1,721,009 research outputs found
Data-driven modelling of vertical dynamic excitation of bridges induced by people running
Full text linkWith increasingly popular marathon events in urban environments, structural designers face a great deal of uncertainty when assessing dynamic performance of bridges occupied and dynamically excited by people running. While the dynamic loads induced by pedestrians walking have been intensively studied since the infamous lateral sway of the London Millennium Bridge in 2000, reliable and practical descriptions of running excitation are still very rare and limited. This interdisciplinary study has addressed the issue by bringing together a database of individual running force signals recorded by two state-of-the-art instrumented treadmills and two attempts to mathematically describe the measurements. The first modelling strategy is adopted from the available design guidelines for human walking excitation of structures, featuring perfectly periodic and deterministic characterisation of pedestrian forces presentable via Fourier series. This modelling approach proved to be inadequate for running loads due to the inherent near-periodic nature of the measured signals, a great inter-personal randomness of the dominant Fourier amplitudes and the lack of strong correlation between the amplitudes and running footfall rate. Hence, utilising the database established and motivated by the existing models of wind and earthquake loading, speech recognition techniques and a method of replicating electrocardiogram signals, this paper finally presents a numerical generator of random near-periodic running force signals which can reliably simulate the measurements. Such a model is an essential prerequisite for future quality models of dynamic loading induced by individuals, groups and crowds running under a wide range of conditions, such as perceptibly vibrating bridges and different combinations of visual, auditory and tactile cues
Dynamic loading of bridges due to people running
No full textWith increasingly popular marathon events in urban environments, dynamic performance due to people running is becoming a critical aspect of bridge design. While the dynamic loads induced by pedestrians walking have been intensively studied since the infamous lateral sway of the London Millennium Bridge in 2000, reliable and practical descriptions of running excitation are still very rare and limited. This study brings together a database of individual vertical running force signals recorded using instrumented treadmill technology and their mathematical characterisation. Motivated by the existing models of wind and earthquake loading, speech recognition techniques and a method of replicating electrocardiogram signals, the model generates random near-periodic signals which can reliably simulate the measurements. Such a model is an essential prerequisite for future quality models of dynamic loading induced by individuals, groups and crowds running under a wide range of conditions, such as perceptibly vibrating bridges and different combinations of visual, auditory and tactile cues
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Sprint Acceleration Mechanics in Masters Athletes
No full textPurpose The best sprint performances are usually reached between the ages of 20 and 30 yr; however, even in well-trained individuals, performance continues to decrease with age. Although this inevitable decrease in performance has been related to reductions in muscular force, velocity, and power capabilities, these measures have not been assessed in the specific context of sprinting. The aim of this study was to investigate the mechanical outputs of sprinting acceleration among masters sprinters to better understand the mechanical underpinnings of the age-related decrease in sprint performance. Methods The study took place during an international masters competition, with testing performed at the end of the warm-up for official sprint races. Horizontal ground reaction force, velocity, mechanical power outputs, and mechanical effectiveness of force application were estimated from running velocity-time data during a 30-m sprint acceleration in 27 male sprinters (39-96 yr). Data were presented in the form of age-related changes and compared with elite young sprinters data. Results Maximal force, velocity, and power outputs decreased linearly with age (all r > 0.84, P < 0.001), at a rate of 1% per year. Maximal power of the oldest subject tested was approximately one-ninth of that of younger world-class sprinters (3.57 vs 32.1 W·kg-1). Although the maximal effectiveness of horizontal force application also decreased with age, its decrease with increasing velocity within the sprint acceleration was not age dependent. Conclusions In addition to lower neuromuscular force, velocity, and power outputs, masters sprinters had a comparatively lower effectiveness of force application, especially at the beginning of the sprint
Optimal force-velocity profile in ballistic movements-Altius: Citius or Fortius?
No full textThe study's purpose was to determine the respective influences of the maximal power (P max) and the force-velocity (F-v) mechanical profile of the lower limb neuromuscular system on performance in ballistic movements. Methods: A theoretical integrative approach was proposed to express ballistic performance as a mathematical function of P max and F-v profile. This equation was (i) validated from experimental data obtained on 14 subjects during lower limb ballistic inclined push-offs and (ii) simulated to quantify the respective influence of P max and F-v profile on performance. Results: The bias between performances predicted and obtained from experimental measurements was 4%-7%, confirming the validity of the proposed theoretical approach. Simulations showed that ballistic performance was mostly influenced not only by P max but also by the balance between force and velocity capabilities as described by the F-v profile. For each individual, there is an optimal F-v profile that maximizes performance, whereas unfavorable F-v balances lead to differences in performance up to 30% for a given P max. This optimal F-v profile, which can be accurately determined, depends on some individual characteristics (limb extension range, P max) and on the afterload involved in the movement (inertia, inclination). The lower the afterload, the more the optimal F-v profile is oriented toward velocity capabilities and the greater the limitation of performance imposed by the maximal velocity of lower limb extension. Conclusions: High ballistic performances are determined by both maximization of the power output capabilities and optimization of the F-v mechanical profile of the lower limb neuromuscular system. Copyright © 2012 by the American College of Sports Medicine
Concurrent validity of GPS for deriving mechanical properties of sprint acceleration
No full textPurpose: The purpose of this study was to test the concurrent validity of data from two different global positioning system (GPS) units for obtaining mechanical properties during sprint acceleration using a field method recently validated by Samozino et al. Methods: Thirty-two athletes performed maximal straight-line sprints, and their running speed was simultaneously measured by GPS units (sampling rate: 20 Hz or 5 Hz) and either a radar or laser device (devices taken as references). Lower limb mechanical properties of sprint acceleration (theoretical maximal force, F0; theoretical maximal speed, V0; maximal power, Pmax) were derived from a modeling of the speed-time curves using an exponential function in both measurements. Comparisons of mechanical properties from 20 Hz and 5 Hz GPS units with those from reference devices were performed for 80 and 62 trials, respectively. Results: The percentage bias showed a wide range of over or underestimation for both systems (-7.9-9.7% and -5.1-2.9% for 20 Hz and 5 Hz GPS), while the ranges of its 90% confidence limits for 20 Hz GPS were markedly smaller than those for 5 Hz GPS. These results were supported by the correlation analyses. Conclusions: Overall, the concurrent validity for all variables derived from 20 Hz GPS measurements was better than that obtained from the 5 Hz GPS units. However, in the current state of GPS devices accuracy for speed-time measurements over a maximal sprint acceleration, we recommend that radar, laser devices and timing gates remain the reference methods for implementing Samozino et al.'s computations
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