Idiosyncratic Characteristics of Postural Sway in Normal and Perturbed Standing

Abstract

ObjectiveAre people with a characteristically large physiological sway rendered particularly unstable when standing on a moving surface? Is postural sway in standing individuals idiosyncratic? In this study, we examine postural sway in individuals standing normally, and when subtle continuous sinusoidal disturbances are applied to their support platform. We calculate consistency between conditions to verify if sway can be considered characteristic of each individual. We also correlate two different aspects of participants’ responses to disturbance; their sway velocity and their regulation of body orientation.MethodsNineteen healthy adults (age 29.2 ± 3.2 years) stood freely on footplates coaxially aligned with their ankles and attached to a motorized platform. They had their eyes closed, and hips and knees locked with a light wooden board attached to their body. Participants either stood quietly on a fixed platform or on a slowly tilting platform (0.1 Hz sinusoid; 0.2 and 0.4 deg). Postural sway size was separated into two entities: (1) the spontaneous sway velocity component (natural random relatively rapid postural adjustments, RMS body angular velocity) and (2) the evoked tilt gain component (much slower 0.1 Hz synchronous tilt induced by the movement of the platform, measured as peak-to-peak (p-p) gain, ratio of body angle to applied footplate rotation).ResultsThere was no correlation between the velocity of an individual’s sway and their evoked tilt gain (r = 0.34, p = 0.15 and r = 0.30, p = 0.22). However, when considered separately, each of the two measurements showed fair to good absolute agreement within conditions. Spontaneous sway velocity consistently increased as participants were subjected to increasing disturbance. Participants who swayed more (or less) did so across all conditions [ICC(3,k) = 0.95]. Evoked tilt gain also showed consistency between conditions [ICC(3,k) = 0.79], but decreased from least to most disturbed conditions.ConclusionThe two measurements remain consistent between conditions. Consistency between conditions of two very distinct unrelated measurements reflects the idiosyncratic nature of postural sway. However, sway velocity and tilt gain are not related, which supports the idea that the short-term regulation of stability and the longer-term regulation of orientation are controlled by different processes.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

“…The data have been collected here and analysed on the basis of one single trial per subject per condition (i.e., the first trial of a series of eight trials), administered in order to investigate the effect of the sensory conditions in the adaptation to repeated stance performances. Given the inter-individual variability in the stance performance (9,176), particularly when standing on foam, this procedure is certainly a limitation however hardly avoidable because repetition of stance trials produces significant adaptation in the balancing pattern (8,127,(177)(178)(179)(180). Alsubaie et al (181) have recently shown that different measures of postural sway are reliable when recorded at two visits 1 week apart, including measures with unstable BoS and sensory conditions.…”

“…Body sway when standing upright on a solid base of support is normally almost negligible in healthy subjects, witnessing accurate and precise neural control ( 6 , 7 ) based on the internal model of gravitational and inertial forces ( 8 ) and on multiple inputs from the receptors detecting the body state. The excursions of the centre of foot pressure (CoP) of subjects standing quietly on the firm ground are approximately contained within the size of a dime, even if there is a large variability in sway across different healthy subjects ( 9 ). In several conditions, though, sway area can significantly increase, such as standing on sloped surfaces or when leaning forward or backward ( 4 , 10 ), or decrease when subjects stand on elevated platforms ( 11 ).…”

Validation, verification, and quality control of computational fluid dynamics analysis for indoor environments using a computer‐simulated person with respiratory tract

“…Based on this information, maximal angular velocity ( ω max ) in the 3.0–5.0°/s range was assumed in this study, to investigate the effect of the general sway intensity (assuming young and healthy adults) on the human microclimate. As the fluctuation of COP, COM and COP minus COM in measured data corresponds to the postural sway frequency, we set 1.0 Hz for the AP direction and 0.5 Hz for the ML direction, based on the previously reported values 52–55 . The angular velocity and sway frequency of the human body were simplified assuming a sine function, as shown in Figure 13A.…”

“…As the fluctuation of COP, COM and COP minus COM in measured data corresponds to the postural sway frequency, we set 1.0 Hz for the AP direction and 0.5 Hz for the ML direction, based on the previously reported values. [52][53][54][55] The angular velocity and sway frequency of the human body were simplified assuming a sine function, as shown in Figure 13A. As a result, the head displacements indicated in Figure 12 for ω max = 3.0, 4.0, and 5.0 °/s were approximately 14.5, 19.3, and 24.1 mm, as summarized in Figure 13B; these values are typical for young and healthy adults.…”

Discrete cortical control during quiet stance revealed by desynchronization and rebound of beta oscillations

“…The body during quiet stance is inclined slightly forward so that the mean vertical projection of the center of mass (CoM) is on average a few centimeters forward of the ankle joint [1]. This causes a tendency of the body to forward fall, due to gravitational toppling torque.…”