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D A Murray

Publications and source records attributed to D A Murray.

5 recordsLinked to original sources
PubMed1986

Optimal filtering of constant velocity torque data.

The purpose of this investigation was to implement an optimal filtering strategy for processing in vivo dynamometric data. The validity of employing commonly accepted analog smoothing methods was also appraised. An inert gravitational model was used to assess the filtering requirements of two Cybex II constant velocity dynamometers at 10 pre-set speeds with three selected loads. Speed settings were recorded as percentages of the servomechanism's maximum tachometer feedback voltage (10 to 100% Vfb max). Spectral analyses of unsmoothed torque and associated angular displacement curves, followed by optimized low-pass digital filtering, revealed the presence of two superimposed contaminating influences: a damped oscillation, representing successive sudden braking and releasing of the servomechanism control system; a relatively stationary oscillatory series, which was attributed to the Cybex motor. The optimal cutoff frequency for any data set was principally a positive function of % Vfb max. This association was represented for each machine by a different, but reliable, third order least-squares polynomial, which could be used to accurately predict the correct smoothing required for any speed setting. Unacceptable errors may be induced, especially when measuring peak torques, if data are inappropriately filtered. Over-smoothing disguises inertial artefacts. The use of Cybex recorder damping settings should be discouraged. Optimal filtering is a minimal requirement of valid data processing.

Biomechanical Phenomena↗
PubMed1986

Constant velocity dynamometer: an appraisal using mechanical loading.

Measurement characteristics of two constant velocity dynamometers (Cybex II, Lumex, Inc., NY) were evaluated at 11 selected speeds (0 to 5 rad X s-1) under various conditions of inert gravitational loading. A linear accelerometer was affixed to the lever arm. Speed settings were standardized percentages of each machine's maximum tachometer feedback voltage (0 to 100% Vfb max). Regression of Cybex transducer-observed voltages (V0) on true, i.e., expected, torques (Me) revealed that, for any static loading condition or angle, the measurement system was essentially linear and valid, although greater variance was evident for smaller torques. However, different calibrations, i.e., regression lines, were required for different speed settings. The error in predicting Me from V0 increased substantially with increased % Vfb max. Methodologies which assume constant velocity dynamic loading induce questionable, often invalid, results and inferences. Dynamic V0 represents both gravitational and inertial components. Considerable fluctuations in lever arm instantaneous velocity were ascribed to the combined influence of load and % Vfb max upon the servomotor. After computing inertia-corrected Me, calibration variability and prediction error were markedly reduced. Unexplained variability may be due to poor proximity of the Cybex transducer to the force application point. We concluded that: Cybex II should be calibrated only statically; use of an optimal filter will avoid false estimates of joint torque; inertial corrections must be applied in order to minimize potentially serious errors due to system angular acceleration; and reported muscle function relationships might be usefully re-assessed in view of these findings.

Biomechanical Phenomena↗
PubMedDate not supplied

Determination of trace volatile organic compounds in fish tissues by gas chromatography.

Several volatile organic compounds associated with petroleum fuels (mainly alkylated benzenes) were extracted from spiked fish tissue samples with a stream of air, trapped on charcoal, eluted with a solvent, and analyzed by gas chromatography. These volatile compounds are among the most water-soluble components of crude oils and petroleum products, and they have been associated with tainting in fish tissues. Recoveries for these compounds were about 90% when spiked directly either onto traps or into fish tissues although naphthalene desorbed poorly from the charcoal; recoveries of this compound were about 50%. Relative standard deviations (RSD) for most recoveries of spiked samples were in the 2-10% range based on 6 samples analyzed in duplicate. However, when live fish were contaminated experimentally by adding the aromatic compounds to the aquarium water, the RSDs were higher (10-30%).

Animals↗