We previously have demonstrated the use of ultrasonic rotational 3D SWEI to measure all three mechanical properties necessary to fully characterize the vastus lateralis muscle in vivo as an incompressible transversely isotropic material: transverse shear modulusmu_{T}, longitudinal shear modulusmu_{L}, and tensile anisotropychi_{E}. In this work we investigate how varying knee flexion angle affects the mechanical properties of the vastus lateralis. As knee flexion angle increased, mu_{L}, \chi_{E}, andchi_((_{L}-_{T})/_{T}) all increased, whilemu_{T} remained consistent across knee angles. At all nine knee flexion angles investigated in a healthy volunteerchi_{E} >chi_, indicating that these are independent parameters.