— The possibility to provide an adequate task assistance using underactuated robots for human-robot tool comanipulation is investigated. This novel approach does not take into account any a priori knowledge about user depending parameters however optimizes the robot-user synergy, for instance during US breast examinations. Results show that the examination time can be reduced and a tendency for increasing scanning accuracy using underactuated robots. I. COMANIPULATION FOR MEDICAL APPLICATIONS Comanipulated devices are systems in which the robot and the user manipulate together the same tool. This paradigm is particularly interesting for medical applications. Indeed, unlike teleoperated systems, comanipulation devices allow the physician to remain with the patient, by directly handling the instruments that perform the gesture. In an ideal coma-nipulated system, the medical gesture is unchanged with the addition of a robot. The robot acts as a device providing active support to improve performance and to make the gesture safer. Comanipulation devices can be sorted in three categories, in function of the task that they perform. A first task is to localize and calibrate the robot with respect to the patient. It is based on a typical force control law. If the desired effort applied by the robot is set to zero when the surgeon applies an effort onto the tool, the robot does not resist and so follows the user's gesture. In [5], a force compliance mode allows the surgeon to move the robot in order to locate centers of three pins implanted in a bone and to compute the appropriate location of the shape to be cut. In [6], a robotic system for skin harvesting is presented. When used in the manual mode, the surgeon can move the robot towards the initial pose on skin, then towards the final pose to define the trajectory the robot must follow. A second class of devices was developed to provide the surgeon with a better feeling of the tool-patient interaction. When the laparoscopic tool holder MC2E is force controlled with a zero desired force, the surgeon can feel the forces exerted on the tissues and organs without being corrupted by the trocar friction forces [8]. In [9], Taylor et al. proposed a steady-hand device for microsurgery. This device allows This work has been partially funded by ANRT under CIFRE grant n o 247/2009. A. Marx is with GE Healthcare,