Shear-wave (SW) ultrasound elastography is both a clinical and research tool that is increasingly being used to quantify the material properties of muscle. However, how SW velocity relates to stiffness changes on the joint- and musclelevels is poorly understood. Therefore, the purpose of this work was to develop a biomechanical model to estimate plantar flexor muscle stiffness, and measure joint stiffness, joint-based estimates of muscle stiffness, and medial gastrocnemius (MG) SW velocity under different activations (0, 20, and 40%) to quantify the relationships between 1) joint stiffness and jointbased estimates of muscle stiffness; 2) joint stiffness and MG SW velocity; and 3) joint-based estimates of muscle stiffness and MG SW velocity. Our main findings include strong relationships between 1) joint stiffness and joint-based estimates of muscle stiffness ( R2= 0.70) and 2) joint stiffness and MG SW velocity ( R2= 0.66), and a weak relationship between joint-based estimates of muscle stiffness and MG SW velocity ( R2= 0.24). These findings further our understanding of SW velocity measures in muscle and provide a biomechanical model to decompose muscle stiffness from joint stiffness.