An instrument system includes a flexible shaft having a proximal portion and a distal portion, a backend mechanism coupled to the proximal portion of the flexible shaft, and a plurality of tendons. The backend mechanism includes a plurality of capstans. Each capstan engages a respective drive motor that rotates to cause rotation of the capstan. A first capstan of the plurality of capstans is engageable with a first respective drive motor while a second capstan of the plurality of capstans is disengaged from a second respective drive motor. Each of the plurality of tendons includes a proximal section coupled to one of the plurality of capstans and a distal section coupled to a member disposed at the distal portion of the flexible shaft.