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The purpose of this study was to validate the percentage of body fat (%BF) values estimated from the BOD POD (BP) with those obtained from hydrostatic weighing (HW) in athletic American high school boys. Additionally, the %BF values measured via near-infrared interactance (NIR), bioelectrical impedance (BIA), and skinfold (SF) were compared to HW to determine the validity of these measures. Thirty white boys (mean age [+ or -] SD = 15.8 [+ or -] 1.0 years) who where currently participating in organized sports volunteered to have their %BF estimated. Measurements were obtained from NIR, BP, BIA, and SF in random order and concluded with HW. The findings from the present study indicated that the NIR and BIA instruments produced significant (P < 0.008) constant error (CE) and total error (TE) values that were too large to be of practical value (TE > 4.0%BF). The BP produced a significantly (P < 0.008) higher CE with acceptable TE values compared to HW, but compared to all three SF estimations, the BP TE values were higher. Two of the SF equations were nonsignificant (P > 0.008) and had the lowest TE values compared to HW. These data suggest that the BP can produce acceptable body fat measures for athletic white boys but is not superior to estimates made by the SF equations used in this study. KEY WORDS air d splacement plethysmography, near-infrared interactance youth, biolectrical impedance, skinfold

The validity of a one- and a two-cylinder model, underlying thoracic impedance cardiography (TIC), was investigated by studying the length dependence of the impedance parameters [Z.sub.0], [(dZ/dt).sub.min], and stroke volume (SV). It can be shown that, within a one-cylinder model, all parameters are directly proportional to the length, whereas, if the volume conduction of the thorax and the neck are modeled separately, [Z.sub.0] and [(dZ/dt).sub.min] are expected to be linear dependent and SV will be nonlinear dependent upon the length. The expectations were compared to results from in vivo measurements. Two electrode arrays were studied, in which the caudal recording electrode position was varied; SV was calculated using Kubicek's equation. Except for small distances, the results showed a nearly linear relation between the parameters and the length. Regression analysis of the linear part revealed statistically significant intercepts (p < 0.05). Neither the intercept nor the nonlinear part can be explained by a one-cylinder model, whereas a model consisting of two cylinders serially connected describes the experimental results accurately. Thus SV estimation based on a one-cylinder model is biased due to the invalid one-cylinder model. Corrections for the Kubicek-equation need to be developed in future research using this two-cylinder model. Index Terms - Cylinder, electrical impedance cardiography, modeling, stroke volume.

We measured transthoracic impedance in nine presumed healthy adult subjects with a two-frequency plethysmograph at 57 kHz and 185 kHz. The measurement protocol included periods of normal breathing without motion and periods of motion without breathing. We analyzed the cross-correlation and the ratio between the signals at both frequencies for all the different maneuvers. The correlation coefficient was between 0.97 and 1 for breathing, the minimal cross-correlation (0.81) was for simulated obstructive apnea. We found that the amplitude ratio between the two-frequency signals was different for normal breathing and for motion. Based on these results, we designed and tested an adaptive filter to increase the signal-to-artifact ratio (SAR). The increase in SAR (mean [+ or -] standard deviation) compared with the signal at 57 kHz was: 183% [+ or -] 117% for arm movement, 133% [+ or -] 93% for leg movement, and 34% [+ or -] 62% for simulated obstructive apnea.

Impedance cardiography (ICG) may be altered by noises as respiration and movement artifacts, mainly during exercise. In this work, a scaled Fourier linear combiner (SFLC) event-related to the R-R interval of ECG is proposed. It estimates the deterministic component of the impedance cardiographic signal and removes the noises uncorrelated to this interval. The impedance cardiographic signal is modeled as Fourier series with the coefficients estimated by the least mean square (LMS) algorithm. Simulations have been carried out to evaluate the filter performance for different noise conditions. Moreover, the method capability to remove uncorrelated noises was also examined in physiological data obtained in rest and exercise, by synchronizing respiration and pedaling with a metronome. Analyzing the ICG power spectrum, it was concluded that the proposed filter could remove the noises that are not synchronized with heart rate.

Classic impedance microbiology (CIM) is based on the measurement of the impedance components that appear between a pair of electrodes submerged in a cell containing inoculated broth. Either a bipolar or a tetrapolar technique can be applied, requiring about 1 x [10.sup.3] to 3 x [10.sup.7] cells/ml to produce detectable changes in the impedance curves. Theoretical analysis of the electrode-electrolyte interface during bacterial growth is lacking, with no generally accepted measuring standards. Besides, there is considerable disagreement. We separated out the interface and medium components using the frequency variation technique (FVT) and also analyzed the interface reactance-resistance diagram, both before and after bacterial growth. Medium resistance [R.sub.m] interface reactance [X.sub.i], and interface resistance [R.sub.i], were quantified as time functions growth curves, from the complex bipolar impedance seen between two electrodes. We took into account the electrical current density, the temperature and the associated circuitry, also explaining the theoretical and experimental bases that justify the proposed dissecting procedure. It was found that, within the working frequency range, [R.sub.m], [R.sub.i], and [X.sub.i] percental growth curves are frequency-independent, i.e., neither [R.sub.m](f), nor [X.sub.i](f) nor [R.sub.i](f) changed their slopes before, during and after bacterial growth. Besides, no [Alpha]-dispersion effect in [R.sub.m] curves was detected. It is concluded that impedance microbiology could become a fertile area for interdisciplinary knowledge; its development might offer new avenues for basic and applied research. Index Terms--Frequency, impedance, interface, medium, microorganism.