187 research outputs found
Somatosensory-evoked potentials as a marker of functional neuroplasticity in athletes: A systematic review
Background: Somatosensory-evoked potentials (SEP) represent a non-invasive tool to assess neural responses elicited by somatosensory stimuli acquired via electrophysiological recordings. To date, there is no comprehensive evaluation of SEPs for the diagnostic investigation of exercise-induced functional neuroplasticity. This systematic review aims at highlighting the potential of SEP measurements as a diagnostic tool to investigate exercise-induced functional neuroplasticity of the sensorimotor system by reviewing studies comparing SEP parameters between athletes and healthy controls who are not involved in organized sports as well as between athlete cohorts of different sport disciplines. Methods: A systematic literature search was conducted across three electronic databases (PubMed, Web of Science, and SPORTDiscus) by two independent researchers. Three hundred and ninety-seven records were identified, of which 10 cross-sectional studies were considered eligible. Results: Differences in SEP amplitudes and latencies between athletes and healthy controls or between athletes of different cohorts as well as associations between SEP parameters and demographic/behavioral variables (years of training, hours of training per week & reaction time) were observed in seven out of 10 included studies. In particular, several studies highlight differences in short- and long-latency SEP parameters, as well as high-frequency oscillations (HFO) when comparing athletes and healthy controls. Neuroplastic differences in athletes appear to be modality-specific as well as dependent on training regimens and sport-specific requirements. This is exemplified by differences in SEP parameters of various athlete populations after stimulation of their primarily trained limb. Conclusion: Taken together, the existing literature suggests that athletes show specific functional neuroplasticity in the somatosensory system. Therefore, this systematic review highlights the potential of SEP measurements as an easy-to-use and inexpensive diagnostic tool to investigate functional neuroplasticity in the sensorimotor system of athletes. However, there are limitations regarding the small sample sizes and inconsistent methodology of SEP measurements in the studies reviewed. Therefore, future intervention studies are needed to verify and extend the conclusions drawn here
Intermittent dislocation flow in viscoplastic deformation
The viscoplastic deformation (creep) of crystalline materials under constant
stress involves the motion of a large number of interacting dislocations.
Analytical methods and sophisticated `dislocation-dynamics' simulations have
proved very effective in the study of dislocation patterning, and have led to
macroscopic constitutive laws of plastic deformation. Yet, a statistical
analysis of the dynamics of an assembly of interacting dislocations has not
hitherto been performed. Here we report acoustic emission measurements on
stressed ice single crystals, the results of which indicate that dislocations
move in a scale-free intermittent fashion. This result is confirmed by
numerical simulations of a model of interacting dislocations that successfully
reproduces the main features of the experiment. We find that dislocations
generate a slowly evolving configuration landscape which coexists with rapid
collective rearrangements. These rearrangements involve a comparatively small
fraction of the dislocations and lead to an intermittent behavior of the net
plastic response. This basic dynamical picture appears to be a generic feature
in the deformation of many other materials. Moreover, it should provide a
framework for discussing fundamental aspects of plasticity, that goes beyond
standard mean-field approaches that see plastic deformation as a smooth laminar
flow
The inverse electromagnetic scattering problem in a piecewise homogeneous medium
This paper is concerned with the problem of scattering of time-harmonic
electromagnetic waves from an impenetrable obstacle in a piecewise homogeneous
medium. The well-posedness of the direct problem is established, employing the
integral equation method. Inspired by a novel idea developed by Hahner [11], we
prove that the penetrable interface between layers can be uniquely determined
from a knowledge of the electric far field pattern for incident plane waves.
Then, using the idea developed by Liu and Zhang [21], a new mixed reciprocity
relation is obtained and used to show that the impenetrable obstacle with its
physical property can also be recovered. Note that the wave numbers in the
corresponding medium may be different and therefore this work can be considered
as a generalization of the uniqueness result of [20].Comment: 19 pages, 2 figures, submitted for publicatio
Impact of hydrogen on the high cycle fatigue behaviour of Inconel 718 in asymmetric pushâpull mode at room temperature
AbstractThe influence of hydrogen on the high cycle fatigue (HCF) behaviour of Inconel 718 has been studied at room temperature in asymmetric pushâpull mode using an ultrasonic HCF test rig. Fatigue tests have been carried out in gaseous hydrogen (GH2) and in Ar at a pressure of 30MPa. Oscillating stresses with amplitudes (Ïa) up to 450MPa and mean stresses (Ïm) up to 600MPa have been applied. For a given Ïa and Ïm, the lifetime in Ar is generally longer than in GH2, which is explained by a hydrogen-induced embrittlement of the material. For a constant Ïa of 218MPa, the lifetime in Ar and in GH2 is very similar for high Ïm, but the difference in lifetime increases as the mean stress decreases. An approach is presented to describe the number of cycles to failure Nf as a function of Ïa and Ïm.Microstructural analysis has been performed on the specimens tested at Ïa=218MPa and two values of Ïm (300MPa and 600MPa). SEM analyses of the fracture surfaces of these samples indicate embrittlement of the material when tested in hydrogen atmosphere
Stable Determination of the Electromagnetic Coefficients by Boundary Measurements
The goal of this paper is to prove a stable determination of the coefficients
for the time-harmonic Maxwell equations, in a Lipschitz domain, by boundary
measurements
Scaling in Plasticity-Induced Cell-Boundary Microstructure: Fragmentation and Rotational Diffusion
We develop a simple computational model for cell boundary evolution in
plastic deformation. We study the cell boundary size distribution and cell
boundary misorientation distribution that experimentally have been found to
have scaling forms that are largely material independent. The cell division
acts as a source term in the misorientation distribution which significantly
alters the scaling form, giving it a linear slope at small misorientation
angles as observed in the experiments. We compare the results of our simulation
to two closely related exactly solvable models which exhibit scaling behavior
at late times: (i) fragmentation theory and (ii) a random walk in rotation
space with a source term. We find that the scaling exponents in our simulation
agree with those of the theories, and that the scaling collapses obey the same
equations, but that the shape of the scaling functions depend upon the methods
used to measure sizes and to weight averages and histograms
Critical Dynamics of Burst Instabilities in the Portevin-Le Chatelier Effect
We investigate the Portevin-Le Chatelier effect (PLC), by compressing Al-Mg
alloys in a very large deformation range, and interpret the results from the
viewpoint of phase transitions and critical phenomena. The system undergoes two
dynamical phase transitions between intermittent (or "jerky") and "laminar"
plastic dynamic phases. Near these two dynamic critical points, the order
parameter 1/\tau of the PLC effect exhibits large fluctuations, and "critical
slowing down" (i.e., the number of bursts, or plastic instabilities, per
unit time slows down considerably).Comment: the published 4-page version is in the PRL web sit
Radiating and non-radiating sources in elasticity
In this work, we study the inverse source problem of a fixed frequency for
the Navier's equation. We investigate that nonradiating external forces. If the
support of such a force has a convex or non-convex corner or edge on their
boundary, the force must be vanishing there. The vanishing property at corners
and edges holds also for sufficiently smooth transmission eigenfunctions in
elasticity. The idea originates from the enclosure method: The energy identity
and new type exponential solutions for the Navier's equation.Comment: 17 page
Mechanisms for slow strengthening in granular materials
Several mechanisms cause a granular material to strengthen over time at low
applied stress. The strength is determined from the maximum frictional force
F_max experienced by a shearing plate in contact with wet or dry granular
material after the layer has been at rest for a waiting time \tau. The layer
strength increases roughly logarithmically with \tau -only- if a shear stress
is applied during the waiting time. The mechanisms of strengthening are
investigated by sensitive displacement measurements and by imaging of particle
motion in the shear zone. Granular matter can strengthen due to a slow shift in
the particle arrangement under shear stress. Humidity also leads to
strengthening, but is found not to be its sole cause. In addition to these time
dependent effects, the static friction coefficient can also be increased by
compaction of the granular material under some circumstances, and by cycling of
the applied shear stress.Comment: 21 pages, 11 figures, submitted to Phys. Rev.
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