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At least 199 records · Page 11Linked to original sources
PubMed1994

Low-back disorders in agricultural tractor drivers exposed to whole-body vibration and postural stress.

The occurrence of low-back pain (LBP) was investigated in a population of 1155 tractor drivers exposed to whole-body vibration (WBV) and postural stress (response rate 91.2%) and in a control group of 220 office workers (response rate 92.2%). The subjects were questioned about several types of low-back symptom (LBP, sciatic pain, acute LBP, transient and chronic LBP) and various work- and individual-related risk factors, by using a standardized questionnaire. Vibration measurements were performed on a representative sample of the vehicles driven by the tractor drivers in the last ten years. Vibration magnitude and duration of exposure were used to calculate a vibration dose for each tractor driver. Perceived postural load was assessed in terms of frequency and/or duration of awkward postures at work. The prevalence of LBP was found to be greater in the tractor drivers than in the controls. After controlling for potential confounders by logistic modelling, low-back disorders were found to be significantly associated with both vibration dose and postural load. Back accidents and age were also significant predictors for LBP. Quantitative regression analysis indicated that vibration exposure and postural load were independent contributors to the increased risk for LBP according to a multiplicative model. The exposure levels for WBV recently recommended by a proposal of European Directive on physical agents seem to be more adequate to prevent long-term health effects on the lower back than the exposure limits suggested by the International Standard ISO 2631/1.

Journal Article↗
PubMed2004

Changes in postural control during a 48-hr. sleep deprivation period.

Sleep deprivation has detrimental effects on cognitive abilities; however, there has been limited investigation into the effects of sleep deprivation on postural control or influence of time of day on postural control measures. Therefore, we measured postural control on a force plate every 6 hr. in 24 subjects (11 women, 13 men; M age 20 +/- 2.1 yr.; M height 1.68+/-0.29 m; body mass 63.3 +/- 28.7 kg) participating in a 48-hr. dance marathon. During the first 24-hr. period, postural control significantly worsened. Rather than a steady decrease in postural control, during the second day of the event a repeating oscillatory trend of fluctuations in postural control was observed for both days of the event. We hypothesize that the repeating oscillation of postural control followed a circadian rhythm pattern, suggesting that sleep deprivation for 48 hr. does not influence measures of postural control in a consistent manner. The apparent influence of time of day on postural control measures warrants consideration of this factor when conducting studies with repeated measures of postural control across different days.

Adult↗
PubMed2004

Tilt table design for rapid and sinusoidal posture change with minimal vestibular stimulation.

INTRODUCTION: Cardiovascular response to fast posture change can be used to model individual orthostatic response under normal circumstances and after spaceflight. We set out to construct a computer-controlled tilt table suitable for repeated sinusoidal tilt motion as well as fast, single head-up tilt (HUT). The movement profile of the table was designed to prevent muscle tensing and limit vestibular stimulation. METHODS: On the new table, 20 healthy subjects underwent a protocol of fast HUT and sinusoidal tilt motion at 2.5 tilts per minute. BP was measured non-invasively (Finapres). Time domain dynamic response to HUT and frequency domain response to sinusoidal tilts were derived from the beat-to-beat BP and from interbeat-interval (IBI) series. RESULTS: Tilt motion did not induce dizziness and was experienced by all subjects as smooth. The systolic BP response to fast HUT correlated mildly with the systolic BP spectral power at the sinusoidal tilt frequency (R = 0.47). The IBI response to fast HUT correlated well with the IBI power at the sinusoidal tilt frequency (R = 0.74). DISCUSSION: In this study we presented a computer controlled tilt table capable of fast posture change and sinusoidal tilts. An exploratory protocol demonstrated that the table is suitable for obtaining cardiovascular response to posture change for modeling purposes.

Adult↗
PubMed1997

Control model of human stance using fuzzy logic.

A control model of human stance is proposed based on knowledge from behavioral experiments and physiological systems. The proposed model is based on the control of global variables specific to body orientation and alignment, rather than on the control of the body's center of mass within the base of support. Furthermore, the proposed control model is not based on purely inverted pendulum body mechanics where only motion at one joint is controlled, as for instance the ankle. In the proposed model, the degrees of freedom are controlled by using reciprocal and synergistic muscle actions at multiple joints. The control model is based on three sets of different global variables which act in parallel: (1) limb length and its derivative, (2) limb orientation and its derivative, and (3) trunk attitude and its derivative. An important feature of the control model is the use of fuzzy logic, which enables us to model experimental findings and physiological knowledge in a meaningful and explicit way using fuzzy if-then rules. In the control model, 36 fuzzy if-then rules are implemented and applied using a four-linked segment model consisting of a trunk, thigh, shank and foot. Uni- and biarticular limb muscles and trunk muscles are represented as torque actuators at each individual joint. In the model, three sets of global variables act in parallel and make corrective and coordinated responses to internal, self-induced perturbations. The data show that the use of global variables and fuzzy logic successfully enables us to model human standing with sway about a point of equilibrium. Small changes in, for example, total body sway are comparable to those seen during natural sway in human stance. The selected controllers--limb length, limb orientation and trunk attitude--seem to be appropriate for human stance control.

Fuzzy Logic↗
PubMed2002

Personalized biomechanical modeling of Boston brace treatment in idiopathic scoliosis.

The aim of this study was to describe how the Boston brace modify the scoliotic curvatures using a finite element (FE) model and experimental measurements. The experimental protocol, applied on 12 scoliotic girls, was composed of the pressure measurement at the brace-torso interface followed by two radiographic acquisitions of the patient's torso with and without brace. A 3D FE model of the trunk was built for each unbraced patient. The brace treatment was represented by two different modeling approaches: 1) using equivalent forces calculated from the measured pressures; 2) by an explicit personalized FE model of the brace (hexahedral elements) and its interface with the torso (contact elements). In the first model, measured brace forces less than 40N and up to 113N induced respectively less than 21% and up to 87% of real correction. Thoracic forces induced the main correction, affecting partially both lumbar and thoracic curves, in agreement with the literature. In the second model, the brace closing reduced the curves up to 35% of real correction. Contact reaction forces (16-79N) were similar to real brace forces (11-72N). The results suggested that other mechanisms than brace pads contribute to the equilibrium of the patients. Postural control by the muscular system remains a problem to address in a future study. The second model represented more realistically the load transfer from the brace to the spine than external forces application. With such model, it is expected to predict the effect of a brace before its design and manufacturing, and also to improve its design.

Adolescent↗
PubMed2006

Virtual time-to-contact of postural stability boundaries as a function of support surface compliance.

This study investigated the hypothesis that virtual time-to-contact, which specifies the time to reach the functional stability boundary, is a variable controlled in the maintenance of upright posture. Three different levels of support surface compliance were used on a force platform (no foam, 5 cm of foam, and 15 cm of foam). The participant's task was to stand still under each surface support condition both with and without vision. The stability boundary was determined for each set of conditions where the participant was required to lean as far as possible in all directions of the horizontal plane without losing stability. The results showed that the no vision conditions had a significantly larger center of pressure displacement than the vision conditions. No vision and increasing support surface compliance also increased the velocity of the center of pressure trajectory. The distribution of the radial displacement of the center of pressure showed relatively equal frequency over spatial location with no central tendency. The virtual time-to-contact with the stability boundary decreased as platform surface support became more compliant. Furthermore, the distribution of virtual time over the effective scaling range was a power law with a larger exponent in the more unstable no vision and increasing surface foam conditions. The findings provide additional evidence for the hypothesis that virtual time-to-contact with stability boundaries is a postural control variable that is regulated rather than the preservation of minimal motion around the center of the stability region as proposed in pendulum models of posture.

Adolescent↗
PubMed2001

Somatosensory loss increases vestibulospinal sensitivity.

To determine whether subjects with somatosensory loss show a compensatory increase in sensitivity to vestibular stimulation, we compared the amplitude of postural lean in response to four different intensities of bipolar galvanic stimulation in subjects with diabetic peripheral neuropathy (PNP) and age-matched control subjects. To determine whether healthy and neuropathic subjects show similar increases in sensitivity to galvanic vestibular stimulation when standing on unstable surfaces, both groups were exposed to galvanic stimulation while standing on a compliant foam surface. In these experiments, a 3-s pulse of galvanic current was administered to subjects standing with eyes closed and their heads turned toward one shoulder (anodal current on the forward mastoid). Anterior body tilt, as measured by center of foot pressure (CoP), increased proportionately with increasing galvanic vestibular stimulation intensity for all subjects. Subjects with peripheral neuropathy showed larger forward CoP displacement in response to galvanic stimulation than control subjects. The largest differences between neuropathy and control subjects were at the highest galvanic intensities, indicating an increased sensitivity to vestibular stimulation. Neuropathy subjects showed a larger increase in sensitivity to vestibular stimulation when standing on compliant foam than control subjects. The effect of galvanic stimulation was larger on the movement of the trunk segment in space than on the body's center of mass (CoM) angle, suggesting that the vestibular system acts to control trunk orientation rather than to control whole body posture. This study provides evidence for an increase in the sensitivity of the postural control system to vestibular stimulation when somatosensory information from the surface is disrupted either by peripheral neuropathy or by standing on an unstable surface. Simulations from a simple model of postural orientation incorporating feedback from the vestibular and somatosensory systems suggest that the increase in body lean in response to galvanic current in subjects with neuropathy could be reproduced only if central vestibular gain was increased when peripheral somatosensory gain was decreased. The larger effects of galvanic vestibular stimulation on the trunk than on the body's CoM suggest that the vestibular system may act to control postural orientation via control of the trunk in space.

Adaptation, Physiological↗
PubMed2001

Enhancing human balance control with galvanic vestibular stimulation.

With galvanic vestibular stimulation (GVS), electrical current is delivered transcutaneously to the vestibular afferents through electrodes placed over the mastoid bones. This serves to modulate the continuous firing levels of the vestibular afferents, and causes a standing subject to lean in different directions depending on the polarity of the current. Our objective in this study was to test the hypothesis that the sway response elicited by GVS can be used to reduce the postural sway resulting from a mechanical perturbation. Nine subjects were tested for their postural responses to both galvanic stimuli and support-surface translations. Transfer-function models were fit to these responses and used to calculate a galvanic stimulus that would act to counteract sway induced by a support-surface translation. The subjects' responses to support-surface translations, without and with the stabilizing galvanic stimulus, were then measured. With the stabilizing galvanic stimulus, all subjects showed significant reductions in both sway amplitude and sway latency. Thus, with GVS, subjects maintained a more erect stance and followed the support-surface displacement more closely. These findings suggest that GVS could possibly form the basis for a vestibular prosthesis by providing a means through which an individual's posture can be systematically controlled.

Adolescent↗
PubMed1989

Spinal cord circuits: are they mirrors of musculoskeletal mechanics?

Over the past decade, research at three different levels of sensorimotor control has revealed a degree of complexity that challenges traditional hypotheses regarding servocontrol of individual muscles: (a) The connectivity of spinal circuits is much more divergent and convergent than expected. (b) The normal and reflex-induced recruitment of individual muscles and compartments of muscles is more finely controlled than was noted previously. (c) The mechanical interactions among linked skeletal segments and their often multiarticular muscles are neither simple nor intuitively obvious. We have developed a mathematical model of the cat hind limb that permits us to examine the influence of individual muscles on posture and gait. We have used linear quadratic control theory to predict the optimal distribution of feedback from a hypothetical set of proprioceptors, given different assumptions about the behavioral goals of the animal. The changes in these predictions that result from changes in the structure and control objectives of the model may provide insights into the functions actually performed by the various circuits in the spinal cord.

Journal Article↗
PubMed1997

Modelling muscle activity in standing with considerations for bone safety.

The functional use of electrical stimulation (FES) for the restoration of movement to paraplegics has been improved in the last decade but questions about the mechanical effect of stimulation on the skeleton have arisen. In intact people, neuromuscular activity not only controls movement, but also minimizes bone and joint tissue loading. Current FES systems do not use feedback and do not even use average natural patterns of muscle activation. FES systems would be safer if muscle activation patterns were synthesized so as to minimize bending in the long bones. By modelling, we have verified that appropriate muscular activity reduces bone bending stresses, an approach we named active unloading of the skeleton. Using this criterion for control is novel. The muscle activation was calculated using measurements from intact people in different postures, and later modelling of the musculoskeletal system. The two-dimensional model of the lower limb includes 23 muscles relevant primarily for movement in the sagittal plane. The muscle model for constraint calculation is divided into first-order activation dynamics and first-order contraction dynamics. Optimization, which includes minimization of net bending moment calculated along the long bones, is static because changes in the observed postures are slow. In the calculated muscle activity patterns, muscle coactivation and cocontraction yield very uniform and low bone loading. Net bending moment values were fairly stable as the posture varies. The moment distribution in the femur was found to be U-shaped, while in the tibia it is sometimes V-shaped. The bones are naturally thicker at the points of peak moment.

Adult↗
PubMed2005

Performing hand actions assists the visual discrimination of similar hand postures.

Recent theoretical work has suggested that internal predictive signals are used for motor control and coordination. The predictive signal - proposed to be the output of a forward model - would be a sensory representation of action. Hence, these sensory representations could potentially influence other sensory processes. We report here how performance of hand actions assisted the visual discrimination of target hand postures presented at random times within an on-going series of hand images. Reaction times to discriminate the targets were significantly shorter when the displayed images were both sequential and congruent with the action being performed. Hence, the planning or execution of action appears to allow better prediction of a displayed series of congruent images. In further control experiments, we show that the motor-visual priming effect is unlikely to be due to differential attentional demands and it is specific to a first person perspective display; it is short lasting, being lost if a 500 ms delay is introduced between successive stimulus presentations. The data are interpreted as evidence supporting the hypothesis that forward models in the motor system provide action-specific sensory predictions that are available to cognitive processes.

Adolescent↗
PubMed2003

Closing an open-loop control system: vestibular substitution through the tongue.

The human postural coordination mechanism is an example of a complex closed-loop control system based on multisensory integration [9,10,13,14]. In models of this process, sensory data from vestibular, visual, tactile and proprioceptive systems are integrated as linearly additive inputs that drive multiple sensory-motor loops to provide effective coordination of body movement, posture and alignment [5-8, 10, 11]. In the absence of normal vestibular (such as from a toxic drug reaction) and other inputs, unstable posture occurs. This instability may be the result of noise in a functionally open-loop control system [9]. Nonetheless, after sensory loss the brain can utilize tactile information from a sensory substitution system for functional compensation [1-4, 12]. Here we have demonstrated that head-body postural coordination can be restored by means of vestibular substitution using a head-mounted accelerometer and a brain-machine interface that employs a unique pattern of electrotactile stimulation on the tongue. Moreover, postural stability persists for a period of time after removing the vestibular substitution, after which the open-loop instability reappears.

Acceleration↗
PubMed1993

Intrasubject variability of selected force-platform parameters in the quantification of postural control.

Platform stabilometry is increasingly applied to monitor or re-educate standing balance in clinical rehabilitation. Consequently, insight is needed into the validity, reliability, and sensitivity of different force-platform parameters. This study focuses on the intrasubject variability as the major source of variance (unreliability) in the study of human motor skills. The intrasubject variability of several, commonly applied force-platform parameters was determined across ten repeated tests of quiet two-legged standing in healthy subjects to identify the most consistent and stable parameters in the quantification of postural control. The variability of the root mean square (RMS) amplitude, peak-to-peak amplitude, mean frequency, and RMS velocity of the fore-aft and lateral components of the center-of-pressure fluctuations was investigated under varying (visual and cognitive) task conditions. The results indicate that all selected parameters show considerable intrasubject variability irrespective of the task context. Nonetheless, both the RMS amplitude and RMS velocity in either direction of sway do not demonstrate a significant trend across repeated tests. Among the selected parameters, the RMS velocity in the fore-aft direction shows the greatest intrasubject consistency, as well as a high sensitivity to, for example, visual deprivation. These findings support the reliability and validity of this parameter in the clinical quantification of postural control.

Adult↗
PubMed1998

Stretch reflex sensitivity: effects of postural and muscle length changes.

In this study, a combination of clinical evaluation, laboratory testing, and model simulation of spasticity is performed under various postural conditions to investigate the changes in the sensitivity and specific mechanisms of spasticity. Fifty-nine multiple sclerosis patients participated in the study and received spasticity evaluation based on both the Ashworth scale and the pendulum test. Spasticity was found to increase in the pendulum test when the subjects were tested in a supine posture, compared to when they were sitting. Three patterns of stretch reflex response were seen for similar leg swing trajectories. While it was clear that the increased stretch of rectus femoris in the supine posture contributed to the increased spasticity, the results of modeling showed that other more complex factors were also involved. The supraspinal descending modulation associated with postural control may play a more dominant role in the severity of spasticity. The results suggest that the biomechanical test of spasticity should be performed for several different postures or ranges of movement with muscle activities monitored simultaneously, so that the effect of various factors can be examined. The work also indicates that a neuromusculoskeletal model with detailed muscle dynamics and stretch reflex loops is a valuable tool for investigating the neural mechanisms of spasticity.

Adult↗
PubMed1986

An automated microcomputer-based system for determining canine paw pressure quantitatively in the dog synovitis model.

The automated system for measuring canine paw pressure is a very useful tool for the evaluation of nonsteroid antiinflammatory drugs such as indomethacin and ibuprofen in the dog synovitis model. The apparatus has been designed to give the operator control over the dog's posture while measuring hind-paw pressure. The stability of the measuring platform permits reproducible measurement of paw pressures. The software package performs direct data recording and data reduction and eliminates tedious manual calculations. Subsequently, the software produces a printout summarizing the experimental results in tabular and graphic form.

Animals↗
PubMedDate not supplied

A controlled study of the effects of mental relaxation on autonomic excitatory responses in healthy subjects.

OBJECTIVE: Circumstantial evidence indicates that, in the presence of a suitable substratum, sudden, behaviorally induced increases in sympathetic drive to the cardiovascular system might play an important physiopathological role in various conditions, ranging from arterial hypertension to sudden coronary death. Accordingly, it might be useful to study the effects of behavioral interventions, such as mental relaxation, that might be capable of blunting excitatory autonomic responses. It would also be preferable to study healthy subjects in whom autonomic control is not modified by the presence of disease, and to use noninvasive approaches to minimize the possible emotional impact produced by invasive recordings. METHODS: We examined healthy subjects who were either subjected to relaxation training (N = 13) or sham relaxation (N = 12). An additional group, treated with beta-adrenergic blockade (N = 12), was also examined. Spectral and cross-spectral analysis of RR interval and systolic arterial pressure (SAP) variabilities provided quantitative markers of sympathovagal balance modulating the sinoatrial (SA) node, of sympathetic vasomotor modulation, and of the gain of the arterial pressure/heart period baroreflex (index alpha). Subjects were studied at rest, during standing, and during mental arithmetic. RESULTS: Data indicate that both beta-adrenergic blockade and relaxation training significantly blunted the excitatory autonomic responses to standing and to mental arithmetic. Indices of sympathetic modulation also seemed reduced by beta blockade at rest. No changes were observed with sham training. CONCLUSIONS: Frequency domain analysis of cardiovascular variabilities, using a totally noninvasive approach, indicates that relaxation training significantly blunts the excitatory autonomic changes produced by standardized behavioral laboratory stimuli.

Adrenergic beta-Antagonists↗
PubMed1992

Postural control, muscle function and psychological factors in rheumatoid arthritis. Are there any relations?

The aim of the present study was to relate postural control, as measured on a quantitative test battery as standing balance on an AMTI force platform, to results of muscle function of the lower extremities, aerobic capacity, disease characteristics, attitudes revealing anxiety, and demographic variables in patients with rheumatoid arthritis (RA). A group of 61 patients with RA was investigated. Multiple regression analyses, using both the stepwise and the backward elimination method, were employed for length of sway path in two-leg standing looking straight ahead and blindfolded as dependent variables. The results revealed postural control in two-leg standing looking straight ahead to be highly related to age and sex but also to isokinetic endurance and to anxiety. In the model, 55% of the variance was explained by these variables at a significance level of p = 0.05. Characteristics of the RA disease, such as c-reactive protein and joint-mobility did not play any major role in explaining postural control.

Adult↗