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The existence of in-phase and anti-phase postural responses to sinusoidal perturbations to the base of support is well known. In this study, we investigate if such coordinated postural responses exist at 'near-sway' perturbations where the perturbation amplitudes are kept within the range of normal sway lengths in healthy adults (n=10). The postural responses are analyzed via bursts of anterior-posterior (AP) 2.5 mm horizontal sinusoidal oscillations of the base of support at sequentially varying frequencies (0.25, 0.375, 0.5, 0.625, 0.75, 1 and 1.25 Hz). The parametric plots of the perturbation signal (platform position) and the response profiles (AP Center of Pressure [APCoP]) show the emergence of elliptical Lissajous patterns as the perturbation frequency is increased from 0.25 Hz to 1.25 Hz. The presence of such characteristic pattern shows the 'lock-in' behavior of APCoP with perturbation signal. These elliptical patterns become more apparent at the center frequencies (0.375 to 0.75 Hz). At the higher frequencies (1 and 1.25 Hz), the Lissajous patterns do exist but are dominated by low-frequency drift. The area and orientation of Lissajous patterns and the phase shifts between perturbation and APCoP show a strong nonlinear decreasing trend with increasing perturbation frequency for both, young (n=5) as well as mature (n=5) adults within the study group. This may suggest that such characteristic, frequency-locked, phased shifted response of healthy posture control could be a fundamental property of a healthy posture control's response to 'near-sway' sinusoidal translations in AP direction.

In a 2-Alternative Forced Choice Interval task (2AFCi), a standing subject is required to press a button once or twice to signal in which of two 4 s sequential intervals that (s)he thought that a short ≤ 16 mm postural perturbation had occurred. The perturbation might or might not result in transient changes of the subject's Anterior-Posterior Center of Pressure (APCOP) or in other measures. This paper used fuzzy inference to explore whether the correctness of a subject's stimulus detection can be gleaned from analyzing changes in one of more metrics related to changes in the APCOP. Also, distinguishing guesses from correct responses is a critical issue in any psychophysical detection paradigm. Biomechanical and psychophysical data are used to design a prediction model based on fuzzy inference that is able to discriminate correct responses from guesses.

To help separate out the contributions of the somatosensory and vestibular systems to postural and sway control, short (1, 4 and 16 mm) anterior translations of lengths less than the normal sway path length were made of a platform upon which blindfolded young adult test subjects (n=12) stood or sat. Acceleration detection thresholds from these short moves were compared in standing vs seated conditions using a 2-Alternative [Interval] Forced-Choice psychophysical test protocol. A negative power law trading relationship was found between peak acceleration threshold and move length and duration for standing subjects. For these same subjects while seated, acceleration thresholds for all lengths were nearly constant, and showed a weak positive power law trade between threshold and move length or duration. This latter observation is consistent with that of Benson et al '86, who also observed a positive power law trade relationship between acceleration threshold and move duration for seated subjects. Thresholds were higher at 1mm for standing vs. seated tests; while at 16 mm, standing tests had lower thresholds compared to those obtained for the seated tests. These results suggest that the vestibular system provides the principal input for detecting these short translations while seated, but not while standing.

Background: A fundamental unsolved problem in psychophysical detection experiments is in discriminating guesses from the correct responses. This paper proposes a coherent solution to this problem by presenting a novel classification method that compares biomechanical and psychological responses., Methods: Subjects (13) stood on a platform that was translated anteriorly 16 mm to find psychophysical detection thresholds through a Adaptive 2-Alternative-Forced-Choice (2AFC) task repeated over 30 separate sequential trials. Anterior-posterior center-of-pressure (APCoP) changes (i.e., the biomechanical response R(B)) were analyzed to determine whether sufficient biomechanical information was available to support a subject's psychophysical selection (R(Ψ)) of interval 1 or 2 as the stimulus interval. A time-series-bitmap approach was used to identify anomalies in interval 1 (a₁) and interval 2 (a₂) that were present in the resultant APCoP signal. If a₁ > a₂ then R(B) = Interval 1. If a₁ < a₂, then R(B)= Interval 2. If a₂-a₁ < 0.1, R(B) was set to 0 (no significant difference present in the anomaly scores of interval 1 and 2)., Results: By considering both biomechanical (R(B)) and psychophysical (R(Ψ)) responses, each trial run could be classified as a: 1) HIT (and True Negative), if R(B) and R(Ψ) both matched the stimulus interval (SI); 2) MISS, if R(B) matched SI but the subject's reported response did not; 3) PSUEDO HIT, if the subject signalled the correct SI, but R(B) was linked to the non-SI; 4) FALSE POSITIVE, if R(B) = R(Ψ), and both associated to non-SI; and 5) GUESS, if R(B) = 0, if insufficient APCoP differences existed to distinguish SI. Ensemble averaging the data for each of the above categories amplified the anomalous behavior of the APCoP response., Conclusions: The major contributions of this novel classification scheme were to define and verify by logistic models a 'GUESS' category in these psychophysical threshold detection experiments, and to add an additional descriptor, "PSEUDO HIT". This improved classification methodology potentially could be applied to psychophysical detection experiments of other sensory modalities.

Background: This study explored the effects of diabetes mellitus (DM) and peripheral neuropathy (PN) on the ability to detect near-threshold postural perturbations., Methods: 83 subjects participated; 32 with type II DM (25 with PN and 7 without PN), 19 with PN without DM, and 32 without DM or PN. Peak acceleration thresholds for detecting anterior platform translations of 1 mm, 4 mm, and 16 mm displacements were determined. A 2(DM) × 2(PN) factorial MANCOVA with weight as a covariate was calculated to compare acceleration detection thresholds among subjects who had DM or did not and who had PN or did not., Results: There was a main effect for DM but not for PN. Post hoc analysis revealed that subjects with DM required higher accelerations to detect a 1 mm and 4 mm displacement., Conclusion: Our findings suggest that PN may not be the only cause of impaired balance in people with DM. Clinicians should be aware that diabetes itself might negatively impact the postural control system.

A phase-locked loop (PLL) model of the response of the postural control system to periodic platform motion is proposed. The PLL model is based on the hypothesis that quiet standing (QS) postural sway can be characterized as a weak sinusoidal oscillation corrupted with noise. Because the signal to noise ratio is quite low, the characteristics of the QS oscillator are not measured directly from the QS sway, instead they are inferred from the response of the oscillator to periodic motion of the platform. When a sinusoidal stimulus is applied, the QS oscillator changes speed as needed until its frequency matches that of the platform, thus achieving phase lock in a manner consistent with a PLL control mechanism. The PLL model is highly effective in representing the frequency, amplitude, and phase shift of the sinusoidal component of the phase-locked response over a range of platform frequencies and amplitudes. Qualitative analysis of the PLL control mechanism indicates that there is a finite range of frequencies over which phase lock is possible, and that the size of this capture range decreases with decreasing platform amplitude. The PLL model was tested experimentally using nine healthy subjects and the results reveal good agreement with a mean phase shift error of 13.7 degrees and a mean amplitude error of 0.8 mm.

This paper presents an innovative technique to study postural control. Our translating platform, the Sliding Linear Investigative Platform For Analyzing Lower Limb Stability and Simultaneous Tracking, EMG and Pressure mapping (SLIP-FALLS-STEPm), makes precise, vibration movements under controlled conditions. We look at the psychophysical thresholds to the perception of a sinusoidally induced sway. In the Sine Lock experiments described, an induced sinusoidal perturbation locks the subject's natural sway pattern at the frequency of the perturbation. The input / output system is treated as an Amplitude Shift Key (ASK) modulated signal modulating a carrier frequency (at or about a subject's natural sway frequency). The Position signal (input) and the Anterior-Posterior Center of Pressure (APCOP) signal (output) or the ankle angle are demodulated by mixing them with the pure sine wave carrier at the frequency of underlying oscillation and then low-pass filtering it to detect the amplitude envelope. These detected envelopes elucidate that the square pulse increase in the position sine wave amplitude yields a triangular increase in APCOP demodulated signal.

One of the common causes of morbidity and mortality in workplaces is related to slips or falls. Reaction to external disturbances, such as slips or falls, requires a process of perturbation detection and control of motion changes. Postural control is a common mechanism to compensate unexpected displacements of the body. The ability of postural control diminishes with ageing or neuropathy. In this study, two controlled groups, diabetics and non-diabetics in the elderly population, were investigated to compare how different postural control mechanisms would relate to the detection of perturbation and regain of balance. The ultra-low-vibration Sliding Linear Investigative Platform for Analyzing Lower Limb Stability SYSTEM was used to measure the biomechanical changes of posture and perturbation detection. In phase 1 of the analysis, static measures during quiet standing were considered to investigate the relationship between postural stability and perturbation detection capability. In phase 2 of the analysis, dynamic measures during an occurrence of perturbation were analysed. Statistical tests and linear logistic regression models were applied to find differences of postural control mechanisms and to build a predictive model for perturbation detection quantitatively. It is anticipated that the results of this study will contribute to more comprehensive understanding of postural control mechanisms and design of slip/fall prevention programmes.

One source of falls in the elderly may be an inability to sufficiently adjust to transient postural perturbations or slips. Identifying useful predictors of fall potential, as well as factors that affect the ability of an individual to detect a movement of the standing support surface may provide insight into postural stability and methods to increase stability in elders. The effects of acceleration, displacement, neurological status, and age on movement detection reaction times were studied in 25 individuals--1 young adults, seven neurologically intact elderly adults, and six elders with (diabetic) peripheral neuropathy. Acceleration detection thresholds for anterior perturbations of 1, 4, and 16 mm of the support surface was previously determined for each subject via a two-alternative forced choice (2AFC) protocol, with longer (16 mm) moves yielding lower 2AFC thresholds (12-39 mm/s(2)) that varied by group. Using the acceleration threshold value determined, and 125% of that threshold (suprathreshold), reaction times to the start of the platform movement were determined for all three displacements. Reaction times to an additional superthreshold movement (4 mm at 100 mm/s(2)) were also measured. Lower acceleration values over longer moves required longer reaction times for motion detection. Reaction times were also influenced by peak energy imparted to the subject through the move. The higher prevalence of falls in the elderly and elderly diabetic may be due to slowing reaction times compounded by larger amounts of imparted energy needed for detection of a slipping event.

Balance control systems have usually been studied under two conditions, during quiet standing or under large postural perturbations of a magnitude that requires a postural adjustment to prevent falling. Between these two extremes lie perturbations that can be repeated and measured while not forcing adaptive strategies from the postural control system. Unlike other studies of postural control, we employed very short translations with varying accelerations at the edge of psychophysical detectability. These perturbations were vibration-free anterior or posterior translations of the platform on which a subject stood. Using a full Latin-square design set of perturbations in the forward or backward direction, with a smooth or jerk acceleration profile, and of length 4 or 20 mm, were presented to five subjects. Perceptual peak acceleration thresholds were determined by an iterative psychophysical method that forced the subjects to choose in which of two sequential intervals that they perceived a stimulus to have been presented. The only factor found that significantly correlated with detection was perturbation length. The 4 mm peak thresholds averaged 14.51 mm/s2 while 20 mm thresholds averaged 8.55 mm/s2. For the short perturbations employed in this study, detection of motion thus was dependent upon the magnitude of the acceleration, but it was independent of the acceleration profile (jerk versus smooth) or movement direction. By understanding the influences on the ability to perceptually detect motion underfoot, we can begin to understand what elements of the postural control system might be involved in the second-to-second control of balance.