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PubMed1995

Galvanic vestibular stimulation for analysis of postural adaptation and stability.

Human postural dynamics was investigated in 12 normal subjects by means of a force platform recording body sway, induced by bipolar transmastoid galvanic stimulation of the vestibular nerve and labyrinth. The model adopted was that of an inverted segmented pendulum, the dynamics of postural control being assumed to be reflected in the stabilizing forces actuated by the feet as a result of complex muscular activity subject to state feedback of body sway and position. Time-series analysis demonstrates that a transfer function from stimulus to sway-force response with specific parameters can be identified. In addition, adaptation to the vestibular stimulus is demonstrated to exist, and we describe this phenomenon using quantification in terms of a postural adaptation time constant in the range of 40-50 s. The results suggest means to evaluate adaptive behavior and postural control in the erect human being which may be useful in the rehabilitation of individuals striving to regain upright stance.

Adaptation, Physiological↗
PubMed2001

Increased risk for falling associated with obesity: mathematical modeling of postural control.

Recent epidemiological studies report that obesity is positively related to fracture incidence. In the present experiment, a model of postural control was used to examine the impact of an abnormal distribution of body fat in the abdominal area upon postural stability. Obese and lightweight humanoids were destabilized by imposing a small initial angular speed from a neutral standing position. To avoid a loss of stability yielding a stepping reaction or a fall, an ankle torque is necessary to counteract the perturbation. Three torque parameters--ankle torque onset, time to peak torque, and muscular ankle torque--were entered in a program to simulate the intrinsic variability of the human postural control system. A loss of stability was detected when the center of pressure exceeded stability margins. The most striking observation is the nonlinear increase of torque needed to stabilize the humanoid when the motor response was characterized by delayed temporal parameters. The effect was more pronounced when an anterior position of the center of mass was included in the simulations. This suggests that, when submitted to daily postural stresses and perturbations, obese persons (particularly those with an abnormal distribution of body fat in the abdominal area) may be at higher risk of falling than lightweight individuals.

Abdomen↗
PubMed2004

Detecting postural responses to sinusoidal sensory inputs: a statistical approach.

A common way for understanding sensory integration in postural control is to provide sinusoidal perturbations to the sensory systems involved in balance. However, not all subjects exhibit a response to the perturbation. Determining whether or not a response has occurred is usually done qualitatively, e.g., by visual inspection of the power spectrum. In this paper, we present the application of a statistical test for quantifying whether or not a postural sway response is present. The test uses an F-statistic for determining if there is significant power in postural sway data at the stimulus frequency. In order to describe the application of this method, 20 subjects viewed sinusoidal anterior-posterior (A-P) optic flow at 0.1 and 0.25 Hz, while their A-P head translation was measured. The test showed that significant postural responses were detected at the stimulus frequency in 12/20 subjects at 0.1 Hz and 13/20 subjects at 0.25 Hz.

Adult↗
PubMed1994

Posture-induced nasal obstruction in patients with allergic rhinitis.

Nasal resistance is affected by posture. In this study, the effects of supine and lateral recumbent positions were investigated in eight normal subjects and 31 patients with allergic rhinitis. Of the 31 patients with allergic rhinitis, five showed unilateral complete nasal obstruction and one patient showed bilateral complete obstruction during the change of posture. Total nasal resistance had a tendency to increase with repeating the change of posture (a paired t-test, P < 0.05). The supine and lateral recumbent positions did not induce variable changes in total nasal resistance in normal subjects. Posture induces complete nasal obstruction in the supine or lateral recumbent positions in some patients with allergic rhinitis.

Adult↗
PubMed2006

Changes in the scrotal temperature of subjects in a sedentary posture over a heated floor.

As Koreans habitually sit on the heated floor in their residential buildings, the male testis is directly exposed, and is therefore affected by the floor surface temperature. The purpose of this study was to investigate the changes in the scrotal temperature of the subjects in a sedentary posture over the heated floor. A rigid screening test was performed to select healthy subjects. Finally, six college students volunteered to participate in the experiments. Two experiments were performed in a controlled environmental chamber. Experiment I was designed for a low metabolism state, with the subjects reading a book in a sedentary posture for 50 min. The floor surface temperature (tf) was controlled by varying the temperature of water (tw) flowing into the floor coil from 15 to 50 degrees C, at 5 degrees C intervals. Experiment I revealed that the final scrotal surface temperature was 32.27, 32.62, 33.51, 33.34, 34.14, 34.28, 34.34 and 35.04 degrees C at the tw 15 degrees C (tf 17.0 degrees C), 20 degrees C (tf 20.8 degrees C), 25 degrees C (tf 24.1 degrees C), 30 degrees C (tf 27.8 degrees C), 35 degrees C (tf 31.7 degrees C), 40 degrees C (tf 35.9 degrees C), 45 degrees C (tf 38.6 degrees C) and 50 degrees C (tf 42.2 degrees C), respectively. At tf 17.0 and 20.8 degrees C, the scrotal temperature exhibited a declining pattern and a low temperature for spermatogenesis. At tf 24.1, 27.8 and 31.7 degrees C, however, the thermal regulatory system of the scrotum and testis was activated appropriately. On the contrary, scrotal temperature ascended at tf 35.9, 38.6 and 42.2 degrees C. Of the six subjects, two subjects demonstrated scrotal temperatures above 35 degrees C at tf 38.6 degrees C and four subjects exhibited scrotal temperatures above 35 degrees C at tf 42.2 degrees C. Experiment II was designed for a high metabolism state, with the subjects playing a card game in a sedentary posture for 180 min. The tf was controlled by varying the tw from 30 to 40 degrees C, at 5 degrees C intervals. Experiment II revealed that the final scrotal temperature was 33.43, 34.78 and 35.61 degrees C, and the difference between the initial and final scrotal temperatures was +0.34, +1.06 and +2.24 degrees C, at tw 30 degrees C (tf 27.8 degrees C), 35 degrees C (tf 31.7 degrees C) and tw 40 degrees C (tf 35.9 degrees C), respectively. The scrotal temperature was affected by the floor surface temperature and by the rate of metabolism of the subject in a sedentary posture. As derived from regression analysis, the recommended surface temperature of a heated floor is within 23-33 degrees C under the assumption that scrotal and consecutively testicular temperature above 35 degrees C impairs spermatogenesis.

Body Temperature↗
PubMed2005

Preliminary rapport on head posture and muscle activity in subjects with class I and II.

Forward head posture may cause alterations in the stomatognathic system, including changes in the muscle activity of the masticatory muscles and dental occlusion alterations. Considering the need for further understanding of the relationship between the stomatognathic system and the cervical region, the purpose of this study was to analyse the head posture and the electromyographic (EMG) activity of the anterior portion of temporal and masseter muscles bilaterally among bruxist's subjects with different dental occlusion classifications using the Angle method. The study consisted of 20 female volunteers, between the ages of 17 and 27 years. They were separated into two groups (class I and class II occlusions) according to a dentist-performed evaluation. An assessment of forward head posture was conducted using a photographic technique (angular calculus) combined with a clinical analysis. In the EMG analyses, active differential surface electrodes (Ag) were utilized and were placed bilaterally on the belly of masseter and temporal muscles, perpendicular to the muscles fibres. The EMG signal recorded during bilateral isotonic mastication, was presented using the Root Mean Square and was processed by Matlab software. The results indicated that the EMG responses of temporal and masseter muscles tend to be modified by occlusion alteration class II. Subjects with class II occlusion tended to present more occurrence of forward head posture with alterations in the muscle activity pattern between masseter and temporal muscles.

Adolescent↗
PubMed1990

Postural stability during long-term treatment of cancer patients with epidural opioids.

Ten cancer patients treated with stable doses of epidural opioids were tested for postural stability. The postural stability was measured using a quantitative Romberg's test, performed on a computerized force-plate system. Sway tendencies in the sagittal and the transverse directions were recorded. The postural stability of the patients was compared with values obtained from healthy controls. Comparing the cancer patients with the controls, postural stability in eight out of ten patients was intact. The study suggests that long-term epidural opioid treatment has little influence on the patient's ability to stand safely.

Adult↗
PubMed1999

Effect of gaze on postural responses to neck proprioceptive and vestibular stimulation in humans.

1. We studied the effect of gaze orientation on postural responses evoked by vibration of neck dorsal muscles or by galvanic stimulation of the vestibular system during quiet standing in healthy humans. Various gaze orientations were obtained by different combinations of horizontal head-on-feet (-90, -45, 0, 45, 90 deg) and eye-in-orbit (-30, 0, 30 deg) positions. The instantaneous centre of foot pressure was recorded with a force platform. 2. With a symmetrical position of the vibrator relative to the spine, neck muscle vibration elicited a body sway in the direction of the head naso-occipital axis when the eyes were aligned with it. The same result was obtained both during head rotations and when the head and trunk were rotated together. 3. For lateral eye deviations, the direction of the body sway was aligned with gaze orientation. The effect of gaze was present both with eyes open and eyes closed. After long-lasting (1 min) lateral fixation of the target the effect of gaze decreased significantly. 4. Postural responses to galvanic vestibular stimulation tended to occur orthogonal to the head naso-occipital axis (towards the anodal ear) but in eight of the 11 subjects the responses were also biased by the direction of gaze. 5. The prominent effect of gaze in reorienting automatic postural reactions indicates that both neck proprioceptive and vestibular stimuli are processed in the context of visual control of posture. The results point out the importance of a viewer-centred frame of reference for processing multisensory information.

Adult↗
PubMed1990

Posture, place, and mood effects on ambulatory blood pressure.

Ambulatory blood pressure was studied as a function of posture, place, and mood in 131 subjects classified according to race, gender, and hypertensive status. The effect of posture was significant and explained a substantial proportion of within-subject variability. After controlling for posture, significant place and mood effects were observed when subjects were sitting but not when they were standing. Home vs. work differences in both systolic and diastolic blood pressure were significantly greater in Whites than in Blacks. Similar differences in systolic blood pressure were greater in mild hypertensive than in normotensive subjects. The results of this study underscore the need to control for effects of posture when interpreting ambulatory blood pressure readings.

Adult↗
PubMed1994

Temperature may be an appropriate sensor for chronotropically incompetent patients with postural syncope.

Chronotropically incompetent patients benefit most from sensor driven rate response during exercise. Postural syncope may occur despite the chronotropic response because of the failure of currently available sensors to respond physiologically to postural changes. Seven chronotropically incompetent patients with postural syncope who had a dual chamber rate adaptive pacemaker (Circadia) that modulates heart rate in response to temperature change were studied with respect to: (1) response to exercise; and (2) head-up tilt (HUT). During exercise, continuous-wave Doppler of aortic velocities and two-dimensional echocardiographic derived measurements of left ventricular systolic function were used to assess cardiac function. Patients exercised longer (by an average of 168 sec) in the DDDR compared to the DDI mode (P = 0.013). Increase in exercise duration was due mostly to the sensor driven increase during DDDR pacing. During DDDR pacing, heart rate increased from 71 +/- 6 to 121 +/- 17 ppm compared to 70 +/- 1 to 103 +/- 21 ppm for the DDI pacing (P = 0.038). Stroke volume as assessed by Doppler derived stroke distance (SD) contributed more significantly to the cardiac output increase during exercise in the DDI mode (SD increased from 13.4 +/- 4 to 18 +/- 7 cm in DDI compared to 13 +/- 4 to 14 +/- 2 cm in DDDR mode), although these mechanisms were insufficient to fully compensate for failure of appropriate chronotropic response. In response to the HUT, right ventricular temperature increased from 36.78 degrees C +/- 0.29 degrees C to 36.89 degrees +/- 0.28 degrees C (P = 0.0002), and heart rate increased from 54 +/- 3 to 71 +/- 8 ppm (P = 0.0003) in the DDDR mode. No significant change in heart rate occurred in the DDI mode in response to the HUT. Strong positive correlation of temperature and heart rate was noted in all patients in response to HUT (P = 0.001, R2 = 0.755-0.976). We conclude that temperature sensor responds physiologically to exercise and HUT. Therefore, temperature sensing rate adaptive dual chamber pacing may be appropriate for chronotropically incompetent patients with posture related syncope.

Aged↗
PubMed1995

Effect of masticatory muscle fatigue on cranio-vertical head posture and rest position of the mandible.

The aim of this study was to investigate whether induced fatigue of the masticatory muscles had any influence on the head posture, and whether this influence is related to the rest position and the movement characteristics of the mandible. The sample consisted of 13 female individuals, aged 23-34 yr. For the evaluation of possible changes in the natural cranio-vertical head posture, standardized facial profile photographs were used. Photographs were also used for the study of the facial characteristics. The freeway space and the opening and closing velocity of the mandibular displacement, as well as the duration of the masticatory cycles, were monitored with an optoelectronic method. A controlled dynamic fatigue was induced by a specially constructed spring-loaded device placed in the premolar region. No significant changes in the mean cranio-vertical postural position of the head were found during the various recording stages, while the freeway space was found to increase significantly after the fatigue test. No significant differences were observed concerning the average values of the mandibular movement characteristics. The analysis of the association between the individual changes showed an increase in the freeway space after the fatigue test in the subjects which exhibited an increase in the duration of the masticatory cycle in that period. No significant associations could be found between the changes in the head posture and the mandibular movement characteristics. Also, no significant correlation could be found between the facial type of the subjects and the variables studied.

Adult↗
PubMed1974

The postural response of normal dogs to sinusoidal displacement.

1. Normal dogs were trained to adopt a laterally symmetric stance on a horizontal platform. Sinusoidal oscillation of the platform in the cephalocaudal direction caused the dogs to adopt a strategy of response which would keep them from falling down during the period of imposed motion or perturbation.2. A Fourier analysis of the response variables provided a quantitative measure of the distortion in the induced movement at the various hind leg joints and in the motion of the body. Certain aspects of the distortion could be accounted for by recognized random events such as drift and panting. The remainder of the distortion was task related and therefore provided evidence that the dog's postural control system behaved essentially as a non-linear system.3. The motion of the body was less distorted than the motion at the joints. The frequency response of the body motion resembled that of a second order linear system, but the amplitude of the body motion did not vary in constant proportion with changes in the amplitude of the input. Further, the phase relation between body motion and platform motion was not constant when the amplitude of the input was changed. Therefore, it was concluded that the control of the body position was probably non-linearly related to the input disturbance but that the low pass filter characteristics tended to minimize the appearance of distortion in that system output.4. Control of the position of the hind limb is related to the control of the torque generated at the hind limb joints. To the extent that joint angle and change in joint angle are related to the torque at a joint, the distortion of the motion at the joints clearly demonstrates that control of the hind limb during this postural task was non-linearly related to the sinusoidal input.5. The uniformity of the response parameters, as assessed from the Fourier coefficients, indicated that all of the tested dogs adopted the same or nearly the same strategy for solving the problem of adjusting their postural control in response to the perturbation. Therefore, a reasonable hypothesis for future testing is that the central programme which generates this particular postural response will be structured similarly from one dog to the next.

Animals↗
PubMed1985

The respiratory activity of human levator costae muscles and its modification by posture.

The activation of the levator costae, a set of axial muscles inserted between each rib and the vertebra immediately rostral, has been studied electromyographically in a human subject, standing upright, with head erect ('neutral' posture). Caudal levator costae muscles (T9, and T10) are active during eupnoea. Periodic activity in phase with inspiration occurs in admixture with different levels of tonic activity that is strongly influenced by posture. Postural changes and various voluntary respiratory manoeuvres which should lengthen or shorten the levator costae muscles increase and decrease, respectively, their activity in the 'neutral' posture. The tonic activity was most readily enhanced by contralateral flexion of the spine, and this could bring to threshold phasic activity not otherwise present. Conversely, ipsilateral flexion could abolish both phasic and tonic components. Active expiratory efforts and static voluntary relaxation of the respiratory muscles could diminish or abolish levator costae activity. Paradoxically, chest-directed inspiratory efforts maintained at elevated lung volume with open glottis could also diminish or abolish levator costae activity. We attribute this to shortening of the levator costae muscles and unloading of their muscle spindles by the elevation of the ribs due to the action of the external intercostal and parasternal muscles. The results are discussed in relation to the mechanical coupling between the diaphragm and the lower rib cage; emphasis is given to the possible independent motions of the lower two or three ribs and the abductor action of their corresponding levator costae muscles, based on anatomico-physiological considerations.

Chin↗
PubMed2002

Numerical simulation of the influence of gravity and posture on cardiac performance.

A numerical model of the cardiovascular system was used to quantify the influences on cardiac function of intrathoracic pressure and intravascular and intraventricular hydrostatic pressure, which are fundamental biomechanical stimuli for orthostatic response. The model included a detailed arterial circulation with lumped parameter models of the atria, ventricles, pulmonary circulation, and venous circulation. The venous circulation was divided into cranial, central, and caudal regions with nonlinear compliance. Changes in intrathoracic pressure and the effects of hydrostatic pressure were simulated in supine, launch, sitting, and standing postures for 0, 1, and 1.8 G. Increasing intrathoracic pressure experienced with increasing gravity caused 12% and 14% decreases in cardiac output for 1 and 1.8 G supine, respectively, compared to 0 G. Similar results were obtained for launch posture, in which the effects of changing intrathoracic pressure dominated those of hydrostatic pressure. Compared to 0 G, cardiac output decreased 0.9% for 1 G launch and 15% for 1.8 G launch. In sitting and standing, the position of the heart above the hydrostatic indifference level caused the effects of changing hydrostatic pressure to dominate those of intrathoracic pressure. Compared to 0 G, cardiac output decreased 13% for 1 G sitting and 23% for 1.8 G sitting, and decreased 17% for 1 G standing and 31% for 1.8 G standing. For a posture change from supine to standing in 1 G, cardiac output decreased, consistent with the trend necessary to explain orthostatic intolerance in some astronauts during postflight stand tests. Simulated lower body negative pressure (LBNP) in 0 G reduced cardiac output and mean aortic pressure similar to I G standing, suggesting that LBNP provides at least some cardiovascular stimuli that may be useful in preventing postflight orthostatic intolerance. A unifying concept, consistent with the Frank-Starling mechanism of the heart, was that cardiac output was proportional to cardiac diastolic transmural pressure for all postures and gravitational accelerations.

Adaptation, Physiological↗
PubMed2004

Pre-impact lower extremity posture and brake pedal force predict foot and ankle forces during an automobile collision.

BACKGROUND: The purpose of this study was to determine how a driver's foot and ankle forces during a frontal vehicle collision depend on initial lower extremity posture and brake pedal force. METHOD OF APPROACH: A 2D musculoskeletal model with seven segments and six right-side muscle groups was used. A simulation of a three-second braking task found 3647 sets of muscle activation levels that resulted in stable braking postures with realistic pedal force. These activation patterns were then used in impact simulations where vehicle deceleration was applied and driver movements and foot and ankle forces were simulated. Peak rearfoot ground reaction force (F(RF)), peak Achilles tendon force (FAT), peak calcaneal force (F(CF)) and peak ankle joint force (F(AJ)) were calculated. RESULTS: Peak forces during the impact simulation were 476 +/- 687 N (F(RF)), 2934 +/- 944 N (F(CF)) and 2449 +/- 918 N (F(AJ)). Many simulations resulted in force levels that could cause fractures. Multivariate quadratic regression determined that the pre-impact brake pedal force (PF), knee angle (KA) and heel distance (HD) explained 72% of the variance in peak FRF, 62% in peak F(CF) and 73% in peak F(AJ). CONCLUSIONS: Foot and ankle forces during a collision depend on initial posture and pedal force. Braking postures with increased knee flexion, while keeping the seat position fixed, are associated with higher foot and ankle forces during a collision.

Acceleration↗
PubMed2000

Vibration-induced postural reaction continues after the contact with additional back support.

We investigated the development of postural reactions induced in standing subjects by Achilles tendon vibration. We compared vibratory reactions in 3 different conditions: normal standing, standing near support, and when the solid support being protracted forward changed the initial posture. Additional support for the back was placed at subject's sacral or shoulder level. In the easy standing condition, the postural vibration reaction consists of progressive backward upper body movement. When the body contacted the additional support on the sacral level during the vibratory reaction, the movement of the upper body continued in most of the subjects. This was accompanied by an increase of pressure on the toes. When the support was applied at the shoulder level, the body motion reversed its direction in half of the subjects. In this case, backward-forward oscillations occurred near the support. The initial change of body-support interaction did not influence the ensuing vibration reaction; namely the reaction was similar to that with the support near to the body at the sacral level. Our data demonstrate that the vibration-induced reaction is not a local reaction limited to one joint, but a complex postural synergy that involves both leg and trunk muscles and integrates the information from touch and pressure afferents of the upper body.

Achilles Tendon↗
PubMed2000

Role of different sensory inputs for maintenance of body posture in sitting rat and rabbit.

In this paper, we describe the postural activity in sitting rats and rabbits. An animal was positioned on the platform that could be tilted in the frontal plane for up to +/-20-30 degrees, and postural corrections were video recorded. We found that in both rat and rabbit, the postural reactions led to stabilization of the dorsal-side-up trunk orientation. The result of this was that the trunk tilt constituted only approximately 50% (rat) and 25% (rabbit) of the platform tilt. In addition, in the rabbit the head orientation was also stabilized. Trunk stabilization persisted in the animals subjected to the bilateral labyrinthectomy and blindfolding, suggesting that the somatosensory input is primarily responsible for trunk stabilization. Trunk stabilization was due to extension of the limbs on the side moving down, and flexion of the opposite limbs. EMG recordings showed that the limb extension was caused by the active contraction of extensor muscles. We argue that signals from the Golgi tendon organs of the extensor muscles may considerably contribute to elicitation of postural corrective responses to the lateral tilt.

Animals↗
PubMed1989

Scaling body support in mammals: limb posture and muscle mechanics.

The scaling of bone and muscle geometry in mammals suggests that peak stresses (ratio of force to cross-sectional area) acting in these two support elements increase with increasing body size. Observations of stresses acting in the limb bones of different sized mammals during strenuous activity, however, indicate that peak bone stress is independent of size (maintaining a safety factor of between 2 and 4). It appears that similar peak bone stresses and muscle stresses in large and small mammals are achieved primarily by a size-dependent change in locomotor limb posture: small animals run with crouched postures, whereas larger species run more upright. By adopting an upright posture, large animals align their limbs more closely with the ground reaction force, substantially reducing the forces that their muscles must exert (proportional to body mass) and hence, the forces that their bones must resist, to counteract joint moments. This change in limb posture to maintain locomotor stresses within safe limits, however, likely limits the maneuverability and accelerative capability of large animals.

Animals↗