User interface devices, systems, and methods can be used for selectively bending of, altering the bend characteristics of, and/or altering the lengths of catheter bodies, guidewires, steerable trocars, and other flexible structures inserted into a patient during use. Optionally, a housing is coupled to a proximal end of a catheter, and movement of the housing by a hand of a system user is sensed and used as a movement command for articulation of the catheter. Alternatively, a sensor can be coupled to an elongate flexible body flexing outside of the patient so as to alter bending of a catheter within the patient. Movements generated through a combination of manual manipulation and powered articulations are facilitated.