New developments in elastography applied to muscle biomechanics

Abstract

Skeletal muscle contraction is a complex phenomenon involving several biochemical, bioelectrical, and biomechanical processes. Such processes occur at different scales, from the micro to the macro level, given as a final result the force production. Electromyography and dynamometry have been the most widely used methods to study the biomechanical properties of skeletal muscle. More recently, elastography has begun to be applied to account for its mechanical properties. Thus, this technique, initially described as a complementary tool for the medical diagnosis of certain pathologies, has added a new dimension to the biomechanical study of this tissue. Particularly, it accounts for the skeletal muscle as a soft solid, thus enabling studying the different processes and physiological phenomena associated with the change of its shear elastic modulus. In this context, the main goal of the present thesis is to provide new theoretical, practical, and methodological frameworks to continue delving into the study of muscle biomechanics through elastography. The text begins by introducing the main concepts related to structural and physiological aspects of muscle contraction. Furthermore, it addresses the principles on which elastography is based to model and measure the shear elasticity of skeletal muscle in vivo and non-invasively. Then, the load-sharing between the elbow flexor synergistic muscles is addressed through the characterization of the dynamics of the relative longitudinal deformation of each muscle, by combining the acousto-elasticity theory, the short-range stiffness principle, and the ligand-binding approach. This provides a new framework to assess skeletal muscle contraction, which involves macro and microscopic aspects of force production. Therefore, it was also incorporated into the development of a static optimization model to calculate the contributions of individual muscle forces to the total torque. In this regard, the thesis contributes significantly to this long-standing challenge in biomechanics. On the other hand, the thesis also assessed the time-dependence of muscle synergism between the elbow flexors muscles. As the reference shear wave elastography methods are not suitable for this purpose, we designed a new version of a non-ultrasound surface wave elastography method (developed previously by our group at the Laboratory of Ultrasound Acoustics of the Faculty of Sciences (UdelaR)), which has a higher sampling rate and allows to measure in more than one muscle simultaneously. In this concern, the thesis shows how this new realization of such a method extends the current experimental conditions to perform elastographic measurements on skeletal muscle. This has the potential to widen the scope of elastography-driven research in biomechanics. In summary, the thesis makes important contributions to muscle biomechanics and opens up future lines of research in this field. In this regard, the text concludes by describing the general conclusions and the perspectives for future studies, based on the findings and developments of the present work.