Currently available foot interfaces to command an external assistive robotic arm for laparoscopic surgery typically consist of a couple of push buttons placed on a planar platform. In this paper, we develop and compare two foot interfaces implementing isotonic vs. elastic isometric control strategies. A four DOF robotic arm is used as the slave robot to evaluate both interfaces and their corresponding control strategies. This robot is controlled by foot's translation/rotation in the isotonic interface and by force/torque in the elastic isometric interface. The isotonic interface is used for position control, and the isometric interface for rate control. An experimental study mimicking a laparoscopic surgery task was conducted to compare the performance with these two systems, in which the slave robot is used as an endoscope holder controlled by the foot in the specific interface. The subjects strongly preferred using the elastic isometric interface, which they found more intuitive and less tiring. The low physical burden of this interface allows the users to concentrate on the main task of manipulation. Also, they preferred the rate control as it provides them with a more accurate control on the slave robot.