1,040 research outputs found
Mechanical and material properties of the plantarflexor muscles and Achilles tendon in children with spastic cerebral palsy and typically developing children
© 2016 The Authors. Background: Children with spastic cerebral palsy (CP) experience secondary musculoskeletal adaptations, affecting the mechanical and material properties of muscles and tendons.
CP-related changes in the spastic muscle are well documented whilst less is known about the tendon. From a clinical perspective, it is important to understand alterations in tendon properties in order to tailor interventions or interpret clinical tests more appropriately. The main purpose of this study was to compare the mechanical and material properties of the Achilles tendon in children with cerebral palsy to those of typically developing children.
Methods:
Using a combination of ultrasonography and motion analysis, we determined tendon mechanical properties in ten children with spastic cerebral palsy and ten aged-matched typically developing children. Specifically, we quantified muscle and tendon stiffness, tendon slack length, tendon strain, cross-sectional area, Young׳s Modulus and the strain rate dependence of tendon stiffness.
Findings:
Children with CP had a greater muscle to tendon stiffness ratio compared to typically developing children. Despite a smaller tendon cross-sectional area and greater tendon slack length, no group differences were observed in tendon stiffness or Young׳s Modulus. The slope describing the stiffness strain-rate response was steeper in children with cerebral palsy.
Interpretation:
These results provide us with a more differentiated understanding of the muscle and tendon mechanical properties, which would be relevant for future research and paediatric clinicians
A model for the current instabilities in GaAs‐AlGaAs heterojunction
A model is proposed for the description of the current instabilities in GaAs-AlGaAs heterojunctions. It consists of three parts: the injection of electrons via the contact into the AlGaAs layer, the partial capture of these electrons in deep centers, and the change with time of the band structure. This last ingredient is crucial, since due to the increase of the total number of electrons in the AlGaAs layer the band bending decreases making real-space transfer from the AlGaAs layer to the two-dimensional electron gas possible. We have performed quasistationary simulations of the time dependence of the current. The velocities, average energies, capture rates, etc. were taken from Monte Carlo simulations. It turned out, that the parameters for the modeling of the contact, which are to a high degree unknown, play an essential role
Plasma instability and amplification of electromagnetic waves in low-dimensional electron systems
A general electrodynamic theory of a grating coupled two dimensional electron
system (2DES) is developed. The 2DES is treated quantum mechanically, the
grating is considered as a periodic system of thin metal strips or as an array
of quantum wires, and the interaction of collective (plasma) excitations in the
system with electromagnetic field is treated within the classical
electrodynamics. It is assumed that a dc current flows in the 2DES. We consider
a propagation of an electromagnetic wave through the structure, and obtain
analytic dependencies of the transmission, reflection, absorption and emission
coefficients on the frequency of light, drift velocity of 2D electrons, and
other physical and geometrical parameters of the system. If the drift velocity
of 2D electrons exceeds a threshold value, a current-driven plasma instability
is developed in the system, and an incident far infrared radiation is
amplified. We show that in the structure with a quantum wire grating the
threshold velocity of the amplification can be essentially reduced, as compared
to the commonly employed metal grating, down to experimentally achievable
values. Physically this is due to a considerable enhancement of the grating
coupler efficiency because of the resonant interaction of plasma modes in the
2DES and in the grating. We show that tunable far infrared emitters, amplifiers
and generators can thus be created at realistic parameters of modern
semiconductor heterostructures.Comment: 28 pages, 15 figures, submitted to Phys. Rev.
Influence of retardation effects on 2D magnetoplasmon spectrum
Within dissipationless limit the magnetic field dependence of magnetoplasmon
spectrum for unbounded 2DEG system found to intersect the cyclotron resonance
line, and, then approaches the frequency given by light dispersion relation.
Recent experiments done for macroscopic disc-shape 2DEG systems confirm theory
expectations.Comment: 2 pages,2 figure
Poststarburst Models of LINERs
Since the discovery of low-ionization nuclear emission-line regions in many
galaxies (LINERs), it has been recognized that they constitute a class of
active galactic nuclei (AGNs) which are thought to be powered by gas accretion
onto a central, supermassive black hole. LINERs are observed in approximately
one third of galaxies in the local universe and it has been often thought that
they harbor an AGN-like central engine with moderate activity. However, some
LINERs show no direct evidence for AGNs such as broad emission lines, radio
jets, hard X-ray emission, spectral energy distributions which are inconsistent
with starlight, and so on. For such LINERs (a subset of type 2 LINERs), we
present new poststarburst models which explain some of their most important
optical narrow emission-line ratios. In these models, the ionization sources
are planetary nebula nuclei (PNNs) with temperature of ~ 10^5 K which appear in
the late-phase evolution of intermediate-mass stars with mass between ~ 3
M_solar and ~ 6 M_solar. Such PNNs left in a typical starburst nucleus can
produce an Halpha luminosity of L(Halpha) ~ 10^38 ergs s^-1 for typical
poststarburst LINERs and ~ 10^39 ergs s^-1 only in exceptionally bright cases.
The PNN phase lasts until the death of the lowest-mass stars formed in the
starburst, which is ~ 5 x 10^8 yr for an assumed lower limit of the initial
mass function of 3 M_solar. This long duration appears consistent with the
observed higher frequency of occurrence of LINERs if every galaxy could
experience the starburst activity several times in its life. We therefore
propose that some LINERs which show no direct evidence for AGNs may be
poststarburst nuclei powered by a cluster of PNNs.Comment: 8 pages, 6 figures, emulateapj.sty; To appear in the September 2000
issue of the Astronomical Journa
Membrane Instantons and de Sitter Vacua
We investigate membrane instanton effects in type IIA strings compactified on
rigid Calabi-Yau manifolds. These effects contribute to the low-energy
effective action of the universal hypermultiplet. In the absence of additional
fivebrane instantons, the quaternionic geometry of this hypermultiplet is
determined by solutions of the three-dimensional Toda equation. We construct
solutions describing membrane instantons, and find perfect agreement with the
string theory prediction. In the context of flux compactifications we discuss
how membrane instantons contribute to the scalar potential and the
stabilization of moduli. Finally, we demonstrate the existence of meta-stable
de Sitter vacua.Comment: v3: minor clarifications, JHEP version, 38 page
Plasmons in coupled bilayer structures
We calculate the collective charge density excitation dispersion and spectral
weight in bilayer semiconductor structures {\it including effects of interlayer
tunneling}. The out-of-phase plasmon mode (the ``acoustic'' plasmon) develops a
long wavelength gap in the presence of tunneling with the gap being
proportional to the square root (linear power) of the tunneling amplitude in
the weak (strong) tunneling limit. The in-phase plasmon mode is qualitatively
unaffected by tunneling. The predicted plasmon gap should be a useful tool for
studying many-body effects.Comment: 10 pages, 6 figures. to appear in Phys. Rev. Let
Pathway to the PiezoElectronic Transduction Logic Device
The information age challenges computer technology to process an
exponentially increasing computational load on a limited energy budget - a
requirement that demands an exponential reduction in energy per operation. In
digital logic circuits, the switching energy of present FET devices is
intimately connected with the switching voltage, and can no longer be lowered
sufficiently, limiting the ability of current technology to address the
challenge. Quantum computing offers a leap forward in capability, but a clear
advantage requires algorithms presently developed for only a small set of
applications. Therefore, a new, general purpose, classical technology based on
a different paradigm is needed to meet the ever increasing demand for data
processing.Comment: in Nano Letters (2015
The linear multiplet and ectoplasm
In the framework of the superconformal tensor calculus for 4D N=2
supergravity, locally supersymmetric actions are often constructed using the
linear multiplet. We provide a superform formulation for the linear multiplet
and derive the corresponding action functional using the ectoplasm method (also
known as the superform approach to the construction of supersymmetric
invariants). We propose a new locally supersymmetric action which makes use of
a deformed linear multiplet. The novel feature of this multiplet is that it
corresponds to the case of a gauged central charge using a one-form potential
not annihilated by the central charge (unlike the standard N=2 vector
multiplet). Such a gauge one-form can be chosen to describe a variant nonlinear
vector-tensor multiplet. As a byproduct of our construction, we also find a
variant realization of the tensor multiplet in supergravity where one of the
auxiliaries is replaced by the field strength of a gauge three-form.Comment: 31 pages; v3: minor corrections and typos fixed, version to appear in
JHE
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