In addition to changes in the central nervous system, many changes can occur in the composition and structure of skeletal muscles after a hemispheric stroke. The mechanical behavior of skeletal muscles is linked to the density and structural arrangement of key constituents. Yet, little is known about changes in post-stroke muscle mechanical properties such as viscoelasticity. The aim of this study was to explore the frequency-dependent changes in shear wave (SW) velocity as a potentially informative feature accompanying changes in muscle viscoelastic properties under passive and active conditions in hemiplegic stroke. We used the ultrasound SuperSonic Imaging technique to induce and measure SW propagation in the biceps brachii muscle for both the paretic and contralateral limbs in three hemiplegic stroke survivors during passive and submaximal voluntary muscle contractions. We found that for all subjects, the muscles on both the paretic and non-paretic sides demonstrated large dispersion (i.e., a change in SW phase velocities as a function of frequency within each contraction level) under both passive and active conditions, although muscles on the paretic side displayed larger dispersion. In addition, for a range of frequencies from 108-756 Hz, the SW phase velocity was higher in active nonparetic muscles compared to those of paretic side with an increase of 42% at 756 Hz. This is in contrast with the muscle response under passive condition where the SW phase velocity exhibited a 97 % increase at 765Hz on the paretic side compared to the non-paretic side. These results suggest the mechanical properties are altered for stroke-affected muscles, which may be a result of changes in the muscle extracellular matrix composition. Further, this study provides evidence that there are changes in tissue mechanical properties and that may consequently influence muscle function.