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PubMed1995

The organization of patterns of multilimb coordination as revealed through reaction time measures.

Simple visual reaction time (RT) during the performance of sagittal movements of the upper and/or lower limbs was investigated. Experiment 1 demonstrated that RTs increased when more limbs were to be moved simultaneously. This effect was more apparent for the upper than for the lower limbs. Experiment 2 allowed a separation of RT into premotor time (PMT) and motor time (MOT) components through analysis of electromyographic activity, and showed that these longer response delays were associated with increased PMTs. This suggests that the time required for the central organization of movements increased as more limbs were to be controlled simultaneously. Compared to single-limb performance conditions, the increases in RT were much larger in the upper limbs (up to 16%) than in the lower limbs (up to 5%) when limb segments were added. During single-limb conditions, RTs in the upper limbs tended to be smaller than in the lower limbs, in accordance with efferent nerve conduction time estimates. Conversely, the lower limb(s) was (were) initiated before the upper limb(s) when both effector types were moved simultaneously. This pattern of activation is reminiscent of the organization of postural control during upright standing, where goal-directed arm activity is preceded by (bilateral) leg activity to anticipate for the upcoming postural destabilization. Finally, hemifield manipulations in experiment 2 revealed faster RTs and PMTs for stimuli presented in the right visual field in comparison with the left field. This advantage was evident for ipsilateral as well as contralateral responses and supports the pre-eminence of the left hemisphere in the complex organization of gross motor responses.

Acoustic Stimulation↗

PubMed1999

Postural muscle responses in the spinal cord injured persons during forward reaching.

To compensate for postural muscle function loss spinal cord injured (SCI) people have to use parts of the sensorimotor system which are still intact. In this study, postural control was investigated in high and low thoracic SCI people and in able-bodied controls, using a bimanual forward-reaching task. Muscle activity was recorded bilaterally from the erector spinae (ES) at level L3, T9 and T3, latissimus dorsi (LD), ascending part of the trapezius muscle (TPA), serratus anterior (SA), sternocostal head of the pectoralis major (PM) and the oblique abdominal muscles (OA) by means of surface electromyography. Sitting balance was monitored by measuring the changes in the location of the centre of pressure (CP) using a force platform. Muscle activity analyzed in different phases of the movement showed that SCI people adopt different postural adjustments to face the balance changes due to the reaching movement. SCI people make alternative use of non-postural muscles like the LD and TPA to maintain their sitting balance.

Adaptation, Physiological↗

PubMed2003

Organization of compensatory postural coordination patterns.

The authors investigated whether compensatory postural coordination patterns are organized according to the same dynamical principles as are nonequilibrium phase transitions. Eight participants were asked to maintain upright balance on a moving platform that was sinusoidally translated in the anterior-posterior direction and was systematically increased and decreased 0.19 Hz as a step function every 10 platform cycles through the frequency range 0.19-1.46 Hz. At low platform frequencies, all participants exhibited small joint angular motions with high variability, and the relative phase between the joint motions exhibited drifting patterns and large fluctuations. As platform frequency increased, the amplitude of joint motion increased systematically and joint-specific oscillatory patterns emerged. The findings provided no evidence for a Hopf bifurcation or hysteresis in the transitions of postural coordination modes, however, or, more generally, a basis for distinguishing the relevance of linear versus nonlinear models of postural control.

Adult↗

PubMed1983

Motor control in man after partial or complete spinal cord injury.

The essential features of motor control in spinal man can be understood in terms of segmental reflexes interacting with, and controlled by, the influence of distant segments and even by the brainstem. Thus, overall motor control in patients with spinal cord lesions can be classified according to structure as: (1) simple segmental stretch and withdrawal reflexes; (2) plurisegmental gross reflex movement of paralyzed muscles; or (3) propriospinal processes with partial brain influence (i.e., severe spasticity and traces of position and postural control). Because of the variable nature of the injuries, there may be exceptions to this rule. However, the basic mechanisms can be understood by studying stretch reflex responses to various stimuli. The segmental reflexes are under a powerful influence of the propriospinal interneuron system which conducts impulses up and down the spinal cord. Finally, the apparently "isolated" spinal cord in which clinical signs indicate complete motor paralysis and lack of sensation below the lesion is not always isolated from supraspinal control of involuntary motor activity. In a significant proportion of the clinically complete spinal injuries we studied, it was possible to demonstrate the presence of preserved bulbospinal influences on spinal reflex responses.

Electromyography↗

PubMed2002

Adaptation of reflexive feedback during arm posture to different environments.

In this study we have examined the ability of the central nervous system (CNS) to use spinal reflexes to minimize displacements during postural control while continuous force perturbations were applied at the hand. The subjects were instructed to minimize the displacements of the hand from a reference position that resulted from the force perturbations. The perturbations were imposed in one direction by means of a hydraulic manipulator of which the virtual mass and damping were varied. Resistance to the perturbations came from intrinsic and reflexive stiffness, and from the virtual environment. It is hypothesized that reflexive feedback during posture maintenance is optimally adjusted such that position deviations are minimal for a given virtual environment. Frequency response functions were estimated, capturing all mechanical properties of the arm at the end point (hand) level. Intrinsic and reflexive parameters were quantified by fitting a linear neuromuscular model to the frequency responses. The reflexive length feedback gain increased strongly with damping and little with the eigenfrequency of the total combined system (i.e. arm plus environment). The reflexive velocity feedback gain decreased slightly with relative damping at the largest eigenfrequency and more markedly at smaller eigenfrequencies. In the case of highest reflex gains, the total system remained stable and sufficiently damped while the responses of only the arm were severely underdamped and sometimes even unstable. To further analyse these results, a model optimization was performed. Intrinsic and reflexive parameters were optimized such that two criterion functions were minimized. The first concerns performance and penalized hand displacements from a reference point. The second one weights afferent control effort to avoid inefficient feedback. The simulations showed good similarities with the estimated values. Length feedback was adequately predicted by the model for all conditions. The predicted velocity feedback gains were larger in all cases, probably indicating a mutual gain limiting relation between length and velocity afferent signals. The results suggest that both reflex gains seem to be adjusted by the CNS, where in particular the length feedback gain was optimal so as to maximize performance at minimum control effort.

Adult↗

PubMed2002

Spinocerebellar ataxia in monozygotic twins.

CONTEXT: Although phenotypic heterogeneity in autosomal dominant spinocerebellar ataxia (SCA) has been explained in part by genotypic heterogeneity, clinical observations suggest the influence of additional factors. OBJECTIVES: To demonstrate, quantitate, and localize physiologic abnormalities attributable to nongenetic factors in the development of hereditary SCA. DESIGN: Quantitative assessments of ocular motor function and postural control in 2 sets of identical twins, one with SCA type 2 and the other with episodic ataxia type 2. SETTING: University laboratory. MAIN OUTCOME MEASURES: Saccadic velocity and amplitude, pursuit gain, and dynamic posturography. RESULTS: We found significant differences in saccade velocity, saccade metrics, and postural stability between each monozygotic twin. The differences point to differential involvement between twins of discrete regions in the cerebellum and brainstem. CONCLUSIONS: These results demonstrate the presence of quantitative differences in the severity, rate of progression, and regional central nervous system involvement in monozygotic twins with SCA that must be owing to the existence of nongermline or external factors.

Adult↗

PubMed2004

Vestibular compensation in lampreys: restoration of symmetry in reticulospinal commands.

Removal of a vestibular organ (unilateral labyrinthectomy, UL) in the lamprey results in a loss of equilibrium, so that the animal rolls (rotates around its longitudinal axis) when swimming. Owing to vestibular compensation, UL animals gradually restore postural equilibrium and, in a few weeks, swim without rolling. Important elements of the postural network in the lamprey are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections to the spinal cord. As shown previously, a loss of equilibrium after UL is associated with disappearance of vestibular responses in the contralateral group of RS neurons. Are these responses restored in animals after compensation? To answer this question, we recorded vestibular responses in RS neurons (elicited by rotation of the compensated animal in the roll plane) by means of chronically implanted electrodes. We found that the responses re-appeared in the compensated animals. This result supports the hypothesis that the loss of equilibrium after UL was caused by asymmetry in supraspinal motor commands, and the recovery of postural control in compensated animals was due to a restoration of symmetry.

Action Potentials↗

PubMed2006

On the introduction of a new parameter for the analysis of posture by acceleration of the centre of pressure.

AIM: With this study we wanted to determine whether the standard parameters for describing posture are sufficient. METHODS: The time-series analysis of statokinesigrams of 50 healthy volunteers and Parkinsonian subjects, the latter representing an excellent example of persons affected by a postural control disorder, were assessed and compared. The tests were carried out using a new parameter ''p'', which is introduced with this work. The novel parameter is adimensional and independent of the subjects' anthropometric data and can be easily computed with elementary mathematics. RESULTS: Parameter ''p'' was found to discriminate healthy subjects from parkinsonians (as an example of subjects affected by a disease of the central control system). CONCLUSIONS: A new tool for the early diagnosis of disease of the central control system could theoretically be developed from our results.

Acceleration↗

PubMed2000

Reorientation of a visually evoked postural response during passive whole body rotation.

Visually evoked postural responses (VEPR) to a roll-motion rotating disk were recorded from normal subjects standing on a yaw axis motorised rotating platform. The disk was fluorescent so that subjects could be tested in an otherwise dark room. Movements of the head and centre of foot pressure were measured while subjects looked at the disk with their eyes and head in the primary position and while the rotating platform moved the subjects randomly to 0, +/-45 degrees and +/-90 degrees angles from the visual stimulus. Subjects were instructed to maintain fixation on the centre of the rotating disk but the amount of horizontal eye and head movement used was not specified. Platform rotational velocity was set near threshold values for perception of self-rotation (approximately 2 degrees/s) so that subjects would find it difficult to reconstruct the angle travelled. The data showed that the VEPR occurred in the plane of disk rotation, regardless of body position with respect to the disk, and despite the subjective spatial disorientation induced by the experiment. Averages of the response revealed a good match (gain=0.95) between disk orientation and sway direction. The horizontal gaze deviation required to fixate the centre of the disk was largely achieved by head motion (head 95%, eye 5%). The results confirm previous results that VEPRs are reoriented according to horizontal gaze angle. In addition, we show that the postural reorientation is independent of cognitively or visually mediated knowledge of the geometry of the experimental conditions. In the current experiments, the main source of gaze position input required for VEPR reorientation was likely to be provided by neck afferents. The results support the notion that vision controls posture effectively at any gaze angle and that this is achieved by combining visual input with proprioceptively mediated gaze-angle signals.

Adult↗

PubMed1998

Vestibular exercises improve central vestibulospinal compensation after vestibular neuritis.

OBJECTIVE AND BACKGROUND: Animal experiments have shown that central vestibular compensation of unilateral peripheral vestibular lesions can be improved by vestibular exercises. There are, however, no equivalent clinical studies on the efficacy of such specific physiotherapy on acute unilateral peripheral vestibular lesions in humans. DESIGN AND METHODS: To quantify the differential effects of specific vestibular exercises on central compensation in patients with an acute/subacute unilateral vestibular lesion (vestibular neuritis), we determined the time course of recovery of 1) the ocular torsion (OT) for the vestibulo-ocular system, 2) the subjective visual vertical (SVV) for perception, and 3) the total sway path (SP) values for postural control in 19 patients with and 20 patients without vestibular exercises. All patients had a persisting peripheral vestibular deficit for at least 30 days (statistical end point). RESULTS: Although normalization of OT and SVV was similar in the control and physiotherapy groups, the total SP values on day 30 after symptom onset differed significantly: 3.2 +/- 1.9 m/min in the physiotherapy group and 16.9 +/- 6.1 m/min in the control group (ANOVA, p < 0.001). CONCLUSIONS: This prospective clinical study suggests that specific vestibular exercises improve vestibulospinal compensation in patients with acute peripheral vestibular lesions.

Adult↗

PubMed2006

Recovery of intentional dynamic balance function after intravenous sedation with midazolam in young and elderly subjects.

BACKGROUND AND OBJECTIVE: Computerized dynamic posturography using an intentional postural-sway task can be used to assess body-leaning ability and postural-control ability to prevent falls. Falls are the leading cause of morbidity and mortality for the elderly. The purpose of the present study was to evaluate the recovery of intentional dynamic balance function after intravenous sedation with midazolam in elderly subjects in comparison with that in young subjects. METHODS: Midazolam was administered in small, divided doses over 4-5 min until the Wilson sedation score reached three in 20 young and 18 elderly male volunteers. The dynamic limits of the stability test, in which subjects leaned their body intentionally as indicated by a cursor moving on a computer screen, was determined before (baseline) and 50, 70, 90, 110, and 130 min after administration of midazolam. RESULTS: The changes from baseline values of path sway and movement time 50 min after the administration of midazolam in elderly subjects (106.8 +/- 101.0%, 4.6 +/- 3.0 s; mean +/- SD) were significantly greater than those (32.9 +/- 87.2%, 1.9 +/- 2.8 s) in young subjects (P = 0.024, P = 0.008), respectively. CONCLUSIONS: The elderly show slower recovery of the intentional dynamic balance function than do young adults after intravenous sedation with midazolam.

Adult↗

PubMed2003

Mediolateral sway in single-leg stance is the best discriminator of balance performance for Tai-Chi practitioners.

OBJECTIVES: To identify a balance measure that can best distinguish Tai-Chi from non-Tai-Chi practitioners and to examine whether longer Tai-Chi practice results in better balance control. DESIGN: Cross-sectional comparative study. SETTING: Community. PARTICIPANTS: Nineteen Tai-Chi practitioners (who practiced Tai Chi for 30-45min at least 3/wk for >1y) and 19 healthy subjects with regular exercise habits (who practiced Tai Chi for 30-45min at least 3/wk for >1y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Scores on 2 clinical tests (functional reach, gait) and 1 laboratory test (postural sway). RESULTS: Tai-Chi practitioners had better clinical test scores for functional reach, gait speed, stride length, and sway parameters during single-leg stance (P<.05). Sway in mediolateral direction during single-leg stance was the balance performance variable that best discriminated the Tai-Chi group from non-Tai-Chi group. More experience practicing Tai Chi was associated with better balance performance. CONCLUSIONS: Tai-Chi practitioners performed better both in clinical and laboratory tests when compared with subjects who did not practice Tai Chi. More Tai-Chi experience was associated with better postural control.

Activities of Daily Living↗

PubMed1994

The initiation of normal walking.

Lower extremity electromyograms (EMGs), ground reaction forces, and body motion were measured during the brisk initiation of forward walking performed by 12 healthy adults, aged 20 to 82 years. Gait was initiated 20 times in response to a visual cue. During gait initiation, the body rotated about the ankles like a flexible inverted pendulum. The muscles of the lower extremities were activated stereotypically so as to create moments of force about the ankles that propelled the body toward the stance foot and into forward motion. All volunteers exhibited similar patterns of gait initiation, which were so reproducible that computer averaging of multiple steps by each person was possible. Gait initiation is a stereotyped sequence of postural shifts that culminates in a forward step. Disturbances of gait initiation could result from abnormalities in postural control, movement, or their integration.

Adolescent↗

PubMed1995

Behavioral and metabolic consequences of neonatal exposure to diazepam in rat pups.

The short-term consequences of a neonatal exposure to diazepam (DZP) on neurobehavioral development and postnatal changes in local cerebral metabolic rates for glucose (LCMRglcs) in selected regions were studied in rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP or of the dissolution vehicle from Postnatal Day 2 (P2) to 21 (P21). DZP did not affect the static righting reflex tested at P4 but increased suspension time at P10 and time to complete a 180 degrees pivoting on an inclined plane at P9. In a locomotor coordination test performed at P20, swimming or climbing on a vertical pole was not affected by DZP while the drug impaired the ability of the rat to place its hind-paws on the horizontal platform after climbing. Likewise, DZP induced marked decreases (19-45%) in LCMRglcs in most structures studied at P10, P14, and P21. The results of the present study show that neonatal DZP treatment induces motor deficits that appear to be quite subtle, to concern mainly posture and body balance. They are not apparent in tasks such as swimming or climbing but become obvious in more difficult tasks such as achieving a horizontal quadruped position on a platform after a climbing phase. Decreases in cerebral energy metabolism appear to be mainly located in areas controlling posture and body balance and are partly correlated with the locomotor impairments recorded in the present study.

Animals↗

PubMed1995

Time-frequency analysis of postural sway.

Postural sway during quiet stance has been used to characterize the postural control system. Most studies have used center of pressure (COP) measurements and have assumed stationarity, however, recent research has indicated that COP is not stationary. The purpose of this study is to introduce and demonstrate a nonstationary spectral estimation technique to examine the time-varying nature of postural sway. Data from two experiments were used to verify the usefulness of the spectral estimator for the analysis of COP. The first data set contains COP recorded from normal subjects swaying about their ankles in response to a metronome as it was gradually changed from 2 to 1 Hz. The time-frequency distribution reveals time-varying spectral changes corresponding to frequency changes made by the subjects. The second set consists of COP from normal subjects and vestibularly impaired patients standing quietly on a force plate with eyes closed for 100 s. The time-frequency distributions for the COP were estimated for both sets of data. The COP's appear to be nonstationary with the energies at a given frequency modulating through time.

Computer Simulation↗

PubMed1999

Functional reach: does it really measure dynamic balance?

BACKGROUND: Functional reach (FR) is a new clinical measurement intended to assess dynamic balance. The purposes of this study were (1) to measure the mean FR distance in healthy elders compared with individuals with known balance impairments, (2) to analyze the extent to which FR measures dynamic balance, and (3) to describe movement strategies used during FR. METHODS: Thirteen healthy elders and 15 individuals with vestibular hypofunction (VH) were tested during FR and free gait. Whole body kinematic and kinetic data including the center of gravity (CG) and center of pressure (CP) using 11 body segments and two force plates, respectively, were collected. RESULTS: There was no difference in FR distance between healthy elders and individuals with VH. FR distance was not correlated to lateral stability measures, but was related to anterior-posterior postural control measures of FR (r = .69 to .84) in both groups. Although FR distance strongly correlated with maximum moment arm during FR in both groups, the correlations were not as strong when the subjects were then classified by movement strategy. The mean moment arm during FR was significantly less than that of free gait. CONCLUSIONS: These data suggest FR does not measure dynamic balance; healthy elders and balance-impaired individuals with vestibular dysfunction attained the same FR distance and did so without increasing the moment arm during or at the end of FR. Recording the strategy used during FR, however, may provide other valuable information necessary in addressing balance control. Clinical implications of assessing movement strategy are discussed.

Adult↗

PubMed2000

The vertical ground reaction force for analysis of balance?

The aim of this study was to investigate if the vertical ground reaction force can be used to detect if a subject has poor balance. The body equilibrium is perturbed by the heartbeats during quiet standing and this is reflected in the vertical ground reaction force. Force plate measurements showed that healthy adults damp the vertical body oscillations caused by the heartbeats more efficiently than subjects with an impaired or not fully developed postural control system do. This indicates that the vertical force has potential to become useful for assessment of balance during quiet standing.

Adult↗

PubMed1992

Growth and motor performance in preterm children at 8 years of age.

At 8 years of age 41 preterm and 24 term children were followed up in a long-term prospective study of 46 unselected infants born before 35 completed weeks of gestation and of 26 term infants. The two groups were comparable in physical growth and there was no significant difference between them in motor performance, as evaluated with the Test of Motor Impairment. Only minor motor problems were found (in 22% of the preterm and 17% of the term group). In the preterm group, motor impairment was correlated to birthweight. The preterm children had less developed postural control and manifested more compensatory movements during the balance tasks than the term children.

Adaptation, Psychological↗