Displacement Standard Deviation Research Articles

The assessment of body sway and the choice of the stability parameter(s)

This methodological study aims at comparison of the practical usefulness of several parameters of body sway derived from recordings of the center of pressure (CoP) with the aid of a static force platform as proposed in the literature. These included: mean displacement velocity, maximal range of movement along x- and y-co-ordinates, movement area, planar deviation, phase plane parameter of Riley and the parameters of the diffusion stabilogram according to Collins. They were compared in over 850 experiments in a group of young healthy subjects (n = 10, age 21-45 years), a group of elderly healthy (n = 38, age 61-78 years) and two groups of elderly subjects (n = 10 and n = 21, age 65-89 years) with stability problems under different conditions known to interfere with stability as compared to standing with open eyes fixing a visual anchoring point: closing the eyes, standing on plastic foam in stead of a firm surface and performing a cognitive task: the modified stroop test. A force platform (Kistler) was used and co-ordinates of the body's center of pressure were recorded during 60 s of quiet barefoot standing with a sampling frequency of 10 Hz. In general, the results show important overlapping among groups and test conditions. Mean displacement velocity shows the most consistent differences between test situations, health conditions and age ranges, but is not affected by an extra cognitive task in healthy old people. Mean maximal sideways sway range is different among groups and test conditions except for the cognitive task in young and elderly subjects. Standardised displacement parameters such as standard deviations of displacements and planar deviation discriminate less well than the actual range of motion or the velocity. The critical time interval derived from the diffusion stabilogram according to Collins et al. seems to add a specific type of information since it shows significant influence from addition of a cognitive task in old subjects standing on a firm surface but not when standing on plastic foam. The critical time interval shows no consistent relation to any other parameter. The influence of cognitive activity on balance merits further exploration. A new parameter, sum of maximal deviation time (SDT) was proposed showing complete discrimination between frail elderly and other old subjects when obtained while visual input was suppressed. It was concluded that mean displacement velocity seems to be the most informative parameter in most situations.

On-Off Decentralized Control of Flexible Structures

A new near minimum fuel method for on-off decentralized control of flexible structures is introduced. Fuel minimization is achieved by turning on actuators when local velocities are in the neighborhood of a maximum and when local displacements are in the neighborhood of a minimum. Maximum velocity and minimum displacement neighborhoods at each actuator location are determined by recursively computing standard deviations of displacements and velocities over running intervals of time. The fuel consumed by on-off control is shown to be 20 percent lower than by linear optimal control for the same reductions in energy over the same periods of time. On-off control of a uniform beam illustrates the method.

Observations on phase-locking within the response of primary muscle spindle afferents to pseudo-random stretch.

In order to uncover encoder properties of primary muscle spindle afferent fibers, time coupling (phase-locking) of action potentials on cyclic muscle stretch was studied by means of pseudo-random noise. In cats Ia action potentials were recorded from dorsal root filaments and the gastrocnemius muscles of one hind leg were stretched. The stimulus time course was a determined sequence of randomly varying muscle length which could be applied repeatedly (sequence duration 0.6 or 20 s). The noise amplitude sigma (standard deviation of displacements) was varied between 5 and 300 micron, the upper cut-off frequency of noise fc was varied between 20 and 100 Hz. The responses to the consecutive pseudo-random noise cycles were displayed as raster diagrams and cycle histograms. Phase-locking characterized the responses at all noise amplitudes outside the near threshold range (sigma greater than 10 micron). The higher sigma and fc, the stronger was the phase-locking of impulses on the stretch. When sigma and fc were selected to achieve high mean stretch velocities of about 500 mm/s, phase-locking was as precise as 0.15 ms, measured as the variability of spike occurrences with respect to stretch. The rasters obtained with low noise amplitudes (less than 40 micron) showed a loose phase-locking and this gave insight into underlying mechanisms: The elicitation of action potentials caused by dynamic stretch can be prevented by a post-spike depression of excitability. This disfacilitation was very effective in counteracting weak stretch components within the random sequence and less effective or even missing when relatively strong stretch components could force the spike elicitation.(ABSTRACT TRUNCATED AT 250 WORDS)

A Quantitative Study of Vertebral Displacement and Angulation in the Normal Cervical Spine Under Axial Load

Cervical traction of one-third the person's weight was applied for 3 minutes to normal subjects. The mean and standard deviation of displacement and rotation between adjacent cervical vertebra was calculated in the sagittal plane. The average separation between adjacent vertebra was 0.7 mm with a standard deviation of 0.5 mm. The average angular motion between adjacent vertebra was 0.9 degrees with a standard deviation of 2.4 degrees.