In vivo assessment of shear modulus along the fibers of pennate muscle during passive lengthening and contraction using steered ultrasound push beams

Ultrasound shear wave elastography (SWE) has emerged as a promising non-invasive method for muscle evaluation by assessing the propagation velocity of an induced shear wavefront. In skeletal muscles, the propagation of shear waves is complex, depending not only on the mechanical and acoustic properties of the tissue but also upon its geometry. This study aimed to comprehensively investigate the influence of muscle pennation angle on the shear wave propagation, which is directly related to the shear modulus. A novel elastography method based on steered pushing beams (SPB) was used to assess the shear modulus along the fibers of the gastrocnemius medialis (pennate) muscle in twenty healthy volunteers. Ultrasound scans were performed during passive muscle lengthening (n = 10) and submaximal isometric contractions (n = 10). The shear modulus along the fibers was compared to the apparent shear modulus, as commonly assessed along the muscle shortening direction using conventional SWE sequences. The shear modulus along the muscle fibers was significantly greater than the apparent shear modulus for passive dorsiflexion angles, while not significantly different throughout the range of plantar flexion angles (i.e., under any or very low tensile loads). The concomitant decrease in pennation angle along with the gradual increase in the shear modulus difference between the two methods as the muscle lengthens, strongly indicates that non-linear elasticity exerts a greater influence on wave propagation than muscle geometry. In addition, significant differences between methods were found across all submaximal contractions, with both shear modulus along the fibers and the pennation angle increasing with the contraction intensity. Specifically, incremental contraction intensity led to a greater bias than passive lengthening, which could be partly explained by distinct changes in pennation angle. Overall, the new SPB sequence provides a rapid and integrated geometrical correction of shear modulus quantification in pennate muscles, thereby eliminating the necessity for specialized systems to align the ultrasound transducer array with the fiber's orientation. We believe that this will contribute for improving the accuracy of SWE in biomechanical and clinical settings.