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Temporal and spatial relationships between head motion and tilt of a visual surround were evaluated in conditions 3 and 6 of the sensory organization test (SOT) to determine the validity of visual stabilization during computerized dynamic platform posturography (CDP). Seven healthy subjects participated (mean age 42.4 years, SD = 19 years, range 25-72 years). Sagittal head translation and head pitch were evaluated separately with respect to displacement of the visual surround. Overall, the mean correlation between sagittal head motion and surround tilt was r = 0.68 for C3 and r = 0.93 for C6. The average phase (aggregate findings supported with trial-by-trial analysis), however, showed that sagittal head displacement leads motion of the visual surround ( + 452 ms for C3 and + 250 ms for C6). The mean correlation between head pitch and surround tilt was r = -0.30 for C3 and r = -0.72 for C6. Head pitch also leads visual surround motion (+567 ms for C3 and +539 ms for C6), but with greater variability than sagittal head translation. The results showed that visual stabilization was not consistently achieved during SOT conditions 3 and 6. The fundamental assumption that posture behaves as an inverted pendulum, with control of equilibrium originating in the lower extremities, does not appear to be applicable to the voluntary corrections of sway required during the SOT.

The quality of sensory information that is necessary for balance and postural stability will depend to a great extent on head stability as the body moves. How older persons coordinate head and body motion for balance during volitional activities is not known. The purposes of this article are to present a basis for understanding the influence of aging on head control during voluntary motion and to discuss some data that demonstrate how elderly people might control head movement to improve gaze and the quality of vestibular inputs. A "top-down" or "head-first" control scheme is proposed as the mechanism that elderly people without disabilities use to maintain head position during self-initiated motion. This type of control ensures that the angular position of the head in space remains relatively constant--through the use of a head-stabilization-in-space (HSS) strategy--regardless of the magnitude or direction of displacements in the body's center of force. The HSS strategy is thought to reduce potential ambiguities in the interpretation of sensory inputs for balance and is derived primarily from a geocentric (orientation to the vertical) frame of reference. Egocentric (orientation of the head with respect to the body) or exocentric (orientation to objects in the environment) frames of reference, however, refine the control of head stabilization. Preliminary research suggests that elderly people use the HSS strategy to control head pitch during difficult balance tasks. These findings, if supported by more definitive studies, may be useful in the treatment of patients with balance disorders. The treatment of patients with balance dysfunction is discussed within the conceptual framework of a "head-first" organization scheme.

Objective: To compare the sensitivity and specificity of platform posturography with other vestibular tests for patients with peripheral vestibular deficits (PVD), Meniere's disease, benign paroxysmal positional vertigo (BPPV), and central nervous system-vestibular impairment (CNS)., Data Sources: A computed search was conducted using the Index Medicus database (1966-1994) and Current Contents Science Editions., Study Selection: Studies were selected for analysis if the article addressed the sensitivity and/or specificity of platform posturography, compared posturography with another objective test of vestibular function, identified the basis for abnormal test results, and reported the data with sufficient detail to calculate an effect size from a 2 x 2 contingency table., Data Extraction: A count of the normal and abnormal test results for posturography and the criterion standard were retrieved from each article, analyzed using a chi 2 statistic, and converted to an effect size. A positive effect size indicated that posturography identified abnormalities in patients who had normal tests on the criterion standard., Data Synthesis: Sensitivity and specificity of posturography were about 50%. The overall effect size was small (0.13) but positive. The diagnostic category had a significant influence on the predictive value of abnormal results (73% for Meniere's disease and BPPV, compared with 41% for PVD, and 44% for mixed CNS and PVD (F2,12 = 5.26, P = .02) and on the magnitude of the effect size (0.41 for mixed CNS and PVD compared with 0.22 for Meniere's disease and BPPV, and -0.10 for PVD (F2,12 = 13.95, P = .001)., Conclusions: Platform posturography provides a measurable supplement to the standard vestibular examination. The enhancement was most notable when the target population included patients with CNS deficits.

Various motor patterns or 'strategies' can be used to maintain balance. The purpose of this study was to determine the responsiveness of a pediatric strategy score (PED-SS) compared to a standard strategy score (SS) as a measure of age-related changes in the force patterns used to maintain stance. Eighty-one healthy children between 3-6 years of age were tested during stance on a force platform while facing a visual surround. The platform, visual surround (or both) moved simultaneously with the child's body sway. Four sensory conditions that altered visual and somatosensory (support surface) inputs were presented. The PED-SS was found to be more responsive to age-related changes in balance behavior compared to the SS. The oldest children (aged 5 and 6 years) showed the greatest ability to utilize horizontal A/P shear force to maintain stance and this finding was reflected only in the PED-SS. The implications of evaluating force strategy as one component of balance in healthy children is discussed with respect to the early developmental assessment of vestibular and developmental coordination disorders.

Sensitive and specific measures are needed to identify patients with vestibular impairments. The purpose of this clinical perspective is to describe the sensitivity and specificity of dynamic and static platform posturography for detecting vestibular disorders. The sensory organization test (SOT) of dynamic posturography (EquiTest), the motor "perturbation" test, and Romberg's tests on a static (fixed) force platform each had over 90% specificity. This finding means that nearly all of the subjects who should have tested negative, did test negative on each type of assessment. The sensitivity of the SOT was evaluated across five studies involving a total of 836 patients with peripheral vestibular deficits (PVDs). Abnormalities in the SOT were detected in only 40% (n = 338) of the cases. Static platform posturography sensitivity was evaluated across six studies involving a total of 571 patients with PVDs, and abnormalities were detected in 53% (n = 302) of these cases. Tests of spontaneous and positional nystagmus and the horizontal component of the vestibuloocular reflex (VOR), by comparison, detected PVDs in 48% of 798 patients with suspected vestibular impairment. For patients with vestibular deficits associated with central nervous system disease, a total of 389 cases were identified in five studies and SOT abnormalities were found in 54% (n = 209) of these cases. The motor perturbation test was abnormal in 35% (n = 41) of 119 patients with central vestibular disease. In conclusion, the sensitivity of static posturography appeared to be slightly better than that of dynamic posturography for detecting PVDs, but the level of sensitivity for each posturography test, as well as for tests of horizontal VOR function, was considered to be low. Combining either type of posturography with other tests of vestibular function, however, increased the overall sensitivity of detecting vestibular deficits to 61% to 89%. It was concluded that dynamic and static platform posturography as well as tests of VOR function lack adequate sensitivity to detect vestibular impairment when applied in isolation. Posturography appears to detect vestibular deficits in some patients who had normal VOR assessments and, therefore, provides supplemental rather than redundant information about vestibular dysfunction.

Vestibular impairments have not been routinely identified in patients with multiple sclerosis (MS), because of the confounding effects of deficits in other neural systems. In this study, 35 patients with MS were evaluated by means of a systematic alteration of the sensory environment (dynamic posturography) in order to identify those patients who became unstable when vestibular inputs were needed to maintain stance. Subjects were assigned to either a high-function (HF) or a low-function (LF) group on the basis of a functional status assessment score obtained prior to the posturography test. For the HF group, 30% (7/23) had abnormal posturography scores. Of those subjects, 3 had a vestibular dysfunction pattern or a somatosensory-vestibular impairment. In contrast, 58% of the LF group (7/12) had abnormal posturography scores. Nearly all of these LF patients (6/7) had a vestibular dysfunction pattern or a combined visual-vestibular or somatosensory-vestibular impairment. Posturography might serve as one method to evaluate the functional consequences of a vestibular deficit in patients with MS.

The purposes of this study were to (1) evaluate the effects of sway-referenced visual and ankle somatosensory inputs on head movement and body sway during stance and (2) determine how sway-referenced sensory inputs might influence the selection of postural patterns. Seventeen healthy adult subjects were evaluated while attempting to maintain standing balance. Ankle somatosensory inputs were altered during stance by creating support surface rotations which were continuously adjusted ("referenced") to the center of force. In selected trials, ankle somatosensory or visual inputs were distorted further by increasing the platform/body sway gain or by increasing the sway-referenced gain of a rotating visual enclosure. The results showed that spontaneous body sway in a given direction was generally associated with head displacement in the opposite direction. The slope of the sway vs. sagittal head position linear regression line was statistically greater at a sway-referenced gain of 1.5 compared to a gain of 1.0. This finding was observed specifically when mechanical compliance of the support surface was increased and occurred whether the eyes were open or closed. Horizontal shear force was generated in the same direction as head displacement and the linear regression slope was greatest with eyes open and at a sway gain of 1.5. These findings indicate that (1) destabilization of the ankles altered displacement of the head independently from visual input, and (2) visual information was necessary to produce specific postural patterns utilizing horizontal shear forces to maintain stance on a mechanically compliant surface. The results provide evidence for a neural mechanism linking ankle somatosensory inputs and head control.

The effect of altered sensory environments on balance was studied in 82 healthy children between 3 and 6 years of age. The purpose was to obtain normative measurements of postural stability during early stages of development for use in clinical posturography. Subjects were tested in three visual conditions (eyes open, blind-folded, and sway-referenced visual enclosure) during stance on either a fixed or sway-referenced force platform (EquiTest). During sway-referenced conditions, spontaneous changes in the subject's center of force (COF) drove rotational displacements of the platform and/or visual surround. The gain of platform/COF or surround/COF displacement was altered from 0.00 to 1.5. An equilibrium score (ES) was calculated based on the amount of postural sway compared to the maximum sway possible without falling. A higher ES reflected greater postural stability. An improvement in postural stability was observed between 4 and 5 years of development when the sensory environment was fixed (sway referenced gain = 0.0). In a dynamic sensory environment, however, age-related improvements in postural stability were not uniform and depended upon the specific sensory condition. For example, while 6 year olds generally showed the greatest postural stability of any age group, all ages showed statistically equivalent equilibrium scores during stance on a compliant surface with eyes closed, at all sway gains. The ability to ignore misleading sensory inputs to maintain balance was observed at 3 years of age; 76% of the 3 year olds (n = 16) were able to maintain stance in altered sensory environments at the 1.0 sway gain. In addition, postural stability was greater when visual inputs were sway-referenced compared to conditions that manipulated the support surface compliance. These data suggest that the predominance of visual-vestibular control of balance gives way to a somatosensory-vestibular dependence by age 3, but that the transition to adult-like balance responses is not complete for all sensory conditions even by age 6. The use of posturography to enhance the assessment of pediatric vestibular and sensory integration deficits is discussed.

Vision and/or ankle somatosensory inputs often do not correlate with the position of the center of gravity. In this case, visual or somatosensory information may be in conflict with other sensory systems that convey a true sense of body orientation. The purpose of this study was to determine how conflicting visual and ankle somatosensory inputs influenced standing balance in elders with a history of falls. Forty-seven community-dwelling elders (8 male, 39 female), between 65 and 96 years of age (mean = 80.5, SD = 9.0), participated in this project. Subjects with two or more falls in the 6 months prior to study were assigned to a fall group (n = 16), whereas those with no history of falling during the same time interval were assigned to a no-fall group (n = 31). In order to remove any bias in the testing procedure, the tester was not aware of group assignments. Subjects were evaluated using a sensory organization test (SOT) for standing balance and a "Get Up and Go" test (GUGT) for general mobility. Analysis of covariance was used to evaluate the SOT scores (by group, vision, and surface condition) and the GUGT scores. Body sway is known to increase with the normal aging process, and for this reason, age was selected as a covariate. The association between the SOT total score and the GUGT score was evaluated using Spearman rank-order correlation coefficients. The results showed a significant interaction between group and surface conditions, which indicated a decreased stance duration for fallers on a compliant surface compared with the stance duration for nonfallers (adjusted mean faller stance duration = 53 seconds, SD = 42; mean nonfaller stance duration = 67 seconds, SD = 32). Subjects in the fall group also had significantly higher GUGT scores (which indicated poor mobility function) than did subjects in the nonfall group (adjusted mean faller GUGT score = 2.65, SD = 1.48; mean nonfaller GUGT score = 1.47, SD = 0.77). The Spearman correlation between total SOT scores and the GUGT scores was greater for fallers (r = -.67) than for nonfallers (r = -.44). Orientation input from the ankle appears to have greater importance for preventing falls compared with a visual reference. The SOT and GUGT may be useful in the field to establish criteria for screening elders in a fall-prevention program.

The onset latency and discharge amplitude of preprogrammed postural responses were evaluated in order to determine if the structure of synergistic activation could be altered by ligamentous laxity at the knee joint. Twelve subjects with unilateral and one subject with bilateral anterior cruciate ligament (ACL) insufficiency were tested while standing on a moveable platform. External balance perturbations (6 cm anterior or posterior horizontal displacements of the platform) were presented at velocities ranging from 15 to 35 cm/s. Perturbations were presented under the following experimental conditions: unilateral and bilateral stance, knees fully straight or flexed, and with ankle motion restricted or free. These stance, knee position, and ankle motion conditions were introduced to alter the stress transmitted to the knee joint during movement of the support surface. The automatic postural response was recorded from the tibialis anterior (T), quadriceps (Q), and medial hamstrings muscles (H) bilaterally. The normal response to an externally induced backward sway involved the automatic activation of T and Q at latencies of 80 ms and 90 ms respectively. Activation of the hamstrings in the non-injured extremity was not coupled with the postural response. Hamstrings are not typically involved in the correction posterior sway because H activation would tend to pull the center of mass further backwards. However, when the response in the ACL-deficient extremity was compared to the non-injured limb: (1) the automatic postural response in the ACL-deficient extremity was restructured to include hamstrings activation (100 ms latency), (2) H activation time was faster and less variable in the ACL-deficient limb, and (3) the ratio of H/Q discharge amplitude integrated over 100 ms and 200 ms from the onset of EMG activation showed a dominance of hamstring activity during unilateral stance on the lax limb. In addition, H/Q ratios integrated over 200 ms showed dominant hamstring activity in the ACL-deficient limb during bilateral stance. (4) Cross-limb comparisons showed greater normalized IEMG amplitudes for T, H, and Q during unilateral stance on the lax limb. These results suggest that a capsular-hamstring reflex is integrated into the existing structure of a preprogrammed postural synergy in order to compensate for ligamentous laxity. Furthermore, the generalized increase of response gain observed during perturbations of unilateral stance on the lax limb indicates that joint afference can modulate central programming to control localized joint hypermobility. A concept of postural control is discussed with respect to the capsular reflex, joint loading and displacement of the center of gravity.

The locus of postural stability during dynamic, visually cued weight redistribution was studied in six hemiplegic and five able-bodied subjects. After a verbal warning with a variable foreperiod, a visual trace appeared on an oscilloscope to indicate a sway magnitude equaling half the subject's maximal voluntary sway. Body movement was cued in three directions--forward, and toward the paretic and nonparetic lower extremities. In order to assess extraneous postural patterns, all three directions of sway were analyzed for each trial. The results showed that with cued lateral sway (right or left) there was a tendency for hemiplegic subjects to reduce the dispersion (or variability) of the sway path. Clustering of the active lateral postural response near the geometric center of the sway path indicated a restricted balance response in this plane of motion. During cued forward sway the variability of the sway path was not statistically different from the able-bodied, but three-dimensional visualization of the sway pattern illustrated a qualitatively different postural response with extraneous, nonfocused balance activity biased toward the nonparetic lower extremity. Short-term practice did not alter the dispersion of the sway path for hemiplegic or able-bodied subjects in any direction. Hemiplegic extraneous movement in one case (forward sway) and restricted movement in another (lateral sway) emphasize the level of specificity which needs to be addressed during rehabilitation of balance.

The temporal and spatial characteristics of rapid voluntary body sway following unilateral cerebrovascular accident were assessed in single and combined planes of motion during a visually cued non-choice reaction time movement. Three distinct directions of intentional body sway were assessed: forward, toward the paretic, and toward the non-paretic lower extremity. Fluctuations in the center of foot pressure served as the basis for calculating body displacement and velocity by combining frontal and sagittal movement in an X-Y coordinate system (a bi-planar analysis). A linear regression analysis was used on individual force output channels to reduce the sway motion into separate component parts. The velocity profile in each direction of intended sway was calculated from the slope of the regression line (a single-plane analysis). Bi-planar velocity analysis showed that hemiplegics swayed as fast as normal subjects in any sway direction. In contrast, single plane analysis revealed a decrease in the velocity of hemiplegic sway when weight shifts were directed toward the paretic extremity. Response velocity in the non-paretic direction was similar to a comparison group of normal individuals. The apparent contradiction between single and bi-planar analyses was attributed to hemiplegic sway in extraneous planes of motion surrounding the intended sway path. During the course of body sway, hemiplegics showed a statistically larger range of extraneous movement away from the plane of intended movement. The directional deficits in sway velocity and excursion are compared to previously reported sway pathology associated with lesions of the human central nervous system.

Patterns of postural adjustment accompanying spontaneous single limb movement in the cat were studied to examine qualitative and quantitative variations in support strategies. A multiple plate dynamometer and electromyograph were used to measure supporting actions during consecutive trials. Weight transfer involving primarily the forelimbs was dominant. A similar tripodal stance with increased bilateral hindlimb weight-bearing characterized a secondary support strategy. Both dominant and secondary balance behavior showed transient reversals to a bipodal postural pattern. Bipodal stance exhibited increased weight-bearing in the stabilizing forelimb and in the diagonally opposite hindlimb. Selection of different postural strategies for identical phasic limb movements could possibly result from modulation of lower level reflex inhibition.

In order to study voluntary and reflexive mechanisms of postural control, young and elderly persons were given large-fast and small-slow ankle-rotation postural disturbances while standing on a movable platform capable of measuring ground reaction forces. Large-fast rotations were employed to activate long-loop reflexes, and small-slow rotations were employed to tap the higher level sensory integration aspects of postural control. Overall, the elderly persons exhibited more perturbation induced sway and showed a slowing in voluntary, as opposed to reflexive mechanisms of correcting postural disturbance. For both age groups, reflexive mechanisms were found to be relatively intact. When small perturbations were given, the elderly persons swayed more than young participants and produced sporadic reflexive activity. Moreover, elderly persons did not adapt to the small perturbations and exhibited increased postural sway to repetitive presentation of the perturbation, whereas young participants substantially decreased their postural sway. These data demonstrate that elderly persons are at some disadvantage when posture is under the control of slower, higher level sensory integrative mechanisms.

An EMG-force measurement system which can be placed inside a sound dampened testing chamber to study subtle postural responses in the standing quadruped is described. The dynamometer is adjustable to accommodate different foot placement strategies and to afford a more natural standing posture. A non-telemeterized method of recording EMG from an unrestrained animal is also discussed.

The purpose of this study was to analyze the effects of prior instruction on automatic postural responses and voluntary postural sways measured about the ankle joint. Ten subjects with right hemiplegia resulting from a cerebrovascular accident (mean age = 56 +/- 14 years) and 5 healthy subjects (mean age = 47 +/- 6 years) stood symmetrically on a movable force platform. During platform-induced sway, the support surface was translated horizontally to induce antero-posterior body sway about the ankle joints. Surface electromyographs were obtained from the tibialis anterior, quadriceps femoris, gastrocnemius, and hamstring muscles bilaterally. Prior knowledge appeared to have no significant influence on healthy subjects' ability to execute postural adjustments more rapidly during AP displacements. Hemiplegic subjects exhibited longer and more variable latencies in the paretic limb than in the nonparetic limb during voluntary AP weight shifts. When hemiplegic subjects had prior knowledge of the platform's movement (time and direction), latencies were significantly shorter in the paretic limb and could be as brief as those seen in the nonparetic limb. The results showed that prior knowledge may be an important treatment consideration for patients with muscle timing disorders.

The purpose of this study was to evaluate body-sway characteristics during anticipatory and reactive postural compensation. Mechanisms of postural control were studied during self-initiated rapid arm movements (RAMs) and unexpected support surface displacements (SSDs). Ten healthy subjects, 5 men and 5 women with a mean age of 28 years, were blindfolded and stood symmetrically on a moveable force platform or with weight shifted right (R) or left (L). Averaged electromyographs were obtained from L anterior deltoid, L ipsilateral biceps femoris (BFi), R contralateral biceps femoris (BFc), R quadriceps (Q), R medial gastrocnemius (G), and R tibialis anterior (TA) muscles. During SSD, the relative timing of distal-proximal synergists was altered during all biased postures. In L biased standing, functional pairs, TA-Q and G-BF, discharged at their shortest latencies. Anterior deltoid onset times during RAM were most preparatory during R weight shift. Sway latencies were shorter also in biased standing. Absolute sway latency, peak sway, and time-to-peak sway were significantly longer in SSD conditions in comparison with RAM. When disturbances in balance cannot be predicted, lower extremity preloading facilitates contralateral muscle-discharge patterns at shorter latencies. In some neurological conditions that result in incorrect body alignment, the mechanical effects of postural bias rather than the abnormal control mechanisms may at least contribute to alterations in the postural response. These results provide a basis for evaluating balance in patients with neurological deficit.