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PubMed2004

The role of anticipatory postural adjustments in a rocking on heels movement.

We studied the anticipatory postural adjustments (APAs) associated with a voluntary rocking on heels movement performed in a self-paced manner. Ground reaction forces and the EMG activity of the leg muscles were recorded. Such a movement shifted the centre of gravity (CG) backwards, due to APAs on the lower limbs. It is generally assumed that APAs only generate corrections in the opposite direction to the perturbation due to the focal movement. However, our study shows that APAs, by creating the initial backward oriented impulse, displace the CG backwards and generate the dynamic conditions required to move.

Adolescent↗

PubMed2004

Influence of velocity on the human global postural strategies during the movement leading up to the vertical upside-down position.

The aim of this study was to examine the influence of the velocity on the control of the final postural equilibrium in the movement of standing up into upside-down vertical posture on the hands. Seven expert gymnasts performed the movement on a force plate of large dimensions from which the kinematic of center of mass (CoM) was studied. The first result showed that the speed of CoM was increased in the vertical axis but did not change in the antero-posterior axis. The second result showed a reduction of the duration of the anticipatory postural adjustments (APA) conversely to the speed. This was in contrast to the movements performed in the natural erect stance. These results suggest firstly that the speed of CoM corresponds to a global strategic response linked to the body's posturo-kinetic capacity, and secondly that the reduction of the APA seems to be linked to the precariousness of the final equilibrium.

Adolescent↗

PubMed2004

Is there interaction between vision and local fatigue of the lower limbs on postural control and postural stability in human posture?

An investigation of the interaction between local fatigue and vision on postural control and postural stability was carried out. Fatigue was effected in a sitting position and was assumed based on a shortening of the exertion time of the soleus muscles (60% of their maximal voluntary contractions). Postural stability was assessed by centre of gravity motion, which was computed from centre of pressure motion, evaluating postural control. Ten healthy male subjects were asked to stand as still as possible with eyes open (EO) and eyes closed (EC) before and after the fatigue protocol. Results showed that fatigue produced similar effects for the two vision conditions on postural control and postural stability analyzed separately, increasing postural control and leaving postural stability unchanged. Local fatigue essentially produced an increase of neuromuscular activity in high frequencies. However, this increase was more pronounced for the EO, as compared to the EC condition.

Adult↗

PubMed2004

Postural control as a function of self- and object-motion perception.

The goals of this study were to examine the visual information influence on body sway as a function of self- and object-motion perception and visual information quality. Participants that were aware (object-motion) and unaware (self-motion) of the movement of a moving room were asked to stand upright at five different distances from its frontal wall. The visual information effect on body sway decreased when participants were aware about the sensory manipulation. Moreover, while the visual influence on body sway decreased as the distance increased in the self-motion perception, no effects were observed in the object-motion mode. The overall results indicate that postural control system functioning can be altered by prior knowledge, and adaptation due to changes in sensory quality seem to occur in the self- but not in the object-motion perception mode.

Adult↗

PubMed2004

Effect of mood state on anticipatory postural adjustments.

Static postural control has been demonstrated to link with psychological state. However, the effect of psychological state on dynamic postural control remains unclear. In this study, we examined the effect of mood state on anticipatory postural adjustment (APA), one of the most important functions for dynamic postural control. Fourteen healthy male subjects performed unilateral arm elevation tasks after completing a Profile of Mood States (POMS) questionnaire. Mood state measured by POMS and the latency or amplitude of the APA in the ventral muscles (rectus femoris, tibialis anterior) of the lower limb showed significant negative correlations. The correlation between the mood state and APA amplitude in the soleus was found to be significantly positive. There were significant negative correlations between the mood state and reaction-time. These findings suggest that it is possible that dynamic postural control is affected by mood state.

Adaptation, Physiological↗

PubMed2005

Exploring coordination dynamics of the postural system with real-time visual feedback.

Differential performance over a wide range of possible postural coordination modes was investigated using 16 ankle-hip relative phase patterns from 0 degrees to 337.5 degrees. Participants were instructed to produce each coordination mode with and without real time visual feedback. Feedback consisted of a Lissajous figure indicating the discrepancy between actual and requested ankle-hip relative phase. The results showed: (1) the presence of a unique attractor around the anti-phase pattern (relative phase approximately 180 degrees); (2) performance was similar with and without visual feedback; (3) the absence of an attractor for the in-phase pattern (relative phase approximately 20 degrees). The third result is not consistent with previous research in which both in-phase and anti-phase patterns emerged when they were not imposed [B.G. Bardy, L. Martin, T.A. Stoffregen, R.J. Bootsma, Postural coordination modes considered as emergent phenomena, J. Exp. Psychol. Hum. Percept. Perform. 25 (1999) 1284-1301; B.G. Bardy, O. Oullier, R.J. Bootsma, T.A. Stoffregen, Dynamics of human postural transitions, J. Exp. Psychol. Hum. Percept. Perform. 28 (1999) 499-514]. This finding indicates the strong dependency to task variation and instructions of postural pattern formation.

Adult↗

PubMed2004

A unified view of quiet and perturbed stance: simultaneous co-existing excitable modes.

When standing quietly, human upright stance is typically approximated as a single segment inverted pendulum. In contrast, investigations which perturb upright stance with support surface translations or visual driving stimuli have shown that the body behaves like a two-segment pendulum, displaying both in-phase and anti-phase patterns between the upper and lower body. Here we present evidence that a single-segment characterization of quiet stance is inadequate. Similar to perturbed stance, quiet stance has simultaneously co-existing in-phase and anti-phase patterns. Subjects stood with eyes closed in three sensory conditions: a fixed surface, a foam surface, and a sway-referenced surface. Spectral analysis showed that the body behaved like a multi-link pendulum with two co-existing modes. The angles of the trunk and leg segments were in-phase for frequencies below 1 Hz and anti-phase for frequencies above 1Hz. The shift from in-phase to anti-phase sway showed an abrupt change for the fixed and foam surfaces, but a gradual change for the sway-referenced condition with the trunk showing a phase lead over the legs. The coexistence of in-phase and anti-phase patterns during quiet stance suggests that the ankle and hip strategies are not extremes along a behavioral continuum of mixed strategies. They are "simultaneously co-existing excitable modes", both always present, but one of which may predominate depending upon the characteristics of the available sensory information, task or perturbation.

Adult↗

PubMed2005

Effects of immersion in virtual reality on postural control.

In the present study, we examined the effects of the time lag between visual scene and the head movement in the virtual reality (VR) world on motion sickness and postural control in healthy volunteers. After immersion in VR with additional time lags (from 0 to 0.8 s) to the inherent delay (about 250 ms), the visual-vestibular conflict induced a slight motion sickness in experimental subjects, but no change was noticed in the body sway path with eyes open and closed. However, Romberg ratio of body sway path with eyes closed divided by that with eyes open after immersion in VR was significantly decreased in comparison with that before immersion in VR. Since Romberg ratio is an index of visual dependency on postural control, this finding indicates that the immersion in VR decreases the visual dependency on postural control. It is suggested that adaptation to visual-vestibular conflict in VR immersion increases the contribution of vestibular and somatosensory inputs to postural control by ignoring the conflicting delayed visual input in the VR world. VR may be a promising treatment for visual vertigo in vestibular patients with unsuccessful compensation by its ability to induce vestibular and somatosensory reweighing for postural control.

Adult↗

PubMed2005

Postural stability and unilateral posterior crossbite: is there a relationship?

The aim of this study was to test the hypothesis that unilateral posterior crossbite influences postural stability of the whole body. Twenty-six subjects (14 males and 12 females) affected with unilateral posterior crossbite were selected and compared with 52 controls matched for age and gender. Postural stability was assessed using a stabilometric platform. The following stabilometric measurements were assessed: weight distribution on foot area and speed of body sway. Tests were performed under two occlusal conditions: teeth in intercuspal position (ICP) and while keeping two cotton rolls between teeth without clenching. The weight distribution on foot area and the speed of body sway were not significantly influenced from crossbite (with and without lateral mandibular slide), occlusal conditions (ICP, cotton rolls), and gender. Therefore, the treatment of this malocclusion in order to prevent or to treat postural disorders is not justified.

Adolescent↗

PubMed2005

TENS to the posterior aspect of the legs decreases postural sway during stance.

The purpose of this study was to examine the effect of Transcutaneous Electrical Nerve Stimulation (TENS) applied to the posterior aspect of the legs, on postural sway during stance. Thirty healthy subjects were tested while standing on a force platform under four stimulation conditions: no TENS, bilateral TENS, and unilateral left and right TENS. Thirty-second long tests, employing detection threshold amplitudes, were performed in three blocks. In each block, the four conditions were applied both with and without vision in a random order. The results indicate that the application of TENS brought about a decrease in postural sway as expressed by average sway velocity, in addition to a decrease in the absolute values of maximal and minimal medio-lateral and anterior-posterior velocity. Thus, similar to sub-threshold random electrical noise, it appears that the application of low-amplitude TENS to the lower limbs decreases postural sway during stance. Considering the ease of TENS application and the high prevalence of postural disorders, the potential clinical significance of this observation is to be determined by further studies.

Adult↗

PubMed2005

Identification of distinct characteristics of postural sway in Parkinson's disease: a feature selection procedure based on principal component analysis.

We selected descriptive measures of the centre of pressure (CoP) displacement in quiet standing, by means of a procedure based on principal component analysis, in two groups particularly different in terms of postural behaviours, such as subjects with Parkinson's disease (PD) in the levodopa off and on states. We computed 14 measures of the CoP: 5 measures of CoP trajectory over the support surface, 3 measures that estimated the area covered by the CoP, 1 measure that estimated the principal CoP sway direction, 1 measure that quantified the CoP total power, 1 measure that estimated the variability of CoP frequency content and 3 measures of characteristic CoP frequencies [L. Rocchi, L. Chiari, A. Cappello, Feature selection of stabilometric parameters based on principal component analysis, Med. Biol. Eng. Comput. 42 (2004) 71-79; L. Rocchi, L. Chiari, F.B. Horak, Effects of deep brain stimulation and levodopa on postural sway in Parkinson's disease, J. Neurol. Neurosurg. Psychiatry, 73 (2002) 267-274]. The feature selection, independently applied to the measures obtained in the two groups, resulted in different principal component (PC) subspaces of the 14-dimension original data set (4 PCs in the off and 3 PCs in the on state to account for over 90% of the original variance), but in the same 5 CoP measures (selected features) needed to describe the different postural behaviours: root mean square distance; mean velocity; principal sway direction; centroidal frequency of the power spectrum; frequency dispersion. The five selected features were found to provide insight into the postural control mechanisms and to describe changes in postural strategies in the two groups of PD subjects, off and on levodopa. Thus, the five selected features may be recommended for use in clinical practice and in research, in the direction toward the definition of a standard protocol in quantitative posturography.

Aged↗

PubMed2006

Postural reactions to soleus muscle vibration in Parkinson's disease: scaling deteriorates as disease progresses.

Previous research has shown that Parkinson's disease (PD) patients, especially those with postural instability, respond hyperactively to visual, vestibular, and neck proprioceptive sensory manipulation. To determine if this impairment of the sensory information scaling holds true for the lower leg proprioceptive system, we studied postural responses to mechanical vibration (which affects the muscle spindle Ia afferents) applied to the soleus muscles of PD subjects and healthy controls. Early-stage and advanced-stage PD patients as well as age-matched control subjects participated. Each group comprised 11 subjects. Nine pulses of 3-s long vibration were applied randomly to both soleus muscles while subjects kept their eyes closed. Postural responses to these stimuli were measured by static posturography. The effect of dopaminergic medication was established by testing patients in both ON and OFF treatment phases. There was no intergroup difference in the pattern or latencies of responses. However, the amplitudes were significantly larger in advanced PD patients; controls did not differ from early-stage PD patients. Dopaminergic medication had no significant effect on any of the measures. The scaling of postural reactions triggered by lower leg proprioception is disturbed in advanced PD. Neither afferent proprioceptive deficits nor inaccurate timing is involved. This study gives further evidence for the generalized impairment of the scaling of postural responses evoked whenever there is a sudden change of sensory conditions, as occurs with the progression of PD. Such impairment could play a significant role in the pathophysiology of postural instability and falls in PD patients.

Accidental Falls↗

PubMed2006

The effect of asymmetry of posture on anticipatory postural adjustments.

The study investigates the effect of body asymmetry on anticipatory postural adjustments (APAs). Subjects performed a task involving a standard load release induced by a shoulder abduction movement while standing symmetrically or in an asymmetrical stance with either their right or left leg in 45 degrees of external rotation. EMG activities of trunk and leg muscles were recorded during the postural perturbation and were quantified within the time intervals typical of APAs. Anticipatory postural adjustments were observed in all experimental conditions. It was found that asymmetrical body positioning was associated with significant asymmetrical patterns of APAs seen in the right and left distal muscles. These APA asymmetries were dependant upon the side in which the body asymmetry was induced: reduced APAs were observed in the leg muscles on the side of leg rotation, while increased APAs were seen in the muscles on the contralateral side. These findings stress the important role that body asymmetries play in the control of upright posture.

Adaptation, Physiological↗

PubMed2006

Static postural control in children with developmental dyslexia.

The present investigation tries to better understand potential association and causal relationship between phonological and postural impairment due to developmental dyslexia. The study included 50 boys with developmental dyslexia and selected on the basis of their overall reading difficulties, and 42 control boys. Body sway during a quite standing posture eye open and eye closed on a force platform were tested in the two groups of subjects that were between 10 and 13 years of age. Analysis of classical parameters quantifying the centre of pressure (CP) displacements along antero-posterior and lateral axes showed a significant difference between the two groups. Dyslexic children showed on average greater instability, with greater length, variability and mean power frequency of CP displacements with or without vision. Our results demonstrate that postural parameters may discriminate between children with dyslexia and age-equivalent controls.

Adolescent↗

PubMed2006

Postural modulation induced by pictures depicting prosocial or dangerous contexts.

The ability to identify safety and danger is critical to survival. However, not much is known about human somatic body reactions in these contexts. We performed a posturographic study comparing body reactions to the sight of pictures of smiling babies and families (affiliative) versus matched neutral people, and to pictures depicting body envelope violations (mutilation) versus matched neutral people. The participants stood on a force platform and heart rate and displacement of the center of pressure were recorded while the pictures were presented. Pictures of mutilation induced a freezing-like reaction consisting of a medial-lateral (M-L) decrease in the amplitude of sway (immobility) and increase of the mean power frequency (rigidity), associated with bradycardia. Affiliative stimuli also induced an immobility and rigidity behavior but in the anterior-posterior (A-P) axis. This resembles the "immobility-without-fear reaction" proposed to occur when, upon detection of safety cues, mammals including humans are involved in pro-social activities. We conclude that the sight of visual cues of affiliation and danger produce distinct body somatic reactions.

Adolescent↗

PubMed2006

Posturographic stabilisation of healthy subjects exposed to full-length mirror image is inversely related to body-image preoccupations.

Affective states, anxiety in particular, have been shown to negatively influence human postural control efficiency as measured by posturographic means, while exposure to a full-length mirror image of one's body exerts a stabilizing effect. We tested the hypothesis that body image concerns and preoccupations would relate negatively to this stabilising effect. Sixty-six healthy students, who screened negative for psychiatric disorders, completed rating scales for anxiety, depression and body image concerns. Posturography recordings of body sway were taken under three conditions: with eyes closed, looking at a vertical bar and looking at a full-length mirror. The Eyes Open/Mirror Stabilometric Quotient [EOMQ=(sway path with eyes closed/sway path looking at the mirror)x100], an index of how much postural control is stabilized by mirror feedback in comparison to the visual vertical bar condition, was significantly inversely related to body image concerns and preoccupations, and to trait anxiety. This finding confirms the impact of emotional factors on human postural control, which warrant further studies. If confirmed in clinical populations characterized by high levels of body image disturbances, e.g. eating disorders, it could lead to developments in the assessment and monitoring of these patients.

Adult↗

PubMed2006

Postural after-effects of stepping on an inclined surface.

In previous studies, blindfolded, healthy subjects exhibited an after-effect of leaning while standing on a horizontal surface after a period of standing on an inclined surface. We investigated whether this kinesthetic after-effect would transfer from one task to another by asking blindfolded subjects to stand on a horizontal surface after stepping-in-place on an incline. Results showed that all subjects demonstrated a forward trunk leaning after-effect lasting from half a minute to over 6 min after stepping on a 10 degrees -toes-up incline for 2.5 min. For 5/7 subjects, the amplitude of the leaning after-effect was very similar following stepping or standing on the inclined surface. The similarity of the post-incline lean between the standing and stepping conditions suggests a common underlying mechanism for the after-effect following standing and walking on a gradient and suggests that prolonged maintenance of a constant ankle or leg posture is not a prerequisite condition for the after-effect. The transfer of a postural effect built-up during a locomotor task to a postural after-effect during a standing task is consistent with a central adaptive mechanism that adjusts the surface-referenced set point for whole body postural orientation for both gait and posture.

Adaptation, Physiological↗

PubMed2006

Postural control after a night without sleep.

The present study analysed the efficiency of postural control after 12 h of nocturnal forced wakefulness using Romberg's test comprising 1 min of recording with eyes-open and 1 min of recording with eyes-closed, with a 1 min break between the two sessions. Our aim was to see if the decreased postural control efficiency after a sleepless night was unspecific (in both eyes-closed and eyes-open conditions) or selective (in only one of the conditions). A total of 55 students spent a whole night awake at our laboratory and were tested at 22:00 and 08:00 h. In general, the results showed that postural sway increased, performing the recording from eyes-open to eyes-closed condition. The statokinesigram length (SL or efficiency of the postural system) increased after the sleepless night, while in eyes-open condition, the length in function of surface (LFS or accuracy of postural control) and Romberg's index (or contribution of vision to maintain posture) significantly decreased. This could indicate that after a night without sleep, there is a slower elaboration of visual inputs in the postural control process. On the basis of these results, the effects of sleep deprivation on cognitive performance were considered from a neuropsychological point of view.

Adult↗