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J Biomech Eng. 1979;101(1):1. doi:10.1115/1.3426218.
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Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J Biomech Eng. 1979;101(1):2-14. doi:10.1115/1.3426221.
Abstract
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):15-22. doi:10.1115/1.3426217.

A simplified constitutive equation is proposed to characterize the nonlinear viscoelastic behavior of the canine anterior cruciate ligament. The theory contains a single relaxation function which was measured experimentally by a series of uniaxial relaxation tests. To test the applicability of the theory, the experimentally determined function was used to attempt to predict the results of a special log-ramp test, as well as a constant strain-rate test. It was found that the simplified equation predicted stress results which were somewhat higher than experiment throughout much of the strain range tested.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):23-27. doi:10.1115/1.3426219.

The mechanical properly of soft tissues is highly nonlinear. Normally, the stress tensor is a nonlinear function of the strain tensor. Correspondingly, the strain energy function is not a quadratic function of the strain. The problem resolved in the present paper is to invert the stress-strain relationship so that the strain tensor can be expressed as a nonlinear function of the stress tensor. Correspondingly, the strain energy function is inverted into the complementary energy function which is a function of stresses. It is shown that these inversions can be done quite simply if the strain energy function is an analytic function of a polynomial of the strain components of the second degree. We have shown previously that experimental results on the skin, the blood vessels, the mesentery, and the lung tissue can be best described by strain energy functions of this type. Therefore, the inversion presented here is applicable to these tissues. On the other hand, a popular strain energy function, a polynomial of third degree or higher, cannot be so inverted.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):28-37. doi:10.1115/1.3426220.

This paper describes an investigation of the frequency-response characteristics of active human skeletal muscle in vivo over the frequency range 1 Hz to 15 Hz. The applied force, forearm position, and surface electromyograms (from biceps, triceps, and brachioradialis) were recorded simultaneously in four normal adult male subjects for small oscillations of the forearm about a mean position of 90 deg flexion. Two modes of oscillatory behavior are discussed: externally forced oscillations under constant muscle force and voluntary oscillations against an elastic resistance. The observed amplitude and phase relations are presented herein and are compared to the response predicted by a simple model for neuromuscular dynamics. It appears that the small amplitude frequency response of normal skeletal muscle in vivo can be represented by a second order model. The main muscle parameters of this model are a muscular stiffness K, two time constants τ1 and τ2 associated with contraction dynamics, and a time delay τ: typical values of these parameters at moderate contraction levels (approximately 20 percent of maximum voluntary effort) are K = 100 N · m/rad, τ1 and τ2 = 50 ms, and τ = 10 ms. Reflex feedback under forced-oscillation conditions was also examined and may be characterized by a gain parameter (ΔE/Δθ), the ratio of the surface EMG amplitude to the angular displacement of the forearm, and the phase by which the EMG leads muscle stretch. The reflex EMG is observed to lead muscle stretch at all frequencies between 1 Hz and 15 Hz. The muscle stiffness K and the reflex gain parameter (ΔE/Δθ) are approximately proportional to the average force of contraction.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):38-45. doi:10.1115/1.3426222.

A constitutive equation for the lung tissue elasticity is formulated under the hypotheses of a simplified alveolar geometry and a pseudo-strain-energy function for the interalveolar septa. The resulting equation contains four material constants. The theoretical result has been tested against published data on uniaxial and triaxial loadings, and is tested critically here with respect to new experimental results on biaxial loading. Comparison between theory and experiments shows that a general agreement is obtained in an approximate sense. The model fits our biaxial experimental data with most correlation coefficients above 0.995. Some details not predicted by the theory are discussed. Since the theory is derived for triaxial loading and the biaxial test is a severe one, the formula should be applicable to the triaxial case at least to the same degree of approximation. The form of the theoretical formula is convenient to use in analytic studies of lung mechanics. Additional key words: mechanical behavior of the lung; stress-strain relationship; strain energy; alveolus model; distortion; interdependence; pressure volume curves.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):46-52. doi:10.1115/1.3426223.

In this first part of a three-part report, the mechanical behavior of 42 fresh human cadaver lumbar motion segments in flexion, extension, lateral bending, and torsion is examined. Motions and intradiskal pressure changes that occurred in response to these loads, with posterior elements both intact and excised, are reported.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):53-57. doi:10.1115/1.3426225.

In this second part of a three-part report, we consider the mechanical behavior of 42 fresh human cadaver lumbar motion segments in compression, and in anterior, posterior, and lateral shear. We report the motions and the intradiskal pressure increases that occurred with posterior elements both intact and destroyed. We also examine to what extent intervertebral disk gross morphology can explain the large scatter in the results presented here, as well as in the results reported in Part I concerning mechanical behavior in flexion, extension, lateral bending, and torsion.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):58-65. doi:10.1115/1.3426226.

The Gibbs’ heated thermocouple technique for measuring a tissue’s thermal conductivity to infer the blood perfusion rate has been synthesized with a model of transient heal transport in perfused tissue. This procedure eliminates the necessity of probe calibration in surrogate tissue. The analytical predictions of the transient temperature behavior of a newly designed spherical probe were compared with experimental temperature transients to deduce thermal conductivities of and perfusion rates in gelatin and dog kidney. The principle heating modality was step heating. Consistent conductivity values for both the gelatin and the kidney were found. The calculated perfusion rates in the kidney were consistent but were higly dependent upon probe size and geometry, and the independently measured tissue’s thermal conductivity.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):66-72. doi:10.1115/1.3426227.

The superficial temperature distribution for various types of dispersive electrodes applied to human subjects and a surrogate medium are presented. Typical temperature distributions on the human thigh display a high temperature perimeter and cooler central area, with the temperature extremes and contours peculiar to the electrode design. These patterns persist for several minutes after electrode removal. A series of experiments was conducted on a surrogate medium to determine the extent of volumetric (ohmic) heating, to evaluate influence of media properties on the temperature distribution, and to evaluate the use of the medium for simulation of the human system. It was found that volumetric heating is appreciable and that appropriate alteration of the medium resistivity with depth produced patterns having similar characteristics to those obtained with the same electrode on a human thigh. A simplified model to analytically predict the temperature distribution is presented and the results are similar to those observed on human subjects.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):73-81. doi:10.1115/1.3426228.
Abstract
Topics: Atherosclerosis
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1979;101(1):82-86. doi:10.1115/1.3426229.

Analytical expressions for transient temperature distributions within a two-zone heating probe and the surrounding infinite, isotropic, homogeneous, perfused medium are obtained. The periphery of the probe simulates the stagnant blood layer which usually forms around a thermistor probe if it is inserted into the tissue. A procedure to estimate thermal properties of tissues in vivo using these results is also outlined.

Commentary by Dr. Valentin Fuster

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