263 research outputs found
Knee and Hip Joint Kinematics Predict Quadriceps and Hamstrings Neuromuscular Activation Patterns in Drop Jump Landings.
PURPOSE: The purpose was to assess if variation in sagittal plane landing kinematics is associated with variation in neuromuscular activation patterns of the quadriceps-hamstrings muscle groups during drop vertical jumps (DVJ). METHODS: Fifty female athletes performed three DVJ. The relationship between peak knee and hip flexion angles and the amplitude of four EMG vectors was investigated with trajectory-level canonical correlation analyses over the entire time period of the landing phase. EMG vectors consisted of the {vastus medialis(VM),vastus lateralis(VL)}, {vastus medialis(VM),hamstring medialis(HM)}, {hamstring medialis(HM),hamstring lateralis(HL)} and the {vastus lateralis(VL),hamstring lateralis(HL)}. To estimate the contribution of each individual muscle, linear regressions were also conducted using one-dimensional statistical parametric mapping. RESULTS: The peak knee flexion angle was significantly positively associated with the amplitudes of the {VM,HM} and {HM,HL} during the preparatory and initial contact phase and with the {VL,HL} vector during the peak loading phase (p<0.05). Small peak knee flexion angles were significantly associated with higher HM amplitudes during the preparatory and initial contact phase (p<0.001). The amplitudes of the {VM,VL} and {VL,HL} were significantly positively associated with the peak hip flexion angle during the peak loading phase (p<0.05). Small peak hip flexion angles were significantly associated with higher VL amplitudes during the peak loading phase (p = 0.001). Higher external knee abduction and flexion moments were found in participants landing with less flexed knee and hip joints (p<0.001). CONCLUSION: This study demonstrated clear associations between neuromuscular activation patterns and landing kinematics in the sagittal plane during specific parts of the landing. These findings have indicated that an erect landing pattern, characterized by less hip and knee flexion, was significantly associated with an increased medial and posterior neuromuscular activation (dominant hamstrings medialis activity) during the preparatory and initial contact phase and an increased lateral neuromuscular activation (dominant vastus lateralis activity) during the peak loading phase
The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies
Silicates are an important component of interstellar dust and the structure
of these grains -- amorphous versus crystalline -- is sensitive to the local
physical conditions. We have studied the infrared spectra of a sample of
ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow
absorption features at 11, 16, 19, 23, and 28 microns, characteristic of
crystalline silicates, superimposed on the broad absorption bands at 10 and 18
microns due to amorphous silicates in a subset of this sample. These features
betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of
the olivines. Previously, crystalline silicates have only been observed in
circumstellar environments. The derived fraction of forsterite to amorphous
silicates is typically 0.1 in these ULIRGs. This is much larger than the upper
limit for this ratio in the interstellar medium of the Milky Way, 0.01. These
results suggest that the timescale for injection of crystalline silicates into
the ISM is short in a merger-driven starburst environment (e.g., as compared to
the total time to dissipate the gas), pointing towards massive stars as a
prominent source of crystalline silicates. Furthermore, amorphization due to
cosmic rays, which is thought to be of prime importance for the local ISM, lags
in vigorous starburst environments.Comment: 7 pages, 5 figures, accepted for publication in Ap
SiO2 glass density to lower-mantle pressures
The convection or settling of matter in the deep Earth’s interior is mostly constrained by density variations between the different reservoirs. Knowledge of the density contrast between solid and molten silicates is thus of prime importance to understand and model the dynamic behavior of the past and present Earth. SiO2 is the main constituent of Earth’s mantle and is the reference model system for the behavior of silicate melts at high pressure. Here, we apply our recently developed x-ray absorption technique to the density of SiO2 glass up to 110 GPa, doubling the pressure range for such measurements. Our density data validate recent molecular dynamics simulations and are in good agreement with previous experimental studies conducted at lower pressure. Silica glass rapidly densifies up to 40 GPa, but the density trend then flattens to become asymptotic to the density of SiO2 minerals above 60 GPa. The density data present two discontinuities at ∼17 and ∼60 GPa that can be related to a silicon coordination increase from 4 to a mixed 5/6 coordination and from 5/6 to sixfold, respectively. SiO2 glass becomes denser than MgSiO3 glass at ∼40 GPa, and its density becomes identical to that of MgSiO3 glass above 80 GPa. Our results on SiO2 glass may suggest that a variation of SiO2 content in a basaltic or pyrolitic melt with pressure has at most a minor effect on the final melt density, and iron partitioning between the melts and residual solids is the predominant factor that controls melt buoyancy in the lowermost mantle
X-ray Emission from Young Stellar Objects in the \epsilon Chamaeleontis Group: the Herbig Ae Star HD 104237 and Associated Low-Mass Stars
We present Chandra-HETGS observations of the Herbig Ae star HD 104237 and the
associated young stars comprising lower mass stars, in the 0.15-1.75\msol mass
range, in their pre-main sequence phase. The brightest X-ray source in the
association is the central system harboring the Herbig Ae primary, and a K3
companion. Its X-ray variability indicates modulation possibly on time scales
of the rotation period of the Herbig Ae star, and this would imply that the
primary significantly contributes to the overall emission. The spectrum of the
Herbig Ae+K3 system shows a soft component significantly more pronounced than
in other K-type young stars. This soft emission is reminiscent of the unusually
soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and
therefore we tentatively attribute it to the Herbig Ae of the binary system.
The HETGS spectrum shows strong emission lines corresponding to a wide range of
plasma temperatures. The He-like triplet of MgXI and NeIX suggest the presence
of plasma at densities of about cm, possibly indicating
accretion related X-ray production mechanism.
The analysis of the zero-order spectra of the other sources indicates X-ray
emission characteristics typical of pre-main sequence stars of similar spectral
type, with the exception of the T Tauri HD104237-D, whose extremely soft
emission is very similar to the emission of the classical T Tauri star TW Hya,
and suggests X-ray production by shocked accreting plasma.Comment: accepted for publication on the Astrophysical Journa
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
Mid-infrared spectra of PAH emission in Herbig AeBe stars
We present spectra of four Herbig AeBe stars obtained with the Infrared
Spectrograph (IRS). on the Spitzer Space Telescope. All four of the sources
show strong emission from polycyclic aromatic hydrocarbons (PAHs), with the 6.2
um emission feature shifted to 6.3 um and the strongest C-C skeletal-mode
feature occuring at 7.9 um instead of at 7.7 um as is often seen. Remarkably,
none of the four stars have silicate emission. The strength of the 7.9 um
feature varies with respect to the 11.3 um feature among the sources,
indicating that we have observed PAHs with a range of ionization fractions. The
ionization fraction is higher for systems with hotter and brighter central
stars. Two sources, HD 34282 and HD 169142, show emission features from
aliphatic hydrocarbons at 6.85 and 7.25 um. The spectrum of HD 141569 shows a
previously undetected emission feature at 12.4 um which may be related to the
12.7 um PAH feature. The spectrum of HD 135344, the coolest star in our sample,
shows an unusual profile in the 7-9 um region, with the peak emission to the
red of 8.0 um and no 8.6 um PAH feature.Comment: Accepted by ApJ 23 June, 2005, 8 pages (emulateapj), 5 figures (3 in
color
ISO spectroscopy of gas and dust: from molecular clouds to protoplanetary disks
Observations of interstellar gas-phase and solid-state species in the 2.4-200
micron range obtained with the spectrometers on board the Infrared Space
Observatory are reviewed. Lines and bands due to ices, polycyclic aromatic
hydrocarbons, silicates and gas-phase atoms and molecules (in particular H2,
CO, H2O, OH and CO2) are summarized and their diagnostic capabilities
illustrated. The results are discussed in the context of the physical and
chemical evolution of star-forming regions, including photon-dominated regions,
shocks, protostellar envelopes and disks around young stars.Comment: 56 pages, 17 figures. To appear in Ann. Rev. Astron. Astrophys. 2004.
Higher resolution version posted at
http://www.strw.leidenuniv.nl/~ewine/araa04.pd
First results of the Herschel Key Program 'Dust, Ice and Gas in Time': Dust and Gas Spectroscopy of HD 100546
We present far-infrared spectroscopic observations, taken with the
Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space
Observatory, of the protoplanetary disk around the pre-main-sequence star HD
100546. These observations are the first within the DIGIT Herschel key program,
which aims to follow the evolution of dust, ice, and gas from young stellar
objects still embedded in their parental molecular cloud core, through the
final pre-main-sequence phases when the circumstellar disks are dissipated.
Our aim is to improve the constraints on temperature and chemical composition
of the crystalline olivines in the disk of HD 100546 and to give an inventory
of the gas lines present in its far-infrared spectrum. The 69 \mu\m feature is
analyzed in terms of position and shape to derive the dust temperature and
composition. Furthermore, we detected 32 emission lines from five gaseous
species and measured their line fluxes. The 69 \mu\m emission comes either from
dust grains with ~70 K at radii larger than 50 AU, as suggested by blackbody
fitting, or it arises from ~200 K dust at ~13 AU, close to the midplane, as
supported by radiative transfer models. We also conclude that the forsterite
crystals have few defects and contain at most a few percent iron by mass.
Forbidden line emission from [CII] at 157 \mu\m and [OI] at 63 and 145 \mu\m,
most likely due to photodissociation by stellar photons, is detected.
Furthermore, five H2O and several OH lines are detected. We also found high-J
rotational transition lines of CO, with rotational temperatures of ~300 K for
the transitions up to J=22-21 and T~800 K for higher transitions
Norovirus gastroenteritis general outbreak associated with raw shellfish consumption in South Italy
BACKGROUND: Despite Noroviruses (NV, previously "Norwalk-like viruses") being a leading cause of acute gastroenteritis outbreaks, the impact of NV infection is at present unknown and little information is available about strains circulating in Italy. In April 2002 an outbreak of gastroenteritis occurred in the province of Bari (South-east Italy), involving several households. METHODS: A retrospective cohort study was performed in order to assess risk factors associated with illness. All households where a case occurred were included in the study. Faecal specimens were collected from ill individuals. NV-specific RT-PCR was performed. Eleven samples of mussels were collected from fish-markets involved in the outbreak. A nested PCR was used for mussel samples. RESULTS: One hundred and three cases, detected by means of active surveillance, met the case definition. Raw shellfish eating was the principal risk factor for the disease, as indicated by the analytic issues (Risk Ratio: 1.50; IC 95%: 1.18 – 1.89; p < 0.001). NVs were found by means of RT-PCR of all the stool specimens from the 24 patients tested. Eleven samples of shellfish from local markets were tested for the presence or NVs; six were positive by nested PCR and genotypes were related to that found in patients' stools. CONCLUSION: This is the first community outbreak caused by NVs related to sea-food consumption described in Italy. The study confirms that the present standards for human faecal contamination do not seem to be a reliable indicator of viral contaminants in mussels
The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry
We report on the properties of pre-main-sequence objects in the Taurus
molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer
Space Telescope. There are 215 previously-identified members of the Taurus
star-forming region in our ~44 square degree map; these members exhibit a range
of Spitzer colors that we take to define young stars still surrounded by
circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no
detectable dust excesses). We looked for new objects in the survey field with
similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet
imaging, and found 148 candidate new members of Taurus. We have obtained
follow-up spectroscopy for about half the candidate sample, thus far confirming
34 new members, 3 probable new members, and 10 possible new members, an
increase of 15-20% in Taurus members. Of the objects for which we have
spectroscopy, 7 are now confirmed extragalactic objects, and one is a
background Be star. The remaining 93 candidate objects await additional
analysis and/or data to be confirmed or rejected as Taurus members. Most of the
new members are Class II M stars and are located along the same cloud filaments
as the previously-identified Taurus members. Among non-members with Spitzer
colors similar to young, dusty stars are evolved Be stars, planetary nebulae,
carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the
preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm
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