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

Abstract

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; <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>25.0</mn> <mo>±</mo> <mn>4.1</mn></mrow> </math>  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 <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>S</mi> <mi>W</mi> <msub><mi>S</mi> <mo>‖</mo></msub> </mrow> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>S</mi> <mi>W</mi> <msub><mi>S</mi> <mo>⊥</mo></msub> </mrow> </math> , yielding zero for isotropic media. Orientation and contraction effects were tested with repeated-measures analyses. At rest, longitudinal SWS exceeded transverse SWS ( <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>2.5</mn> <mo>±</mo> <mn>0.2</mn></mrow> </math> vs. <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>1.4</mn> <mo>±</mo> <mn>0.1</mn></mrow> </math>  m/s; paired t-test <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi> <mo><</mo> <mn>0.01</mn></mrow> </math> ). With contraction, SWS increased to <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>3.2</mn> <mo>±</mo> <mn>0.2</mn></mrow> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>3.8</mn> <mo>±</mo> <mn>0.3</mn></mrow> </math>  m/s (15%, 30% MVC) along fibers, and to <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>1.6</mn> <mo>±</mo> <mn>0.1</mn></mrow> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>1.8</mn> <mo>±</mo> <mn>0.1</mn></mrow> </math>  m/s across fibers (all <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi> <mo><</mo> <mn>0.01</mn></mrow> </math> ). A two-factor repeated-measures ANOVA on SWS showed main effects of orientation and contraction and a significant interaction (all <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi> <mo><</mo> <mn>0.01</mn></mrow> </math> ). FA increased from <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>0.37</mn> <mo>±</mo> <mn>0.04</mn></mrow> </math> at rest to <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>0.43</mn> <mo>±</mo> <mn>0.04</mn></mrow> </math> at 15% and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>0.47</mn> <mo>±</mo> <mn>0.03</mn></mrow> </math> at 30% MVC ( <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi> <mo><</mo> <mn>0.01</mn></mrow> </math> ). 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.