TOKYO—At an office-building construction site in the center of Japan’s capital, 67-year-old Kenichi Saito effortlessly stacks 44-pound boards with the ease of a man half his age.

His secret: a bendable exoskeleton hugging his waist and thighs, with sensors attached to his skin. The sensors detect when Mr. Saito’s muscles start to move and direct the machine to support his motion, cutting his load’s effective weight by 18 pounds.

“I...