Acousto-elasticity of Transversely Isotropic Incompressible Soft Tissues: Characterization of Skeletal Striated Muscle

TL;DR

This study develops an acousto-elastic theory for shear waves in transversely isotropic, incompressible soft tissues and measures how muscle stress affects wave speed, revealing individual variability and potential clinical applications.

Contribution

The paper introduces a novel acousto-elastic model for shear waves in anisotropic soft tissues and provides experimental data linking muscle stress to elastic properties.

Findings

Shear wave speed varies with muscle stress and individual characteristics.

The apparent shear elastic modulus changes inversely with voluntary contraction.

Muscle fiber type influences the elastic response and maximum voluntary contraction.

Abstract

Using shear wave elastography, we measure the changes in the wave speed with the stress produced by a striated muscle during isometric voluntary contraction. To isolate the behaviour of an individual muscle from complementary or antagonistic actions of adjacent muscles, we select the flexor digiti minimi muscle, whose sole function is to extend the little finger. To link the wave speed to the stiffness, we develop an acousto-elastic theory for shear waves in homogeneous, transversely isotropic, incompressible solids subject to uniaxial stress. We then provide measurements of the apparent shear elastic modulus along, and transversely to, the fibre axis for six healthy human volunteers of different age and sex. The results display a great variety across the six subjects. We find that the slope of the apparent shear elastic modulus along the fibre direction changes inversely to the maximum voluntary contraction (MVC) produced by the volunteer. We propose an interpretation of our results by introducing the S (slow) or F (fast) nature of the fibres, which harden the muscle differently and accordingly, produce different MVCs. This work opens the way to measuring the elastic stiffness of muscles in patients with musculoskeletal disorders or neurodegenerative diseases.