They are unlike the prosthetic limbs; they wont replace but will compliment the real ones
Two groups of MIT scientists are working on something which will change what we perceive as reality and what we think of as the world of fiction. These scientists are currently developing robotic limbs, which when worn together can make you look like one famous comic book villain: Doctor Octopus. They are not like the usual prosthetic limbs, the main purpose of which is to replace real ones but the teams' (from MIT's d'Arbeloff Laboratory) Supernumerary Robotic Limbs (SRLs) are designed to supplement your existing arms.
One of these two teams recently showcased its latest shoulder-mounted SRL prototype at the IEEE International Conference on Robotics and Automation in Hong Kong. Members of the team showed how these fake limbs can help you do different tasks while your real arms are busy in other chores. The robotic limbs called the SRLs take data collected by the accelerometer and gyro embedded in the base of the shoulder mount, as well as their accompanying wrist monitors. Following this, the limbs are going to move according to the actions you undertake. If you will raise your own real arms, these limbs would go up and so on.
The second team has been working on waist-mounted SRLs and the project is being sponsored by Boeing so the results can be used by their aging employees. And this is why these mechanical limbs are designed to be used either as arms or legs, in order to help with airplane construction by bracing or supporting workers' weights.