Contractile and Hemodynamic Modulation of Skeletal Muscle Viscoelasticity Quantified In Vivo by Ultrasound Time-Harmonic Elastography.

Skeletal muscle is a living, perfused soft tissue whose viscoelastic behavior is shaped by both voluntary contraction and hemodynamic state. However, the independent and superimposed contributions of contractile loading and blood flow restriction (BFR) have not been quantified simultaneously in real time. Twenty-six healthy adults underwent multi-frequency ultrasound time-harmonic elastography (THE, 60-80 Hz) of the vastus lateralis under six conditions: rest, 15% and 30% maximal voluntary contraction (MVC) before BFR, passive BFR after 4 min of cuff inflation, and 15% and 30% MVC shortly after cuff release. Shear wave speed (SWS), reflecting elasticity, and penetration rate (PR), reflecting inverse viscous damping, were extracted using the k-MDEV inversion algorithm. BFR significantly elevated SWS at all three contraction levels relative to the corresponding pre-BFR measurements (Holm-corrected p ≤ 0.011; dz = 0.54-2.13). PR decreased during resting BFR (dz = 1.34, p < 0.001) and at 15% MVC after cuff release (dz = 0.94, p < 0.001), but not at 30% MVC (dz = 0.21, p = 0.294). BFR-related changes reduced the SWS-force slope by 15% and the PR-force slope by 40%. Men exhibited a greater BFR-induced increase in resting SWS than women. These findings show that THE can distinguish contractile and hemodynamic contributions to skeletal-muscle viscoelasticity and provide complementary information on elastic and dissipative tissue behavior in vivo. STATEMENT OF SIGNIFICANCE: Skeletal muscle is a living, perfused soft tissue whose mechanical behavior reflects the interaction of contractile structures, extracellular matrix, and fluid-containing compartments. Most elastography studies focus primarily on contraction-related stiffness changes. Here, we show that blood flow restriction modifies both shear wave speed and penetration rate even without voluntary contraction, and that these hemodynamic effects remain detectable during the early post-release phase. The distinct responses of the two parameters indicate complementary sensitivity to elastic and dissipative tissue behavior. These findings establish ultrasound time-harmonic elastography as a non-invasive approach for probing coupled contractile and fluid-mediated contributions to skeletal-muscle viscoelasticity in vivo.