BIOMECHANICS, MUSCLE ELASTICITY AND ULTRASOUND ELASTOGRAPHY
Biomechanics is the mechanical study of the biological movements of the human body. Understanding and quantifying the muscular forces at play in order to analyze movement is fundamental in many disciplines such as sport, robotics and medicine, where biomechanics helps to better understand certain pathologies, optimize treatments and design prostheses. During movement, the mechanical properties of muscles and tendons are modified to execute and produce the desired force for the desired task. This allows us to perform movements with a great precision (a 5-year-old child can manipulate a variety of objects with a dexterity superior to that of any robot) and to adapt our bodies to our environment to avoid physical injury, for example. In order to study these properties locally at the level of the recruited muscles, scientists have developed increasingly sophisticated non-invasive measurement methods. Recently, the scientific community has turned its attention to the use of ultrasonic shear-wave elastography. This approach takes advantage of an innovative ultrafast ultrasound technology to provide precise information on the local mechanical properties of tissues, based on the measurement of shear wave propagation velocity, which is directly related to the elasticity of the tissue under study. Elastography has already revealed its potential for the diagnosis of certain pathologies in which tissue rigidity is altered, such as breast cancer or liver fibrosis (see Joliot news of January 2024). However, these developments are based on the assumption that the tissues or organs considered are simple, elastic and isotropic. For complex, anisotropic, viscoelastic and non-linear tissues such as muscles, elastography presents certain limitations.
In the 2 publications presented here, the researchers have taken the performance of elastography a step further, in order to quantify the mechanical parameters involved in the case of muscle: anisotropy and non-linearity.
PUBLICATION 1
To quantify anisotropy [Publication 1], the team started from the observation that the elasticity of a muscle is not the same in all directions in space, and developed a "bias push" technique. By cleverly inclining the ultrasound beam in the muscle through a modification of the electronic focusing laws programmed into the ultrasound scanner, the researchers were able, without moving the probe, to generate shear waves in different directions in space, providing information on the anisotropy of the muscle. They have thus, for the first time, quantified the parallel µ// and perpendicular µꓕ shear moduli to muscle fibers, but also the tensile anisotropy factor χE well known in mechanics.
PUBLICATION 2
To quantify nonlinear elastic properties [Publication 2], the researchers applied the theory of acoustoelasticity in an isotropic transverse medium, a geometric medium describing the muscle. This theory, developed by Bied and Gennisson in 2021, explains why the elasticity (hardness) of tissue or organs changes when subjected to external constraints. Building on the "bias push" techniques developed for anisotropy, they developed an experimental protocol with a specific ultrasound sequence to quantify these parameters in a fusiform muscle (the biceps brachii). For the first time in humans, they have quantified not only linear muscle parameters (µ//, µꓕ and χE) but also non-linear parameters (A, H, K).
@ Ricardo Andrade, MIP, Nantes
Taken together, these multiparametric measurements pave the way for comprehensive muscle characterization, enabling us to quantify the force developed by each individual muscle directly in the body in motion. This should help diagnose neuromuscular pathologies, improve management of aging muscle and enhance sports performance.
Contact : Jean-Luc Gennisson (jean-luc.gennisson@universite-paris-saclay.fr)