A surgical robotic system for use in a surgical procedure, comprising: - a surgical arm comprising a movable arm part , the movable arm part comprising an instrument connector for mounting of a surgical instrument , the surgical instrument having a longitudinal axis , the movable arm part having at least one degree-of-freedom to enable longitudinal movement of the surgical instrument along the longitudinal axis of the surgical instrument towards a surgical target ; - a human machine interface for receiving, using a first input modality or input mode of the human machine interface, positioning commands from a human operator for controlling the longitudinal movement of the surgical instrument; - an actuator configured and arranged for actuating the movable arm part to effect the longitudinal movement of the surgical instrument; - a processor configured for controlling the actuator in accordance with the positioning commands and a virtual bound (132-135), the virtual bound establishing a transition in the control of the longitudinal movement of the surgical instrument in a direction towards the surgical target, wherein the processor is configured for controlling the actuator to disallow longitudinal movement of the surgical instrument towards the surgical target past the virtual bound, and wherein the processor is further configured for, during use, determining the virtual bound based on the positioning commands; characterized in that the human machine interface comprises a further input modality or input mode which, when used by the human operator during the surgical procedure, causes: - the virtual bound to be set to coincide with, or to be at a predefined distance from, a current instrument tip location as indicated by the positioning commands; and - the virtual bound to be incrementally displaced or expanded relative to a current virtual bound. - a surgical arm comprising a movable arm part , the movable arm part comprising an instrument connector for mounting of a surgical instrument , the surgical instrument having a longitudinal axis , the movable arm part having at least one degree-of-freedom to enable longitudinal movement of the surgical instrument along the longitudinal axis of the surgical instrument towards a surgical target ; - a human machine interface for receiving, using a first input modality or input mode of the human machine interface, positioning commands from a human operator for controlling the longitudinal movement of the surgical instrument; - an actuator configured and arranged for actuating the movable arm part to effect the longitudinal movement of the surgical instrument; - a processor configured for controlling the actuator in accordance with the positioning commands and a virtual bound (132-135), the virtual bound establishing a transition in the control of the longitudinal movement of the surgical instrument in a direction towards the surgical target, wherein the processor is configured for controlling the actuator to disallow longitudinal movement of the surgical instrument towards the surgical target past the virtual bound, and wherein the processor is further configured for, during use, determining the virtual bound based on the positioning commands; - the virtual bound to be set to coincide with, or to be at a predefined distance from, a current instrument tip location as indicated by the positioning commands; and - the virtual bound to be incrementally displaced or expanded relative to a current virtual bound. <EOS>