FIELD : medicine. SUBSTANCE : group of inventions relates to medical equipment. Surgical instrument may comprise a surgical end effector that is configured to perform at least one surgical procedure upon application of at least one control motion thereto. Surgical instrument may further include an elongated shaft assembly that defines a longitudinal tool axis and is operably coupled to the surgical end effector, and may operably support at least one control component and include a flexible articulation portion for facilitating articulation of the surgical end effector relative to the longitudinal tool axis. Flexible articulation portion may comprise a longitudinally-extending, elongated articulation spine that includes a plurality of ribs that extend from each lateral side of the articulation spine. EFFECT : each rib may include at least one anti-twist feature for preventing the flexible articulation portion from twisting about the longitudinal tool axis during articulation of the surgical end effector. 17 cl, 19 dwg