In this work, we detail the design and verification of a novel, 3D-printed, flexible knee brace with an embedded magnetic angle sensor for monitoring joint kinematics. The brace's torsional stiffness can be selectively modified by applying elastic bands of varying thickness. Through benchtop tests and finite element analysis simulations, we characterize the mechanical behavior of the knee brace and determine estimates of torsional stiffness across a range of band thicknesses. To demonstrate the ability to modulate knee joint loading in a real-world scenario, we report results of a pilot study in which able-bodied subjects wear the device during treadmill walking and seated flexion-extension tasks.