A surgical instrument is disclosed. It comprises a handle； and an elongated shaft assembly extending distally from the handle. The elongated shaft assembly comprises a first tubular member including a first flexible portion with a first preferential bending direction； a second tubular member including s second flexible portion with a second preferential bending direction； and a third tubular member including a third flexible portion with a third preferential bending direction and a direction of bending resistance. The first tubular member, the second tubular member, and the third tubular member are coaxially located, and the third tubular member is moveable relative to the first tubular member and the second tubular member. The third tubular member prevents articulation of the first tubular member and the second tubular member when movement of the third tubular member aligns the direction of bending resistance with at least one of the first preferential bending direction and the second preferential bending direction.
