of primitive CAD commands, the macros not only decrease the amount of time needed to create workstation components, but they also make the task easier for the user and decrease the risk of errors. Despite the limitation of anthropometric data and manikins, CAD is an effective method for identifying postural stresses and redesigning the workstation to control the identified stresses.
A disturbance of postural control is one of the most invalidating symptoms of Parkinson's disease (PD), and mechanisms underlying its development have not been so far elucidated. To specify the postural control features in different PD forms, clinico-neurophysiological analysis was conducted in 61 patients divided into 3 groups according to PD forms: tremor-rigid, rigid-tremor and akinetic-rigid. Dissociation between clinical expression of postural instability and its stabilometric reflection--the square of statokinesogram was found, indicating importance of differentiated approach in performance of stabilometric analysis in patients with different PD forms. The square of statokinesogram may be regarded as a neurophysiological marker of postural instability only in patients with rigid forms, in tremor parkinsonian phenotypes an increased square of stabilogram being mainly a stabilometric reflection of tremor. The importance of stabilometric test performance with cognitive loading, allowing switching out of voluntary posture control, is shown. Possible neuromediator mechanisms involved in postural instability in PD are discussed.
The role of vision in postural control is crucial and is strictly related to the characteristics of the visual stimulus and to the performance of the visual system. The purpose of this investigation was to evaluate the effects of chronically reduced visual cues upon postural control in patients affected by Congenital Nystagmus (CN). These patients have developed since birth a postural strategy mainly based on vestibular and somatosensorial cues. Fifteen patients affected by CN and 15 normal controls (NC) were enrolled in the study and evaluated by means of dynamic posturography. The overall postural control in CN patients was impaired as demonstrated by the equilibrium score and by the changes of the postural strategy. This impairment was even more enhanced in CN than in NC group when somatosensorial cues were experimentally reduced. An aspecific pattern of visual impairment and a pathological composite score were also present. Our data outline that in patients affected by CN an impairment of the postural balance is present especially when the postural control relies mainly on visual cues. Moreover, a decrease in accuracy of the somatosensory cues has a proportionally greater effect on balance than it has on normal subjects.
Two basic aspects of the neuronal control of bipedal stance and gait have been discussed, namely the interlimb coordination and the antigravity function of leg extensors. During stance and gait both legs act in a cooperative manner as each limb affects the strength of muscle activation and the time-space behaviour of the other. There are indications that interlimb coordination is mediated by spinal interneuronal circuits, which are themselves under supraspinal (e.g. cerebral and cerebellar) control. Proprioceptive reflexes involved in the maintenance of body equilibrium depend on the presence of contract forces opposing gravity. Extensor load receptors are thought to signal changes of the projection of body's centre of mass with respect to the feet. According to recent observations in the spinal cat, this afferent input probably arises from Golgi tendon organs and represents a newly discovered function of these receptors in the regulation of stance and gait.
This overview of three decades of clinical experience with oral therapies complemented by oral devices-vestibular screens (VS) and ISMARs (Innsbruck sensory motor activators and regulators)-tries to put these devices into an oral functional context including postural control of mobile structures both inside and outside the mouth-down to the feet. ISMARs have certain unique features which teach a person not only better postural control of the lower jaw and, hence, hyoid bone and tongue, but also to stimulate movements of lips and tongue to improve salivary control, functions of eating and drinking and the domain of communication, e.g. facial movements and articulation of speech. A review of the relevant postural background is provided and the mechanisms are explained and may serve as models for other oral therapies. The inter-relation and multi-functionality of muscular connections throughout the body explains the uniformity of facial, oral and pharyngeal dysfunctions in different conditions such as cerebral palsy, Moebius syndrome, chronic nasal congestion or developmental retardation and exposes patients with oral dysfunctions to the risks of vicious circles. However, at the same time the opportunity is given to trigger beneficious circles. Three principles of postural control in and for the neck region are postulated. Clinical observations and experiments with volunteers are described and some supporting information is given-hopefully enough to stimulate clinical work and give rise to further experimental work to illuminate this field which is extremely important for both human communication and inter-relation.
A basic aspect of the neuronal control of bipedal stance and gait represents the antigravity function of leg extensors. Proprioceptive reflexes involved in the maintenance of body equilibrium depend on the presence of contact forces opposing gravity. Extensor load receptors are thought to signal changes of the projection of the body's centre of mass with respect to the feet. According to recent observations in the spinal cat, this afferent input probably arises from Golgi tendon organs and represents a newly discovered function of these receptors in the regulation of stance and gait. In humans, evidence for a significant contribution of the load receptor to leg muscle activation has come from immersion experiments. Compensatory leg muscle activation depends on the actual body weight. Furthermore, recent experiments in paraplegic patients showed that the beneficial effects of a locomotor training critically depends on the initial degree of body unloading and reloading during the course of the training period.
BACKGROUND AND PURPOSE: Measures of postural control may be useful for determining fall risk in older people and for determining the outcomes of treatments aimed at improving balance. Commonly used tools measure the output of the postural control system. The purpose of this study was to determine the degree to which one component of postural control (muscle force) contributes to scores on 3 functional balance measures. SUBJECTS: Fifty community-dwelling volunteers between 65 and 91 years of age (mean = 74.82, SD = 6.11) participated. Based on their histories, 11 subjects were classified as being at risk for falling. METHODS: Measures were the Berg Balance Scale (BBS), the Functional Reach Test (FRT), and the Timed Get Up & Go Test (GUG). The force generated by 12 lower-extremity muscle groups was measured using a handheld dynamometer. RESULTS: In the group reporting no falls, dorsiflexor and subtalar evertor force accounted for 58% of the score on the BBS, ankle plantar-flexor and subtalar invertor force accounted for 48.4% of the score on the GUG, and ankle plantar-flexor force accounted for 13% of the score on the FRT. Ankle dorsiflexor and hip extensor forces were lower in subjects reporting falls, and force of the ankle dorsiflexors predicted fall status. CONCLUSION AND DISCUSSION: Distal muscle force measures may be able to contribute to the prediction of functional balance scores; however, the muscles involved in the prediction differ depending on the measure of balance.
Functional ability declines in later life. The purpose of this project was to determine if strength, postural control, and joint pain predict performance of four functional tasks among older adults. A sample of 28 older adults completed assessments of strength, postural control, joint pain, and four functional tasks. The duration to complete the functional tasks of: 1) getting out of bed, going to a chair, and then returning to bed; 2) crossing a street and getting onto a bus; 3) exiting the passenger side of a car; and 4) climbing a flight of 27 stairs was recorded. Step-wise regression equations indicated that seated row strength and dynamic postural control were significant predictors of all of the tasks and accounted for the largest proportion of the variance in each equation. These results indicate that measures of physical fitness may be more important predictors of functional tasks among older adults than chronological age.
We investigated the changes of human posture control of upright stance which occur when vestibular cues (VEST) are absent and visual and somatosensory orientation cues (VIS, SOM) are removed. Postural responses to sinusoidal tilts of a motion platform in the sagittal plane (+/-2 degrees, f=0.05, 0.1, 0.2 and 0.4 Hz) were studied in normal subjects (Ns) and patients with bilateral vestibular loss (Ps). We found that absence of VEST (Ps, visual reference) and removal of VIS (Ns, no visual reference) had little effect on stabilization of upright body posture in space. In the absence of both VEST and VIS (Ps, no visual reference) somatosensory graviception still provided some information on body orientation in space at 0.05 and 0.1 Hz. However, at the higher frequencies Ps qualitatively changed their behavior; they then tended to actively align their bodies with respect to the motion platform. The findings confirm predictions of a novel postural control model.