Hugh Herr, left, started designing prosthetic limbs after a clim# accident.

Hugh Herr, the director of an M.I.T. laboratory that pursues the "merging of body and machine," grew up in a Mennonite family outside Lancaster, Pennsylvania. He and his brothershe was the youngest of five childrenoften helped their father, a builder, lay shingles, install drywall, and strip wires. During the summer, the family visited places like Alaska and the Yukon in their camper van, and the kids frequently set out alone to hike and rock climb. "When I was eleven, I was this clim# prodigy, clim# things most adults couldn't do," Herr told me. "When I was fifteen and sixteen, I started clim# things that no adults had ever done.

And then, when I was seventeen, the accident happened."

Rock climbers call bouldering moves "problems," and the most difficult section of a route is the "crux."The young Herr spent days imagining difficult ascents, plotting a path across slots, cracks, and overhangs as one might work through a complex question in geometry or physics. Then he would go out and become the first person to ascend, say, a rock face on the Shawangunk Ridge. (By tradition, the person who makes the first ascent gets to name the route. The chosen names are often kooky: Moby Grape, They Died Laughing, Lonesome Dove, Millennium Falcon.) Herr sometimes did what's known as free-solo clim#. "I'll never forget the day I climbed two thousand feet without a rope," he told me. "All your sensations are heightened. It's a remarkable feeling of one's physical control and power."