787 research outputs found
The effect of accounting for biarticularity in hip flexor and hip extensor joint torque representations
Subject-specific torque-driven models have ignored biarticular effects at the hip. The aim of this study was to establish the contribution of monoarticular hip flexors and hip extensors to total hip flexor and total hip extensor joint torques for an individual and to investigate whether torque-driven simulation models should consider incorporating biarticular effects at the hip joint. Maximum voluntary isometric and isovelocity hip flexion and hip extension joint torques were measured for a single participant together with surface electromyography. Single-joint and two-joint representations were fitted to the collected torque data and used to determine the maximum voluntary joint torque capacity. When comparing two-joint and single-joint representations, the single-joint representation had the capacity to produce larger maximum voluntary hip flexion torque (larger by around 9% of maximum torque) and smaller maximum voluntary hip extension torque (smaller by around 33% of maximum torque) with the knee extended. Considering the range of kinematics found for jumping movements, the single-joint hip flexors had the capacity to produce around 10% additional torque, while the single joint hip extensors had about 70% of the capacity of the two-joint representation. Two-joint representations may overcome an over-simplification of single-joint representations by accounting for biarticular effects, while building on the strength of determining subject-specific parameters from measurements on the participant
Are joint torque models limited by an assumption of monoarticularity?
This study determines whether maximal voluntary ankle plantar flexor torque could be more accurately
represented using a torque generator that is a function of both knee and ankle kinematics. Isovelocity and
isometric ankle plantar flexor torques were measured on a single participant for knee joint angles of 111°
to 169° (approximately full extension) using a Contrex MJ dynamometer. Maximal voluntary torque was
represented by a 19-parameter two-joint function of ankle and knee joint angles and angular velocities with
the parameters determined by minimizing a weighted root mean square difference between measured torques
and the two-joint function. The weighted root mean square difference between the two-joint function and
the measured torques was 10 N-m or 3% of maximum torque. The two-joint function was a more accurate
representation of maximal voluntary ankle plantar flexor torques than an existing single-joint function where
differences of 19% of maximum torque were found. It is concluded that when the knee is flexed by more than
40°, a two-joint representation is necessary
Neutrino Large Mixing in Universal Yukawa Coupling Model with Small Violation
We have analyzed the possibility that the universal Yukawa coupling
(democratic mass matrix) with small violations of Dirac and Majorana neutrinos
can induce the large mixing of neutrinos through the seesaw mechanism. The
possibility can be achieved by the condition that the violation parameters of
Majorana neutrinos are sufficiently smaller than the violation parameters of
Dirac neutrinos. Allowed regions of the violation parameters producing the
observed neutrino mass hierarchy and large neutrino mixing are not so
restricted at present in contrast to the violation parameters for quark sector.Comment: 14 pages, 4figure
An isovelocity dynamometer method to determine monoarticular and biarticular muscle parameters
This study aimed to determine whether subject-specific individual muscle models for the ankle plantarflexors could be obtained from single joint isometric and isovelocity maximum torque measurements in combination with a model of plantarflexion. Maximum plantarflexion torque measurements were taken on one subject at six knee angles spanning full flexion to full extension. A planar three segment (foot, shank and thigh), two muscle (soleus and gastrocnemius) model of plantarflexion was developed. Seven parameters per muscle were determined by minimizing a weighted root mean square difference (wRMSD) between the model output and the experimental torque data.
Valid individual muscle models were obtained using experimental data from only two knee angles giving a wRMSD score of 16 Nm, with values ranging from 11–17 Nm for each of the six knee angles. The robustness of the methodology was confirmed through repeating the optimization with perturbed experimental torques (±20%) and segment lengths (±10%) resulting in wRMSD scores of between 13–20 Nm. Hence, good representations of maximum torque can be achieved from subject-specific individual muscle models determined from single joint maximum torque measurements. The proposed methodology could be applied to muscle-driven models of human movement with the potential to improve their validity
Hierarchical Neutrino Mass Matrices, CP violation and Leptogenesis
In this work we study examples of hierarchical neutrino mass matrices
inspired by family symmetries, compatible with experiments on neutrino
oscillations, and for which there is a connection among the low energy CP
violation phase associated to neutrino oscillations, the phases appearing in
the amplitude of neutrinoless double beta decay, and the phases relevant for
leptogenesis. In particular, we determine the predictions from a texture based
on an underlying SU(3) family symmetry together with a GUT symmetry, and a
strong hierarchy for the masses of the heavy right handed Majorana masses. We
also give some examples of inverted hierarchies of neutrino masses, which may
be motivated in the context of U(1) family symmetries.Comment: 34 pages. Replaced with published version -typos, corrections and
references adde
Minimal Scenarios for Leptogenesis and CP Violation
The relation between leptogenesis and CP violation at low energies is
analyzed in detail in the framework of the minimal seesaw mechanism. Working,
without loss of generality, in a weak basis where both the charged lepton and
the right-handed Majorana mass matrices are diagonal and real, we consider a
convenient generic parametrization of the Dirac neutrino Yukawa coupling matrix
and identify the necessary condition which has to be satisfied in order to
establish a direct link between leptogenesis and CP violation at low energies.
In the context of the LMA solution of the solar neutrino problem, we present
minimal scenarios which allow for the full determination of the cosmological
baryon asymmetry and the strength of CP violation in neutrino oscillations.
Some specific realizations of these minimal scenarios are considered. The
question of the relative sign between the baryon asymmetry and CP violation at
low energies is also discussed.Comment: 36 pages, 5 figures; minor corrections and references updated. Final
version to appear in Phys. Rev.
One-phonon coherent neutron scattering from certain polycrystalline materials
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32012/1/0000054.pd
Bi-large Neutrino Mixing and Mass of the Lightest Neutrino from Third Generation Dominance in a Democratic Approach
We show that both small mixing in the quark sector and large mixing in the
lepton sector can be obtained from a simple assumption of universality of
Yukawa couplings and the right-handed neutrino Majorana mass matrix in leading
order. We discuss conditions under which bi-large mixing in the lepton sector
is achieved with a minimal amount of fine-tuning requirements for possible
models. From knowledge of the solar and atmospheric mixing angles we determine
the allowed values of sin \theta_{13}. If embedded into grand unified theories,
the third generation Yukawa coupling unification is a generic feature while
masses of the first two generations of charged fermions depend on small
perturbations. In the neutrino sector, the heavier two neutrinos are model
dependent, while the mass of the lightest neutrino in this approach does not
depend on perturbations in the leading order. The right-handed neutrino mass
scale can be identified with the GUT scale in which case the mass of the
lightest neutrino is given as (m_{top}^2/M_{GUT}) sin^2 \theta_{23} sin^2
\theta_{12} in the limit sin \theta_{13} = 0. Discussing symmetries we make a
connection with hierarchical models and show that the basis independent
characteristic of this scenario is a strong dominance of the third generation
right-handed neutrino, M_1, M_2 < 10^{-4} M_3, M_3 = M_{GUT}.Comment: typos correcte
Relating the Lorentzian and exponential: Fermi's approximation,the Fourier transform and causality
The Fourier transform is often used to connect the Lorentzian energy
distribution for resonance scattering to the exponential time dependence for
decaying states. However, to apply the Fourier transform, one has to bend the
rules of standard quantum mechanics; the Lorentzian energy distribution must be
extended to the full real axis instead of being bounded from
below (``Fermi's approximation''). Then the Fourier transform
of the extended Lorentzian becomes the exponential, but only for times , a time asymmetry which is in conflict with the unitary group time evolution
of standard quantum mechanics. Extending the Fourier transform from
distributions to generalized vectors, we are led to Gamow kets, which possess a
Lorentzian energy distribution with and have exponential
time evolution for only. This leads to probability predictions
that do not violate causality.Comment: 23 pages, no figures, accepted by Phys. Rev.
Enlarging the window for radiative leptogenesis
We investigate the scenario of resonant thermal leptogenesis, in which the
leptonic asymmetries are generated through renormalization group corrections
induced at the leptogenesis scale. In the framework of the standard model
extended by three right-handed heavy Majorana neutrinos with masses M1 = M2 <<
M3 at some high scale, we show that the mass splitting and CP-violating effects
induced by renormalization group corrections can lead to values of the CP
asymmetries large enough for a successful leptogenesis. In this scenario, the
low-energy neutrino oscillation data can also be easily accommodated. The
possibility of having an underlying symmetry behind the degeneracy in the
right-handed neutrino mass spectrum is also discussed.Comment: 20 pages, 2 figures, final version to appear in Phys. Lett.
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