Julien Petit

Maximal and sustained isokinetic lower-limb muscle strength in hospitalized older people

Maximal strength decreases with aging whereas sustained strength is less affected. Strength decline may be worsened by hospitalization. The aim of this study was to estimate the maximal and sustained isokinetic muscle strength of lower limbs in hospitalized elderly subjects. We evaluated 43 hospitalized elderly subjects (86 ± 5 years), 28 elderly community‐dwelling control subjects (75.4 ± 6.2 years), and 25 young subjects (28.2 ± 3.7 years). Among hospitalized subjects, 30 underwent isokinetic evaluation at clinical stability (T0) and again 1 month later (T1). Maximal peak torque (MPT) of the plantarflexors was measured at 30° and 60°/s, and knee flexors and extensors at 90°/s. Evolution of the MPT and the endurance coefficient (EC) on 20 repetitions of plantarflexion at 60°/s were calculated. MPT of plantarflexors and knee flexors and extensors had improved at T1 compared with T0, in hospitalized subjects. MPT evolution and EC values during the sustained strength test revealed no decrease in strength over time in hospitalized subjects at T0 and at T1 compared with community‐dwelling control subjects and young subjects. In hospitalized subjects, the absence of an initial phase of fast decrease in muscle strength, which is observed in young subjects during the sustained strength test, could explain this result. It could be related to the modifications of muscle‐fiber composition described in elderly subjects and enhanced by hospitalization. Muscle Nerve, 2007

Break-up dynamics of fluctuating liquid threads

The thinning dynamics of a liquid neck before break-up, as may happen when a drop detaches from a faucet or a capillary, follows different rules and dynamic scaling laws depending on the importance of inertia, viscous stresses, or capillary forces. If now the thinning neck reaches dimensions comparable to the thermally excited interfacial fluctuations, as for nanojet break-up or the fragmentation of thermally annealed nanowires, these fluctuations should play a dominant role according to recent theory and observations. Using near-critical interfaces, we here fully characterize the universal dynamics of this thermal fluctuation-dominated regime and demonstrate that the cross-over from the classical two-fluid pinch-off scenario of a liquid thread to the fluctuation-dominated regime occurs at a well-defined neck radius proportional to the thermal length scale. Investigating satellite drop formation, we also show that at the level of the cross-over between these two regimes it is more probable to produce monodisperse droplets because fluctuation-dominated pinch-off may allow the unique situation where satellite drop formation can be inhibited. Nonetheless, the interplay between the evolution of the neck profiles from the classical to the fluctuation-dominated regime and the satellites’ production remains to be clarified.

A comparison between two fractional multimodels structures for rat muscles modelling

Abstract Peroneus digiti quarti and peroneus brevis muscles responses of the rat are studied for 10 Hz pulses stimulations. A comparison between two multimodels structures is presented. These multimodels include fractional sub-models. The multimodels allow distinguishing contraction and relaxation phases for identification. Fractional orders used in the sub-models lead to minimize the size of transfer functions. The present study develops the multimodels structure earlier established, by including variation functions of extra parameters for IIA and IIB fibres and to explain muscle response for Motor Units (MU) stimulations at 10 Hz. The multimodels explains rat striated muscle responses, and so, allow its inclusion in a future muscle computer model.

Letter to the editor: On the non-singularity of the quasi-Newton-least squares method

We show that, for an affine problem, the approximate Jacobian of the Quasi-Newton-Least Squares method cannot become singular before the solution has been reached.

Delay-induced Turing-like waves for one-species reaction-diffusion model on a network

A one-species time-delay reaction-diffusion system defined on a complex network is studied. Traveling waves are predicted to occur following a symmetry-breaking instability of a homogeneous stationary stable solution, subject to an external nonhomogeneous perturbation. These are generalized Turing-like waves that materialize in a single-species populations dynamics model, as the unexpected byproduct of the imposed delay in the diffusion part. Sufficient conditions for the onset of the instability are mathematically provided by performing a linear stability analysis adapted to time-delayed differential equations. The method here developed exploits the properties of the Lambert W-function. The prediction of the theory are confirmed by direct numerical simulation carried out for a modified version of the classical Fisher model, defined on a Watts-Strogatz network and with the inclusion of the delay.

Eddies and interface deformations induced by optical streaming

We study flows and interface deformations produced by the scattering of a laser beam propagating through non absorbing turbid fluids. Light scattering produces a force density resulting from the transfer of linear momentum from the laser to the scatterers. The flow induced in the direction of the beam propagation, called “optical streaming“, is also able to deform the interface separating the two liquid phases and to produce wide humps. The viscous flow taking place in these two liquid layers is solved analytically, in one of the two liquid layers with a stream function formulation, as well as numerically in both fluids using a Boundary Integral Element Method. Quantitative comparisons are shown between the numerical and analytical flow patterns. Moreover, we present predictive simulations dedicated to the effects of the geometry, of the scattering strength and of the viscosities, on both the flow pattern and the deformation of the interface. Theoretical arguments are finally put forth to explain the robustness of the emergence of secondary flows in a two-layer fluid system.

Equivalence of QN-LS and BQN-LS for affine problems

Previously, we studied methods to solve the coupled system of non-linear equations F ( g ) = p and S ( p ) = g . In this paper we take a closer look at two of them, the Quasi-Newton method with Least Squares Jacobian (QN-LS) and the Block Quasi-Newton method with Least Squares Jacobian (BQN-LS). We show that both are algebraically equivalent if one of the operators ( F or S ) is affine. This implies that for this type of problem there is no reason to use BQN-LS, as the results will be the same but for a higher computational cost.

Pattern formation in a two-component reaction–diffusion system with delayed processes on a network

Reaction–diffusion systems with time-delay defined on complex networks have been studied in the framework of the emergence of Turing instabilities. The use of the Lambert W-function allowed us to get explicit analytic conditions for the onset of patterns as a function of the main involved parameters, the time-delay, the network topology and the diffusion coefficients. Depending on these parameters, the analysis predicts whether the system will evolve towards a stationary Turing pattern or rather to a wave pattern associated to a Hopf bifurcation. The possible outcomes of the linear analysis overcome the respective limitations of the single-species case with delay, and that of the classical activator–inhibitor variant without delay. Numerical results gained from the Mimura–Murray model support the theoretical approach.

Analysis of muscle length effect on an S type motor-unit fractional multi-model

Muscle models are useful in bio-inspired robotic, because they allow to reproduce accurately natural motion. When they are used for robotic issue, they need to be compact and embeddable. The non-integer model order has the advantage to ensure a parametrical parsimony that permits to implant it easily on an embedded system. Thus, a fractional multi-model of muscle was identified and presented in later paper. This model is able to predict the response of a motor unit to an electrical stimulation, considering isometric contractions (that is to say, muscle length is constant). There are three different physiological types of motor unit (FR, FF, and S). The aim of this work is to study muscle length impact on the multi-model and the limitations of the linear multi-model. Previous paper was published using FR type motor unit. In this paper, the results of the study using S type motor unit are presented.

Study of the peroneus digiti quarti muscle length effect on a motor unit fractional multi-model

Skeletal muscle models have several applications. In medicine, they can improve the understanding of these organs, and if inverted they can also be used for muscle rehabilitation by functional electrical stimulation. Moreover they are also useful in bio-inspired robotic, because they allow to reproduce accurately natural motion. When they are used for robotic issue, they need to be compact and embeddable. Thus, a multi-model of muscle was identified and presented in previous papers. This model is able to predict the response of a motor unit to an electrical stimulation, considering isometric contractions (that is to say, muscle length is constant). In this paper, muscle length effect on the multi-model is studied and the limitations of the linear multi model are presented.