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PubMed1992

A power equation for the sprint in speed skating.

An analysis of the start of the 500 m speed skating races during the 1988 Olympic Winter Games showed a remarkably high correlation between the acceleration of the skater in the first second of the sprint and the final time (r = -0.75). In this study a power equation is used to explain this high coefficient of correlation. The performance in speed skating is determined by the capability of external power production by the speed skater. This power is necessary to overcome the air and ice friction and to increase the kinetic energy of the skater. Numerical values of the power dissipated to air and ice friction, both dependent on speed, are obtained from ice friction and wind tunnel experiments. Using aerobic and anaerobic power production as measured during supra maximal bicycle tests of international-level speed skaters, a model of the kinetics of power production is obtained. Simulation of power production and power dissipation yields values of speed and acceleration and, finally, the performance time of the sprint during speed skating. The mean split time at 100 m and the final time at 500 m in these races, derived from simulation, were 10.57 s (+/- 0.31) and 37.82 s (+/- 0.96), respectively. The coefficient of correlation between the simulated 500 m times and the actual 500 m times was 0.90. From the results of this study it can be concluded that the distribution of the available anaerobic energy is an important factor in the short lasting events. For the same amount of anaerobic energy the better sprinters appear to be able to liberate considerably more energy at the onset of the race than skaters of lower performance level.

Biomechanical Phenomena↗
PubMed1993

Effects of porous coating, with and without collar support, on early relative motion for a cementless hip prosthesis.

In theory, porous or rough coatings could be used to reduce early post-operative relative motion about cementless hip prostheses. To investigate this theory, we used detailed, non-linear finite element analysis to compare early relative motion about a well-fit Anatomical Medullary Locking (AML) prosthesis for different amounts of porous coating (full, proximal 2/3, and no coating), both with and without collar support. Details of the model included quantitative computed tomography-derived (QCT-derived) geometric and material properties for the bone, and a no-tension interface condition at all bone-prosthesis interfaces, with Coulomb friction (mu = 1.73) over coated surfaces and zero friction elsewhere. Predicted values of relative motion for this well-fit device were in the range of approximately 1-550 microns. The distribution of relative motion was relatively insensitive to the amount of porous coating but was sensitive to collar support, while the magnitude of relative motion was sensitive to the porous coating and collar support. In addition, a reduction in the porous coating caused larger increases in relative motion when there was no collar support, indicating an interaction between the effects of porous coating and collar support. For example, distal twist increased (full vs 2/3 partial coating) by 38% with collar support and by 58% without collar support. These data suggest that porous coating, or other surface treatments which result in a high coefficient of friction at the bone-prosthesis interface, may well be used to control the magnitude of early relative motion, particularly when there is no collar support.

Biomechanical Phenomena↗
PubMed1993

Ultrasound scattering properties of Albunex microspheres.

Albunex is an ultrasound contrast agent used in echocardiography and in other areas, it consists of microspheres of which more than 95% have a diameter in the range 1-10 microns. The scattering properties of this agent as functions of ultrasound frequency and microsphere diameter and concentration are investigated. A model of the Albunex microspheres has been previously described, considering the individual microspheres as air bubbles surrounded by a thin elastic shell. In the present study this model is extended by including into it the internal friction in the shell when the microsphere vibrates. Acoustic scattering and transmission are measured in the frequency range from 700 kHz to 12.5 MHz. The measured transmitted power is used to estimate the two parameters in the theoretical model: the shell elasticity parameter, Sp and the shell friction, Sf. Introduction of the shell friction into the model improves the agreement between theory and measurements. For the scattered power, differences between measured and calculated values lie within 3 dB. It is concluded that for the frequencies 2.5 and 5 MHz, microspheres with a diameter between 5 and 12 microns are preferred as these deliver the most significant contribution to the total scattered power and cause relatively little attenuation.

Albumins↗
PubMed1991

Soft layered prostheses for arthritic hip joints: a study of materials degradation.

A great deal of interest is being shown in complaint bearing surfaces for artificial joints. These produce very low friction because of the fluid-film lubrication that they exhibit, and therefore should produce lower wear than current prosthetic materials as the two surfaces of the joint are completely separated by a film of synovial fluid. However, one problem with soft elastic materials in vivo is that the elastomers may degrade with time. Specimens of four polyurethanes were kept in Ringer's solution at 37 degrees C for about 4 months to investigate changes in mechanical properties and the mode of lubrication with time of immersion. The materials tested were a polyether-urethane (E57), a polyester-urethane (E58), an aromatic polyether-urethane (P1) and an aliphatic polyether-urethane (A1). Samples of E57 showed a decrease in hardness and elastic modulus of 21% and 28% respectively. This was much better than E58 which showed decreases of 32% and 42% respectively. Better results were achieved with A1 (hardness change 2%) and P1 (decrease in hardness of 14% and modulus 17%). When tested in a hip-function simulator, A1 showed virtually no change in its very low coefficient of friction (0.004), P1 showed an increase of 60%, while E57 and and E58 both showed a doubling in friction over the course of the study. The modes of in vitro degradation were considered and surface effects determined to be most damaging. Currently the best candidate for a compliant material in hip replacement is the aliphatic polyether-urethane which maintains fluid-film lubrication.

Compliance↗
PubMed1993

Three dimensional shape reconstruction and finite element analysis of femur before and after the cementless type of total hip replacement.

Computerized tomography was used to reconstruct a shape, and stresses in three-dimensional objects were analysed. The human femur, which has a very irregular shape, was chosen as an object. CT image data of a cadaver femur were transferred to a computer, and an edge extraction program generated the cross-section of bone by specifying a range of CT values for each slice. Pixel data from the CT scan are converted into a vector of points (x, y, z) which can specify the boundaries of bone. Lateral surfaces are defined by stacking up the slices and making use of the vectorized data. Intermediate and oblique cross-sections can be obtained by an interpolation technique. The constructed model was used as input data for the finite element analysis. To understand the stress distributions before and after the cementless type of total hip replacement, a three-dimensional finite element stress analysis of the bone-implant system was carried out, assuming micromotions between the stem and the femur. The analysis was done for both frictionless and friction cases, modelling the contact point with a gap element having isotropic friction. The analysis shows that the stress is not concentrated on the femoral calcar when the friction coefficient is large.

Bone Cements↗
PubMed1993

The size, shape and stability of complement component C9.

Electron microscopy of specimens of C9 tilted through 90 degrees visualized this protein to be a globular ellipsoid with dimensions of 77 x 70 x 52 A. To check the congruence of this observation with physical properties of the molecule, hydrodynamic parameters for C9 were determined. From this work a frictional ratio of 1.32 was calculated. C9 was compared with several other proteins of similar frictional ratios whose tertiary structures are known. All examples found of such proteins whose frictional ratios were between 1.26 and 1.37 are either heart-shaped or globular ellipsoids, but none are prolate ellipsoids. By comparison the size and shape of C9 determined by electron microscopy are congruent with its hydrodynamic parameters. Both electron microscopy and physical measurements suggest that the length (110-120 A) of C9 determined by neutron and X-ray scattering experiments is an overestimate. The source of the discrepancy was identified by the demonstration that the high concns of C9 employed in neutron and X-ray scattering work lead to aggregation of the protein. Thus, investigations involving neutron and X-ray scattering were measuring polydisperse solutions of C9. The deduced value of the radius of gyration from that work (33-35 A) is now recognized as being statistical and significantly higher than the correct value of monomeric C9 (26 A), which was calculated from electron microscopy measurements. Also high-resolution electron microscopy clearly visualized poly(C9) to be a barrel-stave construct. These results suggest that monomeric C9 must undergo a major conformational alteration to extend by 55-70 A in order to self-associate laterally in order to fashion the cylindrical poly(C9).

Complement C9↗
PubMed1994

Coupling between external viscosity and the intramolecular dynamics of ribonuclease T1: a two-phase model for the quenching of protein fluorescence.

Our recent equilibrium dialysis studies showed that proteins are able to interact preferentially with acrylamide (Punyiczki et al. (1993) Biophys. Chem. 47, 9-19). The presence of considerable amounts of acrylamide--albeit weakly bound--in the protein volume, coupled with the failure of a simple gating model of quenching to rationalise viscosity dependence of the quenching of tryptophan (Trp) fluorescence in Ribonuclease T1 (RNase T1) has prompted us to explore a new model, the two-phase model for quenching. According to this model, the dynamic quenching is accomplished by quencher molecules already in the protein phase at the moment of excitation. Some of the molecules may, at this moment, form an encounter complex with the fluorophore and thus be responsible for the observed static contribution. We use the rate equation derived from our model to study the viscosity dependence of acrylamide quenching of Trp fluorescence in RNase T1. The model allows us to separate co-solvent effects: the chemical effect on the protein and on the distribution of quencher molecules between the bulk and the protein phases and, further, the viscosity effect due to coupling between the bulk viscosity and the local friction affecting intramolecular fluctuations of the protein matrix. We express local friction in terms of bulk viscosity, eta, and a coupling constant kappa (friction = eta kappa). Addition of glycerol up to 65% is characterised by a kappa of 0.50. The viscosity dependence of the apparent bimolecular quenching constant is a combination of two compensating effects: changes in chemical activity and changes in patterns of structural fluctuations.

Acrylamide↗
PubMed1992

Towards a new approach for estimating indirect costs of disease.

Many researchers in the field of evaluation of health care doubt the usefulness of estimates of indirect costs of disease in setting priorities in health care. This paper attempts to meet part of the criticism on the concept of indirect costs, which are defined as the value of production lost to society due to disease. Thus far in cost of illness studies and cost-effectiveness analyses the potential indirect costs of disease were calculated. In the following a first step will be taken towards a new method for estimating indirect costs which are expected to be effectuated in reality: the friction cost method. This method explicitly takes into account short and long run processes in the economy which reduce the production losses substantially as compared with the potential losses. According to this method production losses will be confined to the period needed to replace a sick worker: the so called friction period. The length of this period and the resulting indirect costs depend on the situation on the labour market. Some preliminary results are presented for the indirect costs of the incidence of cardiovascular disease in the Netherlands for 1988, both for the friction costs and the potential costs. The proposed methodology for estimating indirect costs is promising, but needs further development. The consequences of illness in people without a paid job need to be incorporated in the analysis. Also the relation between internal labour reserves and costs of disease should be further investigated. Next to this, more refined labour market assumptions, allowing for diverging situations on different segments of the labour market are necessary.

Absenteeism↗
PubMed2005

Relationship between hamstring activation rate and heel contact velocity: factors influencing age-related slip-induced falls.

The purpose of this research was to determine whether a decreased hamstring activation rate among the elderly is responsible for a higher horizontal heel contact velocity and increased likelihood of slip-induced falls compared to their younger counterparts. Twenty-eight subjects from two age groups (14 young and 14 old) walked across a linear walking track with embedded force platforms while wearing a fall arresting harness attached to an overhead arresting rig for safety. In order to obtain realistic unexpected slip-induced fall data, a soapy vinyl floor surface was hidden from the subjects and unexpectedly introduced. Synchronized kinematics, kinetic and electromyography (EMG) analyses during the heel contact phase of the gait cycle while walking over slippery and non-slippery floor surfaces were examined in the study. Normalized EMG data were examined in terms of hamstring activation rate and evaluated with heel contact velocity and friction demand characteristic (as measured by peak required coefficient of friction (RCOF)) on the dry vinyl floor surface. Furthermore, slip parameters (i.e. slip distances and slipping velocity) were assessed on the soapy vinyl floor surface. The results indicated that younger adults' hamstring activation rate was higher than older adults, whereas younger adults' heel contact velocity was not different from older adults. These results suggested that heel contact velocity in younger adults was sufficiently reduced before the heel contact phase of the gait cycle. This could be due to the outcome of higher hamstring activation rate in younger adults in comparison to older adults. However, lower friction demand (peak RCOF), shorter slip distances, slower peak sliding heel velocity and more falls among older adults suggested that the slip initiation characteristics were not the only factors contributing to slip-induced falls among the elderly.

Accidental Falls↗
PubMed2005

Pressure-derived measurement of coronary flow reserve.

OBJECTIVES: We aimed to validate the technique of measuring the coronary flow reserve (CFR) with coronary pressure measurements against an established thermodilution technique. BACKGROUND: The CFR has traditionally required measurement of coronary blood flow velocity with the Doppler wire and, more recently, using a thermodilution technique with the coronary pressure wire. However, recent work has suggested that the CFR may be derived from pressure measurements alone (the ratio of the square root of the pressure drop across an epicardial stenosis during hyperemia to that value at rest). This depends on the assumption that friction losses across a coronary stenosis are negligible. METHODS: We compared pressure-derived CFR values with those obtained by the thermodilution technique using the intracoronary pressure wire in 38 stenoses in 34 patients with significant coronary stenoses undergoing percutaneous intervention. We also compared these two techniques of measuring CFR in 25 stenoses (6 vessels) artificially created by inflating small balloons within a stented coronary artery after percutaneous intervention. RESULTS: There is a close linear relationship between pressure-derived and thermodilution CFR in native (r(2) = 0.52; p < 0.001) and artificial stenoses (r(2) = 0.54; p < 0.05), although the pressure-derived technique appears to systematically underestimate CFR values in both situations. This applies to native and artificial stenoses. CONCLUSIONS: Coronary flow reserve cannot be measured merely with pressure alone, and it cannot be safely assumed that friction losses are negligible across a native coronary stenosis. These data suggest that friction loss is an important determinant of the pressure gradient along an atherosclerotic coronary artery.

Aged↗
PubMed2006

Minimizing errors during in vitro testing of multisegmental spine specimens: considerations for component selection and kinematic measurement.

Apparatus-induced artifacts may invalidate standard spine testing protocols. Kinematic measurements may be compromised by the configuration of motion capture equipment. This study has determined: (1) the influence of machine design (component friction) on in vitro spinal kinetics; (2) the sensitivity of kinematic measurements to variations in the placement of motion capture markers. A spinal loading simulator has been developed to dynamically apply pure bending moments (three axes) with or without a simultaneous compressive preload. Two linear slider types with different friction coefficients, one with caged ball bearings and one with high-precision roller bearings on rails, were mounted and specimen response compared in sequential tests. Three different optoelectronic marker cluster configurations were mounted on the specimen and motion data was captured simultaneously from all clusters during testing. A polymer tube with a uniform bending stiffness approximately equivalent to a polysegmental lumbar spine specimen was selected to allow reproducible behavior over multiple tests. The selection of sliders for linear degrees of freedom had a marked influence on parasitic shear forces. Higher shear forces were recorded with the caged-bearing design than with the high-precision rollers and consequently a higher moment was required to achieve a given rotation. Kinematic accuracy varied with each marker configuration, but in general higher accuracy was achieved with larger marker spacings and situations where markers moved predominantly parallel to the camera's imaging plane. Relatively common alternatives in the mechanical components used in an apparatus for in vitro spine testing can have a significant influence on the measured kinematic and kinetics. Low-magnitude parasitic shear forces due to friction in sliders induces a linearly increasing moment along the length of the specimen, precluding the ideal of pure moment application. This effect is compounded in polysegmental specimens. Kinematic measurements are highly sensitive to marker design and placement, despite equivalent absolute precision of individual marker measurements, however marker configurations can be designed to minimize errors related to spatial distribution and system bias.

Artifacts↗
PubMed2006

Studies on electrochemical characterization and performance prediction of cellulose acetate and Zeocarb-225 composite membranes in aqueous NaCl solutions.

We have mixed cellulose acetate and Zeocarb-225 in different ratios, leading to the preparations of Membrane-1 and Membrane-2. Membrane potential, water content, and conductance measurements have been carried out to estimate and analyze the data in terms of equilibria and important electrochemical parameters. The Donnan equilibrium has been incorporated to estimate the activity coefficient of counterions, y(p)M, and solute, y(+/-)M in the membrane phase along with the parameter, so called varphi expressing non-ideality. Dependence of the extent of hydrophilicity of both membranes on mean electrolyte concentrations has been examined. Selectivity in membranes is discussed in terms of dissociation equilibria, K(d)s and K(d)f. It has been found that membrane surface charge density sigma(s) increases with increasing of external NaCl concentration. Dependence of water transport number and cationic transport number on electrolyte concentration shows a similar trend of variation. At higher mean concentration of electrolyte, water transport number in Membrane-2 has a negative value. Membrane-2 has a higher value of water transport number than Membrane-1. The entropy production due to solute and water transport has been quantified for both the membranes in the light of nonequilibrium thermodynamics. The various type of interactions such as solute-membrane, solute-water, and water-membrane are analyzed in terms of friction coefficients (f(ij)) of Spiegler's frictional pore model. In our case, an f(wm) < f(sm) < f(sw)-like trend is observed in both membranes. These frictional coefficients show close dependence on external electrolyte concentrations. Pore potential values of Membrane-1 and Membrane-2 have been worked out using the Poisson-Boltzmann equation. In both systems pore potential values increase with increasing mean electrolyte concentrations. The transport through Membrane-1 and Membrane-2 tends to follow diffusion-control criteria, i.e., (D(+/-) . C. d/D(+/-)M C(M) . delta) >> 2. A slightly higher value of solute rejection is found in Membrane-2.

Cellulose↗
PubMed2006

AFM interaction study of alpha-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions.

Ionic strength dependence of interaction and friction forces between hydrophilic alpha-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force-distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10(-3) M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the alpha-alumina surfaces at pH 11.

Journal Article↗
PubMed2006

The influence of wire positioning upon the initial stability of scaphoid fractures fixed using Kirschner wires A finite element study.

A finite element model of the carpal scaphoid and its joints was developed to study how wire positioning affects the initial stability of the fixation of scaphoid waist fractures using Kirschner wires. A transverse fracture of the scaphoid waist was simulated along with its fixation using five different two-wire configurations. The resulting models were subjected to a load simulating a 200N force passing through the wrist. Friction between bony fragments was taken into account; as the friction coefficient of cancellous bone is unknown, three different values were analysed. For each of these friction coefficient values, the smallest transverse interfragmentary displacements, and consequently maximum initial stability, were obtained for the model that simulated the maximum gap between wires in the plane of fracture. Results also show that for a similar gap in the plane of fracture, more stable fixation can be achieved when wires cross each other not only in the frontal plane of the hand, but also perpendicularly to it.

Bone Wires↗
PubMed2004

Degradation of postural control system as a consequence of Parkinson's disease and ageing.

In this study we have shown how Parkinson's disease and ageing affect the postural control system. Our investigation of that system has involved analysis of quiet-standing center of pressure (COP) trajectories. Using the method derived from Langevin equation, we have found disease-specific and age-specific changes in the dynamics of the COP. These findings were obtained from a comparison of the diffusion matrix, the friction coefficient and the matrix of the fluctuation strength for healthy young, healthy elderly and parkinsonian (elderly) subjects under eyes-open and eyes-closed conditions. Especially the analysis of the friction coefficient and the matrix of the fluctuation strength have allowed the more detailed study of the postural control system of the parkinsonian and elderly subjects. We have found that the healthy elderly as well as parkinsonian subjects exhibit larger level of the muscular stochastic activity, compared to the young subjects. Furthermore, closure their eyes causes further increment of that activity. We have also shown that for the healthy elderly subjects a compensating mechanism, which can be described by the friction coefficient, acts under eyes-closed conditions, whereas for parkinsonian subjects this mechanism was not observed. Thus, the increase of the muscular activity is an age-specific indicator, whereas the absence of the compensation of that increase is a disease-specific indicator.

Adaptation, Physiological↗
PubMed2006

Superimposed ultrasonic oscillations in compression tests of aluminium.

The application of ultrasonics in metal forming applications has been shown to reduce the forming load significantly in many research studies. The load reduction has been related to the stress superposition effect, rise in temperature and change in the friction condition between the specimen and die interfaces. This paper reports an investigation into the effects of superimposed ultrasonic oscillation of the die in compression tests on aluminium specimens. In particular, a finite element model is developed to simulate uniaxial compression and to model the effects of a change in the friction boundary condition when ultrasonic excitation is applied to the lower platen. The model predictions of the stress-strain relationship can be compared with test data to provide some insights into the effects of the interfacial condition. The paper shows how the analysis of the test data, combined with finite element models of ultrasonic compression test simulations, allows some initial conclusions to be drawn regarding the influence of the interfacial friction during ultrasonic compression.

Journal Article↗
PubMed2005

Energy demand in sludge dewatering.

This work investigates the energy required to dewater a suspension, i.e., activated sludge dewatered by centrifugation or consolidation. Total energy input to the suspension from the dewatering device, bond strength between adjacent water and solid surface, and intra-cake friction loss were evaluated for original and flocculated sludges. In centrifugal dewatering, most energy input during the initial stage was consumed by overcoming process irreversibility other than intra-cake friction, and, thereby, had a low energy efficiency. To increase centrifuge speed or to flocculate the sludge at optimal flocculant dosage would yield a high-energy input. In the consolidation test, most energy input at the initial stage was consumed in breaking down the bond strength until the moisture content reduced to less than the critical content. During subsequent dewatering stages, friction loss became the dominant source of energy loss. Dewatering sludge with high-energy efficiency is beneficial to optimally operate a dewatering process.

Centrifugation↗
PubMed2002

Size-distribution analysis of proteins by analytical ultracentrifugation: strategies and application to model systems.

Strategies for the deconvolution of diffusion in the determination of size-distributions from sedimentation velocity experiments were examined and developed. On the basis of four different model systems, we studied the differential apparent sedimentation coefficient distributions by the time-derivative method, g(s*), and by least-squares direct boundary modeling, ls-g*(s), the integral sedimentation coefficient distribution by the van Holde-Weischet method, G(s), and the previously introduced differential distribution of Lamm equation solutions, c(s). It is shown that the least-squares approach ls-g*(s) can be extrapolated to infinite time by considering area divisions analogous to boundary divisions in the van Holde-Weischet method, thus allowing the transformation of interference optical data into an integral sedimentation coefficient distribution G(s). However, despite the model-free approach of G(s), for the systems considered, the direct boundary modeling with a distribution of Lamm equation solutions c(s) exhibited the highest resolution and sensitivity. The c(s) approach requires an estimate for the size-dependent diffusion coefficients D(s), which is usually incorporated in the form of a weight-average frictional ratio of all species, or in the form of prior knowledge of the molar mass of the main species. We studied the influence of the weight-average frictional ratio on the quality of the fit, and found that it is well-determined by the data. As a direct boundary model, the calculated c(s) distribution can be combined with a nonlinear regression to optimize distribution parameters, such as the exact meniscus position, and the weight-average frictional ratio. Although c(s) is computationally the most complex, it has the potential for the highest resolution and sensitivity of the methods described.

Animals↗