An apparatus for operating on tissue, the apparatus comprising: (a) a body assembly; (b) a shaft assembly extending distally from the body assembly, wherein the shaft assembly defines a longitudinal axis; (c) an acoustic waveguide, wherein the waveguide comprises a flexible portion; (d) an articulation section coupled with the shaft assembly, wherein a portion of the articulation section encompasses the flexible portion of the waveguide, wherein the articulation section further comprises: (i) a first member, and (ii) a second member, wherein the second member is longitudinally translatable relative to the first member; (e) an articulation control assembly configured to move relative to the shaft assembly in order to longitudinally translate the second member relative to the first member; (f) an end effector comprising an ultrasonic blade in acoustic communication with the waveguide; and (g) an articulation lock, wherein the articulation lock is configured to prevent movement of the articulation control assembly relative to the shaft assembly, wherein the shaft assembly comprises an outer sheath configured to translate from a first position to a second position, wherein articulation lock comprises: (i) a plurality of teeth located on the articulation control assembly, and (ii) a pin fixed to the outer sheath of the shaft assembly, wherein the pin is configured to mesh with the plurality of teeth when the outer sheath is in the second position, wherein the body assembly further comprises a trigger, wherein the trigger is configured to pivot toward or away from the body assembly from a first pivotal position to a second pivotal position, and wherein the outer sheath is configured to translate from the first position to the second position in response to the trigger pivoting from the first pivotal position to the second pivotal position. (a) a body assembly; (b) a shaft assembly extending distally from the body assembly, wherein the shaft assembly defines a longitudinal axis; (c) an acoustic waveguide, wherein the waveguide comprises a flexible portion; (d) an articulation section coupled with the shaft assembly, wherein a portion of the articulation section encompasses the flexible portion of the waveguide, wherein the articulation section further comprises: (i) a first member, and (ii) a second member, wherein the second member is longitudinally translatable relative to the first member; (i) a first member, and (ii) a second member, wherein the second member is longitudinally translatable relative to the first member; (e) an articulation control assembly configured to move relative to the shaft assembly in order to longitudinally translate the second member relative to the first member; (f) an end effector comprising an ultrasonic blade in acoustic communication with the waveguide; and (g) an articulation lock, wherein the articulation lock is configured to prevent movement of the articulation control assembly relative to the shaft assembly, wherein the shaft assembly comprises an outer sheath configured to translate from a first position to a second position, wherein articulation lock comprises: (i) a plurality of teeth located on the articulation control assembly, and (ii) a pin fixed to the outer sheath of the shaft assembly, (i) a plurality of teeth located on the articulation control assembly, and (ii) a pin fixed to the outer sheath of the shaft assembly,