A Bionic Leg Controlled by the Brain

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 brothers—he was the youngest of five children—often 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 climbing prodigy, climbing things most adults couldn’t do,” Herr told me. “When I was fifteen and sixteen, I started climbing 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 climbing. “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.”

In January, 1982, when Herr was a junior in high school, he and a friend, Jeff Batzer, set off to ice-climb Huntington Ravine, in the White Mountains of New Hampshire. They then planned to hike an additional mile or so to the peak of Mt. Washington, a popular destination that is notorious for fast-changing conditions. (It has been said to have the worst weather in the world.) According to the Mount Washington Avalanche Center, about twenty-five people have to be rescued each year, and more than a hundred people have died on the mountain.

Herr and Batzer, who were experienced and very fit, ascended the ice of Huntington Ravine without much difficulty. They figured that if they had trouble on Mt. Washington they could always turn around. A few hundred feet along the trail, however, they encountered winds of up to ninety miles per hour. Visibility was poor. They had to shout to hear each other. While retreating downhill, they lost their way. Several times, Herr’s feet broke through ice into a freezing stream.

The two spent three nights in an isolated valley, in extreme cold. Herr didn’t think that they would survive for even one night. But he wasn’t accounting for the benefits of being with another person. “You can hug them,” Herr has said. “You dramatically reduce the surface area of the dual body, but you double the heat source.” The boys had all but given up when, on the fourth day, a woman out snowshoeing, following what she thought were moose tracks, came across the two young men. They wondered if they were hallucinating; she gave them raisins and a sweater and hurried to get help. They later learned that a search party had spent days out in the snow looking for them.

To stave off gangrene, Batzer had a thumb, several fingers, a foot, and a portion of one leg amputated. Herr had both legs removed below the knee. He was fitted with leg prostheses that had sockets made of plaster; he was warned that putting too much stress on them could crack them. “I was, like, ‘We’ve gone to the moon, and this is it?’ ” he told me. When a prosthetist brought out a box of feet for Herr to choose from, Herr asked if there were any narrow enough to fit inside climbing shoes. “I wanted to get back on the horse,” he told me. “Climbing was my life.”

In rehab, Herr got in trouble for, literally, climbing the walls. A few months later, when he was on crutches, he and his brother Tony made their way to the base of a familiar mossy rock face in Pennsylvania, and Herr scaled it. He felt free and strong. And without his lower legs he was, he said, “like, fourteen pounds lighter.” Climbing was easier for him than walking with his prostheses; it was also much less painful. “My dad was, like, If you want to climb, climb. Invent stuff.”

A traditional amputation stifles the signals between the brain and a residual limb, but a breakthrough surgery can channel them into a prosthesis.

Herr had studied metal fabrication at a nearby vocational school—he had chosen his high-school classes in a way that would leave his schedule open for climbing. He started building prostheses with custom features, such as feet that could grip ice and pointed toes that could be wedged into cracks. “One year after the accident, I was climbing as well as I did before,” he said. Jim Ewing, a climbing friend who was Herr’s roommate around that time, told me, “I watched him become one of the strongest rock climbers in the world—and that was with bilateral amputations.” Herr made a first ascent of a route up Sky Top Ridge, in the Shawangunks, and he christened it Footloose and Fancy Free. (One of his prosthetic feet broke off when he fell and was caught by a rope.) He took on increasingly difficult climbs: City Park, in Washington State, and Liquid Sky and Stage Fright, in New Hampshire. In May, 1983, he appeared on the cover of Outside magazine. In the photo, he’s wearing a bandanna tied around his head; his prosthetic legs are painted with red-and-blue stripes reminiscent of athletic socks. Two pairs of feet, neither attached, are nearby.

But, by 1985, Herr was worrying about the strain on his body. He thought about going to college, which would give him the education he needed to build even more advanced prostheses. “I was imagining limbs where I could run faster than a person with biological limbs,” he told me. “I was imagining non-anthropomorphic structures like legs with wings, and I could fly. I had no idea, obviously, how to do that.” He enrolled at Millersville University, a public school near Lancaster. Herr said that as a teen-ager he’d had such a limited grasp of math that he couldn’t have calculated ten per cent of a hundred. After two years of relentless studying, however, he had advanced to quantum mechanics. “What had been a climbing obsession became an academic obsession,” he said. He thought that maybe his aptitude for mathematics had come in part from all the problem-solving he did as a climber.

One summer, Herr started developing an adjustable socket for his prostheses, which tended to loosen as swelling in his legs ebbed in the course of a day. He and a prosthetist friend, Barry Gosthnian, who had been a mechanic in the Air Force, had talked about using inflatable bladders, and Herr tried to make some from various materials. Finally, after the seventeenth prototype, he had something that worked. Herr ultimately got a master’s degree in mechanical engineering, from M.I.T., and a Ph.D. in biophysics, from Harvard. He started the M.I.T. Media Lab’s Biomechatronics Group, which uses neurology and engineering to “restore function to individuals who have impaired mobility.” (He is also a director of the K. Lisa Yang Center for Bionics.) At M.I.T., he led the development of a robotic foot-and-ankle prosthesis called the BiOM, which has three microprocessors and six sensors and is tuned to a user’s gait. He started using one himself; in 2011, Time named him the “Leader of the Bionic Age.” Swapping out the powered ankle for a passive spring device, Herr said, felt like stepping off a moving walkway at the airport.

Still, the sophistication of prostheses was limited by the way leg amputations were performed. Surgeons traditionally sew down residual muscles when they amputate a limb. There are good reasons for this—padding the bone is one—but it also severely restricts how much the muscles can move, leading to atrophy. Herr feels as if his legs are still there, but locked into rigid ski boots. The movements of his prostheses and his phantom limbs are out of synch.

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More recently, however, Herr’s group has been working on a new type of prosthesis—one that is controlled by the brain. Not long ago, in his clutterless, white-walled office at M.I.T., he showed me a video of a woman who had undergone a novel kind of amputation that better preserves the ability of remaining muscles to contract and stretch. The signals that the brain sends to those muscles can be communicated, by way of numerous electrodes on the skin and external wires, to microprocessors in the prosthesis. I watched as the woman, simply by thinking, smoothly flexed and pointed her carbon-fibre toes. Herr told me that, when he walks with the robotic ankle he designed, “it feels like I’m in the back seat of a car. She feels like she is the car.”

More than a hundred people, including at least twenty at M.I.T., have now tried these brain-controlled prostheses. “They don’t typically get emotional at first,” Herr told me. “They giggle.” The prostheses are being used only for research, since they require more testing to be considered for F.D.A. approval, but research participants have already achieved a “biomimetic gait.” This makes it the first leg design that allows users to walk approximately as quickly and unthinkingly as anyone else—a feat that Herr described as “more than I had expected in my wildest dreams.” Several labs have developed neurally controlled arm prostheses, and the Utah Bionic Leg, created by a University of Utah team led by Tommaso Lenzi, continually adjusts to a person’s gait. The M.I.T. prosthesis, however, is under full neural control. These accomplishments are not just about microprocessors, carbon fibre, and titanium. They required the engineering of much more familiar materials: muscles, tendons, and bones.

When I first met Matthew Carty, a tall plastic surgeon with gray hair and bright-blue eyes, he had just returned from a twelve-hour breast-reconstruction surgery, and I could still see the imprint of magnification glasses on his face. We were across the Charles River from Herr’s office, in the Hale Building for Transformative Medicine, at Brigham and Women’s Hospital, where Carty works. Carty is an expert in the field of microsurgery, which involves especially fine work, including the reattachment of vessels that can be smaller than a millimetre in diameter. Like many surgeons, he radiates the confidence that a person would want to see before going under the knife.

Herr had wondered whether the muscles severed in an amputation could be meaningfully connected again. (“I didn’t know if it was even possible,” he told me.) Someone he knew at a nearby rehabilitation center suggested that he speak with Carty. Carty, in turn, had been asking around about experts on advanced leg prostheses, and was told that he ought to meet Herr. Over dinner at a no longer extant Italian restaurant, they weighed the idea of leg transplants, which have never been done in the United States, against leg amputations combined with advanced prostheses. Many conversations followed.

Carty told me that, early in his career, he was troubled that amputation remained relatively primitive despite major advances in most other surgeries; the procedure not only led to muscle atrophy but also caused chronic pain, blistering, infections, and arthritis. It seemed improvable. “If I showed you a textbook from the Civil War describing the technique for a below-knee-level amputation and I showed you a textbook from right now, they would look almost the same,” he told me. “How many things can you think of that have not evolved in two hundred-plus years?”

Other physicians were also rethinking the details of amputation. Severed nerves sometimes regenerate chaotically in residual limbs, forming painful clumps of tissue called neuromas. Gregory Dumanian, at Northwestern University, and Paul Cederna, at the University of Michigan, each developed techniques for embedding the ends of nerves into muscles, which is said to give the nerves somewhere to go and something to do; in practice, it alleviated some types of chronic pain. Carty had a further thought: a large amount of intact muscle, nerves, vasculature, and bone was often disposed of during amputations. In other procedures done by plastic surgeons, “material” was commonly repurposed for reconstruction. Couldn’t a similar inventiveness be applied systematically to amputations?

The human nervous system can be spooky. If an artery in your heart is occluded by a clot, pain may be felt in distant places such as your neck and your arm. Your gallbladder is in your abdomen, yet the pain from gallstones is sometimes felt as far away as your shoulder. Phantom-limb sensations are a very different variety of confusing messages of the nervous system. I spoke with a woman whose right leg was amputated up to the hip at Brigham and Women’s; shortly thereafter, on a hot day, she felt sweat in the pit behind the knee that she no longer had. Another patient said that pain from his phantom ankle “drives me bananas” and keeps him up at night. These sensations are not solely nuisances. They can also be useful.

Carty remembers asking Herr, early in their conversations, about his height. Herr was about five feet eleven before the accident, but his prostheses made him six feet two. Things didn’t have to be just as they were; there was room to play. Eventually, Herr and Carty began discussing the idea of amputation surgeries that would reëngineer parts of the residual limb.

The key insight that emerged from their collaboration centered on muscle pairs made up of agonists and antagonists. When you bend your elbow, the biceps, an agonist, contracts; the triceps, an antagonist, stretches. When you raise your heel to walk, part of the largest muscle in your calf (the gastrocnemius) contracts, and a muscle right next to the shinbone (the tibialis anterior) stretches; both muscles connect to bones in the foot and, in this way, move the leg.

One problem with traditional amputations is that they leave agonists and antagonists without that bone connection. What was in essence the muscles’ means of communication or coördination is gone. But Carty and Herr, in close collaboration with Shriya Srinivasan and Tyler Clites, who were then graduate students in Herr’s lab, started to envision ways of functionally reconnecting those agonist and antagonist muscles.

After a traditional amputation, the neural signals from severed muscles are only about three per cent of what they once were—insufficient for effective communication with a neurally controllable prosthesis. If the connections between agonist and antagonist muscles could be restored, however, the neural signals might be strengthened and clarified. The limb could then keep the brain informed about where it is and what it’s doing; the brain in turn might become better at controlling the muscles in a natural way. In other words, a person’s prosthetic limb could potentially be brought into close alignment with their phantom limb.

Many years of research went into this new approach to amputation. At one point, Clites spent about a year developing a way of stitching together agonist and antagonist muscles with tendons, which could slide back and forth along a bone-mounted titanium pulley. Clites told me that he had “all the ‘i’s dotted and all the ‘t’s crossed, and then that didn’t work at all.” In an experimental animal surgery, the muscles scarred down and became immobilized. “We had to ask ourselves, first, is the concept even good?” Clites told me. The titanium, which was not native to the body, seemed a likely culprit for the failure.

After many discussions, Herr, Carty, Srinivasan, and Clites went with a design that fashioned a pulley from a part of the ankle joint which in traditional amputations is basically tossed out. The idea looked good in theory, and the team presented it at a plastic-surgery conference. “The predominant feedback from surgeons was ‘That’s a really cool idea, but it will scar down and it will not move,’ ” Clites recalled.

The team tried out the surgery on human cadavers and animal models and thought that it might be working. “But you can’t get a rat to tell you what they are feeling,” Carty said. Did movement feel natural? How much could the animal sense its phantom limb or prosthesis? Did the prosthesis move in accordance with its thoughts? To answer these questions, the researchers needed a human—someone who was healthy but needed an amputation, and who was willing to receive a novel procedure. As Carty put it, they were looking for a “first astronaut.”

In the years after Herr’s accident, he had done numerous climbs with Ewing, his roommate. When they were about twenty, Ewing had “Life sucks” written on his left shoe; on the right, he had “And then you die.” On one climb, part of the way up the rock face, Herr asked him, “Does life really suck, Jim?” Ewing eventually married, had a child, and became a mechanical engineer, but he kept mountaineering. One day, in 2014, he was scaling a limestone cliff in the Cayman Islands with his daughter. He slipped, fell, stopped a couple of times, and then fell again—this time all the way down, about fifty feet. Somehow, he survived.

Ewing’s left foot was so badly injured that putting any pressure on it caused excruciating pain, even a year later. “As an engineer, I was researching all kinds of different things to rebuild my ankle,” he said. But he couldn’t find anything that would let him climb again. Even walking was very difficult. “I was super depressed,” he said. He knew that Herr was leading a biomechatronics lab, so he got back in touch to inquire about anything new and experimental—and, alternatively, to hear what life with an amputation might be like. He remembers Herr saying, “Well, funny that you ask—we’ve just developed this new amputation protocol.” Herr directed him to Carty, and a couple of months later Ewing decided on amputation. He would be the first person with an “agonist-antagonist myoneural interface”—AMI for short.

Precisely when people began to make and use prostheses is unknown. A prosthetic leg, fashioned from poplar and tipped with a horse’s hoof, was found in a two-millennia-old grave in present-day China, along the Silk Road. A Roman general is said to have received an iron replacement for his right hand, to allow him to hold a shield. Ambroise Paré, a sixteenth-century French military barber-surgeon, devised a mostly metal leg with a knee joint, which could bend when a person was walking and lock when he was standing. Paré also worked on innovative surgical approaches to amputation, such as saving skin and muscle.

Throughout the years, prostheses have been reimagined in creative ways. Still, for a long time, the main difficulty for soldiers who needed amputations was surviving long after the operation. During the American Civil War, when infections killed more soldiers than artillery did, it was said that a soldier was lucky to have a limb shot off rather than cut off by a battlefield surgeon; field surgeons were unlikely to be working with a clean blade. (Advertisements from the time offered a type of ankle prosthesis that contained no metal and had a socket made from polished ivory and vulcanized rubber. It was touted as “EXTREMELY LIGHT; MUCH LIGHTER THAN ANY OTHER.”) For people who needed amputations, the greatest advance in care arguably was not superior prostheses but more modern surgical practices.

In the century that followed, amputation remained a neglected area of medicine. “When I was a medical student, amputations were sometimes given to the most junior member of a surgical team, and it was a contest to see how fast you could get the limb off,” David Crandell, a physiatrist in the Department of Physical Medicine and Rehabilitation at Harvard Medical School, told me. To this day, surgeons performing amputations too often have little sense of what happens to a patient in the years after recovery.

“Part of the problem was that amputation was thought of as a failure,” Carty told me. “The thinking was, Either you salvage the limb or you fail to salvage the limb.” He brought up Ewing’s case to demonstrate how that approach can be misguided. “He had this preserved foot, but it’s painful all the time,” Carty said. “He stops climbing—he stops doing all these things that matter most to him.” For Ewing, an amputation and a fitting with a prosthesis could be more restorative than keeping the foot.

Carty and his colleagues were confident that the AMI amputation would be safe, and that Ewing would be able to use a conventional prosthesis without trouble. “Still, when you’re doing something for the first time, you’re freaking out the whole time, because you’re wondering what you’ll find,” Carty said. They were not sure that the surgery would allow the muscles to move more freely, which was essential for a strong neural connection to a prosthesis.

According to a description of what would become known as the Ewing amputation, the surgeon makes a “stairstep incision” over the shin using a scalpel. The relevant part of the limb is “exsanguinated.” A flap of skin is peeled back to expose the leg muscles. Care is to be taken, the account notes, to isolate the saphenous vein and a nearby nerve. This is only the beginning of what is simultaneously a delicate, gruesome, and revolutionary surgical procedure; one of the required tools is a bone saw.

On July 19, 2016, Ewing spent more than five hours in the operating room. “Things went pretty well for me,” he recalled. Two weeks after the surgery, even before he had healed enough to have a prosthesis fitted, he went to a local climbing gym. “I remember feeling very liberated,” he said. “I was using just one leg, but I felt free from pain. I could propel myself up that wall dynamically.”

A few weeks later, Ewing went to the lab at M.I.T. The first thing the team wanted to know was whether the connected agonist and antagonist muscles in the amputated limb could move. An ultrasound probe showed that they could. “For a scientist, that’s Christmas morning,” Clites, who is now an assistant professor at the U.C.L.A. School of Engineering, said. “That was the big wow.” The research team then worked on picking up electrical signals from the muscle, measuring the strength of those signals, and using them to guide the movement of a prosthetic leg.

Ewing amputations are now the standard of care at Brigham and Women’s, and are performed at many hospitals. Carty frequently teaches the method to other surgeons, sometimes even by Zoom. Footage from one of Ewing’s later visits to the lab shows the first time that the research team connected the prosthesis directly to his leg. “It’s really cool to feel it through my knee,” he says in the video. “Feels like there’s a foot there.” At first, he moves the prosthesis slowly. Later, he observes, “Literally within minutes of having it all connected, it starts becoming part of me.” We see him sitting cross-legged, with the prosthesis on top, fidgeting the foot by flexing and pointing it repeatedly—a moment Carty remembers as astonishing. “I said, ‘Jim, do you know you’re doing that?’ ” Carty recalled. Ewing replied, “No, I was just hanging out.”

One of the many eerie elegances of our bodies is that we manage to walk without thinking much about it. We never have to study a user’s guide to our legs in order to coördinate the contraction of one muscle and the relaxation of another. Almost all of that labor is done unconsciously. I sometimes think of the conscious mind as a clueless factory boss who spends her time daydreaming while the workers on the floor operate all the necessary machinery. Every so often, the self-important boss is startled into action and sends down a message like “Step around that puddle!” or “Run faster!” But only the workers know all the detailed adjustments required to carry out the order. “Even now, we don’t fully understand walking—which surprises people,” Herr told me. His lab has spent thousands of hours filming, assessing, scanning, and mathematically modelling people as they walk.

Even the most sophisticated robotic leg prostheses are engaged in merely a rough approximation of human locomotion; they “know” only what the current science knows about how we walk or run or jump, which leaves out a considerable amount. They have microprocessors that make thousands of calculations a second, and they can convey a burst of energy that, even in the absence of a calf muscle, enables a person to lift their prosthetic heel with the appropriate amount of energy. But on uneven ground, for example, they don’t allow a person to move in a truly biomimetic way. This means that an almost incomprehensibly complex technology effectively knows less than a child.

When Clites was a Ph.D. student in Herr’s lab, he worked closely with Ewing to “tune” the prosthesis to Ewing’s perception of movement. The sensors for electromyography (EMG), which is like an EKG for muscles outside the heart, were taped to his residual limb and detected the electrical activity in his leg muscles. (The team is also researching an approach to detecting muscle movements that involves small implanted metal spheres.) If Ewing was asked to lightly flex his foot but the prosthetic foot flexed intensely, the system could be adjusted. “Maybe one philosophical concept here is that, if the amputation is done well and the interface is done well, then the best possible prosthetic device is a really stupid one,” Clites told me. “It is one that doesn’t have to think very much at all . . . because the person’s brain and spinal cord are doing all the thinking.” Herr described this in a clarifying way: “There’s no real algorithm on the robot. It’s all from biological computation. That’s cool, because the person is in control.”

When I visited the lab, in July, I met Amy, a sporty woman with auburn hair whose leg had been severely burned in a work accident. Her turn as an astronaut came shortly after Ewing’s. The surgery alone was a breakthrough, Amy said, because “it gives back so much proprioception”—the ability to detect one’s body in space. She became very active with traditional prostheses; she runs races and rows crew. But trying the neurally controlled prosthesis in the lab, she said, was a kind of revelation.

“It’s only a conspiracy theory now, but with the right marketing it could become a widely held belief.”

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Amy described for me how, after EMG electrodes were taped along her residual limb, she was given instructions about how to move her no-longer-there leg. “At first, the prosthesis was mounted on the table, while I sat in a nearby chair,” she said. “It was wild. The fact that my leg is here, and this thing is over there, and then all I do is think . . . and now this thing is moving.” The researchers would ask her to point the foot quickly, and then slowly; to turn her phantom ankle inward as far as she could, and then outward as far as she could, and then inward just a little, and so on. It was like telekinesis, and she was having fun. Sometimes, just to mess with the team, she would start tapping her phantom foot, as if with impatience.

Eventually, the bionic prosthetic leg was hooked up directly to Amy’s body. Her movements were studied as she walked with it—on ramps, up and down stairs, around obstacles. If it was out of tune, she might find herself walking on tiptoes. Whenever she went home, the bionic leg remained in the lab. “You do miss it when you leave,” she told me. “You just put it on and you start walking and doing everything that you could do before. It’s amazing. You feel whole again.” Then she revised her phrasing: “It’s not that I don’t feel whole. I love myself and I love being an amputee and I love my legs.” But her connection to the neurally controlled prosthesis is of another order. “It’s almost like I’ve imprinted on it,” she said.

In the United States alone, more than a million and a half people use prostheses. They have run marathons, swum for miles, climbed Mt. Everest, and danced the foxtrot. In the U.S., a typical leg prosthesis can cost eight to ten thousand dollars, and custom prosthetic sockets made from materials such as carbon fibre have become reasonably common. But robotic prostheses that use microprocessors, like Herr’s bionic ankle, still cost more than thirty thousand dollars and remain unusual. It will probably take five years for neurally controlled leg prostheses to be proved safe and effective and to become commercially available. (If a prosthetic arm fails to grab a cup, that’s not too bad; a failure with a prosthetic leg can mean a disastrous fall.) Even then, the price is likely to be prohibitive for many.

Crandell, the Harvard physiatrist, is a funny, brainy, and upbeat clinician who treats people with amputations in the Spaulding Rehabilitation Center, a building overlooking Boston Harbor. He told me that until recently, and to some extent even now, medical students had little exposure to or knowledge of his specialty. “I would say the field changed eleven years ago, especially here in Boston, after the Boston Marathon bombing,” he told me. “There were a lot of donations. We became less invisible.” Still, many advances in the field have not yet reached people who need them. “If you’re a veteran or if you were hurt at work, then insurance tends to cover the price of a high-end prosthesis,” he said. But most others can’t afford one. (Crandell is part of a group that lobbies the Massachusetts legislature to require insurance to cover more.)

On a recent Wednesday, Crandell’s first patient of the day was a former trumpet player from Russia who was in his seventies. The man’s wife had to translate for him because he spoke little English. He’d had his right leg amputated below the knee four years earlier, after falling from a ladder while trimming a pine tree. “The leg is no longer fitting well,” she said of his prosthesis. “He needs a smaller one.”

The patient pulled up the leg of his jeans and disconnected his metal ankle prosthesis from the socket. He then removed the socket, a sort of fitted stocking that covers the residual limb. Next, he took off a large sock from the limb, then another, and then several more, finally pulling off a soft liner that was the last layer. He needed all the layers to keep the socket from falling off. “He also has foam he puts inside the socket, as padding,” his wife said. The reason for the poor fit was that his residiual limb had diminished in size as it lost muscle.

Crandell and the couple discussed the cost of a replacement socket, organizations that might be able to help with the co-pay, the wife’s job at Whole Foods, and her husband’s former work as a music teacher. With a dismissive hand gesture, he cheerfully indicated that he had quit. He couldn’t adjust to the American style of back-patting and gentle encouragement, his wife said. “He wanted to push them,” she said. “And they said, ‘You have to say, “It’s O.K., it’s very good.” ’ He said, ‘I cannot.’ ” The couple laughed at the memory.

“Next time, he has to come with the trumpet,” Crandell said.

Another patient that day, a father of two boys, had lost his leg to bone cancer. The cancer had returned in the form of lung nodules, but he was in remission. He had one prosthetic leg for swimming and another for playing basketball in the driveway, and he had recently gone downhill skiing with his kids. He was seeing Crandell because he wanted a replacement for his running leg.

A man who had once suffered a severe stroke arrived in a wheelchair, accompanied by an aide from his nursing home. His right leg had been amputated above the knee, but he did not want a prosthesis. He was there because of pain in the residual limb. He had trouble forming words but said a couple of times, at appropriate moments, “Awesome” and “Shit happens.” Throughout the day, the mood in the office was bright, pragmatic, and unhurried. Seven patients came through. Only one had a prosthesis that contained a microprocessor. The world of neuroprostheses, which Herr was developing just a few miles away, seemed like far-off science fiction.

In 2018, almost two years after Ewing’s surgery, he returned to the Cayman Islands, where he had fallen. A graduate student in Herr’s lab, Emily Rogers, had designed a neurally controlled leg for Ewing, tailored to the demands of rock climbers. The bionic ankle of the climbing prosthesis had “two planes of motion,” meaning it could point and flex and also move inward and outward; it was also lighter (but less powerful) than what he used in the lab. Herr and Clites joined Ewing on what was to be an emotional, and also instructional, trip: after months of laboratory tests, they wanted to assess the naturalness of Ewing’s movements when climbing.

From the base of the wall, Ewing, wearing sunglasses with red lenses, looked up. He pointed out to Clites the limestone formations, called tufas, near where he had fallen. He remarked on how high up he had been—and he laughed.

With the blue water of the Caribbean behind him, he ascended a pale, craggy cliff. Herr was waiting for him at the top, where the rock formations were bleached and pocked with fossils.

“I was not terribly surprised that it worked,” Ewing told me. “I was surprised at how well it worked.”

As a young man, Herr spoke to almost no one of the physical pain he experienced; in pictures of his early climbs with prostheses, he appears to be in high spirits, even mischievous, with his prosthetic legs pin-striped in one photo. Another leg was polka-dotted. When he is asked about his bilateral amputations, Herr often says things like “These scratches don’t bother me,” and that bionic legs are interesting, and powerful, and cool. He did not tell his two daughters how he lost his legs until the younger one became a passionate hiker. He regrets getting lost on Mt. Washington only because Albert Dow, a young man who was part of the search team, died in an avalanche. Herr always felt that to respect Dow he needed to eschew self-pity, and to devote his life to something worthwhile.

Herr, with his traditional amputations, cannot use the magical-seeming neuroprosthesis that he helped invent. The electrical signals in his residual muscles would be too confused and too weak for the neural-control system, unless his amputation could be modified. (So far, seven people have had their amputations retroactively changed into AMI amputations, and the results have been encouraging.) “I am green with envy of Jim,” Herr said. He often talks about “embodiment,” the feeling that a device is not a tool but a part of one’s self—or that “mechatronics are their body.”

Herr told me that his innovations are “not about rebuilding me as much as about loving technological augmentation.” He has designed artificial ankles and knees and also bionic shoes that “reduce the metabolic cost” of movement by more than twenty per cent. Instead of talking in terms of ability and disability, he speaks with boyish excitement of a future in which “we will be able to sculpt our own brains and bodies, and therefore our own identities and experiences.” Such advances, he told me, offer both tremendous possibility and tremendous risk. “If we give keys to future parents to design their future baby, that would be horrific,” he said. “Humans have too narrow a view of beauty, and of intelligence.” What appeals to him is the idea of many more kinds of bodies out there, each with its own capacities and charisma. “If I’m correct, humans will be unrecognizable in a hundred years from what we are today,” he said.

Such visions of the future can have a curiously retro feel to them. In the nineteen-seventies TV show “The Six Million Dollar Man,” a hunky former astronaut is augmented after a crash to have extraordinary levels of strength and speed. In “The Empire Strikes Back,” from 1980, a medical robot fits Luke Skywalker with a biomimetic bionic arm after a lightsabre fight. In the 1984 film “The Terminator,” Arnold Schwarzenegger, who already seemed superhuman, plays an almost unkillable cyborg. Today’s neuroprostheses are not about superhuman powers. They’re about human ones: moving without thinking much about it, experiencing one’s body as one’s self. The marvel is more David Attenborough than Isaac Asimov.

When I called Ewing, he was on vacation in Honduras, just back from a scuba dive. He is relentlessly active; he and his family sometimes take ski trips with Carty’s family. It’s climbing, though, that still means the most to him. I asked him why. “Other than the adventure and the physical challenge and the problem-solving . . .” He trailed off. He’s been climbing for nearly fifty years, he said, and he is still trying to find words to explain it. “I realized recently—the only time my brain is quiet is when I’m climbing,” he said. “I yearn for that.” ♦