Changes In Postural Strategy Research Articles

Sensory organization of postural control after long term space flight.

Alterations in motor control systems is an inevitable consequence of space flights of any duration. After the flight, the crew-members have significant difficulties with maintaining upright balance and locomotion, which last several days following landing. At the same time, the specific mechanisms of these effects remain unclear. The aim of the study was to assess effects of long-term space flight on postural control and to define the changes of sensory organization caused by microgravity. 33 cosmonauts of Russian Space Agency, the members of International Space Station (ISS) flights of duration between 166 and 196 days took part in this study. Computerized Dynamic Posturography (CDP) tests, which include assessment of visual, proprioceptive and vestibular function in postural stability, was performed twice before the flight and on the 3rd, 7th, and 10th days after landing. The video analysis of ankle and hip joints fluctuations was performed to investigate the basis of postural changes. Exposure to long-term space flight was followed by considerable changes of postural stability (-27% of Equilibrium Score value in the most complicated test, SOT5m). Changes in postural strategies to maintain balance were observed in the tests which provide the challenge for vestibular system. In particular, increased hip joint involvement (+100% in median value and +135% in 3rd quartile of hip angle fluctuation RMS in SOT5m) into postural control process was revealed. Decrease of postural stability after long-term space flight was associated with alterations in vestibular system and biomechanically was revealed by increased hip strategy which is less accurate, but simpler in terms of the central control.

Changes in postural strategy of the lower limb under mechanical knee constraint on an unsteady stance surface.

Joint constraint could limit the available degrees of freedom in a kinematic chain for maintaining postural stability. This study investigated adaptive changes in postural synergy due to bracing of bilateral knee joints, usually thought to have a trifling impact on upright stance. Twenty-four young adults were requested to maintain balance on a stabilometer plate as steadily as possible while wearing a pair of knee orthoses, either unlocked (the non-constraint (NC) condition) or locked to restrict knee motion (the knee constraint (KC) condition). Knee constraint led to a significant increase in the regularity of the stabilometer angular velocity. More than 95% of the variance properties of the joint angular velocities in the lower limb were explained by the first and second principal components (PC1 and PC2), which represented the ankle strategy and the combined knee and hip strategy, respectively. In addition to the increase trend in PC1 regularity, knee constraint enhanced the mutual information of the stabilometer angular velocity and PC1 (MISTBV-PC1) but reduced the mutual information of the stabilometer angular velocity and PC2 (MISTBV-PC2). The MISTBV-PC1 was also positively correlated to stance steadiness on the stabilometer in the KC condition. In summary, in the knee constraint condition, postural synergy on the stabilometer was reorganized to increase reliance on ankle strategies to maintain equilibrium. In particular, a stable stabilometer stance under knee constraint is associated with a high level of coherent ankle–stabilometer interaction.

Functional dizziness: from phobic postural vertigo and chronic subjective dizziness to persistent postural-perceptual dizziness.

Functional dizziness is the new term for somatoform or psychogenic dizziness. The aim of this study is to review arguments for the new nomenclature, clinical features, possible pathomechanisms, and comorbidities of functional dizziness. The prevalence of functional dizziness as a primary cause of vestibular symptoms amounts to 10% in neuro-otology centers. Rates of psychiatric comorbidity in patients with structural vestibular syndromes are much higher with nearly 50% and with highest rates in patients with vestibular migraine, vestibular paroxysmia, and Ménière's disease. Pathophysiologic processes seem to include precipitating events that trigger anxiety-related changes in postural strategies with an increased attention to head and body motion and a cocontraction of leg muscles. Personality traits with high levels of neuroticism and low levels of extraversion appear as risk factors for anxiety and depressive disorders and increased morbidity in functional disorders. Correct and early diagnosis of functional dizziness, as primary cause or secondary disorder after a structural vestibular syndrome, is very important to prevent further chronification and enable adequate treatment. Treatment plans that include patient education, vestibular rehabilitation, cognitive and behavioral therapies, and medications substantially reduce morbidity and offer the potential for sustained remission when applied systematically.

Changes in postural strategy during exercise against perturbation using the balance exercise assist robot: a pilot study.

[Purpose] To clarify the changes in postural strategy by evaluating leg joint motion and muscle activity before and after continuous exercise against perturbation using the Balance Exercise Assist Robot (BEAR). [Subjects and Methods] Nine healthy subjects (male 7, female 2; mean age 23 ± 1 years) performed a postural perturbation coping exercise only. In the task, the robot leaned and moved automatically. Participants were instructed to maintain their default upright position and they performed the exercise five times in a row (1 minute/trial). Changes in total movement distance, range of motion of each joint (hip, knee, ankle), and mean activity of each muscle for the first and fifth trials were compared. [Results] The total movement distance of BEAR and range of motion in the hip decreased significantly from the first trial to the last trial. No change in muscle activity was observed in the rectus femoris, biceps femoris, tibialis anterior or gastrocnemius. [Conclusion] The results for exercise against perturbation using BEAR in this study suggest that BEAR may be a promising method to improve the ankle strategy for maintaining a standing posture.

The effect of aging on vertical postural control during the forward and backward shift of the center of pressure

Preventing fall-related injuries is becoming a priority as the world population ages. This study's purpose was to examine the effect of aging on vertical postural control in the community-dwelling elderly. Thirty-six elderly individuals and twenty-two healthy young adults were asked to shift their centers of pressure (COPs) as far as possible while standing. The COP position, angle of each lower leg joint, and postural muscle activities were measured using a force plate, three-dimensional motion analyzer, and electromyogram, respectively. The vertical position of the center of mass (COM) was also measured to assess the change in vertical postural control. The backward COP shift in the elderly group was significantly smaller than that in the young group, and both the forward and backward COM shifts were significantly smaller in elders relative to those in youths. The COM position in the elderly group during the backward COP shift was also significantly lower than that in the young group. Knee and ankle joint movements differed between the two groups during the backward COP shift. Factor analysis indicated that dorsal and ventral muscle groups were involved in the COP shift. Specifically, the relationship between the biceps femoris muscle and the voluntary COP shift was reinforced in the elderly group. These findings suggest that the vertical postural strategy changes in the elderly during the backward COP shift.

Vestibular compensation following vestibular neurotomy

ObjectivesFour studies assessing vestibular compensation in Menière's disease patients undergoing unilateral vestibular neurotomy, using different analysis methods, are reviewed, with a focus on the different strategies used by patients according to their preoperative sensory preference. Material and methodsFour prospective studies performed in a university tertiary referral center were reviewed, measuring the pattern of vestibular compensation in Menière's disease patients before and after unilateral vestibular neurotomy on various assessment protocols: postural syndrome assessed on static posturography and gait analysis; perceptual syndrome assessed on subjective visual vertical perception; and oculomotor syndrome assessed on ocular cyclotorsion. ResultsVestibular compensation occurred at variable intervals depending on the parameter investigated. Open-eye postural control and gait/walking returned to normal one month after neurotomy. Fine balance analysis found that visual perception of the vertical and ocular cyclotorsion impairment persisted at long-term follow-up. Clinical postural disturbance persisted only when visual afferents were cut off (eyes closed). These impairments were the expression of a postoperative change in postural strategy related to the new use of visual and non-visual references. ConclusionsUnderstanding pre-operative interindividual variation in balance strategy is critical to screening for postural instability and tailoring vestibular rehabilitation.

Electrically-induced muscle fatigue affects feedforward mechanisms of control

ObjectiveTo investigate the effects of focal muscle fatigue induced by electromyostimulation (EMS) on Anticipatory Postural Adjustments (APAs) during arm flexions performed at maximal velocity. MethodsFifteen healthy subjects performed self-paced arm flexions at maximal velocity before and after the completion of fatiguing electromyostimulation programs involving the medial and anterior deltoids and aiming to degrade movement peak acceleration. APA timing and magnitude were measured using surface electromyography. ResultsFollowing muscle fatigue, despite a lower mechanical disturbance evidenced by significant decreased peak accelerations (−12%, p<.001), APAs remained unchanged as compared to control trials (p>.11 for all analyses). ConclusionThe fatigue signals evoked by externally-generated contractions seem to be gated by the Central Nervous System and result in postural strategy changes which aim to increase the postural safety margin. SignificanceEMS is widely used in rehabilitation and training programs for its neuromuscular function-related benefits. However and from a motor control viewpoint, the present results show that the use of EMS can lead to acute inaccuracies in predictive motor control. We propose that clinicians should investigate the chronic and global effects of EMS on motor control.

Postural responses of young adults to collision in virtual world combined with horizontal translation of haptic floor.

Balance and postural response strategies change when subjects are exposed to horizontal translations of the floor or virtual reality or both. This may impact the balance training strategy and balance capabilities assessment in the future telerehabilitation. In the study 15 neurologically intact volunteers participated. Balance standing frame with virtual reality tasks and our novel haptic floor able to generate horizontal translations were used. The postural responses were measured with center of gravity and muscle electromyography of plantar-dorsiflexors, quadriceps, hamstrings, hip and spine muscles in three scenarios. The results demonstrated that center of gravity and electromyographic activity were comparable; with low latency at translation only, longer latency at combination with virtual reality and long latency when only virtual reality was applied. Soleus and semimembranousis demonstrated lower latency at back-right horizontal translations when virtual reality was present. The outcomes suggests that the postural strategy changes from ankle to ankle-hip strategy with availability of additional sensory systems which may be an important issue for objective balance evaluation in the clinical environment and remote telerehabilitation.

Acute effects of cryotherapy on postural control

To investigate the acute effects of whole-body cryotherapy (WBC) on postural control, we measured postural sway (COP) in a quiet stance with eyes open in four consecutive 20-second tests: before and 1, 6 and 11min after the WBC. Twenty-four healthy young subjects aged 19.3±0.9 were exposed to WBC (−110°C) for 2min. The time series recorded with a sampling rate of 100Hz was used to evaluate postural performance (COP variability) and strategies (COP frequency and entropy). There were no differences between the pre- and post-WBC values of these measurements in the frontal plane; however, in the sagittal plane postural sway increased immediately after WBC (p<0.05) and remained elevated throughout the experiment. Deteriorated performance brought about lagged changes in postural strategies, including a decrease in frequency and entropy. These changes remained sustained until the end of the experiment. In conclusion, the WBC caused a drop in complexity, adaptability, and automaticity in postural control, which accounted for specific constraints imposed on the postural system due to cooling.

Impairment in postural control is greater when ankle plantarflexors and dorsiflexors are fatigued simultaneously than when fatigued separately

The aim of the present study was to compare the effects of ankle muscle fatigue on postural control when plantarflexors (PFs) and dorsiflexors (DFs) are fatigued simultaneously compared with separately. This study also investigated the recovery of postural control after fatigue. Sixteen adults (eight women and eight men) performed postural trials before and after an isokinetic fatigue task involving either (i) only PFs (30° s −1), (ii) only DFs (120° s −1), or (iii) both PFs and DFs simultaneously. The fatigue task involved maximal contractions repeated until the torque produced decreased below 50% of the maximal torque. Postural trials lasted 30 s and were performed on one leg with eyes open (EO) or eyes closed (EC). Sway area, medio-lateral (ML) and antero-posterior (AP) positions and velocities were calculated from the center of pressure displacements. With EO, no effect of fatigue was found on postural variables. With EC, sway area and AP velocity increased only when both PFs and DFs were fatigued simultaneously. An effect of fatigue present only when both muscle groups are fatigued simultaneously could be due to impairment in the compensatory activity between agonist and antagonist muscles and/or a greater decrease in proprioception due to a greater number of fatigued muscles. In addition, when PFs and DFs were fatigued simultaneously, sway area and AP velocity returned to pre-fatigue values within 2 min, whereas a posterior shift in AP position persisted for 10 min. This last result may suggest a longer-lasting change in postural strategy needed for optimal postural control.

Comparing postural strategy changes following adapted versus non-adapted responses in subjects with and without spinal stenosis

Musculoskeletal evaluation skills are needed to examine postural compensation techniques, but little is known about ground reaction forces (GRF) in standing posture. Even though a number of studies have evaluated GRF in patients with low back pain (LBP) during vertical perturbations, it would be important to consider compensation characteristics which might be associated with abnormal patterns of postural responses. The vertical excursions of the body center of mass (BCOM) were measured with delay time and normalized amplitude of GRF. Overall, there was no difference based on the vertical excursion of the BCOM (F=0.12, p=0.90), amplitude of the normalized GRF (F=0.16, p=0.74), or response time (F=1.98, p=0.17) between subjects with and without spinal stenosis.There was a gender difference based on the vertical excursion of the normalized BCOM (F=5.92, p=0.02) as well as the normalized amplitude of GRF (F=4.17, p=0.04). It was shown that male subjects implemented better adjustment strategies during adapted and non-adapted responses in order to improve body stability. In this way, manual therapists should be aware that gender differences exist in patients with spinal stenosis since the condition may change the individual's postural adjustment ability.

P03-41 Improved postural control after behavioural short term intervention in patients with psychiatric dizziness

Background:Patients with psychiatric dizziness often report subjective instability of stance and gait and fear of falls. They showed increased activity of their body sway in static posturography compared to normals (1, 2). Aim of the present study was to evaluate the efficiency of a behavioural therapy by static posturography.Methods:14 patients with psychiatric dizziness were included in the prospective study on static posturography to quantify the postural sway (sway path) during upright stance under different conditions. the psychosomatic examination comprised of standardized interviews (SCID-Interview) and a psychometric examination battery (SCL-90, VSS, VHQ). Results of static posturography were compared to those of an age-matched control group (n=23), baseline measurements were compared to the results after behavioural short term intervention.Results:At baseline patients revealed significantly lower sway-path values in the anterior-posterior and lateral planes (p&lt; 0.001 - p=0.042) but significantly elevated values in the vertical plane (p=0.015 - p=0.042). They had to be supported more often in the examination setting to prevent falls (p&lt; 0.001). after the short term intervention therapy a normalization of sway path values was present, especially the increased vertical sway activity was reduced (p=0.028).Conclusion:Patients with psychiatric dizziness activate significantly more anti-gravity muscles which reflects a change in postural strategy with an increased “stiffness” and a consecutively higher rate of “falls”. the patient's conscious control of stance augments coactivation of anti-gravity muscles. This typical pattern of postural control could be normalized by a short term behavioural therapy.1. Krafczyk, 1999; 2. Querner, 2000.

How experienced alpine-skiers cope with restrictions of ankle degrees-of-freedom when wearing ski-boots in postural exercises

The present study investigates the mechanisms underlying changes in postural strategy that occur to compensate for mechanical ankle joint restrictions induced by wearing ski-boots during postural exercises. Fourteen experienced skiers were asked to stand as still as possible in a stable (STA) posture and in 2 postures with instability in the medio/lateral and antero/posterior (ML and AP postures) direction. Postural tasks were performed with eyes open or closed and while wearing or not wearing ski-boots. The electromyographic (EMG) activity of representative lower limb muscles and positions of centre-of-foot pressure (COP) were recorded and analyzed. Our results illustrated enhanced postural performances with ski-boots in the STA posture, whereas postural performances remained unchanged when wearing ski-boots in the ML and AP postures. Analysis of COP sways in the frequency domain did not illustrate any modification in the contribution of different neuronal loops when the study subjects wore ski-boots. EMG showed that the mechanical effects of wearing ski-boots were compensated by changes in postural strategy through the reorganization of muscle coordination, made possible by inherent redundancies in the human body. The preservation of postural performances, despite restrictions of ankle degrees-of-freedom induced by ski-boots, emphasizes the subjects’ capacity to exploit the additional support provided by ski-boots by adequately adjusting muscle coordination to control posture in different balance conditions.

Muscle synergies involved in preparation to a step made under the self-paced and reaction time instructions

Objective To investigate multi-muscle synergies during preparation to making a step and their changes with speed of stepping and method of step initiation, self-paced vs. reaction-time. Methods Standing healthy persons performed stepping tasks under 3 instructions, ‘comfortably, self-paced’, ‘very quick, self-paced’, and ‘as fast as possible to a visual signal’. Electromyographic (EMG) signals of 10 postural muscles were recorded and analyzed. The participants also performed standard tasks associated with releasing a load. These trials were used to identify muscle groupings (M-modes) associated with shifts of the center of pressure (COP) and relations between small changes in the M-modes and COP shifts. The framework of the uncontrolled manifold hypothesis was used to partition the EMG variance across stepping trials into two components that kept constant and changed the COP coordinates in the anterior–posterior (AP) and medio-lateral (ML) directions. Results Muscle synergies stabilizing COP trajectories in both AP and ML directions were observed in both the stepping and supporting legs. The synergies tended to weaken and disappear close to the moment of take-off. They showed only minor differences across the 3 tasks. In contrast, mechanical and EMG characteristics of postural adjustments to stepping were significantly larger and showed higher rates of change during quick steps and steps under the reaction-time instruction. Conclusions COP shifts associated with preparation to making a step are stabilized by multi-muscle synergies. These synergies are robust across stepping speed and methods of step initiation, self-paced and reaction-time. Significance Studies of multi-muscle postural synergies are important for understanding postural disorders in neurological patients and changes in postural strategies with age.

Changes in coordination of postural control during dynamic stance in chronic low back pain patients

The human postural system operates on the basis of integrated information from three independent sources: vestibular, visual and somatosensory. It is conceivable that a derangement of any of these systems will influence the overall output of the postural system. The peripheral proprioceptive system or the central processing of proprioceptive information may be altered in chronic low back pain (CLBP). We therefore investigated whether patients with CLBP exhibited an altered postural control during quiet standing. Dynamic posturography was performed by 12 CLBP patients and 12 age-matched controls. Subject's task was to stand quietly on a computer-controlled movable platform under six sensory conditions that altered the available visual and proprioceptive information. While the control of balance was comparable between the two groups across stabilized support surface conditions (1-3), CLBP patients oscillated much more than controls in the anterior-posterior (AP) direction in platform sway-referenced conditions (4-6). Control experiments ruled out that increased sway was due to pain interference. In CLBP patients, postural stability under challenging conditions is maintained by an increased sway in AP direction. This change in postural strategy may underlie a dysfunction of the peripheral proprioceptive system or the central integration of proprioceptive information.

Postural strategy changes with fatigue of the lumbar extensor muscles

The purpose of this study was to investigate the effect of lumbar extensor fatigue on postural strategy in response to a balance perturbation. Anteriorly-directed force perturbations were applied to the upper back with a padded pendulum and attempted to challenge the postural control system without eliciting a stepping response. In three separate sessions, subjects were perturbed both before and after a fatiguing protocol that induced lumbar extensor fatigue to one of three different fatigue levels. Postural strategy was quantified using center of pressure position along with joint angles and joint torques for the ankle, knee, hip, and "low back" joints. Results showed both proactive and reactive changes in postural strategy. Proactive changes involved a slight anterior lean prior to the perturbation, and reactive changes were consistent with a shift toward more of a hip strategy with fatigue. In addition, results suggested that subjects classified as moving mostly at the hip prior to fatigue were more affected by fatigue compared to subjects classified as moving roughly equal amounts at the ankle and hip prior to fatigue. Increasing fatigue level exaggerated some, but not all, of the changes in postural strategy with fatigue. These findings illustrate that neuromuscular fatigue can influence postural strategy in response to a balance perturbation.

Visual Influence on Postural Control, With and Without Visual Motion Feedback

Body sway was investigated in 20 healthy subjects to determine whether visual input must contain motion feedback information from the surroundings in order to influence postural control. Posturography was used to record body sway under the following visual conditions: eyes open with or without a restricted visual field; eyes open in ganzfield white light; eyes open in darkness with a head-fixed visual target; eyes open in darkness; and eyes closed in darkness. Stance was perturbed by means of a pseudorandomly applied vibratory stimulation to the calf muscles. Least sway was found with eyes open in an unrestricted visual field but increased in a restricted visual field. Greatest sway was found without visual motion feedback, i.e. under the following conditions: eyes closed; eyes open in darkness; eyes open in ganzfield white light; and with a head-mounted fixation point. Sway was significantly ( p < 0.05) greater with eyes open in darkness compared with eyes closed during the initial 50 s with perturbations. After 150 s, sway was almost identical under the four test conditions without visual motion feedback. Standing with eyes open in darkness was initially a disadvantage compared with having the eyes closed. The postural control system may be programmed to expect visual feedback information when the eyes are open, which may delay changes in postural strategy.

Increased body sway at 3.5–8 Hz in patients with phobic postural vertigo

Postural sway during upright stance was analyzed in 12 patients with phobic postural vertigo (PPV) and in 12 age-matched healthy volunteers. Recordings were made under different conditions (with the eyes open or closed): when standing on a foam rubber pad with the head upright, turned 30 degrees to the right or left, or during 1 Hz horizontal head oscillations. Sway analysis included calculation of sway path, of root mean square values of sway and of the power spectrum of sway in fore/aft and lateral directions. There was a significant increase in sway activity in the 3.53–8 Hz frequency band in patients with PPV. This did not impair objective postural stability. Increase in higher frequency sway activity may simply reflect a change in postural strategy rather than a sensorimotor dysfunction. The patients' conscious control of stance may augment co-activation of anti-gravity muscles, a strategy applied by normal subjects when performing demanding balancing tasks.

Postural Control Reduced by Subanesthetic Nitrous Oxide Narcosis

The effects of subanesthetic (21%) nitrous oxide (N2O) narcosis on postural control, as measured by posturography were studied in 12 subjects. Vibration induced body sway, with open and closed eyes, and body sway induced by a visual stimulus were evaluated. Adaptation was measured as the quotient of body sway variance between the second and first halves of each trial. Change in postural strategy was evaluated as change in the variance of shear forces relative to change in body sway. Subjective evaluation of narcosis was recorded. Body sway variance increased significantly during exposure to N2O, and equally for all body sway inducing stimuli. With the eyes open, adaptation to vibratory perturbation was significantly reduced by N2O. Postural strategy was unaffected by N2O, but differed significantly between stimuli, with relatively less involvement of shear forces during vibratory perturbation in the eyes open condition than during the other body sway inducing stimuli. Subjective evaluation of narcosis correlated with body sway variance during the visual stimulus. The present findings indicate that subanesthetic N2O narcosis reduces postural control. Adaptation to a balance disturbing stimulus, with undisturbed vision, is decreased by N2O. Visual clues are shown to be crucial to the postural strategy adopted to maintain balance. The effects of N2O are assumed to be due to impairment of sensorimotor integration in the CNS.