To characterize muscle tissue as a viscoelastic transversely isotropic material, we use rotational volumetric shear wave elasticity imaging (SWEI) and group speed viscoelastic analysis. We performed these measurements in vivo in the vastus lateralis of two healthy volunteers, both at rest and in varying degrees of contraction. We found a high degree of repeatability in our at rest elasticity measurements, determining that the stiffness along the fibers is higher than across the fibers. The viscosity at rest is approximately equal and low in both directions. In our contraction analysis, we found that stiffness increases substantially along the fibers and increases slightly across the fibers with increasing contraction. No clear trends were observed between viscosity and contraction level. Future work will improve the method of model-based viscoelastic characterization within the con of transversely isotropic materials and expand this analysis to more subjects.