John Dickerson

Spectral characteristics of aging postural control

Abstract The purpose of this study was to apply techniques of spectral analysis to the study of postural control and determine if aging results in changes to spectral parameters obtained from postural forces. Subjects were young adults and elderly who live independently and had no difficulty performing tasks of daily living. Ground reaction forces were obtained from a measurement plat-form during repeated trials of static standing. Forces were analyzed using spectral analysis and selected parameters (central tendency and dispersion) were extracted from averaged spectral data. Significant differences were observed between age groups in all parameters extracted from spectra obtained from medial-lateral postural forces. Similar results were not obtained for data in the anterior-posterior direction. Results suggest that aging affects the spectral characteristics of postural forces used to maintain stability in the m-l direction.

Helical flow couplets in submarine gravity underflows

Active and relic meandering channels are common on the seafloor adjacent to continental margins. These channels and their associated submarine fan deposits are products of the density-driven gravity flows known as turbidity currents. The tie between channel curvature and its effects on these gravity flows has been an enigma. This paper records the results of both large-scale laboratory measurements and a numerical simulation that captures the three-dimensional flow field of a gravity underflow at a channel bend. These findings reveal that channel curvature drives two helical flow cells, one stacked upon the other. The lower cell forms near the channel bed surface and has a circulation pattern similar to that observed in fluvial channels, i.e., with a near-bed flow directed inward. The other circulation cell forms in the upper part of the gravity flow and has a streamwise vorticity with the opposite sense of the lower cell.

Spectral characteristics of ageing postural control

Abstract The purpose of this study was to apply techniques of spectral analysis to the study of postural control and determine if ageing results in changes to spectral parameters obtained from postural forces. Subjects were young adults and elderly who live independently and had no difficulty performing tasks of daily living. Ground reaction forces were obtained from a measurement platform during repeated trials of static standing. Forces were analysed using spectral analysis and selected parameters (central tendency and dispersion) were extracted from averaged spectral data. Significant differences were observed between age groups in all parameters extracted from spectra obtained from medial-lateral postural forces. Similar results were not obtained for data in the anterior-posterior direction. Results suggest that ageing affects the spectral characteristics of postural forces used to maintain stability in the m-I direction.

Performance of able-bodied subjects on a text-typing task using a head-operated device and expanded membrane cursor keys.

Children and adults with disabilities frequently rely on computers to complete written tasks. Those with significant motor limitations typically use alternative computer-input devices since the traditional keyboard and mouse are insufficient to accommodate their abilities. For persons unable to isolate their fingers, input devices controlled by movements of the head or whole hand or arm movements may be among the options considered. This study investigated the performance of a head-operated device and expanded membrane cursor keys for text entry. Data from 24 young adults indicated the head-operated device performed significantly faster given reduced cognitive demands for device operation, increased stimulus–response compatibility, and simplicity of movement. Use of the expanded membrane cursor keys resulted in significantly lower error rates. No significant differences in comfort or ease of use were reported for the two devices. The relative performance of device options for users sharing similar motor challenges provides rehabilitation specialists with important clinical information.

Microprocessor‐based data‐acquisition system

This article describes the development of an automated microprocessor‐based data‐acquisition system and, in particular, the use of an inexpensive microcomputer to automate an acoustic experiment. Specifically, it explains the use of a Motorola MC6801 in the single‐chip mode to control a rather complex experimental process. The system is configured to provide for automated, two‐dimensional data acquisition of ultrasonic transmissibility through a specimen surface. Results to evaluate the accuracy and the performance of the system with known analytical predictions will be presented.

Stability of nonautonomous second-order partial differential equations

Existence, uniqueness, and three sufficient stability criteria will be established for a class of differential equations of the form: d2u(t)dt2 + C(t)du(t)dt + K(t) u(t) = 0. The stability criteria emphasize the situations where the nonautonomous portion of the linear operators C(t) and K(t): (a) have small magnitudes, (b) have small derivatives (slowly varying), and (c) have small integrals (rapidly varying). Although this paper directs attention only to the second-order equation, corresponding stability results for the first-order equation could be obtained utilizing similar arguments.