A robotic surgical instrument comprising: a shaft; a first end effector element; an articulation connecting the first end effector element to a distal end of the shaft, the first end effector element movable relative to the articulation; and a drive mechanism at a proximal end of the shaft comprising a first actuator, the first actuator being connected to the first end effector element by a first pair of driving elements (409a, 409b), the first pair of driving elements being configured such that a tension force applied by the first actuator to a first one of the first pair of driving elements causes the first end effector element to move relative to the articulation in a first motion direction, and the tension force applied to a second one of the first pair of driving elements causes the first end effector element to move relative to the articulation in a second motion direction; wherein the first one of the first pair of driving elements has a first path between the first actuator and the first end effector element, and the second one of the first pair of driving elements has a second path between the first actuator and the first end effector element; the first path having a lower tension force loss than the second path such that the first motion direction transfers a greater proportion of the tension force to the first end effector element than the second motion direction; wherein the robotic surgical instrument is characterised by the first one of the first pair of driving elements being constructed of a smaller number of thicker strands than the second one of the first pair of driving elements. a shaft; a first end effector element; an articulation connecting the first end effector element to a distal end of the shaft, the first end effector element movable relative to the articulation; and a drive mechanism at a proximal end of the shaft comprising a first actuator, the first actuator being connected to the first end effector element by a first pair of driving elements (409a, 409b), the first pair of driving elements being configured such that a tension force applied by the first actuator to a first one of the first pair of driving elements causes the first end effector element to move relative to the articulation in a first motion direction, and the tension force applied to a second one of the first pair of driving elements causes the first end effector element to move relative to the articulation in a second motion direction; wherein the first one of the first pair of driving elements has a first path between the first actuator and the first end effector element, and the second one of the first pair of driving elements has a second path between the first actuator and the first end effector element; the first path having a lower tension force loss than the second path such that the first motion direction transfers a greater proportion of the tension force to the first end effector element than the second motion direction; wherein the robotic surgical instrument is characterised by the first one of the first pair of driving elements being constructed of a smaller number of thicker strands than the second one of the first pair of driving elements.