Shear-Wave Anisotropy of the Vastus Lateralis During Low-Level Isometric Contraction Measured with Ultrasound...

Skeletal muscle is commonly modeled as a transversely isotropic medium; however, the behavior of its anisotropy under active loading remains insufficiently characterized. In this study, we used ultrasound time-harmonic elastography (THE) to quantify direction-dependent shear-wave speed (SWS) in the vastus lateralis (VL) muscle at rest and during low isometric contraction intensities.

Twenty-six healthy adults (15 men, 11 women; $25.0\pm 4.1$  y) underwent multi-frequency THE (60-80 Hz). The transducer was aligned parallel (longitudinal) and perpendicular (transverse) to VL fascicles, and measurements were acquired at rest and at 15% and 30% of maximal voluntary contraction (MVC). Fractional anisotropy (FA) was computed from $SW{S}_{\Vert }$ and $SW{S}_{\perp }$ , yielding zero for isotropic media. Orientation and contraction effects were tested with repeated-measures analyses.

At rest, longitudinal SWS exceeded transverse SWS ( $2.5\pm 0.2$ vs. $1.4\pm 0.1$  m/s; paired t-test $p<0.01$ ). With contraction, SWS increased to $3.2\pm 0.2$ and $3.8\pm 0.3$  m/s (15%, 30% MVC) along fibers, and to $1.6\pm 0.1$ and $1.8\pm 0.1$  m/s across fibers (all $p<0.01$ ). A two-factor repeated-measures ANOVA on SWS showed main effects of orientation and contraction and a significant interaction (all $p<0.01$ ). FA increased from $0.37\pm 0.04$ at rest to $0.43\pm 0.04$ at 15% and $0.47\pm 0.03$ at 30% MVC ( $p<0.01$ ). No sex- or BMI-related effects were detected.

VL exhibited marked shear-wave anisotropy at rest that increased with low-level contraction intensities, indicating disproportionate stiffening along the fiber direction. THE provides a rapid, cost-effective, orientation-sensitive readout of muscle mechanics that may support studies of neuromuscular function and pathology.