A robotic surgical instrument comprising: a shaft; a first end effector element (401, 815); an articulation connecting the first end effector element to a distal end of the shaft, the first end effector element (401, 815) 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 (401, 815) by a first pair of driving elements (409a, 409b, 809a, 809b), the first pair of driving elements (409a, 409b, 809a, 809b) being configured such that a tension force applied by the first actuator to a first one of the first pair of driving elements (409a, 809a) causes the first end effector element (401, 815) 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 (409b, 809b) causes the first end effector element (401, 815) to move relative to the articulation in a second motion direction; wherein the first one of the first pair of driving elements (409a, 809a) has a first path between the first actuator and the first end effector element (401, 815), and the second one of the first pair of driving elements (409b, 809b) has a second path between the first actuator and the first end effector element (401, 815), 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 (401, 815) than the second motion direction; wherein the first one of the first pair of driving elements (409a, 809a) is constrained to move around M pulleys (411a, 412a, 502, 503, 810, 811), and the second one of the first pair of driving elements (409b, 809b) is constrained to move around N pulleys (411b, 412b, 413b, 504, 505, 812, 813, 816), where M < N. a shaft; a first end effector element (401, 815); an articulation connecting the first end effector element to a distal end of the shaft, the first end effector element (401, 815) 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 (401, 815) by a first pair of driving elements (409a, 409b, 809a, 809b), the first pair of driving elements (409a, 409b, 809a, 809b) being configured such that a tension force applied by the first actuator to a first one of the first pair of driving elements (409a, 809a) causes the first end effector element (401, 815) 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 (409b, 809b) causes the first end effector element (401, 815) to move relative to the articulation in a second motion direction; wherein the first one of the first pair of driving elements (409a, 809a) has a first path between the first actuator and the first end effector element (401, 815), and the second one of the first pair of driving elements (409b, 809b) has a second path between the first actuator and the first end effector element (401, 815), 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 (401, 815) than the second motion direction; wherein the first one of the first pair of driving elements (409a, 809a) is constrained to move around M pulleys (411a, 412a, 502, 503, 810, 811), and the second one of the first pair of driving elements (409b, 809b) is constrained to move around N pulleys (411b, 412b, 413b, 504, 505, 812, 813, 816), where M < N.