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PubMed1999

Kinematic characteristics of standing disequilibrium: reliability and validity of a posturographic protocol.

OBJECTIVE: To investigate retest reliability and concurrent validity of the fundamental measurements made of a posturographic protocol that employs quiet standing to quantify the severity and the nature of patients' postural disturbances. STUDY DESIGN: Retrospective complete block design. SETTING: Geriatric rehabilitation department. PARTICIPANTS: Thirty-six participants (age range, 67 to 86 yrs) having normal, moderate, or severe levels of disequilibrium. METHODS: Quiet standing was evaluated on three occasions using a three-dimensional motion analysis system and a force platform. Eight testing conditions, designed to vary task difficulty by controlling the contributions of vision, foot proprioception, and base-of-support width, were administered. MAIN OUTCOME MEASURES: Retest reliability of body sway, joint alignment, body position, and motor coordination indicators were evaluated by intraclass correlation coefficients (ICCs). Concurrent validity of protocol measures was evaluated by the prediction of disequilibrium from a stepwise linear discriminant analysis. RESULTS: ICCs indicated high level of retest reliability for all variables but those of motor coordination, which was not influenced by testing conditions. Discriminant analysis resulted in a four-factor discriminator that included measures of body sway, position, alignment, and motor coordination. The derived linear discriminate function correctly classified 96% of the patients' level of disequilibrium. CONCLUSIONS: The posturographic protocol has the potential to be a useful tool for evaluating severity and nature of postural instability and the effects of pharmacologic and rehabilitative treatment. Results also indicate that combining direct body measurements with force-plate data has the potential to expose the underlying impairments that cause disequilibrium, determine their pathogenesis, and evaluate compensatory strategies.

Aged↗
PubMed1999

Age differences in postural sway during volitional head movement.

OBJECTIVE: To examine the role of a volitional self-paced head-turn movement on the postural sway characteristics of healthy young and elderly subjects. DESIGN: Cross-sectional design. SETTING: Motor control research laboratory. SUBJECTS: Ten young adults and 10 elderly subjects. MAIN OUTCOME MEASURES: Postural sway characteristics of each subject were examined using a Kistler force platform. Each subject was tested under four experimental conditions: (1) static postural sway with vision; (2) static postural sway without vision; (3) postural sway with vision and self-paced head-turn movement; and (4) postural sway with no vision and a self-paced head-turn movement. Subjects performed six 15-second trials in each experimental condition. Dependent variables analyzed on each trial were mean sway amplitude (in millimeters), sagittal sway standard deviation, lateral sway standard deviation, and frequency of sway (in hertz). RESULTS: During the static conditions (e.g., no voluntary movement), the young subjects produced significantly less postural sway than the elderly in both the vision condition (sway amplitude in the young, 3.80 mm; in the elderly, 4.89 mm) and the no-vision condition (young, 5.44 mm; elderly, 5.95 mm). This increased sway was the result of greater lateral sway in the elderly for the vision condition (3.73 vs. 2.68 mm), and greater sagittal sway for the elderly in the no-vision condition (5.55 vs. 4.70 mm). There were no significant differences between the groups in the frequency of sway. When asked to initiate and complete the head-turn, elderly subjects significantly increased their mean sway amplitude and decreased their frequency of sway, whereas the young subjects did not significantly alter their postural sway profiles. CONCLUSIONS: These results demonstrate different postural sway control strategies for young and elderly subjects when asked to perform volitional movements.

Accidental Falls↗
PubMed2000

Adaptative effects of loss of vision on upright undisturbed stance.

In order to evaluate the long term effect of blindness on an undisturbed postural task, seven blind individuals (BLI) were compared to twelve healthy individuals (REF) who were required to close their eyes. To this aim, centre of pressure (CP) trajectories were recorded through a force platform on which the subjects remained upright. Through a low-pass filter, the vertical projection of the centre of gravity (CG(v)) and consequently the difference CP-CG(v) were estimated. The former represents the controlled variable whilst the latter is assumed to express the resultant joint stiffness. A first analysis consisted in studying the frequency distribution of these motions. Increased mean amplitudes in both elementary motions and no difference between frequency distributions were observed for the BLI when compared to the REF group. These trajectories were then modelled as a fractional Brownian motion process in order to gain insight about the nature of the control mechanisms called into play and their spatio-temporal organisation. Moreover, the main characteristic observed in the BLI group is a decreased in CP-CG(v) motions, which, according to Winter's model, may be related to joint stiffness. In addition, reduced spatial transition point co-ordinates and more deterministic control mechanisms aimed at returning the estimated CG to an equilibrium point during the longest Deltat were observed. This phenomenon is more pronounced in the antero-posterior direction and also allows the blind subject to significantly diminish their CG sways. Finally, these features are discussed in relation to other similarly analysed investigations.

Adult↗
PubMed2001

Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats.

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20-60 nA caused an inhibition of tilt-induced firing of alpha-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of beta-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20-60 nA), a 5-HT1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT1A receptor to inhibit transmission of otolith information to alpha- and beta-type MVN neurons.

Action Potentials↗
PubMed2003

Development of levodopa-induced dyskinesias in parkinsonian monkeys may depend upon rate of symptom onset and/or duration of symptoms.

Levodopa-induced dyskinesias (LIDs) present a major problem for the long-term management of Parkinson's disease (PD) patients. Due to the interdependence of risk factors in clinical populations, it is difficult to independently examine factors that may influence the development of LIDs. Using macaque monkeys with different types of MPTP-induced parkinsonism, the current study evaluated the degree to which rate of symptom progression, symptom severity, and response to and duration of levodopa therapy may be involved in the development of LIDs. Monkeys with acute (short-term) MPTP exposure, rapid symptom onset and short symptom duration prior to initiation of levodopa therapy developed dyskinesia between 11 and 24 days of daily levodopa administration. In contrast, monkeys with long-term MPTP exposure, slow symptom progression and/or long symptom duration prior to initiation of levodopa therapy were more resistant to developing LIDs (e.g., dyskinesia developed no sooner than 146 days of chronic levodopa administration). All animals were similarly symptomatic at the start of levodopa treatment and had similar therapeutic responses to the drug. These data suggest distinct differences in the propensity to develop LIDs in monkeys with different rates of symptom progression or symptom durations prior to levodopa and demonstrate the value of these models for further studying the pathophysiology of LIDs.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1997

Flexibility of anticipatory postural adjustments revealed by self-paced and reaction-time arm movements.

In standing humans, it is not certain whether anticipatory postural adjustments associated with rapid, voluntary elbow flexion movements (focal movements) originate as a selection from preset synergies or as the result of specific planning of motor commands. We studied these muscle recruitment patterns when the same focal movement was made under behavioral conditions of a self-paced task (SPT) and a reaction-time task (RTT). While standing still, eight normal subjects performed focal movements under the SPT and RTT behavioral conditions and under three different biomechanical conditions: (1) unloaded-upright, (2) loaded-upright (holding a 3800-g metal bar), and (3) unloaded-forward leaning. Anticipatory postural adjustments were quantified using the latency and duration of electromyographic (EMG) data and the center of pressure (COP) displacement data. Postural-kinematic [joint and body center of gravity (COG) motion] data were used to quantify the anticipatory postural program's effectiveness at preventing postural movement. Focal movement kinematics and associated EMG activity differed due to biomechanical but not behavioral (SPT vs. RTT) conditions. The maximum and net displacement of the body COG measurements did not differ between the behavioral conditions. The amplitude, timing, and net movements of lower extremity joints were influenced by the behavioral conditions. However, the behavioral conditions significantly affected the phasing (including order of activation) and duration of anticipatory postural EMG activity and the phasing of COP displacements under certain biomechanical conditions. These findings support the theory that anticipatory postural adjustments are planned in detail.

Adolescent↗
PubMed1999

The timing of galvanic vestibular stimulation affects responses to platform translation.

We compared the effects of galvanic vestibular stimulation applied at 0, 0.5, 1.5 and 2.5 s prior to a backward platform translation on postural responses. The effect of the galvanic stimulation was largest on the final equilibrium position of the center of pressure (CoP). The largest effects occurred for the 0.5 and 0-s pre-period, when the dynamic CoP pressure changes in response to both the galvanic stimulus and the platform translation coincided. The shift in the final equilibrium position was also larger than the sum of the shifts for the galvanic stimulus and the platform translation alone for the 0.5 and 0-s pre-periods. The initial rate of change of the CoP response to the platform translation was not significantly affected in any condition. Changes in the peak CoP position could be accounted for by local interaction of CoP velocity changes induced by the galvanic and translation responses alone, but the changes in final equilibrium position could only be accounted for by a change in global body orientation. These findings suggest that the contribution of vestibulospinal information is greatest during the dynamic phase of the postural response, and that the vestibular system contributes most to the later components of the postural response, particularly to the final equilibrium position. These findings suggest that a nonlinear interaction between the vestibular signal induced by the galvanic current and the sensory stimuli produced by the platform translation occurs when the two stimuli are presented within 1 s, during the dynamic phase of the postural response to the galvanic stimulus. When presented at greater separations in time, the stimuli appear to be treated as independent events, such that no interaction occurs.

Adult↗
PubMed1999

Arm raising in humans under loaded vs. unloaded and bipedal vs. unipedal conditions.

The aim of the present experiment was to study the central organization of equilibrium control during arm raising in the frontal plane. Nine adult subjects (five seniors and four young adults) were asked to raise their right arm to a horizontal position in the frontal plane in two support conditions (bipedal vs. unipedal) and two load conditions (unloaded vs. a 3.5-kg load added on the moving hand). No instructions were given concerning the movement speed. The movements were performed at about half the maximum speed achievable under reaction time conditions. The final lateral center of mass (CM) position 1 s after the movement offset, and the time course of the CM shift during the movement were studied in the four experimental conditions, using a CM compensation index. The electromyographic (EMG) pattern of the main muscles involved in the movement performance and in the postural control were studied in three out of nine subjects during movements performed at two velocities (at the preferred speed and as fast as possible). The results indicate that (1) the CM shift remains minimized in the frontal plane during the time course of the arm movement and during the final stabilization of the arm regardless of the stance and load conditions; (2) the time course of the CM compensation index remains stable during the first 400 ms after the movement onset, decreasing late in the movement and increasing again at the end of the stabilization stage. A modelisation suggests that the time course is the result of the interaction of two controls: a first one, putative feedforward, starting early and decreasing with time and a second one, putative feedback, starting late in the movement and increasing with time; (3) both early and late index values are influenced by the support and load conditions, the highest index values being observed during unipedal stance and load conditions; (4) activation of quadratus lomborum (QL) contralateral to the raising arm is time locked with the deltoidus activation of the raising arm in both fast and slow movements: this contralateral QL activation corresponds to an anticipatory postural adjustment (APA) aimed at minimizing the CM shift.

Adult↗
PubMed2000

Simulating the response of a standing operator to vibration stress by means of a biomechanical model.

Under vibration stress the compressive forces transmitted in the joints of a standing operator are composed of nearly static and oscillating force parts. Because these forces can hardly be measured they were assessed by means of a biomechanical model. In the model, 27 rigid bodies with 103 degrees of freedom represent the segments of the human body. 106 force elements imitate the muscles of the trunk and the legs. At first, the model parameter were varied so that for the simulated sitting posture the model fits the seat-to-head transmissibility given in the literature and in ISO/CD 5982. For the standing posture, the transfer functions between the ground acceleration and the oscillating forces in the ankle, the knee, the hip, and the motion segment L3-L4 were computed. According to the moduli of these functions the forces in the ankles are higher than those in the knees or the hips and they nearly come up to the forces in the lumbar spine. Further the results of the simulation indicate that under equal vibration stress in the standing and the sitting posture the differences between the compressive forces in the lumbar spine are small.

Biomechanical Phenomena↗
PubMed2003

The mechanics of back-extensor torque production about the lumbar spine.

The purpose of this study was to develop and evaluate a biomechanical model of lumbar back extension over a wide range of positions for the lumbar spine, incorporating the latest information on muscle geometry and intra-abdominal pressure (IAP). Analysis of the Visible Human data was utilised in order to obtain anatomical information unavailable from the literature and magnetic resonance imaging was used to generate subject-specific anatomical descriptions. The model was evaluated by comparisons with measured maximal voluntary static back-extension torques. Predicted maximal specific muscle tensions agreed well with in vitro measurements from the literature. When modelling the maximal static back-extension torque production, it was possible to come fairly close to simultaneous equilibrium about all the lumbar discs simply by a uniform muscle activation of all back-extensor muscles (the caudal part showed, however, less agreement). This indicates that equilibrium in the lumbar spine is mainly regulated by passive mechanical properties, e.g. muscle length changes due to postural changes, rather than due to complex muscle coordination, as earlier proposed. The model showed that IAP (measured during torque exertions) contributes about 10% of the total maximal voluntary back-extensor torque and that it can unload the spine from compression. The spinal unloading effect from the IAP was greatest with the spine held in a flexed position. This is in opposition to the effects of changed muscle lever arm lengths, which for a given load would give the largest spinal unloading in the extended position. These findings have implications for the evaluation of optimal lifting techniques.

Abdomen↗
PubMed2003

Muscle mechanics and neuromuscular control.

The purpose of this paper is to demonstrate that the properties of the mechanical system, especially muscle elasticity and limb mass, to a large degree determine force output and movement. This makes the control demands of the central nervous system simpler and more robust. In human triceps surae, a muscle with short fibres and a long tendon, the time courses of the total muscle+tendon length and of the length of the contractile component (CC) alone in running are completely different. The muscle tendon complex shows first an eccentric phase with negative work, followed by a concentric phase. The CC, on the other hand, is concentric all the time. Moreover, the work that is done, is done at a speed that guarantees a high energetic efficiency. It is argued that this high efficiency is an in-built property of the muscle mechanics for muscles with a compliant tendon and a low v(max). When a muscle, or a set of muscles, moves a mass, and the duration of the action is short with respect to the isometric time constant of the muscle, we may call it an 'elastic bounce contraction'. In such a case the mass-spring interaction largely determines the time course of the force, and the efficiency of muscle contraction is most of the time close to optimum. In a similar way, whole limbs can be modelled as springs, with a stiffness that can be modulated by flexing the joints more or less. The motor control task of the central nervous system is simple for such elastic bounce contractions: a block-like activation is sufficient, in which timing is critical, but activation level is not. It seems possible that a whole class of actions can be generated by an identical timing sequence, with only a modulation in activation amplitude. An example is walking or running at different speeds.

Adaptation, Physiological↗
PubMed2003

Finger force vectors in multi-finger prehension.

In a majority of studies on grasp, only normal forces were measured and only when a zero torque was exerted on a hand-held object. This study concerns finger force vectors during the torque production tasks. Subjects (n=8) stabilized a handle with an attachment that allowed for change of external torque from -1.5 to 1.5 Nm. Forces and moments exerted by the digit tips on the object were recorded. At the large (>-0.375 Nm) supination torques the index/middle and ring/little pairs of fingers generated oppositely directed tangential forces. The index and middle finger produced forces in a downward direction and therefore did not support the load. At a zero torque and pronation torques, the middle, ring and little fingers produced forces along nearly the same direction. The vector of the index finger force was always directed differently from the vectors of other finger forces, the angles ranged from 19 degrees 30' to 47 degrees 40'. The points of force application were systematically displaced with the torque, with the exception of the little finger. Tangential finger forces contributed substantially to the total torque exerted on the hand-held object.

Adult↗
PubMed2003

Coping with perturbations to a layout somersault in tumbling.

Tumbling is a dynamic movement requiring control of the linear and angular momenta generated during the approach and takeoff phases. Both of these phases are subject to some variability even when the gymnast is trying to perform a given movement repeatedly. This paper used a simulation model of tumbling takeoff to establish how well gymnasts can cope with perturbations of the approach and takeoff phases. A five segment planar simulation model with torque generators at each joint was developed to simulate tumbling takeoffs. The model was customised to an elite gymnast by determining subject specific inertia and torque parameters and a simulation was produced which closely matched a performance of a layout somersault by the gymnast. The performance of a layout somersault was found to be sensitive to the approach characteristics and the activation timings but relatively insensitive to the elasticity of the track and maximum muscle strength. Appropriate variation of the activation timings used during the takeoff phase was capable of coping with moderate perturbations of the approach characteristics. A model of aerial movement established that variation of body configuration in the flight phase was capable of adjusting for takeoff perturbations that would lead to rotation errors of up to 8%. Providing the errors in perceiving approach characteristics are less than 5% or 5 degrees and the errors in timing activations are less than 7ms, perturbations in the approach can be accommodated using adjustments during takeoff and flight.

Adaptation, Physiological↗
PubMed2003

Predicting the dynamic postural control response from quiet-stance behavior in elderly adults.

Human postural sway, as measured by fluctuations of the center of pressure (COP) under the feet of a quietly standing individual, can be characterized as a stochastic process. The fluctuation-dissipation theorem (FDT) provides a linear relationship between the fluctuations of a quasi-static, stochastic system to the same system's relaxation to equilibrium following a perturbation. We applied a similar linear relationship, based on the FDT, to the human postural control system to explore whether anterior-posterior (AP) fluctuations of the COP during quiet stance can be used to predict the AP response of the postural control system to a weak posteriorly directed mechanical perturbation (tug or pull at the waist). We tested 10 healthy elderly (mean age of 69yr) and 10 healthy young (mean age of 25yr) adult subjects. We found that this linear relationship was applicable to the postural control system of all 10 young and eight of the 10 elderly adult subjects. These results suggest that it is possible to predict an individual's dynamic response to a mild perturbation using quiet-stance data, regardless of age. The existence of this FDT-based linear relationship with respect to the human postural control system suggests that, for a given individual, the postural control system may use the same control mechanisms during quiet stance and mild-perturbation conditions, regardless of age.

Adult↗
PubMed2003

Biomechanical modeling and optimal control of human posture.

The present work describes the biomechanical modeling of human postural mechanics in the saggital plane and the use of optimal control to generate open-loop raising-up movements from a squatting position. The biomechanical model comprises 10 equivalent musculotendon actuators, based on a 40 muscles model, and three links (shank, thigh and HAT-Head, Arms and Trunk). Optimal control solutions are achieved through algorithms based on the Consistent Approximations Theory (Schwartz and Polak, 1996), where the continuous non-linear dynamics is represented in a discrete space by means of a Runge-Kutta integration and the control signals in a spline-coefficient functional space. This leads to non-linear programming problems solved by a sequential quadratic programming (SQP) method. Due to the highly non-linear and unstable nature of the posture dynamics, numerical convergence is difficult, and specific strategies must be implemented in order to allow convergence. Results for control (muscular excitations) and angular trajectories are shown using two final simulation times, as well as specific control strategies are discussed.

Algorithms↗
PubMed2003

Inverse dynamic investigation of voluntary trunk movements in weightlessness: a new microgravity-specific strategy.

Present investigation faces the question of quantitative assessment of exchanged forces and torques at the restraints during whole body posture exercises in long-term microgravity. Inverse dynamic modelling and total angular momentum at the ankle joint were used in order to reconstruct movement dynamics at the restraining point, represented by the ankle joint. The hypothesis is that the minimisation of the torques at the interface point assumes a key role in movement planning in 0 g. This hypothesis would respond to an optimisation of muscles activity, a minimisation of energy expenditure and therefore an accurate control of body movement. Results show that the 0 g movement strategy adopted ensures that the integral of the net ankle moment between the beginning and the end of the movement is zero. This expected mechanical constraint is not satisfied when 0 g movement dynamics is simulated using terrestrial kinematics. This accounts for a significant imposed change of movement strategy. Particularly, the efficient compensation of the inertial effects of the segments in terms of total angular momentum at the ankle joint was evidenced. These results explain the exaggerated axial synergies, observed on kinematics and which moved centre of mass (CM) backward from its already backward initial positioning, as a tool for enhancing the compensation and achieving the desired minimisation of the torques exchanges at the restraints.

Abdomen↗
PubMed2003

Evidence for a role of antagonistic cocontraction in controlling trunk stiffness during lifting.

Activity of the abdominal muscles during symmetric lifting has been a consistent finding in many studies. It has been hypothesized that this antagonistic coactivation increases trunk stiffness to provide stability to the spine. To test this, we investigated whether abdominal activity in lifting is increased in response to destabilizing conditions. Ten healthy male subjects lifted 35 l containers containing 15 l of water (unstable condition), or ice (stable condition). 3D-kinematics, ground reaction forces, and EMG of selected trunk muscles were recorded. Euler angles of the thorax relative to the pelvis were determined. Inverse dynamics was used to calculate moments about L5S1. Averaged normalized abdominal EMG activity was calculated to express coactivation and an EMG-driven trunk muscle model was used to estimate the flexor moment produced by these muscles and to estimate the L5S1 compression force. Abdominal coactivation was significantly higher when lifting the unstable load. This coincided with significant increases in estimated moments produced by the antagonist muscles and in estimated compression forces on the L5S1 disc, except at the instant of the peak moment about L5S1. The lifting style was not affected by load instability as evidenced by the absence of effects on moments about L5S1 and angles of the thorax relative to the pelvis. The data support the interpretation of abdominal cocontraction during lifting as subserving spinal stability. An alternative function of the increased trunk stiffness due to cocontraction might be to achieve more precise control over the trajectory of lifted weight in order to avoid sloshing of the water mass in the box and the consequent perturbations.

Abdomen↗