Factory workers may soon be able to do heavy lifting without throwing their backs out, and ordinary citizens may get some help with the yard work. Last summer, mechanical engineer Homayoon Kazerooni at the University of California at Berkeley finished building what he calls an extender for human arms. Half robot, half garment, it’s designed to amplify the strength and agility of human arms while doing dangerous or difficult tasks. It does this by measuring the force and motion of the wearer’s arms and using those measurements to control the action of a pair of robot arms; a computer keeps the robotic force at a constant multiple of the human one and transmits appropriate feedback to the wearer. Kazerooni expects his arm extenders to hit the marketplace sometime in 1997. The legs, he says, will follow in a few years.