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RESEARCH PAPERS

J Biomech Eng. 1983;105(1):1-5. doi:10.1115/1.3138378.

The effects of impact timing during the cardiac cycle on the sensitivity of the heart to impact-induced rupture was investigated in an open-chest animal model. Direct mechanical impacts were applied to two adjacent sites on the exposed left ventricular surface at the end of systole or diastole. Impacts at 5 m/s and a contact stroke of 5 cm at the end of systole resulted in no cardiac rupture in seven animals, whereas similar impacts at the end of diastole resulted in six cardiac ruptures. Direct impact at 15 m/s and a contact stroke of 2 cm at the end of either systole or diastole resulted in perforationlike cardiac rupture in all attempts. At low-impact velocity the heart was observed in high-speed movie to bounce away from the impact interface during a systolic impact, but deform around the impactor during a diastolic impact. The heart generally remained motionless during the downward impact stroke at high-impact velocity in either a systolic or diastolic impact. The lower ventricular pressure, reduced muscle stiffness, thinner myocardial wall and larger mass of the filled ventricle probably contributed to a greater sensitivity of the heart to rupture in diastole at low-impact velocity. However, the same factors had no role at high-impact velocity.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):6-11. doi:10.1115/1.3138387.

A new versatile apparatus for investigating the neuromusculoskeletal dynamics of the upper limb, termed the Programmable Limb Testing System (PLTS) has been designed, built, and tested. The planning, construction, and operation of this system are described in this paper. Results are presented from a limited set of experiments, measuring intrinsic muscular and reflex-mediated stiffnesses of the triceps brachii, which exhibit the system’s performance and some of its capabilities.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):12-19. doi:10.1115/1.3138377.

Filling of a thin-walled, highly compliant tube in a partially collapsed condition is studied. The theory, based on one-dimensional flow, takes account of friction, longitudinal tension, and the highly nonlinear pressure-area law for the tube. Various aspects of filling behavior are revealed by alternative calculations using: (i) the method of characteristics; (ii) numerical integration of the continuity, momentum, and tube-law equations; and (iii) a crude but simple lumped-element capacitance-inertance-resistance model. Varied phenomena appear. At high Reynolds number, these include dispersive wave trains associated with circumferential bending stiffness and longitudinal tension, nonlinear changes of wave form, development of highly asymmetrical wave reflections, and sloshing. At low Reynolds number, the area changes with time in a diffusivelike manner. The experiments exhibited the dispersive phenomena predicted by the theory.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):20-23. doi:10.1115/1.3138379.

The soft tissue attenuation of acoustic emission signals was measured by transmitting pulses through volunteers and measuring the decay of the waveform characteristics of the pulse as a function of the thickness of the interposed tissue. Waveform characteristics of the received signal (signal duration, number of counts, peak amplitude, energy, and rise time) demonstrated an exponential decrease with increasing tissue thickness. The decrease appeared insensitive to the frequency of the pulse within the range of 50 to 600 KHz.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):24-30. doi:10.1115/1.3138380.

Heat and mass transfer mechanisms have been characterized in physical models of human cadaver airways to simulated depths of 305 m with various gas mixtures. Such characterizations offer a detailed understanding of the effects of environmental pressures, gas composition, and respiratory rates (RMV) on the body cooling capacity of the respiratory airways. Empirical heat transfer relationships in the form

N̄u = AReNPr1/3
are derived for the oral and nasal passageways during inhalation and exhalation flows. N̄u, Re, and Pr are the dimensionless Nusselt, Reynolds, and Prandtl numbers, respectively. The Nusselt and Reynolds numbers are based on the diameter and gas flow rate in the trachea and are applicable to Reynolds number values up to 70,000.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):31-38. doi:10.1115/1.3138381.

Experimental and analytical methods are presented which enable one to examine the local rheological properties of biological tissues which can be captured as flat sheets between matching pressure manifolds and deformed under experimentally prescribed hydrostatic loading conditions. In spite of the fact that most biological tissues, including arteries, are nonlinearly elastic when considered over wide ranges of strain, it was found that the deformation of swine and canine arterial wall specimens in the physiological range of wall strain can be approximated by an isotropic, linearily elastic membrane model. In view of this, the elastic behavior was characterized approximately by an incremental modulus over the range of 0.45 to 0.65 strain. The incremental modulus in both species was shown to increase by a factor of three along the descending thoracic aorta from the ductus scar to the celiac orifice.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):39-46. doi:10.1115/1.3138382.

Static analysis of the left ventricle is developed to estimate the local stresses and deformations that occur during the heart cycle. The left ventricle is represented as a thick hollow tube composed of solid fibers embedded in an inviscid fluid matrix. A finite deformation analysis is developed to estimate the variation of the pressure, fiber tension and fiber extension across the thickness of the left ventricle. Pressure-volume relations are obtained for the diastolic and the systolic peak isovolumetric phases. The fiber stress distribution and pressure variation are estimated as a function of the initial fiber orientation distribution, relative thickness of the ventricle, inner volume of the ventricle and the various tension-extension relations proposed for the fibers of the heart muscle. It is concluded that the diastolic pressure-volume relation is not very sensitive to either the fiber orientation distribution or the thickness of the ventricle. However, the pumping efficiency of the modeled ventricle is shown to increase with increasing thickness of the modeled left ventricle and with increasing contractility of the heart muscle fibers.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):47-50. doi:10.1115/1.3138383.

An analysis is made of the motion of a contact lens normal to the eye through a tear film. The eye is modeled as a rigid sphere and the lens as a deformable spherical cap. Graphs of pressure distribution and tear film thickness variations are obtained. It is found that the pressures are very markedly reduced and film thicknesses increased by decreasing the modulus of the lens materials.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):51-54. doi:10.1115/1.3138384.

A viscoelastic model of the shear response of the Rhesus monkey intervertebral disk, represented by a series chain of four Kelvin units is presented. Two types of investigations are carried out to assess the validity of the model: 1) determination of material properties by comparison with experimental strain creep behavior of the disk; and 2) validation of this viscoelastic model by accurately predicting the experimental results of stress relaxation tests. The use of the series Kelvin units approach provides the first analytical mechanical model capable of predicting the creep and relaxation functions for the intervertebral disk in shear.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):55-62. doi:10.1115/1.3138385.

When the motion associated with an anatomical joint is to be measured, a kinematic model for the joint must first be established. The joint model will have from one to six degrees of freedom, and both the measurement technique and the means used to describe the motion will be influenced by the model and its degrees of freedom. This paper discusses the modeling and measurement of anatomical joint motion from a kinematics viewpoint. A review of the literature pertaining to measurement techniques, kinematic assumptions, and motion descriptions for anatomical joint motion is presented. One, two, three and six degree-of-freedom models for various anatomical joints have appeared in the literature, and the applicability of these models is compared and discussed.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):63-70. doi:10.1115/1.3138386.

A second-generation mechanical head-neck model was constructed, instrumented and subjected to pendulum impact tests against both the head and torso and directed from the front, rear and side. The response history of the system was measured by thirty channels of instrumentation including disk pressure transducers and muscle displacement gages in the neck, and a central accelerometer, intracranial pressure transducers and skull strain gages for the cranium and its contents. The kinematics of the unit was observed by an intermediate speed framing camera and the input was determined by a calibrated force transducer located at the contact point. It was found that peak head linear acceleration and velocity occur either during or immediately after the impact, with corresponding peak rotational values manifested somewhat later, but well before maximum head displacement. Head accelerations were similar, albeit slightly lower than in corresponding cases for an earlier model and displacement values were also similar until large extensions were reached. For rear head or frontal base impact, the head experienced a significant period of translation without rotation immediately after loading, and the system appears to respond more violently to side than to corresponding front or rear impact. The muscle beahvior, which support the findings from the head kinematics, is analyzed in detail and shows its strong influence on limiting head excursions, with strain values up to 40 percent. Disk pressure histories were similar to those found in tests on an earlier model with the highest values between T2 and C4, while the intercranial pressure exhibited more realistic values, about an order of larger magnitude.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):71-76. doi:10.1115/1.3138388.

The nonlinear mechanical behavior of fetal dura mater was tested experimentally and compared to two published nonlinear material strain energy functions, the Mooney-Rivlin and the Skalak, Tozeren, Zarda, and Chien (STZC). The STZC constitutive relations best fit the behavior of the dura mater and were used to describe quantitatively its stiffness. Runge-Kutta numerical procedures were used to fit the theoretical data to the experimental results. The material’s stiffness was positively correlated with fetal weight (r = 0.67, p <0.05). These results are discussed and directions for future research indicated.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):77-83. doi:10.1115/1.3138389.

A method to determine local endothelial erosion stress is presented. Freshly excised segments of middle descending thoracic aortas of dogs were slit open longitudinally and stretched in a specially designed rack to various circumferential and longitudinal dimensions. Jets of physiological saline were impinged normally on the endothelium of these segments. Lesions caused by the jet were made visible by staining the tissue with Evans blue dye. The dye solution was absorbed by regions where the endothelial layer had been damaged or destroyed. Characteristically, the lesions were annular in shape. This indicates that the endothelium can withstand large normal stresses where the jet impinges, but is eroded by the shear stress resulting from the jet efflux. Erosion stress of the endothelium was determined by correlating the external radius of a lesion with the shear stress expected at that radial distance from the center of the jet. Results from 185 lesions created in 17 aortic segments indicate that: 1) the in-vitro endothelial erosion stress as measured by a 30-s duration of exposure to shear stress is 7761 ± 155 (SEM) dynes/cm2 at room temperature (23° C) and 2645 ± 155 (SEM) dynes/cm2 at body temperature (37°C); 2) the erosion stress decreases markedly with an increase in the duration of exposure to shear stress; 3) the results for long-duration exposure are consistent with those of the well-known in-vivo study of Fry on endothelial erosion stress. The importance of the method is its ability to measure local endothelial erosion stress which is of particular significance in the study of the discrete process of atherogenesis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1983;105(1):84-91. doi:10.1115/1.3138390.

Bond graphs are used to model the acoustic behavior of the respiratory system. The model includes the distributed dynamics of the upper airways while the lower passage generations are represented by “lumping” of resistance and compliance effects. The lower airway representation is terminated with ten lung segments. The model is accurate for frequencies as high as 8500 Hz. The model is currently capable of predicting system eigenvalues as a function of system parameters and geometry for a “nonbreathing” lung. Future plans include modifying the model to include lung segment expansion and contraction as well as turbulence generaton at airway bifurcations.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

Commentary by Dr. Valentin Fuster

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