Sway Amplitude Research Articles

Non-linear wave-induced motions of cylindrical-shaped floaters of fish farms

This paper addresses the two-dimensional hydrodynamical problem of a floating circular cylinder in waves by means of model tests and numerical simulations. The problem is relevant for floaters of fish farms. Dedicated model tests and computational fluid dynamics (CFD)-simulations, using a presently developed numerical wave tank are presented. Large amplitude sway motion of the cylinder at a wave frequency equal to half the natural sway frequency was observed, both experimentally and numerically. This is argued to be associated with non-linear hydrodynamic effects and instabilities. Further, linear potential flow theory is shown to overpredict the sway motion at resonance of about 500 per cent compared with experiments and simulations. This discrepancy is explained to be mainly attributable to viscous damping caused by flow separation. Higher-order harmonic components of the hydrodynamic forces are significant and should be considered in fatigue life analyses of fish farms.

Influence of Feedback Parameters on Performance of a Vibrotactile Balance Prosthesis

We investigated the influence of feedback conditions on the effectiveness of a balance prosthesis. The balance prosthesis used an array of 12 tactile vibrators (tactors) placed on the anterior and posterior surfaces of the torso to provide body orientation feedback related to several different combinations of angular position and velocity of body sway in the sagittal plane. Control tests were performed with no tactor activation. Body sway was evoked in subjects with normal sensory function by rotating the support surface upon which subjects stood with eyes closed. Body sway was analyzed by computing root mean square sway measures and by a frequency-response function analysis that characterized the amplitude (gain) and timing (phase) of body sway over a frequency range of 0.017-2.2 Hz. Root mean square sway measures showed a reduction of surface stimulus evoked body sway for most vibrotactile feedback settings compared to control conditions. However, frequency-response function analysis showed that the sway reduction was due primarily to a reduction in sway below about 0.5 Hz, whereas there was actually an enhancement of sway above 0.6 Hz. Finally, we created a postural model that accounted for the experimental results and gave insight into how vibrotactile information was incorporated into the postural control system.

H-reflex Modulation During Different Head Movements

Head movement stimulates the vestibulospinal tract, which contributes to the balance system. Head movement increased our postural sway amplitude. The amplitude of the soleus spinal stretch reflex was decreased by contralateral side head movement. However, it has yet to be investigated if presynaptic inhibition to the soleus Ia afferents is modulated by the head movement. PURPOSE: To examine the modulation of the soleus H-reflex and presynaptic inhibition (PI) to the soleus Ia afferents during standing with head movement. METHODS: Seven young active subjects (mean: 23.3 yrs) were examined. The H-wave and M-responses of the right soleus were determined for each subject during a static standing position. Each subject turned the head up and contralaterally or ipsilaterally with or without hands on table for support while the H-reflexes were examined. To examine H-reflex modulation, a PI H-reflex conditioning protocol was administered. The control H-reflex stimulus was set at 10% of M-max. PI was induced via conditioning of the soleus H-reflex with peroneal nerve stimulation at 100 msec prior to the test stimulus on the tibial nerve. The conditioning stimulus was delivered once the EMG activity of sternocleidomastoid muscle occurred as a trigger signal for lateral head movements. For the head up movement, an electrogoniometer was placed on the cervical spine and this was used as a trigger signal. Each mean of PI H-reflexes was normalized by the amplitude of control H-reflex (%). For data analyses, a 2 x 4 repeated measures ANOVA was used to determine each mean of H-reflex and PI-H reflex. RESULTS: The H/M-max ratio during standing without hands on the table was 41%. The ratio of PI H-reflex to the control H-reflex was 68.5% and 85.7% during standing with and without hands on the table, respectively. The mean ratio of PI H-reflex to control H-reflexes was decreased when the head was turned contralaterally by 8.4% and 13.5% with and without hands on the table, respectively. The ratio was decreased by 27.0% when the head was turned ipsilaterally with hands on the table, whereas it was not changed without hands on the table. CONCLUSIONS: The soleus H-reflex was modulated during head movements. PI to the soleus Ia afferents was decreased when the head was turned laterally, yet the modulation was dependant on the head movement and body stability.

Rocker bottom soles alter the postural response to backward translation during stance

Shoes with rocker bottom soles are utilized by persons with diabetic peripheral neuropathy to reduce plantar pressures during gait. This population also has a high risk for falls. This study analyzed the effects of shoes with rocker bottom soles on the postural response during perturbed stance. Participants were 20 healthy subjects (16 women, 4 men) ages 22–25 years. Canvas shoes were modified by the addition of crepe sole material to represent two forms of rocker bottom shoes and a control shoe. Subjects stood on a dynamic force plate programmed to move backward at a velocity that produced an automatic postural response without stepping. Force plate data were collected for five trials per shoe type. Sway variables for center of pressure (COP) and center of mass (COM) included: mean sway amplitude, sway variance, time to peak, anterior and posterior peak velocities, functional stability margin, and peak duration time. Compared to control, both the experimental shoes had significantly larger COP and COM values for mean sway amplitude, sway variance and peak duration. The functional stability margins were significantly smaller for the experimental shoes while their anterior and posterior peak velocities were slower and time to peaks were significantly longer. In young healthy adults, shoes with rocker bottom soles had a destabilizing effect to perturbed stance, thereby increasing the potential for imbalance. These results raise concerns that footwear with rocker bottom sole modifications to accommodate an insensate foot may increase the risk of falls.

Comparison of two methods of postural control analysis during upright and quiet stance

The goal of this study was to compare two methods of analysis, the use of force platform and based on video system, to investigate the postural control of young adults during upright stance on three bases of support. Fifteen young adults (30±4.96 years old) were asked to maintain an upright and quiet stance on a force platform for 30 s on bipedal, semi-tandem stance, and reduced bipedal bases of support. One reflective marker was placed on their back to acquire the space coordinates. Mean sway amplitude and mean velocity in anterior-posterior (AP) and medial-lateral (ML) directions and the area of stabilogram were calculated and used to compare the two methods of analysis and the effects of bases of support. Coefficient of correlation values indicated strong relation between trajectories of center of pressure and reflective marker in both directions (AP and ML), and statistical analysis of both methods indicated similar results in terms of effects of base of support. According to these results it might be suggested that both methods of analysis to investigate the control of upright and quiet stance in young adults can be used.

A quantitative characterization of postural sway during human quiet standing using a thin pressure distribution measurement system

The accuracy of the center of pressure (CoP) measurement during human quiet standing using an ultra-thin film pressure measurement system (Tekscan, I-Scan) was examined. To this end, CoP sway was measured simultaneously by I-Scan and a force platform (FP) to quantify differences in CoP obtained by the two methods. The sway amplitudes of I-Scan were slightly smaller than those of FP. The differences were systematic, allowing us to identify a filter that could bring every CoP trajectory of I-Scan close to that of FP. We concluded that, with the use of the filter, I-Scan could be used for accurate measurement of CoP sway during quiet standing.

Delayed visual feedback reveals distinct time scales in balance control

We performed an experiment in which we challenged postural stability in 12 healthy subjects by providing artificial delayed visual feedback. A monitor at eye-height presented subjects with a visual representation of the location of their center-of-pressure (COP) and they were instructed to position their COP as accurately as possible on a small target. Visual feedback of the COP was displayed either in real-time, or delayed by 250, 500, 750, or 1000 ms. In a control condition, no visual feedback was provided. As expected, stability increased during real-time visual feedback compared to when feedback was absent. To identify time scales at which postural control during quiet stance takes place we sought to distinguish between different frequencies. Low frequencies, i.e. slow components of postural sway, showed a monotonic increase in sway amplitude with increasing delay, whereas high frequencies, i.e. fast components of postural sway, showed significantly reduced sway amplitude for delays of 500–750 ms compared to the other delay conditions. Low- and high-frequency components of postural sway thus exhibited differential susceptibility to artificial delays, thereby supporting the notion of postural control taking place on two distinct time scales.

Efeito de informação sensorial adicional na propriocepção e equilíbrio de indivíduos com lesão do LCA

OBJETIVO: Investigar o efeito da utilização de informação sensorial adicional na propriocepção e no equilíbrio de indivíduos com lesão do Ligamento Cruzado Anterior (LCA). MÉTODOS: Participaram deste estudo 28 indivíduos com lesão unilateral do LCA e 28 indivíduos com joelhos sadios. A propriocepção foi avaliada por meio do limiar para detecção de movimento passivo da articulação do joelho nas posições de 15 e 45 graus, para as direções de flexão e de extensão. O equilíbrio foi avaliado em posição unipodal sem visão, sobre uma plataforma de força e investigado por meio da amplitude e da velocidade média de oscilação do centro de pressão. As condições de informação sensorial utilizadas foram: informação normal, bandagem infra-patelar e faixa infra-patelar. RESULTADOS: Indivíduos com lesão do LCA apresentam um prejuízo na propriocepção e no equilíbrio quando comparados a indivíduos com joelhos sadios (p<0,05). Entretanto, com adição de informação sensorial, tanto a capacidade proprioceptiva quanto o equilíbrio de indivíduos com lesão do LCA é melhorado (p<0,05). Indivíduos com joelhos sadios não apresentam benefício na utilização de informação sensorial adicional (p>0,05). CONCLUSÃO: A lesão do LCA acarreta um prejuízo da propriocepção e do equilíbrio, porém estes efeitos são minimizados com utilização de informação sensorial adicional.

Effect of visual field locus and oscillation frequencies on posture control in an ecological environment

To examine the respective roles of central and peripheral vision in the control of posture, body sway amplitude (BSA) and postural perturbations (given by velocity root mean square or vRMS) were calculated in a group of 19 healthy young adults. The stimulus was a 3D tunnel, either static or moving sinusoidally in the anterior-posterior direction. There were nine visual field conditions: four central conditions (4, 7, 15, and 30 degrees); four peripheral conditions (central occlusions of 4, 7, 15, and 30 degrees); and a full visual field condition (FF). The virtual tunnel respected all the aspects of a real physical tunnel (i.e., stereoscopy and size increase with proximity). The results show that, under static conditions, central and peripheral visual fields appear to have equal importance for the control of stance. In the presence of an optic flow, peripheral vision plays a crucial role in the control of stance, since it is responsible for a compensatory sway, whereas central vision has an accessory role that seems to be related to spatial orientation.

Control and stability of the complex inverted pendulum models in application to postural sway analysis of the vertical human stance

AbstractMechanism of balance control is investigated on computerized posturography data on the vertical two–legged and one–legged stances measured on healthy subjects and the patients with osteohondrosis and coxarthrosis. Oscillations of the center of mass over 30 s of keeping the quiet stance and the trajectories for a step forward off the force platform have been computed. It was shown that during the one–legged stance the balance control strategy includes ‘scanning’ of the larger area with bigger sway amplitudes in the vicinity of the stable state as compared to the two–legged stance. A generalized model of the body as a multi–link system is proposed. Mass and inertia of each body segment and torques in joints are taken into consideration. The calculated own and forced frequencies of the model correspond to the spectral analysis of the posturography data. One–legged stance is proposed as a proper test for revealing the balance disorders. It is shown that investigation of the oscillations and trajectories of the center of mass for the step forward off the force platform is perspective for medical diagnostics to distinguish between the spine and joint pathologies. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Visual feedback training improves postural adjustments associated with moving obstacle avoidance in elderly women

The study examined the impact of visually guided weight shifting (WS) practice on the postural adjustments evoked by elderly women when avoiding collision with a moving obstacle while standing. Fifty-six healthy elderly women (70.9 ± 5.7 years, 87.5 ± 9.6 kg) were randomly assigned into one of three groups: a group that completed 12 sessions (25 min, 3 s/week) of WS practice in the Anterior/Posterior direction (A/P group, n = 20), a group that performed the same practice in the medio/lateral direction (M/L group, n = 20) and a control group ( n = 16). Pre- and post-training, participants were tested in a moving obstacle avoidance task. As a result of practice, postural response onset shifted closer to the time of collision with the obstacle. Side-to-side WS resulted in a reduction of the M/L sway amplitude and an increase of the trunk's velocity during avoidance. It is concluded that visually guided WS practice enhances elderly's ability for on-line visuo-motor processing when avoiding collision eliminating reliance on anticipatory scaling. Specifying the direction of WS seems to be critical for optimizing the transfer of training adaptations.

Identifying deficits in balance control following vestibular or proprioceptive loss using posturographic analysis of stance tasks

Objective To distinguish between normal and deficient balance control due to vestibular loss (VL) or proprioceptive loss (PL) using pelvis and shoulder sway measures. Methods Body-worn gyroscopes measured pelvis and shoulder sway in pitch (anterior–posterior) and roll (side-to-side) directions in 6 VL, 6 PL and 26 control subjects during 4 stance tasks. Sway amplitudes were compared between groups, and were used to select optimal measures that could distinguish between these groups. Results VL and PL patients had greater sway amplitudes than controls when standing on foam with eyes closed. PL patients also swayed more when standing with eyes closed on firm support and eyes open on foam. Standard sensory analysis techniques only differentiated VL patients from controls. Stepwise discriminate analysis showed that differentiation required pitch measures for VL patients, roll measures for PL patients, and both measures for all three groups. Pelvis measures yielded better discrimination than shoulder measures. Conclusions Distinguishing between normal and deficient balance control due to VL or PL required pitch and roll pelvis sway measures. Significance Accurate identification of balance deficits due to VL or PL may be useful in clinical practice as a functional diagnostic tool or to monitor balance improvements in VL or PL patients.

Asymmetric adaptation with functional advantage in human sensorimotor control

Human movement control is inherently stochastic, requiring continuous estimation of self-motion based upon noisy sensory inputs. The nervous system must determine which sensory signals are relevant on a time scale that enables successful behavior. In human stance control, failure to effectively adapt to changing sensory contexts could lead to injurious falls. Nonlinear changes in postural sway amplitude in response to changes in sensory environmental motion have indicated a dynamic changing of the weighting of the nervous system's multiple sensory inputs so that estimates are based upon the most relevant and accurate information available. However, the time scale of these changes is virtually unknown. Results here show systematic changes in postural gain when visual scene motion amplitude is increased or decreased abruptly, consistent with sensory re-weighting. However, this re-weighting displayed a temporal asymmetry. When visual motion increased, gain decreased within 5 s to a value near its asymptotic value. In contrast, when visual motion decreased, it took an additional 5 s for gain to increase by a similar absolute amount. Suddenly increasing visual motion amplitude threatens balance if gain remains high, and rapid down-weighting of the sensory signal is required to avoid falling. By contrast, slow up-weighting suggests a conservative CNS strategy. It may not be functional to rapidly up-weight with transient changes in the sensory environment. Only sustained changes necessitate the slower up-weighting process. Such results add to our understanding of adaptive processing, identifying a temporal asymmetry in sensory re-weighting dynamics that could be a general property of adaptive estimation in the nervous system.

Additional sensory information reduces body sway of individuals with anterior cruciate ligament injury

The purpose of this study was to investigate whether the additional sensory information could improve postural control in individuals with unilateral anterior cruciate ligament (ACL) injury. Twenty-eight individuals with unilateral ACL injury (mean age 23.6, 26 males, 2 females) and 28 healthy young control subjects (mean age 22.1 years, 26 males, 2 females) participated in this study. Postural control was evaluated with subjects single-leg standing on a force platform with eyes closed under two sensory conditions: normal sensory information and light touch to a stationary bar (applied force below 1N). Three trials of 30s were performed in each single-leg stance and in each sensory condition. Mean sway amplitude and predominant frequency of center of pressure were calculated for both anterior-posterior and medial-lateral directions. Individuals with ACL injury showed greater mean sway amplitude than healthy control individuals even though the predominant frequency was similar for both groups. Additional sensory information improved postural control performance in individuals with ACL injury and healthy control, with a greater effect observed for the ACL group. Based on these results, we suggest that reduction in postural control performance in individuals with ACL injury would be due to the reduction of sensory information provided by the ACL, but when sensory information is enhanced, postural control performance improves. These results have implications for novel approaches to improve stability in individuals with ACL injury.

The periodic motion of lodgepole pine trees as affected by collisions with neighbors

The periodic sways of a group of ten Pinus contorta var. latifolia (lodgepole pine) trees with slender stems from the Two Creeks site (TW) and ten stout trees from the Chickadee site (CH) both in Alberta, Canada were quantified. Tree displacement at TW was measured during periods of consistent wind direction with three mean wind speeds (1.9, 4.6, and 5.4 m/s) and for two mean wind speeds at CH (5.0 and 7.9 m/s). Spectral analysis of sway displacement data showed a decrease in the frequency with wind speed for trees at TW, but remained unchanged for trees at CH. Significant correlations between tree sway frequency and amplitude during high winds at TW indicate a loss of sway energy concomitant with the occurrence of high collision intensity. These observations support the hypothesis that inter-crown collisions have an important influence on the sway frequency of trees and should be incorporated into efforts to model their sway dynamics. We also present a theoretical collision-damped sway model which supports our empirical findings.

Low Visual Acuity is Associated with the Decrease in Postural Sway

Vision contributes to upright postural control by providing afferent feedback to the cerebellum. Vision is generally classified into central and peripheral vision, but little is known about the respective role of central and peripheral vision for postural control with different visual acuity levels. This study examined the influence of visual acuity and visual field conditions on upright posture. Eleven males (21.1 +/- 2.0 yrs) and 15 females (22.2 +/- 2.2 yrs) were classified into high (above 1.0 binocular vision) and low (below 0.3) visual acuity groups. Postural sway was measured for 1 min in each of three visual field conditions (central vision, full vision, and no vision). Participants were given only central visual information (central vision), central and peripheral visual information (full vision), or no visual information (no vision). The effect of central vision on postural sway was detected as a difference between no vision and central vision conditions, and the effect of peripheral vision was assessed as a difference between central vision and full vision conditions. The low visual acuity group decreased their sway amplitude in antero-posterior direction using central plus peripheral visual information, but the high visual-acuity group did not. The high frequency sway was significantly smaller in the low visual-acuity group than that in the high visual-acuity group under the no vision and central vision conditions. These findings suggest the necessity of considering participants' visual acuity in examining the role of the visual information from the central and peripheral visual fields.

The development of infant upright posture: sway less or sway differently?

Postural control is an important factor for early motor development; however, compared with adults, little is known about how infants control their unperturbed upright posture. This lack of knowledge, particularly with respect to spatial and temporal characteristics of infants' unperturbed independent standing, represents a significant gap in the understanding of human postural control and its development. Therefore, our first analysis offers a thorough longitudinal characterization of infants' quiet stance through the 9 months following the onset of independent walking. Second, we examined the influence of sensory-mechanical context, light touch contact, on infants' postural control. Nine typically developing infants were tested monthly as they stood on a small pedestal either independently or with the right hand lightly touching a stationary contact surface. In addition to the longitudinal study design, an age-constant sample was analyzed to verify the influence of walking experience in infant postural development without the confounding effect of chronological age. Center of pressure excursions were recorded and characterized by distance-related, velocity, and frequency domain measures. The results indicated that, with increasing experience in the upright, as indexed by walk age, infants' postural sway exhibited shifts in rate-related characteristics toward lower frequency and slower, less variable velocity oscillations without changing the spatial characteristics of sway. Additional touch contact stabilized infants' postural sway as revealed by decrease in sway position variance, amplitude, and area as well as lower frequency and velocity. These results were confirmed by the age-constant analysis. Taken together, our findings suggest that instead of progressively reducing the sway magnitude, infants sway differently with increasing upright experience or with additional somatosensory information. These differences suggest that early development of upright stance, particularly as it relates to increasing postural and locomotor experience, involves a refinement of sensorimotor dynamics that enhances estimation of self-motion for controlling upright stance.

Influences of Arm Proprioception and Degrees of Freedom on Postural Control With Light Touch Feedback

Lightly touching a stable surface with one fingertip strongly stabilizes standing posture. The three main features of this phenomenon are fingertip contact forces maintained at levels too low to provide mechanical support, attenuation of postural sway relative to conditions without fingertip touch, and center of pressure (CP) lags changes in fingertip shear forces by approximately 250 ms. In the experiments presented here, we tested whether accurate arm proprioception and also whether the precision fingertip contact afforded by the arm's many degrees of freedom are necessary for postural stabilization by finger contact. In our first experiment, we perturbed arm proprioception and control with biceps brachii vibration (120-Hz, 2-mm amplitude). This degraded postural control, resulting in greater postural sway amplitudes. In a second study, we immobilized the touching arm with a splint. This prevented precision fingertip contact but had no effect on postural sway amplitude. In both experiments, the correlation and latency of fingertip contact forces to postural sway were unaffected. We conclude that postural control is executed based on information about arm orientation as well as tactile feedback from light touch, although precision fingertip contact is not essential. The consistent correlation and timing of CP movement and fingertip forces across conditions in which postural sway amplitude and fingertip contact are differentially disrupted suggests posture and the fingertip are controlled in parallel with feedback from the fingertip in this task.

Effect of cognitive demand during training on acquisition, retention and transfer of a postural skill

The study examined whether attentional demands of a concurrent cognitive task during balance training affect the acquisition, retention and transfer of a postural control skill. Single-leg balance was evaluated in 64 volunteers (mean age 24.0years, SD 3.10years) while performing either a cognitive task requiring little attention (forward counting) or a highly demanding cognitive task (arithmetic manipulation) following three days of training. Skill retention was evaluated two days following the cessation of training, and transfer was determined by changes in the untrained extremity. Three training sessions induced decreases in mean sway velocity and amplitude variability. Skill retention was enhanced in the group trained under conditions with greater attentional demands, suggesting that diverting attention away from the postural task and allowing learning to involve more automatic processes may enhance the learning of such tasks. Practice induced similar changes in the trained and untrained extremities following both training protocols.

Experimental Study of Blown Sand in a Vegetated Area

Abstract Wind-tunnel experiments were conducted to investigate the effects of vegetation, in particular beach grass, on blown sand in terms of elevation changes of a mobile sand bed and distributions of mean wind velocity for various vegetation canopies. For flexible vegetation layer displacement amplitude of vegetation leaf sway was also measured. The results indicate that a vegetation canopy with low height, high density, and vegetation flexibility is effective in reducing the sand-transport rate. Within and above a canopy of flexible vegetation, the vibrations of leaves increases the air turbulence, which creates a complex interaction between the mean wind velocity and the air turbulence, and thereby reduces the wind velocity in the vegetation area; consequently, the sand-transport rate decreases.