2,470 research outputs found
Fatiguing Effects of Indirect Vibration Stimulation in Upper Limb Muscles- pre, post and during Isometric Contractions Superimposed on Upper Limb Vibration
© 2019 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/ , which permits unrestricted use, provided the original author and source are credited.Whole-body vibration and upper limb vibration (ULV) continue to gain popularity as exercise intervention for rehabilitation and sports applications. However, the fatiguing effects of indirect vibration stimulation are not yet fully understood. We investigated the effects of ULV stimulation superimposed on fatiguing isometric contractions using a purpose developed upper limb stimulation device. Thirteen healthy volunteers were exposed to both ULV superimposed to fatiguing isometric contractions (V) and isometric contractions alone Control (C). Both Vibration (V) and Control (C) exercises were performed at 80% of the maximum voluntary contractions. The stimulation used was 30 Hz frequency of 0.4 mm amplitude. Surface-electromyographic (EMG) activity of the Biceps Brachii, Triceps Brachii and Flexor Carpi Radialis were measured. EMG amplitude (EMGrms) and mean frequency (MEF) were computed to quantify muscle activity and fatigue levels. All muscles displayed significantly higher reduction in MEFs and a corresponding significant increase in EMGrms with the V than the Control, during fatiguing contractions (p < 0.05). Post vibration, all muscles showed higher levels of MEFs after recovery compared to the control. Our results show that near-maximal isometric fatiguing contractions superimposed on vibration stimulation lead to a higher rate of fatigue development compared to the isometric contraction alone in the upper limb muscles. Results also show higher manifestation of mechanical fatigue post treatment with vibration compared to the control. Vibration superimposed on isometric contraction not only seems to alter the neuromuscular function during fatiguing efforts by inducing higher neuromuscular load but also post vibration treatment.Peer reviewedFinal Published versio
The Cepheid mass discrepancy and pulsation-driven mass loss
Context. A longstanding challenge for understanding classical Cepheids is the
Cepheid mass discrepancy, where theoretical mass estimates using stellar
evolution and stellar pulsation calculations have been found to differ by
approximately 10 - 20%. Aims. We study the role of pulsation-driven mass loss
during the Cepheid stage of evolution as a possible solution to this mass
discrepancy. Methods. We computed stellar evolution models with a Cepheid
mass-loss prescription and various amounts of convective core overshooting. The
contribution of mass loss towards the mass discrepancy is determined using
these models, Results. Pulsation-driven mass loss is found to trap Cepheid
evolution on the instability strip, allowing them to lose about 5 - 10% of
their total mass when moderate convective core overshooting, an amount
consistent with observations of other stars, is included in the stellar models.
Conclusions. We find that the combination of moderate convective core
overshooting and pulsation-driven mass loss can solve the Cepheid mass
discrepancy.Comment: 4 pages, 2 figures and 2 tables. Accepted for publication A&A Letter
Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres
Context. One challenge for measuring the Hubble constant using Classical
Cepheids is the calibration of the Leavitt Law or period-luminosity
relationship. The Baade-Wesselink method for distance determination to Cepheids
relies on the ratio of the measured radial velocity and pulsation velocity, the
so-called projection factor and the ability to measure the stellar angular
diameters. Aims. We use spherically-symmetric model stellar atmospheres to
explore the dependence of the p-factor and angular diameter corrections as a
function of pulsation period. Methods. Intensity profiles are computed from a
grid of plane-parallel and spherically-symmetric model stellar atmospheres
using the SAtlas code. Projection factors and angular diameter corrections are
determined from these intensity profiles and compared to previous results.
Results. Our predicted geometric period-projection factor relation including
previously published state-of-the-art hydrodynamical predictions is not with
recent observational constraints. We suggest a number of potential resolutions
to this discrepancy. The model atmosphere geometry also affects predictions for
angular diameter corrections used to interpret interferometric observations,
suggesting corrections used in the past underestimated Cepheid angular
diameters by 3 - 5%. Conclusions. While spherically-symmetric hydrostatic model
atmospheres cannot resolve differences between projection factors from theory
and observations, they do help constrain underlying physics that must be
included, including chromospheres and mass loss. The models also predict more
physically-based limb-darkening corrections for interferometric observations.Comment: 8 pages, 6 figures, 2 tables, accepted for publication in A&
The occurrence of classical Cepheids in binary systems
Classical Cepheids, like binary stars, are laboratories for stellar evolution
and Cepheids in binary systems are especially powerful ones. About one-third of
Galactic Cepheids are known to have companions and Cepheids in eclipsing binary
systems have recently been discovered in the Large Magellanic Cloud. However,
there are no known Galactic binary Cepheids with orbital periods less than one
year. We compute population synthesis models of binary Cepheids to compare to
the observed period and eccentricity distributions of Galactic Cepheids as well
as to the number of observed eclipsing binary Cepheids in the LMC. We find that
our population synthesis models are consistent with observed binary properties
of Cepheids. Furthermore, we show that binary interaction on the red giant
branch prevents some red giant stars from becoming classical Cepheids. Such
interactions suggest that the binary fraction of Cepheids should be
significantly less than that of their main-sequence progenitors, and that
almost all binary Cepheids have orbital periods longer than one year. If the
Galactic Cepheid spectroscopic binary fraction is about 35%, then the
spectroscopic binary fraction of their intermediate mass main sequence
progenitors is about 40-45%.Comment: 7 pages, 3 figures, resubmitted to A&
Calibrating the projection factor for Galactic Cepheids
The projection factor (p), which converts the radial velocity to pulsational
velocity, is an important parameter in the Baade-Wesselink (BW) type analysis
and distance scale work. The p-factor is either adopted as a constant or
linearly depending on the logarithmic of pulsating periods. The aim of this
work is to calibrate the p-factor if a Cepheid has both the BW distance and an
independent distance measurement, and examine the p-factor for delta Cephei --
the prototype of classical Cepheids. We calibrated the p-factor for several
Galactic Cepheids that have both the latest BW distances and independent
distances either from Hipparcos parallaxes or main-sequence fitting distances
to Cepheid-hosted stellar clusters. Based on 25 Cepheids, the calibrated
p-factor relation is consistent with latest p-factor relation in literature.
The calibrated p-factor relation also indicates that this relation may not be
linear and may exhibit an intrinsic scatter. We also examined the discrepancy
of empirical p-factors for delta Cephei, and found that the reasons for this
discrepancy include the disagreement of angular diameters, the treatment of
radial velocity data, and the phase interval adopted during the fitting
procedure. Finally, we investigated the impact of the input p-factor in two BW
methodologies for delta Cephei, and found that different p-factors can be
adopted in these BW methodologies and yet result in the same angular diameters.Comment: 6 pages, 6 figures and 2 tables. A&A accepte
Is there a mass discrepancy in the Cepheid binary OGLE-LMC-CEP0227?
Context. The Cepheid mass discrepancy, the difference between masses
predicted from stellar evolution and stellar pulsation calculations, is a
challenge for the understanding of stellar astrophysics. Recent models of the
eclipsing binary Cepheid OGLE-LMC-CEP-0227 have suggested that the discrepancy
may be resolved. Aims. We explore for what physical parameters do stellar
evolution models agree with the measured properties of OGLE-LMC-CEP0227 and
compare to canonical stellar evolution models assuming no convective core
overshooting. Methods. We construct state-of-the-art stellar evolution models
for varying mass, metallicity, and convective core overshooting and compare the
stellar evolution predictions with the observed properties. Results. The
observed mass, effective temperature, and radius of the two stars in the binary
system are well fit by numerous combinations of physical parameters, suggesting
a Cepheid mass discrepancy of 10-20% relative to canonical stellar evolution
models. Conclusions. The properties of the observed binary Cepheid suggest that
the Cepheid mass discrepancy is still a challenge and requires more specific
observations, such as the rate of period change, to better constrain and
understand the necessary physics for stellar evolution models to resolve the
discrepancy.Comment: 5 pages, 3 figures, A&A accepte
A Multi-Moded RF Delay Line Distribution System for the Next Linear Collider
The Delay Line Distribution System (DLDS) is an alternative to conventional
pulse compression, which enhances the peak power of rf sources while matching
the long pulse of those sources to the shorter filling time of accelerator
structures. We present an implementation of this scheme that combines pairs of
parallel delay lines of the system into single lines. The power of several
sources is combined into a single waveguide delay line using a multi-mode
launcher. The output mode of the launcher is determined by the phase coding of
the input signals. The combined power is extracted from the delay line using
mode-selective extractors, each of which extracts a single mode. Hence, the
phase coding of the sources controls the output port of the combined power. The
power is then fed to the local accelerator structures. We present a detailed
design of such a system, including several implementation methods for the
launchers, extractors, and ancillary high power rf components. The system is
designed so that it can handle the 600 MW peak power required by the NLC design
while maintaining high efficiency.Comment: 25 pages, 11 figure
Universal Features of Terahertz Absorption in Disordered Materials
Using an analytical theory, experimental terahertz time-domain spectroscopy
data and numerical evidence, we demonstrate that the frequency dependence of
the absorption coupling coefficient between far-infrared photons and atomic
vibrations in disordered materials has the universal functional form, C(omega)
= A + B*omega^2, where the material-specific constants A and B are related to
the distributions of fluctuating charges obeying global and local charge
neutrality, respectively.Comment: 5 pages, 3 fig
Barriers to healthcare and a âtriple empathy problemâ may lead to adverse outcomes for autistic adults: A qualitative study
Autistic people experience more co-occurring health conditions and, on average, die younger than non-autistic
people. Despite growing awareness of health inequities, autistic people still report barriers to accessing healthcare. We aimed to explore the experiences of autistic people accessing healthcare, shining a light on the complex interplay of relevant factors and to explain, at least in part, the possible reasons underling health disparities and adverse health outcomes. This is a qualitative study from an autistic research team. Data were collected from 1248 autistic adults as part of a large, mixed-methods, international survey exploring barriers to primary healthcare. This article reports the qualitative findings, following a thematic analysis. Using our exploratory findings, we then constructed a model to explain the reported experiences. Respondents reported a variety of barriers. Here, our article gives voice to their stories, in their own words. Themes included: early barriers; communication mismatch; doubt â in oneself and from doctors; helplessness and fear; and healthcare avoidance and serious adverse health outcomes. Our constructed model outlines a chronological journey through which healthcare access barriers may lead to adverse health outcomes. Our findings also build on the double empathy problem, situating this in a medical context, proposing a triple empathy problem
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