We investigated alterations in material properties such as elasticity and viscoelasticity of stroke-affected muscles using ultrasound induced shear waves and mechanical models. We used acoustic radiation force to generate shear waves along fascicles of biceps muscles and measured their propagation velocity. The shear wave data were collected in muscles of thirteen hemiplegic stroke survivors under passive conditions at 90°, 120°, and 150° elbow flexion angles. In a viscoelastic medium, as opposed to a purely elastic medium, shear wave propagation velocity depends on the frequency content of the induced wave. Therefore, in addition to shear wave group velocity (GpV), we also estimated frequency dependent phase velocity (PhV). We found significantly higher GpVs and PhVs in stroke-affected muscles (p<0.05). The velocity data were used to estimate shear elasticity and viscosity using elastic and viscoelastic material models. A pure elastic model showed increased shear elasticity in stroke-affected muscles (p < 0.05). The Voigt model estimates of viscoelastic properties were also significantly different between stroke-impaired and non-impaired muscles. We observed significantly larger model-estimated viscosity values on the stroke-affected side at elbow flexion angles of 120° and 150°. Furthermore, the creep behavior (tissue strain resulting from the application of sudden constant stress) of the model was also different between muscles of the paretic and non-paretic side. We speculate that these changes are associated with structural disruption of muscles after stroke and may potentially affect force generation from muscle fibers as well as transmission of force to tendons.
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