Supernumerary robotic limbs (e.g. a second pair of arms) are wearable collaborative robots that enhance human performances and could reduce employee’s fatigue. One of the most challenging feat with collaborative robots is their interaction with human, which requires an excellent level of transparency (i.e. the ability of the robotic system to not impede the natural motion of the user interacting with the machine). In turn, this entails a combination of high force bandwidth and good back-drivability.