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

Abstract

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; $$25.0\pm 4.1$$ 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). Fractional anisotropy (FA) was computed from $$SWS_{\parallel }$$ S W S ‖ and $$SWS_{\perp }$$ S W S ⊥ , yielding zero for isotropic media. Orientation and contraction effects were tested with repeated-measures analyses. Results At rest, longitudinal SWS exceeded transverse SWS ( $$2.5\pm 0.2$$ 2.5 ± 0.2 vs. $$1.4\pm 0.1$$ 1.4 ± 0.1 m/s; paired t-test $$p p 0.01 ). With contraction, SWS increased to $$3.2\pm 0.2$$ 3.2 ± 0.2 and $$3.8\pm 0.3$$ 3.8 ± 0.3 m/s (15%, 30% MVC) along fibers, and to $$1.6\pm 0.1$$ 1.6 ± 0.1 and $$1.8\pm 0.1$$ 1.8 ± 0.1 m/s across fibers (all $$p p 0.01 ). A two-factor repeated-measures ANOVA on SWS showed main effects of orientation and contraction and a significant interaction (all $$p p 0.01 ). FA increased from $$0.37\pm 0.04$$ 0.37 ± 0.04 at rest to $$0.43\pm 0.04$$ 0.43 ± 0.04 at 15% and $$0.47\pm 0.03$$ 0.47 ± 0.03 at 30% MVC ( $$p p 0.01 ). No sex- or BMI-related effects were detected. Conclusion 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.

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