Search

This study compared lower extremity, trunk, and upper extremity kinematics between tee and front toss hitting in youth baseball athletes. Twenty youth baseball athletes (14.3±2.9 yrs) performed three maximal effort swings off front toss and tee. Kinematic data were collected during the preparatory and acceleration phases. Lower extremity, trunk, and upper extremity kinematics were compared between tee and front toss hitting using 1-dimensional statistical parametric mapping (SPM). There was a significant difference in trunk kinematics between tee and front toss during the preparatory phase (p=.001); the trunk rotated more toward the back side when hitting off a tee compared to front toss (p<0.001). There was also a significant difference in trunk kinematics between tee and front toss for 67% of the acceleration phase; the trunk rotated more towards the back side from 0 to 67% when hitting off the tee (p<0.001). Significant differences were found in trunk kinematics between tee and front toss hitting in youth baseball players, where the trunk is less rotated toward the pitcher in the tee than in the front toss. Coaches utilize various training modalities to enhance hitting performance; however, differences in trunk kinematics should be considered between modalities when developing fundamental hitting techiques in youth baseball athletes. [ABSTRACT FROM AUTHOR]

Thermodynamics is a fundamental topic of continuum mechanics and biomechanics, with a wide range of applications to physiological and biological processes. This study addresses two fundamental limitations of current thermodynamic treatments. First, thermodynamics tables distributed online by the U.S. National Institute of Standards and Technology (NIST) report properties of fluids as a function of absolute temperature T and absolute pressure P. These properties include mass density ρ, specific internal energy u, enthalpy h=u+P/ρ, and entropy s. However, formulations of jump conditions across phase boundaries derived from Newton's second law of motion and the first law of thermodynamics employ the gauge pressure p=P-Pr, where Pr is an arbitrarily selected referential absolute pressure. Interchanging p with P is not innocuous as it alters tabulated NIST values for u while keeping h and s unchanged. Using p for functions of state and governing equations solves the problem with using NIST entries for the specific internal energy u in standard thermodynamics tables and analyses of phase transformation in continuum mechanics. Second, constitutive models for the free energy of fluids, such as water and air, are not typically provided in standard thermodynamics treatments. This study proposes a set of constitutive models and validates them against suitably modified NIST data., (Copyright © 2024 by ASME.)

The Cascadia subduction zone in the Pacific Northwest of the United States of America capable of producing magnitude ∼9 earthquakes, likely often accompanied by tsunamis. An outstanding question in this region is the degree and spatial extent of interseismic strain accumulation on the subduction megathrust. Seafloor geodetic methods combining GNSS and underwater acoustic ranging (GNSS-A) are capable of imaging this strain accumulation on the offshore portion of the subduction zone and therefore anticipating the potential size and rupture pattern of a future earthquake. However, the high cost of seafloor geodesy means that only a limited number of stations may be deployed and monitored. To facilitate expansion of current geodetic networks offshore, we develop a quantitative recommendation of optimal locations for future seafloor geodetic observations, based on the amount of new information provided by that observation. The optimal network depends on the problem that one is trying to solve with those observations (mapping subduction locking rates, coupling rates, constraining total moment rate, etc.), and on a number of modelling and data uncertainty assumptions. In particular, data uncertainty assumptions will change over time, as more position observations reduce velocity uncertainties. We find that near-trench observations on the megathrust hangingwall, distributed along-strike, consistently provide significant reduction in differential entropy over a large suite of assumptions, and that a well-placed seafloor observation can provide up to ∼30 times the information gain of the most optimal onshore observation. [ABSTRACT FROM AUTHOR]

The proportions of the limb bones of tetrapods contribute to the energetic costs of locomotion. Consequently, they can be used to classify locomotor type and can inform hypotheses about changes in community structure. We examined mechanical advantage in the limbs of North American Oligocene and Miocene ungulates (Artiodactyla and Perissodactyla), with the expectation that the distribution of limb morphologies would change with the spread of grass-dominated plant communities; that is, we expect to find morphological optimization for efficient high-speed locomotion with more open habitats. We assess changes in the occupation of locomotor ecomorphospace (LEMS) through time using measurements of the lengths of stylopods and zeugopods of North American ungulate species. Analysis of LEMS reveals significant changes in dominant ungulate locomotor morphology between Oligocene and Miocene faunas. Oligocene ungulates show less variation in LEMS than Miocene ungulates. Artiodactyls show a significant selection towards cursorial morphologies (p < 0.01) in the Miocene. A majority of artiodactyls are non-cursorial in the Oligocene, but only one family occupies the non-cursorial morphospace in the Miocene. Perissodactyls maintain similar levels of disparity in limb proportions in the Miocene as in the Oligocene (p = 0.72), but more perissodactyls occupy the cursorial morphospace in the Miocene than the Oligocene. Our results suggest that the shift from closed to relatively open habitat structure from the Oligocene to the Miocene is associated with a significant change in the proximal limb locomotor anatomy and associated limb mechanics of North American ungulate species. [ABSTRACT FROM AUTHOR]

In order to improve the survival ability of armored vehicles in the war, the protecting technology has been attached importance and developed completely by the world military powers. The paper presented the research state and protecting level of modern armor, and enumerated protecting furnishment of advanced armored vehicles, and summarized the research state and level of key protecting technology-armor material. After introducing situation of armor protecting technology, taking the research content of U. S. and U. K., the development trend to the modern armor technology-synthesis protection system were analyzed. [ABSTRACT FROM AUTHOR]

To help instructors in evaluating innovations in biomechanics instruction, a standardised test of the key concepts taught in the introductory biomechanics course was developed. The Biomechanics Concept Inventory (BCI) consists of 24 questions that test four prerequisite competencies and eight biomechanics competencies. Three hundred and sixty seven students from ten universities throughout the United States took the test at the beginning and the end of the introductory biomechanics course. Analysis of a sub-sample of the students showed that the BCI was reliable with typical errors in internal consistency and test-retest conditions of 1.4 and 2.0 questions, respectively. Mean pre-test scores (8.5 ± 2.0) significantly (p < 0.0001) improved to 10.5 ± 3.2 in the post- test (n = 305). Typical biomechanics students could correctly answer half of the prerequisite questions on the pre-test. Instruction resulted in a mean normalised gain (g) of 13.0% of maximum possible improvement that was consistent with research on traditional instruction in introductory physics courses. It was concluded that the BCI could be an effective tool to evaluate the overall effect of pedagogical strategies on student learning of key biomechanical concepts in the introductory biomechanics course. [ABSTRACT FROM AUTHOR]

Objectives: The objective of this study was to compare the physiological determinants of ejection fraction (EF)-ventricular size, contractile function, and ventricular-arterial (VA) interaction-and their associations with clinical outcomes in chronic heart failure (HF)., Background: EF is a potent predictor of HF outcomes, but represents a complex summary measure that integrates several components including left ventricular size, contractile function, and VA coupling. The relative importance of each of these parameters in determining prognosis is unknown., Methods: In 466 participants with chronic systolic HF, we derived quantitative echocardiographic measures of EF: cardiac size (end-diastolic volume [EDV]); contractile function (the end-systolic pressure volume relationship slope [Eessb] and intercept [V0]); and VA coupling (arterial elastance [Ea]/Eessb). We determined the association between these parameters and the following adverse outcomes: 1) the combined endpoint of death, cardiac transplantation, or ventricular assist device (VAD) placement; and 2) cardiac hospitalization., Results: Over a median follow-up of 3.4 years, there were 76 deaths, 52 transplantations, 14 VAD placements, and 684 cardiac hospitalizations. EF was independently associated with death, transplantation, and VAD placement (adjusted hazard ratio [HR]: 3.0; 95% confidence interval [CI]: 1.8 to 5.0 comparing third and first tertiles), as were EDV (HR: 2.6; 95% CI: 1.5 to 4.2); V0 (HR: 3.6; 95% CI: 2.1 to 6.1); and Ea/Eessb (HR: 2.1; 95% CI: 1.3 to 3.3). EDV, V0, and Ea/Eessb were also associated with risk of cardiac hospitalization. Eessb was not significantly associated with any adverse outcomes in adjusted analyses., Conclusions: Left ventricular size, V0, and VA coupling are associated with prognosis in systolic HF, but end-systolic elastance (Eessb) is not. Assessment of VA coupling via Ea/Eessb is an additional noninvasively derived metric that can be used to gauge prognosis in human HF., (Copyright © 2013 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)

The different ethical perspectives of dentists and auto mechanics include primary concern, billing procedures, advertising, emergency care, the level of autonomy granted to their patients/ clients, the amount of disclosure given to their patients/clients, the ability to judge the work of others, and the freedom to pursue romantic relationships with their patients/clients. In analyzing these differences, one finds dentists to have much greater ethical obligations than auto mechanics. There are subtle differences between the ethical expectations of Canadian and United States dentists.

Coal-burning power plants in the United States (US) generate more than 70 million tons of fly ash as a by-product annually. Recycling large volumes of fly ash in geotechnical applications may offer an attractive alternative to the disposal problem as most of it is currently dumped in ponds or landfills. Class-F fly ash, resulting from burning of bituminous or anthracite coals, is the most common type of fly ash in the US. In the present study, the mechanical characteristics (compaction response, compressibility, and shear strength) of class-F fly ash were investigated by performing various laboratory tests (compaction test, one-dimensional compression test, direct shear test and consolidated-drained triaxial compression test) on fly ash samples collected from three power plants in the state of Indiana (US). Test results have shown that despite some morphological differences, class-F fly ash exhibits mechanical properties that are, in general, comparable to those observed in natural sandy soils.

We analyzed cumulative lifetime exposure to chrysotile asbestos experienced by brake mechanics in the US during the period 1950-2000. Using Monte Carlo methods, cumulative exposures were calculated using the distribution of 8-h time-weighted average exposure concentrations for brake mechanics and the distribution of job tenure data for automobile mechanics. The median estimated cumulative exposures for these mechanics, as predicted by three probabilistic models, ranged from 0.16 to 0.41 fibers per cubic centimeter (f/cm(3)) year for facilities with no dust-control procedures (1970s), and from 0.010 to 0.012 f/cm(3) year for those employing engineering controls (1980s). Upper-bound (95%) estimates for the 1970s and 1980s were 1.96 to 2.79 and 0.07-0.10 f/cm(3) year, respectively. These estimates for US brake mechanics are consistent with, but generally slightly lower than, those reported for European mechanics. The values are all substantially lower than the cumulative exposure of 4.5 f/cm(3) year associated with occupational exposure to 0.1 f/cm(3) of asbestos for 45 years that is currently permitted under the current occupational exposure limits in the US. Cumulative exposures were usually about 100- to 1,000-fold less than those of other occupational groups with asbestos exposure for similar time periods. The cumulative lifetime exposure estimates presented here, combined with the negative epidemiology data for brake mechanics, could be used to refine the risk assessments for chrysotile-exposed populations.

Empirical studies have established that when pairs of similar cars collide, the relative injury risk between pairs of different size is inversely related to their mass ratio. Further empirical studies have shown that in frontal collisions between dissimilar cars, relative injury risk is inversely proportional to mass ratio raised to the power of n. The value of the exponent n increases with impact speed, with n approximately 1 at low speeds and n ranging from 2.81 to 3.74 for fatalities. In this paper a theory is derived which explains relative injury risk in terms of three parameters: length (or size) ratio, mass ratio and the ratio of collision energy absorption between the colliding vehicles. It is proposed that the ratio of collision energy absorption between colliding vehicles is a function of the structural collapse forces imposed at maximum dynamic crush. The theory shows that the fundamental factor in collisions between pairs of similar cars is size, i.e. length. For collisions between two dissimilar cars, Monte-Carlo simulations using generalised characterisations for the car population yield theoretical predictions that match empirical findings ranging from minor injuries (AISI +) to fatal (AIS6) injuries.

In this paper the passive safety of cars is examined in the context of the influence of car size and mass on the relative safety of cars. The fundamental relationships of Newtonian mechanics are used to derive a generalized equation for the relative safety of cars of different sizes when involved in frontal collisions. Further equations are derived for collisions between cars of similar size and for single vehicle crashes. These are combined with overall injury criteria to give a series of predicted Relative Injury Risk relationships. Theory shows that in collisions between cars of similar size and in single vehicle accidents the fundamental parameter which determines Relative Injury Risk is the size, i.e. the length of the car whereas in collisions between dissimilar sized cars the fundamental parameters are the masses and the structural energy absorption properties of the cars. The predictions from the theoretical models are compared with the results of field evaluations of Relative Injury Risk to car occupants carried out in the U.S.A. and in Europe for car to car and single vehicle collisions. There is a high level of correlation between the theory and the field evaluations of Relative Injury Risk. Finally the overall structural crush behaviour of cars is reviewed and ways of further improving the overall passive safety of cars proposed.

Team sports at multiple levels of competition have steadily grown more popular and more economically significant in the United States and Canada during the past 150 years. Year-round competition across these large countries requires continual, reliable, and safe transportation of teams and their supporters, often by air, with the potential albeit small for loss of athletes, coaches, and entire teams in air crashes and other accidents. This paper reviews potential and actual accidents involving United States athletic teams from the 1930s to the present, including four fatal air charter crashes from 1960 through 1977. Common factors in crashes included inclement weather, component failure, human factors, and overloading or misloading aircraft. The safety of commercial and charter aviation has been significantly improved. However, weather, human factors, and ongoing maintenance and inspection of aircraft for safety remain significant problems which call for continued vigilance.

The purpose of this paper is to determine the error introduced by ultrasonic hydrophones used to measure current or proposed Mechanical (MI) and Cavitation (CI) Indices, assuming that the hydrophones meet bandwidth specifications contained in US and IEC measurement standards. These indices are based on the peak rarefactional pressure, pr. Since the portion of the pressure waveform where pr occurs is dominated by low frequency components, attention was placed on the low frequency hydrophone response specifications. Both simulated and actual diagnostic pressure pulses (with center frequency fc) were subjected to single-pole high-pass filtering for a range of -3 dB cut-off frequencies (fa). The error in the indices introduced by this filtration was evaluated. At both fa = 0.5fc (the US requirement) and fa = 0.86fc (calculated from the IEC -6dB bandwidth specification at 0.5fc), results showed that errors exceeding -30% could be expected. Furthermore, to reduce errors to less than 5%, the low frequency hydrophone response should extend at least an order of magnitude below the center frequency of the pressure wave. For example, for a 3.5 MHz transducer, the hydrophone should have a lower cut-off frequency of less than 350 kHz, which at present constitutes a challenge because of the lack of commercial hydrophones calibrated below 1 MHz.