Modern multifunctional hand prostheses have many degrees of freedom, but strong limitations on weight and size. The actuators commonly used in these systems are relatively large and heavy, so their number should be kept as low as possible. This is often accomplished by underactuation, which causes a natural motion of the fingers when grasping an object but reduces the ability to execute a variety of grasps. To remedy this, a series of locking mechanisms can be implemented to fix the position of one or more joints. This paper focuses on the development of such a joint locking system that could be used in anthropomorphic prosthetic fingers. Two lock concepts are implemented in a single-joint test setup and evaluated. A gear-based concept is tested, though its actuation requirements prove too high for viable implementation in a prosthesis. A mechanism based on friction amplification is shown to exhibit self-locking properties, which allows for a minimal lock actuation force while withstanding joint torques of over 2 Nm. The friction amplification mechanism is found suitable for prosthesis use, and will be developed further for implementation in a future prosthesis prototype.