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PubMed1992

Postural proprioceptive reflexes in standing human subjects: bandwidth of response and transmission characteristics.

1. This study investigated the reflex control of postural sway during human bipedal stance. The experiments were designed to: (i) find evidence for the operation of 'stretch reflex' pathways during quiet stance, (ii) determine the bandwidth of the reflex response, (iii) describe the reflex transmission characteristics in standing subjects, and (iv) assess the ability of subjects to make a task-dependent change in the reflex. 2. A continuous random perturbation that did not threaten stability was applied at waist level to nine standing subjects. The effects of the perturbation on ankle torque, ankle movement and soleus electromyographic activity (EMG) were identified by cross-correlation. The bandwidth of the reflex response and the transmission characteristics of reflexes that respond to ankle movement were identified by spectral analysis. Changes in these reflex responses were investigated when subjects attempted to stand as still as possible, had their eyes closed, or balanced a load equivalent to their own body in a situation in which neither visual nor vestibular reflexes would be activated. 3. When standing, a reflex response coherent with the perturbation was seen in soleus EMG at frequencies up to 5 Hz, with maximal coherence at 1.0-2.0 Hz. Reflex gain increased with frequency, and there was a frequency-dependent phase advance of soleus EMG on ankle movement reaching 135 deg at 3 Hz. When attempting to minimize sway, subjects produced a more coherent reflex response and significantly increased reflex gain. 4. The response and transmission characteristics of the lower limb proprioceptive reflex in freely standing subjects were similar to those in subjects balancing a load at the ankle, a situation in which vestibular and visual inputs could not contribute. 5. It is concluded that reflex feedback related to ankle movement contributes significantly to maintaining stance, and that much of the reflex response originates from lower limb mechanoreceptors stimulated by ankle rotation. Although reflex gain may be relatively low during quiet stance it can be increased when necessary to maintain stability.

Adult↗
PubMed2005

Sensory organization determinants of postural stability in trained ballet dancers.

Trained ballet dancers and nondancer controls completed six balance tests using computerized dynamic posturography. The tests facilitated assessment of the type of sensory organization used to maintain postural control under conditions ranging from quiet standing to a situation in which visual and/or somatosensory information was systematically removed or made unreliable. Results indicated that ballet dancers and controls have comparable balance ability during eyes open and eyes closed conditions. However, when somatosensory information alone or in combination with visual information was made unreliable, dancers were significantly less stable than controls and utilized a hip strategy to maintain postural control.

Adult↗
PubMed1986

Comparison of electromyographic response patterns during posture and stress tasks in chronic low back pain patterns and control.

Activity of the paraspinals, abdominals, and hip extensor muscles, heart rate, and skin conductance were measured in 13 chronic low back pain patients (CLBP) and 13 controls during static postures and psychological stress. After 10 min of baseline-rest, a subject was tested during standing, 45 degrees bend, maximum bend, posterior and anterior pelvic tilts, sitting, and supine with knees up, followed by mental arithmetic and the cold pressor. The postures were grouped into supported, balanced, and unbalanced categories. Multivariate analyses of variance revealed differences due to posture categories, but not due to group membership. It was concluded that while work is needed to understand normal muscle function during common movements, there is no compelling evidence that back muscle functioning in CLBP patients differs significantly from control subjects.

Adult↗
PubMed1976

Brain stem and cerebellar lesions in female rats. I. Tests of posture and movement.

Lesions were placed in a number of brain stem and cerebellar structures to determine their importance to the control of posture and movement in rats. Lesions of the lateral vestibular nucleus, superior vestibular nucleus, or inferior olive produced severe disturbances of posture and movement. Rats with such lesions had pronounced tremor, abnormal head posture and asymmetrical trunk posture, and failed in a balance beam test. Lesions of the medial medullary reticular formation and medial vestibular nucleus had less overall effect on posture and movement, while animals with lesions of the deep cerebellar nuclei were relatively unimpaired. All lesion groups showed some recovery of function during the course of postoperative testing.

Animals↗
PubMed2003

Predictable postural perturbation in premotor and motor components of reaction time.

The aim of the present experiment was to study the central control of posture and movement coordination. Subjects' task was voluntarily to raise their arms in response to a visual signal, this movement generating an unloading in which the weight of the load (0 gm, 1500 gm, 3000 gm, or 4500 gm) was known in advance. Thus, subjects could predict the postural perturbation generated by the unloading. Analysis showed an effect of the unloading conditions: in the 0-gm and 1500-gm conditions, reaction time (RT) was longer than in the 3000-gm and 4500-gm conditions. This decrease in RT was exclusively due to a reduction of the premotor time, confirming the posturokinetic coordination in the programming processes. The implications of the decrease in premotor time with the increase of the perturbation for the posture/movement coordination strategies are also discussed.

Adult↗
PubMed1983

Learning to walk is still an "old" problem: a reply to Zelazo (1983).

Zelazo (1983) proposes that two cognitive prerequisites are necessary for the development of locomotion: The conversion of neo-natal reflexive stepping into instrumental behavior and a shift from stereotypical to relational play. I argue that these cognitive assumptions are unnecessary. The disappearance of neo-natal stepping can be explained as a result of increasing leg mass, and the retention of this reflex with practice may be simply an exercise effect. Comparative and evolutionary evidence suggests that locomotion is not associated with abstract reasoning ability. Learning to walk is a complex, gradual process of maturation of motivation, the integration of subcortical pattern-generating centers with the neural substrate for control of posture and balance, and important changes in body proportions and bone and muscle strength. The control of walking is likely the province of mechanism phylogenetically more primitive than the human cerebral cortex. There is no need to invoke cortical explanations when more simple ones are sufficient.

Journal Article↗
PubMed1997

Model Predictive Impedance Control: A Model for Joint Movement.

Impedance control has been suggested as the strategy employed by the central nervous system to control human postures and movements. A realization of this strategy is presented that uses a model predictive control algorithm as a higher motor controller. External disturbances are explicitly included in the model. The combination of 3 key factors-joint impedance control, model predictive controller, and external disturbance input-forms the basis for the generality of this model. The model was applied to 3 different types of joint movements: a tracking movement with an unpredicted disturbance, a rhythmic movement, and an unstable biped model of human walking. Computer simulation results showed excellent performance of the model in all 3 cases for optimal values of active joint impedances and an exact match between the musculoskeletal system and the model internal to the model predictive controller. The controller was also able to maintain acceptable performance in the presence of a 25% mismatch between the musculoskeletal system and its internal model.

equilibrium-point hypothesis↗
PubMed2004

Effects of immersion water temperature on whole-body fluid distribution in humans.

AIM: In this study, we quantified acute changes in the intracellular and extracellular fluid compartments during upright neutral- and cold-water immersion. We hypothesized that, during short-term cold immersion, fluid shifts would be wholly restricted to the extracellular space. METHODS: Seven males were immersed 30 days apart: control (33.3 degrees SD 0.6 degrees C); and cold (18.1 degrees SD 0.3 degrees C). Posture was controlled for 4 h prior to a 60-min seated immersion. RESULTS: Significant reductions in terminal oesophageal (36.9 degrees +/- 0.1 degrees -36.3 degrees +/- 0.1 degrees C) and mean skin temperatures (30.3 degrees +/- 0.3 degrees -23.0 degrees +/- 0.3 degrees C) were observed during the cold, but not the control immersion. Both immersions elicited a reduction in intracellular fluid [20.17 +/- 6.02 mL kg(-1) (control) vs. 22.72 +/- 9.90 mL kg(-1)], while total body water (TBW) remained stable. However, significant plasma volume (PV) divergence was apparent between the trials at 60 min [12.5 +/- 1.0% (control) vs. 6.1 +/- 3.1%; P < 0.05], along with a significant haemodilution in the control state (P < 0.05). Plasma atrial natriuretic peptide concentration increased from 18.0 +/- 1.6 to 58.7 +/- 15.1 ng L(-1) (P < 0.05) during cold immersion, consistent with its role in PV regulation. We observed that, regardless of the direction of the PV change, both upright immersions elicited reductions in intracellular fluid. CONCLUSION: These observations have two implications. First, one cannot assume that PV changes reflect those of the entire extracellular compartment. Second, since immersion also increases interstitial fluid pressure, fluid leaving the interstitium must have been rapidly replaced by intracellular water.

Adult↗
PubMed1987

Plasma catecholamines and alpha- and beta-adrenoceptors in circulating blood cells in patients on continuous ambulatory peritoneal dialysis.

Autonomic system dysfunction could be the cause of postural hypotension seen in patients on continuous ambulatory peritoneal dialysis (CAPD). To verify this hypothesis, we examined the alpha 2- and beta 2-adrenoceptors on blood cells after 1/2 h in the resting supine position with the peritoneal cavity filled for 2-3 h, as well as the response of plasma norepinephrine (NE), heart rate (HR) and mean arterial blood pressure (MAP) to 10 min of standing. Supine free and particularly conjugated NE levels were significantly higher in all uremic patients compared with controls. The postural test induced similar increases of MAP and HR in 8 diabetic, 11 nondiabetic patients and 23 controls, whereas 4 diabetic patients became hypotensive. Orthostasis caused a mean free NE increment of only 0.5 nmol/l in the latter patient group with mean NE responses of 1.45-1.65 nmol/l in the former 3 groups. The densities of platelet alpha 2-adrenoceptors (assessed by [3H] yohimbine binding) and of mononuclear leucocyte (MNL) beta 2-adrenoceptors determined by (-) (125I) iodocyanopindolol binding amounted to 160 +/- 50 and 1600 +/- 520 binding sites/cell, respectively, in controls and were unchanged in patients without postural hypotension. The 4 diabetic patients suffering from postural hypotension showed numerically higher beta 2-receptor numbers (2080 binding sites/cell), significantly increased alpha 2-receptor densities (280 binding sites/cell, p less than 0.05) and significantly increased MNL isoproterenol-stimulated adenylate cyclase activities (38 vs 24 pmol cAMP/10(6) MNL/10 min in controls, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Diabetes Mellitus, Type 1↗
PubMed1999

Postural synergies associated with a stepping task.

BACKGROUND AND PURPOSE: Synergistic relationships among multiple muscle components are thought to exist to simplify control of posture and movement. The purpose of this study was to examine the extent to which children, young adults, and older adults exhibit consistent sequences of postural muscle activation when lifting the right foot onto a step from a standing position. SUBJECTS: Twenty subjects without known impairments of the neuromuscular system (10 male, 10 female) in each of 3 age groups--children (8-12 years), young adults (25-35 years), and older adults (65-73 years)--participated. METHODS: A pressure switch taped to the subject's right foot was used to determine movement onset and offset. Latencies of muscle activation were determined using surface electromyography. A preferred postural synergy was defined as the sequence of postural muscle activation observed during the majority of trials for each subject. RESULTS: Mean movement times did not differ among age groups. Although the left tibialis anterior (TA) muscle was the first of the postural muscles activated in 93% of the trials, subjects displayed considerable variability in the subsequent order of postural muscle activation. Across subjects, a total of 14 different preferred postural synergies were observed. Age groups did not differ in the number of different synergies. CONCLUSION AND DISCUSSION: Early TA activation may reflect biomechanical constraints of the stepping task, producing forward displacement of the center of mass over the changing base of support. The fact that subjects of all ages were quite variable in the specific sequences of muscles activated subsequent to the TA suggests that, for this type of task, therapists should not focus their interventions on facilitating execution of particular synergy patterns.

Adolescent↗
PubMed1991

Human postural dynamics.

The study of posture dynamics is important not only to understand disorders of impaired equilibrium and protective reactions to unexpected displacements of the human body, but also to the design of prosthesis and functional neuromuscular stimulation as aids to patients with impaired postural stability and locomotion. Coordinated control of the body segments is a complex aspect of motor behavior, owing to the multiple degrees of freedom of the controlled system. Several interacting subsystems are involved in the dynamics of human posture and locomotion, including the skeletal, neuromuscular, and sensory systems. Human posture control is maintained by somatosensory, vestibular, and visual feedback, integrated within the locomotor and central nervous systems. Studies of posture dynamics and stability therefore entail the study of mechanical aspects of the human body, its sensory systems, and the principles governing coordination in motion control. In this paper is reviewed some of the research done in the field of human posture dynamics, including such topics as biomechanics, equilibrium, stability, motion coordination, neural feedback, neural control systems modeling, motion strategies, optimality of motion, and adaptation. We consider experimental approaches and theoretical models, as well as the gap between them. Principles for the experimental investigation of control systems are considered.

Animals↗
PubMed1976

Aldosterone and renin-angiotensin responses to stimuli in patients with treated congestive heart failure.

Aldosterone responses to physiologic doses of known regulatory factors have been studied in 8 patients with treated congestive heart failure under standard conditions of electrolyte balance and controlled body posture. The response of plasma aldosterone, plasma renin activity (PRA), and in some instances angiotension II, to the brief (120 minute) intravenous administration of ACTH (1.25 mug beta1-24 ACTH in 60 minute and 5 mug in 60 minutes), angiotensin II (30 mug) and potassium (30 mEq.), and to 2 hours in the upright posture, was measured. The plasma aldosterone increase was greatest in response to ACTH (mean increment 20.8 ng. per 100 ml.) followed by that with upright posture (mean rise 17.1 ng. per 100 ml.), potassium (mean 6.7 ng. per 100 ml.) and angiotensin II (5.8 ng. per 100 ml.). PRA rose vigorously following 2 hours of upright posture and fell with angiotensin II infusion, but showed no definite change in response to potassium or ACTH. A diurnal pattern of PRA, angiotensin II, and plasma aldosterone on control days was observed in these patients, with higher prenoon levels falling to a nadir at midnight. The electrolyte and hormone response to administered aldosterone (75 mug over 2 hours) was also studied in the same patients. During aldosterone infusions, plasma aldosterone increased (increment range 36 to 110 ng. per 100 ml.), urine sodium excretion decreased, but no significant change in urine potassium excretion or in PRA was observed. It is concluded that small fluctuations in ACTH secretion, as well as change in body posture, produce marked effects on plasma aldosterone in patients with controlled congestive heart failure. Aldosterone responses to ACTH in these patients is similar to that observed in normal subjects.

Adrenocorticotropic Hormone↗
PubMed2002

Lung function during and after prolonged head-down bed rest.

We determined the effects of prolonged head-down tilt bed rest (HDT) on lung mechanics and gas exchange. Six subjects were studied in supine and upright postures before (control), during [day 113 (D113)], and after (R + number of days of recovery) 120 days of HDT. Peak expiratory flow (PF) never differed between positions at any time and never differed from controls. Maximal midexpiratory flow (FEF(25-75%)) was lower in the supine than in the upright posture before HDT and was reduced in the supine posture by about 20% between baseline and D113, R + 0, and R + 3. The diffusing capacity for carbon monoxide corrected to a standardized alveolar volume (volume-corrected DL(CO)) was lower in the upright than in the supine posture and decreased in both postures by 20% between baseline and R + 0 and by 15% between baseline and R + 15. Pulmonary blood flow (Q(C)) increased from R + 0 to R + 3 by 20 (supine) and 35% (upright). As PF is mostly effort dependent, our data speak against major respiratory muscle deconditioning after 120 days of HDT. The decrease in FEF(25-75%) suggests a reduction in elastic recoil. Time courses of volume-corrected DL(CO) and Q(C) could be explained by a decrease in central blood volume during and immediately after HDT.

Adult↗
PubMed2002

Midbrain control of three-dimensional head orientation.

Little is known about the neural mechanisms controlling head posture and why they fail in clinical syndromes like torticollis. It is well established, however, that the brain controls eye position by integrating eye velocity commands. By electrically stimulating and reversibly inactivating midbrain sites in the head-free (nonimmobilized) monkey, we found that the interstitial nucleus of Cajal functions as a neural integrator for head posture. We suggest that a bilateral imbalance in this structure, through either direct damage or inappropriate input, could be one of the mechanisms underlying torticollis.

Animals↗
PubMed2005

The effect of backpack weight on the standing posture and balance of schoolgirls with adolescent idiopathic scoliosis and normal controls.

Concerns have been raised regarding the effect of carrying a backpack on adolescent posture and balance, but the effect of backpack loading combined with other factors affecting balance, such as adolescent idiopathic scoliosis (AIS), has not been determined. This study examines the effects of backpack load on the posture and balance of schoolgirls with AIS and normal controls. The standing posture of 26 schoolgirls with mild AIS (mean age 13, Cobb angle 10-25 degrees ) and 20 age-matched normal schoolgirls were recorded without a backpack and while carrying a standard dual-strap backpack loaded at 7.5%, 10%, 12.5% and 15% of the subject's bodyweight (BW). Kinematics of the pelvis, trunk and head were recorded using a motion analysis system and centre of pressure (COP) data were recorded using a force platform. Reliable COP data could only be derived for 13 of the subjects with AIS. Increasing backpack load causes a significantly increased flexion of the trunk in relation to the pelvis and extension of the head in relation to the trunk, as well as increased antero-posterior range of COP motion. While backpack load appears to affect balance predominantly in the antero-posterior direction, differences between groups were more evident in the medio-lateral direction, with AIS subjects showing poor balance in this direction. Overall, carrying a backpack causes similar sagittal plane changes in posture and balance in both normal and AIS groups. Load size or subject group did not influence balance, but the additive effect of backpack carrying and AIS on postural control alters the risk of fall in this population. Therefore, load limit recommendations based on normal subjects should not be applicable to subjects with AIS.

Adolescent↗
PubMed1991

Norepinephrine and epinephrine responses to postural stimulus in orthostatic hypotension due to brainstem ischemic lesions.

Norepinephrine (NE) and epinephrine (E) responses to upright posture were investigated in 25 patients with orthostatic hypotension due to brainstem ischemic lesions and in 25 control subjects. In controls the postural stimulus induced constantly a rise in NE urinary excretion and a reduction in E excretion, while in patients with orthostatic hypotension it caused a depression in NE urinary excretion and a rise in E urinary excretion; the last alteration was noticed in all but one patient. The E discharge induced by posture in patients with orthostatic hypotension may be involved in the reduction of vascular peripheral resistance and then in postural fall of blood pressure displayed by such patients.

Adult↗
PubMed2005

High prevalence and severity of cardiac autonomic neuropathy in Vietnamese diabetic patients.

AIMS: Malnutrition is frequent in Vietnamese people and may influence cardiac autonomic neuropathy (CAN). The aim of the present study was to investigate cardiac autonomic function in healthy subjects living in Vietnam and the prevalence of CAN in Vietnamese diabetic patients. METHODS: One hundred and five diabetic patients (BMI = 19.8 +/- 0.3 kg/m(2)), 50 Type 1 and 55 Type 2, living in Hué (Vietnam) were selected and compared with 60 non-diabetic healthy Vietnamese controls (BMI = 20.8 +/- 0.2 kg/m(2)) and also European controls. CAN function was evaluated by five standardized tests: three tests for heart rate variations (HRV) which depend mainly on parasympathetic activity, and two tests for blood pressure (BP) response which depend mainly on sympathetic activity. RESULTS: With age taken into account, 41 of the 60 Vietnamese controls had at least mild CAN, as defined by one abnormal test for HRV when compared with the European control series, and 11 of them had two or three abnormal tests. Among the Vietnamese control men, those with abnormal HRV had lower BMI than those without (P = 0.036). Seven Vietnamese controls had postural hypotension and 16 had an abnormal BP response to the handgrip test. Compared with the Vietnamese controls, 71 diabetic patients (67.6%), 40 Type 1 and 31 Type 2, had at least mild CAN, 37 of them had two or three abnormal HRV tests, and 56 diabetic patients (53.3%) had an abnormal BP response to the sympathetic tests. Abnormal HRV were associated with significantly lower BMI, waist and hip circumferences, longer diabetes duration and higher fasting blood glucose. In the logistic regression analyses, abnormal HRV were associated significantly with duration of diabetes and BMI in patients with Type 2 diabetes. CONCLUSIONS: Cardiac autonomic dysfunction is frequent in normal Vietnamese subjects. CAN appears to be a more frequent complication of diabetes in Vietnam than in Western countries and diabetic parasympathetic dysfunction is frequently associated with sympathetic disorders. This confirms the deleterious effect a poor nutritional state has on cardiac autonomic function.

Adult↗