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

Purpose: 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. Methods: Twenty-six healthy adults (15 men, 11 women; 25.0 +/- 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). The anisotropy index (AI) was defined as AI = SWS_parallel / SWS_perpendicular. Orientation and contraction effects were tested with repeated-measures analyses. Results: At rest, longitudinal SWS exceeded transverse SWS (2.5 +/- 0.2 vs. 1.4 +/- 0.1 m/s; paired t-test p < 0.01). With contraction, SWS increased to 3.2 +/- 0.2 and 3.8 +/- 0.3 m/s (15%, 30% MVC) along fibers, and to 1.6 +/- 0.1 and 1.8 +/- 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). AI increased from 1.7 +/- 0.1 at rest to 2.0 +/- 0.1 at 15% and 2.1 +/- 0.1 at 30% MVC (p < 0.01). No sex- or BMI-related effects were detected. Conclusion: The 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.