Coexistence of stability and mobility in postural control: evidence from postural compensation for respiration

This study evaluated the extent to which movement of the lower limbs and pelvis may compensate for the disturbance to posture that results from respiratory movement of the thorax and abdomen. Motion of the neck, pelvis, leg and centre of pressure (COP) were recorded with high resolution in conjunction with electromyographic activity (EMG) of flexor and extensor muscles of the trunk and hip. Respiration was measured from ribcage motion. Subjects breathed quietly, and with increased volume due to hypercapnoea (as a result of breathing with increased dead-space) and a voluntary increase in respiration. Additional recordings were made during apnoea. The relationship between respiration and other parameters was measured from the correlation between data in the frequency domain (i.e. coherence) and from time-locked averages triggered from respiration. In quiet standing, small angular displacements ( approximately 0.5 degrees ) of the trunk and leg were identified in raw data. Correspondingly, there were peaks in the power spectra of the angular movements and EMG. While body movement and EMG were coherent with respiration (>0.5), the coherence between respiration and COP displacement was low (<0.2). The amplitude of movement and coherence was increased when respiration was increased. The present data suggest that the postural disturbance that results from respiratory movement is matched, at least partly, and counteracted by small angular displacements of the lower trunk and lower limbs. Thus, stability in quiet stance is dependent on movement of multiple body segments and control of equilibrium cannot be reduced to control of a single joint.     

Relative contributions of the ribcage and abdomen to lung volume displacement during speech production

We tested the hypothesis that the pattern of chest wall configuration during speech production correlates with the pattern of chest wall motion during resting breathing. Twenty-one men (age 40 ± 8 years) with ankylosing spondylitis and varied degrees of ribcage involvement participated in the study. None of the patients had an obvious speech abnormality. Ribcage and abdominal displacements during quiet breathing and during reading were measured with a respiratory plethysmograph. Measurements were taken in the sitting and standing body positions. In each body position, ribcage or abdominal displacements during quiet breathing correlated with the corresponding chest wall displacements recorded during reading (P < 0.001). In addition, linear regression analysis showed that the slope of the chest wall motion loop during quiet breathing correlated with the ratio of ribcage to abdomen contribution to lung volume displacement during reading (r = 0.78, P < 0.001 for sitting and r = 0.64, P = 0.002 for standing position). The slopes of the regression lines did not differ between the sitting and standing body position (P > 0.05). We conclude that the relative contribution of the ribcage and abdomen to lung volume displacement during speech production correlates with the relative ribcage and abdomen contribution to tidal volume during quiet breathing; our data support the notion that the pattern of chest wall configuration during quiet breathing largely predicts the pattern of ribcage and abdomen displacement during speech.     

Effect of experimentally induced low back pain on postural sway with breathing

Although breathing perturbs balance, in healthy individuals little sway is detected in ground reaction forces because small movements of the spine and lower limbs compensate for the postural disturbance. When people have chronic low back pain (LBP), sway at the ground is increased, possibly as a result of reduced compensatory motion of the trunk. The aim of this study was to determine whether postural compensation for breathing is reduced during experimentally induced pain. Subjects stood on a force plate with eyes open, eyes closed, and while breathing with hypercapnoea before and after injection of hypertonic saline into the right lumbar longissimus muscle to induce LBP. Motion of the lumbar spine, pelvis, and lower limbs was measured with four inclinometers fixed over bony landmarks. During experimental pain, motion of the trunk in association with breathing was reduced. However, despite this reduction in motion, there was no increase in postural sway with breathing. These data suggest that increased body sway with breathing in people with chronic LBP is not simply because of reduced trunk movement, but instead, indicates changes in coordination by the central nervous system that are not replicated by experimental nociceptor stimulation.     

Reciprocal angular acceleration of the ankle and hip joints during quiet standing in humans

Human quiet standing is often modeled as a single inverted pendulum rotating around the ankle joint, under the assumption that movement around the hip joint is quite small. However, several recent studies have shown that movement around the hip joint can play a significant role in the efficient maintenance of the center of body mass (COM) above the support area. The aim of this study was to investigate how coordination between the hip and ankle joints is controlled during human quiet standing. Subjects stood quietly for 30 s with their eyes either opened (EO) or closed (EC), and we measured subtle angular displacements around the ankle (thetaa) and hip (thetah) joints using three highly sensitive CCD laser displacement sensors. Reliable data were obtained for both angular displacement and angular velocity (the first derivative of the angular displacement). Further, measurement error was not predominant, even among the angular acceleration data, which were obtained by taking the second derivative of the angular displacement. The angular displacement, velocity, and acceleration of the hip were found to be significantly greater (P<0.001) than those of the ankle, confirming that hip-joint motion cannot be ignored, even during quiet standing. We also found that a consistent reciprocal relationship exists between the angular accelerations of the hip and ankle joints, namely positive or negative angular acceleration of ankle joint is compensated for by oppositely directed angular acceleration of the hip joint. Principal component analysis revealed that this relationship can be expressed as: thetah=gammathetaa with gamma=-3.15+/-1.24 and gamma=-3.12+/-1.46 (mean +/-SD) for EO and EC, respectively, where theta is the angular acceleration. There was no significant difference in the values of y for EO and EC, and these values were in agreement with the theoretical value calculated assuming the acceleration of COM was zero. On the other hand, such a consistent relationship was never observed for angular displacement itself. These results suggest that the angular motions around the hip and ankle joints are not to keep the COM at a constant position, but rather to minimize acceleration of the COM.     

Respiratory disturbance to posture varies according to the respiratory mode

The purpose of this study was to determine whether respiratory disturbance to posture varies as a function of the respiratory mode, i.e. thoracic or abdominal. To this aim, 10 healthy male subjects underwent a posturographic examination associated with a measurement of respiratory kinematics. Experimental conditions varied posture (sitting, standing) respiratory amplitude (quiet breathing, deep breathing) and respiratory mode (thoracic, abdominal). In addition to classical posturographic parameters, original peak detection algorithm and emergence parameter calculated from the Fast Fourier Transform were used to assess the respiratory component in CP displacements. Results showed that along the antero-posterior axis, time domain and frequency domain parameters were both significantly greater in thoracic breathing mode than in abdominal mode. It was concluded that respiratory kinematics have a more prominent disturbing effect on posture when they involve the rib cage rather than the abdomen.     

Posture control development in children aged 2-7 years old, based on the changes of repeatability of the stability indices

Precision manual tasks require a stable postural background which might be facilitated by respiratory modulations. We investigated the influence of performing a manual precision aiming task on respiratory rate and dynamics, and the coherence between respiration and center of pressure (COP) fluctuations (i.e., the postural-respiratory synergy). Participants aimed a pointer at targets of different sizes while seated or standing. Respiratory rate increased during the aiming period compared to a pre-task phase, but did not vary as a function of aiming difficulty. Recurrence quantification analysis revealed an increased incidence of slowly changing periods of chest movements during the most difficult aiming condition, which required the highest level of manual precision. Aiming, irrespective of difficulty, led to increases in the regularity of the respiratory pattern. Increases in respiratory rate during aiming were accompanied by an increased level of coherence for the seated but not the standing posture, suggesting that task demands affect the organization of coordination across the postural-respiratory synergy. Functional demands of the task likely shape the effectiveness of compensation for respiration during precision aiming.     

Effect of the hip motion on the body kinematics in the sagittal plane during human quiet standing

Human quiet stance is often modeled as a single-link inverted pendulum pivoting only around the ankle joints in the sagittal plane. However, several recent studies have shown that movement around the hip joint cannot be negligible, and the body behaves like a double-link inverted pendulum. The purpose of this study was to examine how the hip motion affects the body kinematics in the sagittal plane during quiet standing. Ten healthy subjects were requested to keep a quiet stance for 30 s on a force platform. The angular displacements of the ankle and hip joints were measured using two highly sensitive CCD laser sensors. By taking the second derivative of the angular displacements, the angular accelerations of both joints were obtained. As for the angular displacements, there was no clear correlation between the ankle and hip joints. On the other hand, the angular accelerations of both joints were found to be modulated in a consistent anti-phase pattern. Then we estimated the anterior–posterior (A–P) acceleration of the center of mass (CoM) as a linear summation of the angular acceleration data. Simultaneously, we derived the actual CoM acceleration by dividing A–P share force by body mass. When we estimated CoM acceleration using only the angular acceleration of the ankle joint under the assumption that movement of the CoM is merely a scaled reflection of the motion of the ankle, it was largely overestimated as compared to the actual CoM acceleration. Whereas, when we take the angular acceleration of the hip joint into the calculation, it showed good coincidence with the actual CoM acceleration. These results indicate that the movement around the hip joint has a substantial effect on the body kinematics in the sagittal plane even during quiet standing.     

静态姿势下均匀负重和非负重时躯干和表面肌电活动

目的观察躯干均匀负重和非负重状态下静态前屈和后伸时腰背部、腹部及臀中肌的肌电活动规律和运动学特征。方法 6位正常健康的男性受试者直腿站立于特殊设计的试验框架和平台内做静态前屈和后伸的负重和非负重动作,每次试验持续4 s,重复3次。动作时,记录双侧腰臀部的10块肌肉:腹直肌、腹外斜肌、竖脊肌、多裂肌、臀中肌的肌电活动,以及三维角度运动轨迹和足底力系。计算标准化肌电、腰部躯干角和足底中心压力的位移。将所得数据作常规的统计分析。结果负重和非负重前屈时,都是背侧肌活动较大(10.47～16.94)。非负重后伸时,腹侧肌活动也较大;负重后,背侧肌(3.70～17.95)和臀中肌(6.64～11.52)活动增加,腹肌活动减少(10.66～4.18)。后伸时,躯干的闪动次数随负重增加而增多,在3D角上增加1.55次;负重后,足底中心压力向前后的移动多于侧方移动,前屈的前后方移动(14.60)多于后伸的移动(7.65)。结论后伸状态增加了背侧肌的活动,而且多裂肌更明显;同时,腰部角位移度和闪动次数增加,特别在后伸提重时更为显著。     

Motor strategies for initiating downward-oriented movements during standing in adults

Sitting down and squatting are routine activities in daily living that lower the body mass by flexing the trunk and legs, but they obviously require different motor strategies for each goal posture. The former action must transfer the supporting surface onto a seat, whereas the latter must maintain the center of mass within the same base of both feet. By comparing the performance of both maneuvers, the mechanisms involved in initiating the downward-oriented movements and the process of optimizing the performance during their repetitions were studied. Twelve healthy subjects were asked to perform sitting-down and squatting actions immediately when a light cue was given, but at a natural speed. Electromyograms, angular movements of the joints of the right leg, and center of pressure (COP) displacement were recorded before and during each task. The initial mechanisms to initiate the break from the upright posture and the changes of postural adjustments during repetitive movements were analyzed separately. The sitting-down movement was achieved by a stereotyped motor strategy characterized by a gastrocnemius muscle burst coupled with deactivation of the erector spinae muscle. The former produced a transient COP displacement in the forward direction, and simultaneous unlocking of the trunk prevented a fall backward. By contrast, because of the absence of any need to produce momentum in a given direction, a variety of motor strategies were available to initiate squatting. The direction of initial COP displacement to initiate squatting varied with the muscles involved in unlocking the upright posture. During repetition of sitting down, the average COP position of the initial standing posture in the preparatory period was immediately shifted forward after the second trial. Simultaneously, the erector spinae muscle was deactivated earlier in the later trials. These resulted in a decreased momentum in the backward direction while the subjects were transferring themselves onto the seat. In the squatting task, however, these changes could not be identified, except for a slight flexed position of the knee during standing in the first trial. These findings suggest that in the case of transferring the body-mass to another supporting base the central nervous system immediately adjusts the size of the initial impetus to optimize the performance.     

Open-loop and closed-loop control of posture: A random-walk analysis of center-of-pressure trajectories

A new conceptual and theoretical framework for studying the human postural control system is introduced. Mathematical techniques from statistical mechanics are developed and applied to the analysis and interpretation of stabilograms. This work was based on the assumption that the act of maintaining an erect posture could be viewed, in part, as a stochastic process. Twenty-five healthy young subjects were studied under quiet-standing conditions. Center-of-pressure (COP) trajectories were analyzed as one-dimensional and two dimensional random walks. This novel approach led to the extraction of repeatable, physiologically meaningful parameters from stabilograms. It is shown that although individual stabilograms for a single subject were highly variable and random in appearance, a consistent, subject-specific pattern emerged with the generation of averaged stabilogram-diffusion plots (mean square COP displacement vs time interval). In addition, significant inter-subject differences were found in the calculated results. This suggests that the steady-state behavior of the control mechanisms involved in maintaining erect posture can be quite variable even amongst a population of age-matched, anthropometrically similar, healthy individuals. These posturographic analyses also demonstrated that COP trajectories could be modelled as fractional Brownian motion and that at least two control systems — a shortterm mechanism and a long-term mechanism — were operating during quiet standing. More specifically, the present results suggest that over short-term intervals open-loop control schemes are utilized by the postural control system, whereas over long-term intervals closed-loop control mechanisms are called into play. This work strongly supports the position that much can be learned about the functional organization of the postural control system by studying the steady-state behavior of the human body during periods of undisturbed stance.     

Relationship between morphologic somatotypes and standing posture equilibrium

Previous studies have identified height and weight as important factors affecting quiet standing stability but studies have not addressed body morphology as a global factor. Using anthropometric measurements, the morphologic somatotypes were defined in terms of body composition and structure. The aim of this study was to test the hypothesis that morphologic somatotypes were related to standing posture equilibrium in able-bodied girls. A total of 43 able-bodied girls having a mean age of 13.8 &#45 2.2 years participated in this study. Somatotype measurements were taken to determine their endomorphic, mesomorphic or ectomorphic components. Then, subjects were asked to stand still on a force platform for 64 s with their eyes opened, feet about 23 cm apart and arms aligned with the trunk. Afterwards, subjects were grouped based on the highest value of their somatotype component. There was no statistical difference in age, height and weight among the groups. The surface area of an ellipse delineated by the displacement of the centre of pressure (COP) was statistically larger (236.9 &#45 134.3 mm 2 ) for the ectomorphs than for the endomorphs (137.7 &#45 71.4 mm 2 ). The minor axis was longer (8.1 &#45 2.9 mm) for the ectomorphs than for the endomorphs (5.7 &#45 2.2 mm). The decrease in standing posture stability of the ectomorphic group was attributed to a relatively low muscle component, a high height-weight ratio and an elevated position of the body centre of mass in this population of girls. Somatotypes should be considered when assessing standing posture in both able-bodied subjects and patients.     

Changes in sitting posture induce multiplanar changes in chest wall shape and motion with breathing

This study examined the effect of sitting posture on regional chest wall shape in three dimensions, chest wall motion (measured with electromagnetic motion analysis system), and relative contributions of the ribcage and abdomen to tidal volume (%RC/V_t) (measured with inductance plethysmography) in 7 healthy volunteers. In seven seated postures, increased dead space breathing automatically increased V_t (to 1.5 V_t) to match volume between conditions and study the effects of posture independent of volume changes. %RC/V_t (p < 0.05), chest wall shape (p < 0.05) and motion during breathing differed between postures. Compared to a reference posture, movement at the 9th rib lateral diameter increased in the thoracolumbar extension posture (p < 0.008). In slumped posture movement at the AP diameters at T1 and axilla increased (p < 0.00001). Rotation postures decreased movement in the lateral diameter at the axilla (p < 0.0007). The data show that single plane changes in sitting posture alter three-dimensional ribcage configuration and chest wall kinematics during breathing, while maintaining constant respiratory function.     

Can Quiet Standing Posture Predict Compensatory Postural Adjustment?

OBJECTIVE

The aim of this study was to analyze whether quiet standing posture is related to compensatory postural adjustment.

INTRODUCTION

The latest data in clinical practice suggests that static posture may play a significant role in musculoskeletal function, even in dynamic activities. However, no evidence exists regarding whether static posture during quiet standing is related to postural adjustment.

METHODS

Twenty healthy participants standing on a movable surface underwent unexpected, standardized backward and forward postural perturbations while kinematic data were acquired; ankle, knee, pelvis and trunk positions were then calculated. An initial and a final video frame representing quiet standing posture and the end of the postural perturbation were selected in such a way that postural adjustments had occurred between these frames. The positions of the body segments were calculated in these initial and final frames, together with the displacement of body segments during postural adjustments between the initial and final frames. The relationship between the positions of body segments in the initial and final frames and their displacements over this time period was analyzed using multiple regressions with a significance level of p ≤ 0.05.

RESULTS

We failed to identify a relationship between the position of the body segments in the initial and final frames and the associated displacement of the body segments.

DISCUSSION

The motion pattern during compensatory postural adjustment is not related to quiet standing posture or to the final posture of compensatory postural adjustment. This fact should be considered when treating balance disturbances and musculoskeletal abnormalities.

CONCLUSION

Static posture cannot predict how body segments will behave during compensatory postural adjustment.     

Mechanical effect of muscle spindles in the canine external intercostal muscles

High-frequency mechanical vibration of the ribcage increases afferent activity from external intercostal muscle spindles, but the effect of this procedure on the mechanical behaviour of the respiratory system is unknown. In the present study, we have measured the changes in external intercostal muscle length and the craniocaudal displacement of the ribs during ribcage vibration (40 Hz) in anaesthetized dogs. With vibration, external intercostal inspiratory activity increased by ∼50 %, but the respiratory changes in muscle length and rib displacement were unaltered. A similar response was obtained after the muscles in the caudal segments of the ribcage were sectioned and the caudally oriented force exerted by these muscles on the rib was removed, thus suggesting that activation of external intercostal muscle spindles by vibration generates little tension. Prompted by this observation, we also examined the role played by the external intercostal muscle spindles in determining the respiratory displacement of the ribs during breathing against high inspiratory airflow resistances. Although resistances consistently elicited prominent reflex increases in external intercostal inspiratory activity, the normal inspiratory cranial displacement of the ribs was reversed into an inspiratory caudal displacement. Also, this caudal rib displacement was essentially unchanged after section of the external intercostal muscles, whereas it was clearly enhanced after denervation of the parasternal intercostals. These findings indicate that stretch reflexes in external intercostal muscles confer insufficient tension on the muscles to significantly modify the mechanical behaviour of the respiratory system.     

Inverse relations in the patterns of muscle and center of pressure dynamics during standing still and movement postures

The aim of this study was to investigate the postural center of pressure (COP) and surface muscle (EMG) dynamics of young adult participants under conditions where they were required to voluntarily produce random and regular sway motions in contrast to that of standing still. Frequency, amplitude and regularity measures of the COP excursion and EMG activity were assessed, as were measures of the coupling relations between the COP and EMG outputs. The results demonstrated that, even when standing still, there was a high degree of regularity in the COP output, with little difference in the modal frequency dynamics between standing still and preferred motion. Only during random conditions was a significantly greater degree of irregularity observed in the COP measures. The random-like movements were also characterized by a decrease in the level of synchrony between COP motion on the anterior-posterior (AP) and medio-lateral (ML) axes. In contrast, at muscle level, the random task resulted in the highest level of regularity (decreased ApEn) for the EMG output for soleus and tibialis anterior. The ability of individuals to produce a random motion was achieved through the decoupling of the COP motion in each dimension. This decoupling strategy was reflected by increased regularity of the EMG output as opposed to any significant change in the synchrony in the firing patterns of the muscles examined. Increased regularity across the individual muscles was accompanied by increased irregularity in COP dynamics, which can be characterized as a complexity tradeoff. Collectively, these findings support the view that the dynamics of muscle firing patterns does not necessarily map directly to the dynamics at the movement task level and vice versa.     

The hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) increases cortical extracellular glutamate levels in rats

We examined how young and older adults adapt their posture to static balance tasks of increasing difficulty. Participants stood barefoot on a force platform in normal quiet, Romberg-sharpened and one-legged stance. Center of pressure (CoP) variations, electromyographic (EMG) activity of ankle and hip muscles and kinematic data were recorded. Both groups increased postural sway as a result of narrowing the base of support. Greater CoP excursions, EMG activity and joint displacements were noted in old compared to younger adults. Older adults displayed increased hip movement accompanied by higher hip EMG activity, whereas no similar increase was noted in the younger group. It is concluded that older adults rely more on their hip muscles when responding to self induced perturbations introduced by increased task constraints during quiet standing.     

Identifying the control of physically and perceptually evoked sway responses with coincident visual scene velocities and tilt of the base of support

In this study, we have explored whether the impact of visual information on postural reactions is due to the same perceptual mechanisms that produce vection. Pitch motion of the visual field was presented at varying velocities to eight healthy subjects (29.9 ± 2.8 years) standing quietly on a stationary base of support or receiving a 3° toes-up tilt of the base of support. An infrared motion system recorded markers placed on body segments to record angular displacement of head and ankle and calculate whole body center of mass. Onset of the visual field motion and base of support movement were synchronized in all trials. We found that in the first 2 s following onset of visual field motion, both direction and amplitude of the linear displacement of whole body center of mass and angular displacement of the head, hip, and ankle were modulated by the velocity of visual scene motion. When the visual scene rotated in upward pitch, subjects overshot their initial vertical position with amplitudes that increased as velocity of the visual field increased. This behavior was even more evident when the base of support was tilted. These responses were much shorter than those observed in studies of vection. The dependence of the postural response amplitudes on the velocity of the visual field suggests, however, that there might be well-shared control pathways for visual influences on postural reactions and postural sway elicited by an illusion of self-motion.     

Recurrence Quantification Analysis of Human Postural Fluctuations in Older Fallers and Non-fallers

We investigate postural sway data dynamics in older adult fallers and non-fallers. Center of pressure (COP) signals were recorded during quiet standing in 28 older adults. The subjects were divided in two groups: with and without history of falls. COP time series were analyzed using recurrence quantification analysis (RQA) in both anteroposterior and mediolateral (ML) directions. Classical stabilometric variables (path length and range) were also computed. The results showed that RQA outputs quantifying predictability of COP fluctuations and Shannon entropy of recurrence plot diagonal line length distribution, were significantly higher in fallers, only for ML direction. In addition, the range of ML COP signals was also significantly higher in fallers. This result is in accordance with some findings of the literature and could be interpreted as an increased hip strategy in fallers. The RQA results seem coherent with the theory of loss of complexity with aging and disease. Our results suggest that RQA is a promising approach for the investigation of COP fluctuations in a frail population.     

The many roles of vision during walking

Vision can improve bipedal upright stability during standing and locomotion. However, during locomotion, vision supports additional behaviors such as gait cycle modulation, navigation, and obstacle avoidance. Here, we investigate how the multiple roles of vision are reflected in the dynamics of trunk control as the neural control problem changes from a fixed to a moving base of support. Subjects were presented with either low- or high-amplitude broadband visual stimuli during standing posture or while walking on a treadmill at 1 km/h and 5 km/h. Frequency response functions between visual scene motion (input) and trunk kinematics (output) revealed little or no change in the gain of trunk orientation in the standing posture and walking conditions. However, a dramatic increase in gain was observed in trunk (hip and shoulder) horizontal displacement from posture to locomotion. Such increases in gain may be interpreted as an increased coupling to visual scene motion. However, we believe that the increased gain reflects a decrease in stability due to a change of the control problem from standing to locomotion. Indeed, keeping the body upright with the use of vision during walking is complicated by the additional locomotor processes at work. Unlike during standing, vision plays many roles during locomotion, providing information for upright stability as well as body position relative to the external environment.     

Gaze stabilization during dynamic posturography in normal and vestibulopathic humans

Dynamic posturography by measurement of center of pressure (COP) is a widely employed technique for evaluating the vestibular system. However, the relationship of COP motion to vestibulo-ocular reflex (VOR) function and image stability on the retina has not been determined previously. To assess these relationships, we report gaze, head, and trunk stability during dynamic posturography in 11 normal volunteers, 7 subjects with unilateral vestibular lesions, and 3 subjects with bilateral vestibular lesions. Posturographic tasks consisted of standing still and standing on a platform that was sliding (0.2 Hz), tilting (0.1 Hz), or covered with a foam cushion 6 cm thick while tilting (0.1 Hz). Each perturbation was imposed in the anterior-posterior and repeated in the medial-lateral direction, in both light and darkness. Subjects viewed (or in darkness remembered) a target located 50, 100, or 500 cm distant. COP, angular eye position, and angular and linear orbit and trunk positions were measured using magnetic search coils and flux gate magnetometer sensors. With the target visible, the velocity of image motion on the retina was on average always less than 1°/s, well within the range consistent with high visual acuity. In darkness, gaze velocity increased for normal and vestibulopathic subjects. During tilt, vestibulopathic subjects had a significantly greater gaze velocity than controls. Gain of the angular VOR (eye velocity/head velocity) was significantly lower in darkness than in light and in vestibulopathic as compared to control subjects. Gain of the VOR was significantly correlated with gaze instability, but variation in VOR gain accounted for only 20–40% of the variance. In darkness, the velocity of the COP was significantly greater in vestibulopathic than control subjects for every condition tested. In light, this difference was small and often not significant. Although spectral analysis of the COP indicated frequencies above 1 Hz that were not observed in motion of the trunk and orbit, root mean square (RMS) velocities of the trunk and orbit in the horizontal plane were higher in darkness and in vestibulopathic subjects, mirroring COP findings. Only in vestibulopathic subjects tested in darkness was there a correlation between COP velocity and gaze velocity; COP velocity was otherwise uncorrelated with gaze. Gaze velocity was greater with near than with distant targets. Vertical VOR gain was higher with near targets. No other significant effects of target distance were found. Head movement strategy, VOR gain, and COP were all unaffected by target proximity. These data show that gaze velocity measurements during dynamic posturography in darkness are sensitive to vestibular loss. With a visible target, both COP and gaze stability of vestibulopathic subjects are difficult to distinguish from normal. During visual feedback, it is likely that image stabilization over the range of frequencies tested is achieved through better head stability and through visual tracking, allowing vestibulopathic subjects to maintain adequate visual acuity. Received: 25 November 1997 / Accepted: 24 April 1998