"Once we combine the most significant behavioral modes we are able to control the robot such that, from the wearer's perspective, it behaves like an extension of his own body," says Baldin Llorens-Bonilla, a researcher at the d'Arbeloff Laboratory at MIT.
In addition to these extra shoulder arms, other researchers at the d'Arbeloff Lab, led by Professor Harry Asada, are constructing waist arms that can be used for holding objects as well as for bracing the user, much like an extra pair of legs.
Much of this research was sponsored by Boeing, which is interested in finding ways to keep its aging human workforce of aircraft builders from getting injured by repeatedly lifting heavy things, stooping, or crouching repeatedly over long periods of time. The waist-mounted SRLs can be used as two extra arms, two extra legs, or one of each, if that's what would work best.
So why not just go straight to a exoskeleton-nears-production">full-body exoskeleton instead of just additional arms? The researchers say that the constraint of an exoskeleton is that by definition it's bound to the body of the user: no matter what the most advantageous orientation for your limbs might be, the exoskeleton is putting all of its force wherever you decide to put your arms and legs. Having limbs that are powered yet completely separate gives the system many more option for helping you out.