A lower limb prosthesis is able to restore mobility to patients who have lost a limb; however, no current replacement is as moveable and adaptable as the limb that was lost. Therefore, all amputees suffer from a reduction in function at some level. Movement in the transverse plane of a lower limb prosthesis is often negated in a traditional prosthesis, and those devices that do allow for transverse plane motion are set to a single, fixed stiffness, and incapable of adapting to the varying activities of the user. A prototype device has been created that allows for varying stiffness in the transverse plane of a lower limb prosthesis. The variable stiffness torsion adapter (VSTA) functions by way of a movable pivot lever arm that can actively modify the mechanical advantage between the outer housing and the internal spring. The motion of the pivot is perpendicular to the external torque allowing for low power adjustments of the stiffness. Bench tests were performed and demonstrate the ability of the VSTA to vary torsional stiffness between 0.12 and 0.91 N m/deg over a ±30 deg rotational range of motion. The device also includes a mode for fully locked operation. The VSTA may improve the mobility of lower limb amputees by allowing for activity-dependent transverse plane stiffness.