Certain aspects relate to systems and techniques for robotic medical instrument system. The medical system can include an elongated shaft configured for insertion into a patient. The system can also include a pull wire extending along the elongated shaft. The pull wire can be actuatable to articulate the elongated shaft. The medical system can include a dynamic pulley coupled to the pull wire, the dynamic pulley configured to collapse during rotation in a first direction and to expand during rotation in a second direction opposite the first direction.