Support Surface Motion Research Articles

Effort to perceive the position of one visual horizontal line relative to another appearing close causes an earlier postural response to backward perturbation.

Humans make an effort to stabilize the body when a stable eye position is required to improve visibility. Perceiving the position of one visual object relative to another appearing close is relatively difficult compared with perceiving those objects appearing at a distance. Thus, humans must make an effort to stabilize their body to improve visibility when they attempt to perceive the position of one object relative to another appearing close. This process may enhance the response to postural perturbation. The present study tested this hypothesis. A total of 15 healthy participants maintained a position standing over the platform and gazed at a horizontal fixation line in front of them. One of the two warning sound cues was given in each trial; one predicted the forthcoming appearance of a horizontal line 2 mm above or below the horizontal fixation line (indistinct condition), and another predicted the forthcoming appearance of the horizontal line 2 cm above or 2 cm below the horizontal fixation line (distinct condition). After the warning sound cue, the support surface moved in the forward or backward direction. A horizontal line appeared 40 ms after the onset of the support surface motion at the level above or below the horizontal fixation line with the distance from the horizontal fixation line as the warning cue predicted. The participants verbally stated the position of the horizontal line relative to the horizontal fixation line after the offset of the support surface movement. The displacement of the head and sacrum was measured with acceleration sensors. A test of the simple main effect following an analysis of variance revealed that the peak downward displacement of the pelvis in the indistinct condition was significantly earlier than that in the distinct condition. This finding indicated that the effort to stabilize the body for improvement of visibility enhances the response of the pelvis to the forward movement of the support surface. This effect may be used for improvement of the postural response in patients with problems with postural control.

Postural control strategy in response to cyclic motion of support surface

Postural control strategy in response to cyclic motion of support surface

Postural control strategy in response to cyclic motion of support surface

Postural control strategy in response to cyclic motion of support surface

Directional acuity of whole-body perturbations during standing balance

The ability to perceive the direction of whole-body motion during standing may be critical to maintaining balance and preventing a fall. Our first goal was to quantify kinesthetic perception of whole-body motion by estimating directional acuity thresholds of support-surface perturbations during standing. The directional acuity threshold to lateral deviations in backward support-surface motion in healthy, young adults was quantified as 9.5±2.4° using the psychometric method (n=25 subjects). However, inherent limitations in the psychometric method, such as a large number of required trials and the predetermined stimulus set, may preclude wider use of this method in clinical populations. Our second goal was to validate an adaptive algorithm known as parameter estimation by sequential testing (PEST) as an alternative threshold estimation technique to minimize the required trial count without predetermined knowledge of the relevant stimulus space. The directional acuity threshold was estimated at 11.7±3.8° from the PEST method (n=11 of 25 subjects, psychometric threshold=10.1±3.1°) using only one-third the number of trials compared to the psychometric method. Furthermore, PEST estimates of the direction acuity threshold were highly correlated with the psychometric estimates across subjects (r=0.93) suggesting that both methods provide comparable estimates of the perceptual threshold. Computational modeling of both techniques revealed similar variance in the estimated thresholds across simulations of about 1°. Our results suggest that the PEST algorithm can be used to more quickly quantify whole-body directional acuity during standing in individuals with balance impairments.

Asymmetric Sensory Reweighting in Human Upright Stance

To investigate sensory reweighting as a fundamental property of sensor fusion during standing, we probed postural control with simultaneous rotations of the visual scene and surface of support. Nineteen subjects were presented with pseudo-random pitch rotations of visual scene and platform at the ankle to test for amplitude dependencies in the following conditions: low amplitude vision: high amplitude platform, low amplitude vision: low amplitude platform, and high amplitude vision: low amplitude platform. Gain and phase of frequency response functions (FRFs) to each stimulus were computed for two body sway angles and a single weighted EMG signal recorded from seven muscles. When platform stimulus amplitude was increased while visual stimulus amplitude remained constant, gain to vision increased, providing strong evidence for inter-modal reweighting between vision and somatosensation during standing. Intra-modal reweighting of vision was also observed as gains to vision decreased as visual stimulus amplitude increased. Such intra-modal and inter-modal amplitude dependent changes in gain were also observed in muscular activity. Gains of leg segment angle and muscular activity relative to the platform, on the other hand, showed only intra-modal reweighting. That is, changing platform motion amplitude altered the responses to both visual and support surface motion whereas changing visual scene motion amplitude did not significantly affect responses to support surface motion, indicating that the sensory integration scheme between somatosensation (at the support surface) and vision is asymmetric.

Adaptation in Locomotor Stability, Cognition, and Metabolic Cost During Sensory Discordance

Locomotor instability may affect planetary extravehicular activities during the initial adaptation to the new gravitational environment. The goal of this study was to quantify the locomotor, cognitive, and metabolic effects of exposure to a discordant sensory environment. A treadmill mounted on a 6-degree-of-freedom motion base was used to present 15 healthy subjects with a destabilizing support surface while they walked. Dependent measures of locomotor stability, cognitive load, and metabolic cost were stride frequency (SF), reaction time (RT), and the volume of oxygen consumed (Vo2), respectively. Subjects completed an 8-min baseline walk followed by 20 min of walking with a continuous, sinusoidal, laterally oscillating support-surface perturbation. Data for minutes 1, 7, 13, and 20 of the support-surface perturbation period were compared with the baseline. SF, RT, and Vo2 were significantly greater during support-surface motion than during the baseline walking condition and showed a trend toward recovery to baseline levels during the perturbation period. Results demonstrated that adaptation to walking in a discordant sensory environment has quantifiable and significant costs in SF, RT, and Vo2 as shown by mean increases of 9%, 20%, and 4%, respectively, collected during the first minute of exposure. By the fourth minute of exposure, mean Vo2 consumption had increased to 20% over its baseline. We believe that preflight sensorimotor adaptation training paradigms will impart gains in stability and the ability to multitask, and might increase productive mission time by extending work time in extravehicular activity suits where metabolic expenditure is a limiting factor.

Postural coordination patterns as a function of rhythmical dynamics of the surface of support

This study investigated the organization of postural coordination patterns as a function of the rhythmical dynamics of the surface of support. We examined how the number and nature of the dynamical degrees of freedom in the movement coordination patterns changed as a function of the amplitude and frequency of support surface motion. Young adult subjects stood on a moving platform that was translated sinusoidally in anterior-posterior (AP) direction with the task goal to maintain upright bipedal postural balance. A force platform measured the kinetics at the surface of support and a 3D motion analysis system recorded torso and joint kinematics. Principal components analysis (PCA) identified four components overall, but increasing the average velocity of the support surface reduced the modal number of components of the postural coordination pattern from three to two. The analysis of joint motion loadings on the components revealed that organizational properties of the postural pattern also changed as a function of platform dynamics. PC1 (61.6-73.2 %) was accounted for by ankle, knee, and hip motion at the lowest velocity conditions, but as the velocity increased, ankle and hip variance dominated. In PC2 (24.2-20.2 %), the contribution of knee motion significantly increased while that of ankle motion decreased. In PC3 (9.7-5.1 %) neck motion contributed significantly at the highest velocity condition. Collectively, the findings show that the amplitude and frequency of the motion of the surface of support maps redundantly though preferentially to a small set of postural coordination patterns. The higher platform average velocities led to a reduction in the number of dynamical degrees of freedom of the coordination mode and different weightings of joint motion contributions to each component.

Posturography and locomotor tests of dynamic balance after long-duration spaceflight

The currently approved objective clinical measure of standing balance in astronauts after space flight is the Sensory Organization Test battery of computerized dynamic posturography. No tests of walking balance are currently approved for standard clinical testing of astronauts. This study determined the sensitivity and specificity of standing and walking balance tests for astronauts before and after long-duration space flight. Astronauts were tested on an obstacle avoidance test known as the Functional Mobility Test (FMT) and on the Sensory Organization Test using sway-referenced support surface motion with eyes closed (SOT 5) before and six months after (n=15) space flight on the International Space Station. They were tested two to seven days after landing. Scores on SOT tests decreased and scores on FMT increased significantly from pre- to post-flight. In other words, post-flight scores were worse than pre-flight scores. SOT and FMT scores were not significantly related. ROC analyses indicated supra-clinical cut-points for SOT 5 and for FMT. The standard clinical cut-point for SOT 5 had low sensitivity to post-flight astronauts. Higher cut-points increased sensitivity to post-flight astronauts but decreased specificity to pre-flight astronauts. Using an FMT cut-point that was moderately highly sensitive and highly specific plus SOT 5 at the standard clinical cut-point was no more sensitive than SOT 5, alone. FMT plus SOT 5 at higher cut-points was more specific and more sensitive. The total correctly classified was highest for FMT, alone, and for FMT plus SOT 5 at the highest cut-point. These findings indicate that standard clinical comparisons are not useful for identifying problems. Testing both standing and walking balance will be more likely to identify balance deficits.

Walking on an Oscillating Treadmill: Strategies of Stride-Time Adaptation

Gains in mobility, balance, and gait quality have been observed in clinical populations after treadmill training. In our laboratory, we introduce healthy adults to treadmill training programs in an effort to boost their adaptive generalization skills. We hypothesized that some individuals are more naturally predisposed to gait adaptation than others. Stride-time data from 20 healthy adults were collected while they walked on a laterally oscillating treadmill for the first time. We predicted that the more plastic adapters would rapidly synchronize their stride frequencies to the treadmill's oscillation frequency. A treadmill mounted on a 6-degrees-of-freedom motion base platform was programmed to move laterally in a continuous, sinusoidal pattern for 20 min while participants walked at 1.1 m/s. Twenty-five percent of our participants showed a prolonged uninstructed entrained response. Those who did not entrain were still able to adapt to the novel locomotor challenge by applying various alternative strategies. Although entrainment was not required for adaptation, we report compelling evidence that the frequency of the support-surface motion influenced the adapted strategies of 80% of our participants.

Biologically-inspired humanoid postural control

This article presents a biologically-inspired framework for humanoid postural control. It complies with the main features of human postural control that are extracted from recent studies. In this article, the human body is abstracted as a single-inverted pendulum jointed with a foot that rests freely on a supporting surface. In particular, disturbances affecting posture are addressed and accommodated within the proposed framework. Among these are external forces and motion of support surface on which the body stands. The main components of this framework are: 1. A state-feedback mechanism for stabilizing the unstable dynamics of the body. 2. A tracking loop for robustly achieving desired voluntary orientations. 3. A feed-forward control primarily for improving the response to voluntary motions. 4. A stand-alone vestibular sensory fusion algorithm for estimating body orientation. 5. An external-disturbance estimator and a corresponding compensation for minimizing the effect of external disturbances. These components are interconnected in a way that qualifies this framework to modularly address the multi-segment body postural control problem. Although no postural stability measure is explicitly incorporated, experiments run on a special-purpose humanoid demonstrate the stability and the performance merits of the presented framework.

Computerized posturography balance assessment of patients with bilateral ventralis intermedius nuclei deep brain stimulation

Bilateral ventralis intermedius nuclei (Vim) deep brain stimulation (DBS) improves tremor in patients with both essential tremor (ET) and Parkinson's disease (PD). In each condition, patients have individually noted both subjective improvement and worsening in balance. Computerized posturography (CP) is able to quantify some aspects of balance. Twenty-one patients (8 with PD and 13 with ET) with bilateral Vim DBS were recruited to undergo randomized-order identical CP testing (EquiTest system) while their DBS devices were both activated and deactivated. One PD patient could not complete any OFF assessment and is not included. Three PD patients could not tolerate portions of the OFF testing. Overall, sensory organization testing was improved by DBS activation in conditions that involved quiet standing with eyes open with no motion of the support surface, or with sway-referenced support surface motion, but worsened during quiet standing with eyes closed only in ET patients. Falls lessened with activation in ET patients. Motor control testing was not changed. Bilateral Vim DBS activation mostly improved balance, but may modestly worsen other specific features.

Thresholds for inducing protective stepping to force-controlled perturbations in young and old adults

In previous studies of stairway handrails, data were derived from static experiments which characterized the influence of the handrail design on ability to generate stabilizing force. This paper describes a novel and safe experimental approach developed to study the biomechanical efficacy of handrail use under dynamic conditions, wherein support-surface motion is used to perturb the balance of the subject who stands on a small (three step), heavily padded mock staircase. A primary objective of this initial study was to determine the influence of factors such as perturbation magnitude, stance leg (left or right), proximity to the handrail, initial hand position (on or off the rail), and ability to complete a step while grabbing the rail. The study was also intended to address a more basic issue: is it even possible to grab a handrail with sufficient speed and accuracy to prevent a fall after losing balance on a stairway? Testing of four healthy young adults demonstrated that sizeable stabilizing handrail force can be generated very quickly (up to 60% of body weight in less than 1 second) in response to loss of balance. Furthermore, these grabbing responses were clearly of functional significance, resulting in a marked reduction in the incidence of ‘falls’ (i.e. landing on the padded surface) compared to trials where the handrail was absent. The most consistent aspect of the force generation was the tendency to exert a forward axial force along the rail. An unexpected finding was that this force often appears to be exerted through a pulling, rather than pushing, action, because of the posterior location of the grip relative to the body. Although most of the force components tended to increase with perturbation magnitude, the lateral forces appeared to be most dependent on whether a step was taken. Stance leg had few effects, but variation in proximity to the rail was found to influence the angle at which the hand approached the rail. Gripping the rail prior to perturbation led to a greater tendency to pull upwards. Implications of these findings for safer handrail design are discussed.

Employing a virtual environment in postural research and rehabilitation to reveal the impact of visual information

We have united an immersive virtual environment with support surface motion to record biomechanical and physiological responses to combined visual, vestibular, and proprioceptive inputs. We have examined age-related differences during peripheral visual field motion and with a focal image projected on to the moving virtual scene. Our data suggest that the postural response is modulated by all existing sensory signals in a non-additive fashion. An individual’s perception of the sensory structure appears to be a significant component of the postural response in these protocols. We will discuss the implications of these results to clinical interventions for balance disorders.

Postural responses exhibit multisensory dependencies with discordant visual and support surface motion

The purpose of this study was to identify how the postural system weights coincident yet discordant disturbances of the visual and proprioceptive/vestibular systems. Eleven healthy subjects (25-38 yrs) received either fore-aft translations of an immersive, wide field-of-view visual environment (0.1 Hz, +/- 3.7 m/sec), or anterior-posterior translations of the support surface (0.25 Hz, +/- 15 cm/sec), or both concurrently. Kinematics of the head, trunk, and shank were collected with an Optotrak system and angular motion of each segment plotted across time. With only support surface translation, segmental responses were small (1 degrees -2 degrees ) and mostly opposed the direction of sled translation. When only the visual scene was moving, segmental responses increased as the trial progressed. When the inputs were presented coincidentally, response amplitudes were large even at the onset of the trial. Mean RMS values across subjects were significantly greater with combined stimuli than for either stimulus presented alone and areas under the power curve across subjects were significantly increased at the frequency of the visual input when both inputs were presented. Thus, intra-modality dependencies were observed, such that responses to the visual inputs significantly increased and responses to the somatosensory signals reflected the stimulus amplitude only when the two inputs were combined. We believe it unlikely that the role of any single pathway contributing to postural control can be accurately characterized in a static environment if the function of that pathway is context dependent.

Age‐dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man

We investigated the effects of ageing on balance corrections induced by sudden stance perturbations in different directions. Effects were examined in biomechanical and electromyographic (EMG) recordings from a total of 36 healthy subjects divided equally into three age groups (20-34, 35-55 and 60-75 years old). Perturbations consisted of six combinations of support-surface roll (laterally) and pitch (forward-backward) each with 7.5 deg amplitude (2 pure pitch, and 4 roll and pitch) delivered randomly. To reduce stimulus predictability further and to investigate scaling effects, perturbations were at either 30 or 60 deg s(-1). In the legs, trunk and arms we observed age-related changes in balance corrections. The changes that appeared in the lower leg responses included smaller stretch reflexes in soleus and larger reflexes in tibialis anterior of the elderly compared with the young. For all perturbation directions, onsets of balance correcting responses in these ankle muscles were delayed by 20-30 ms and initially had smaller amplitudes (between 120-220 ms) in the elderly. This reduced early activity was compensated by increased lower leg activity after 240 ms. These EMG changes were paralleled by comparable differences in ankle torque responses, which were initially (after 160 ms) smaller in the elderly, but subsequently greater (after 280 ms). Findings in the middle-aged group were generally intermediate between the young and the elderly groups. Comparable results were obtained for the two different stimulus velocities. Stimulus-induced trunk roll, but not trunk pitch, changed dramatically with increasing age. Young subjects responded with early large roll movements of the trunk in the opposite direction to platform roll. A similarly directed but reduced amplitude of trunk roll was observed in the middle-aged. The elderly had very little initial roll modulation and also had smaller stretch reflexes in paraspinals. Balance-correcting responses (over 120-220 ms) in gluteus medius and paraspinals were equally well tuned to roll in the elderly, as in the young, but were reduced in amplitude. Onset latencies were delayed with age in gluteus medius muscles. Following the onset of trunk and hip balance corrections, trunk roll was in the same direction as support-surface motion for all age groups and resulted in overall trunk roll towards the fall side in the elderly, but not in the young. Protective arm movements also changed with age. Initial arm roll movements were largest in the young, smaller in the middle aged, and smallest in the elderly. Initial arm roll movements were in the same direction as initial trunk motion in the young and middle aged. Thus initial roll arm movements in the elderly were directed oppositely to those in the young. Initial pitch motion of the arms was similar across age groups. Subsequent arm movements were related to the amplitude of deltoid muscle responses which commenced at 100 ms in the young and 20-30 ms later in the elderly. These deltoid muscle responses preceded additional arm roll motion which left the arms directed 'downhill' (in the direction of the fall) in the elderly, but 'uphill' (to counterbalance motion of the pelvis) in the young. We conclude that increased trunk roll stiffness is a key biomechanical change with age. This interferes with early compensatory trunk movements and leads to trunk displacements in the direction of the impending fall. The reversal of protective arm movements in the elderly may reflect an adaptive strategy to cushion the fall. The uniform delay and amplitude reduction of balance-correcting responses across many segments (legs, hips and arms) suggests a neurally based alteration in processing times and response modulation with age. Interestingly, the elderly compensated for these 'early abnormalities' with enlarged later responses in the legs, but no similar adaptation was noted in the arms and trunk. These changes with age provide an insight into possible mechanisms underlying falls in the elderly.

Efficacy of handrails in preventing stairway falls: a new experimental approach

In previous studies of stairway handrails, data were derived from static experiments which characterized the influence of the handrail design on ability to generate stabilizing force. This paper describes a novel and safe experimental approach developed to study the biomechanical efficacy of handrail use under dynamic conditions, wherein support-surface motion is used to perturb the balance of the subject who stands on a small (three step), heavily padded mock staircase. A primary objective of this initial study was to determine the influence of factors such as perturbation magnitude, stance leg (left or right), proximity to the handrail, initial hand position (on or off the rail), and ability to complete a step while grabbing the rail. The study was also intended to address a more basic issue: is it even possible to grab a handrail with sufficient speed and accuracy to prevent a fall after losing balance on a stairway? Testing of four healthy young adults demonstrated that sizeable stabilizing handrail force can be generated very quickly (up to 60% of body weight in less than 1 second) in response to loss of balance. Furthermore, these grabbing responses were clearly of functional significance, resulting in a marked reduction in the incidence of ‘falls’ (i.e. landing on the padded surface) compared to trials where the handrail was absent. The most consistent aspect of the force generation was the tendency to exert a forward axial force along the rail. An unexpected finding was that this force often appears to be exerted through a pulling, rather than pushing, action, because of the posterior location of the grip relative to the body. Although most of the force components tended to increase with perturbation magnitude, the lateral forces appeared to be most dependent on whether a step was taken. Stance leg had few effects, but variation in proximity to the rail was found to influence the angle at which the hand approached the rail. Gripping the rail prior to perturbation led to a greater tendency to pull upwards. Implications of these findings for safer handrail design are discussed.

Interactions Between Automatic Postural Adjustments and Anticipatory Postural Patterns Accompanying Voluntary Movement

In order to examine interactions between centrally initiated postural activity preceding voluntary arm movements and compensatory postural activity, we studied patterns of postural muscle activity preceding a vigorous bilateral reach and grasp task or triggered by support surface motion. The reaching task required movement onset to be coincident with a predictable stimulus, and in some trials a brief backward platform perturbation was timed to occur before, during, or after the reach onset. Centrally initiated anticipatory postural activity was subject-specific and was very often absent when perturbation-induced postural activity was elicited just prior to movement onset. Likewise, compensatory postural activity patterns elicited by the platform perturbation did not occur when they would have coincided with the anticipatory postural activity. These data support the idea that the central neural processes which determine the specific activation pattern of the supporting limb musculature are influenced by both the intended dynamic outcome and the current dynamic status of the body.