If New Metal Legs Let You Run 20 Miles/Hour, Would You Amputate Your Own?

A patient with below-knee AMI amputation walks up the stairs.

(Photo credit: Matthew Orr, STAT News)

"Here's a question for you," I say to our dinner guests, dodging a knowing glance from my wife. "Imagine a future in which you could surgically replace your legs with robotic substitutes that had all the functionality and sensation of their biological counterparts. Let's say these new legs would allow you to run all day at 20 miles per hour without getting tired. Would you have the surgery?"

Why are we so married to the arbitrary distinction between rehabilitating and augmenting?

Like most people I pose this question to, our guests respond with some variation on the theme of "no way"; the idea of undergoing a surgical procedure with the sole purpose of augmenting performance beyond traditional human limits borders on the unthinkable.

"Would your answer change if you had arthritis in your knees?" This is where things get interesting. People think differently about intervention when injury or illness is involved. The idea of a major surgery becomes more tractable to us in the setting of rehabilitation.

Consider the simplistic example of human walking speed. The average human walks at a baseline three miles per hour. If someone is only able to walk at one mile per hour, we do everything we can to increase their walking ability. However, to take a person who is already able to walk at three miles per hour and surgically alter their body so that they can walk twice as fast seems, to us, unreasonable.

What fascinates me about this is that the three-mile-per-hour baseline is set by arbitrary limitations of the healthy human body. If we ignore this reference point altogether, and consider that each case simply offers an improvement in walking ability, the line between augmentation and rehabilitation all but disappears. Why, then, are we so married to this arbitrary distinction between rehabilitating and augmenting? What makes us hold so tightly to baseline human function?

Where We Stand Now

As the functionality of advanced prosthetic devices continues to increase at an astounding rate, questions like these are becoming more relevant. Experimental prostheses, intended for the rehabilitation of people with amputation, are now able to replicate the motions of biological limbs with high fidelity. Neural interfacing technologies enable a person with amputation to control these devices with their brain and nervous system. Before long, synthetic body parts will outperform biological ones.

Our approach allows people to not only control a prosthesis with their brain, but also to feel its movements as if it were their own limb.

Against this backdrop, my colleagues and I developed a methodology to improve the connection between the biological body and a synthetic limb. Our approach, known as the agonist-antagonist myoneural interface ("AMI" for short), enables us to reflect joint movement sensations from a prosthetic limb onto the human nervous system. In other words, the AMI allows people to not only control a prosthesis with their brain, but also to feel its movements as if it were their own limb. The AMI involves a reimagining of the amputation surgery, so that the resultant residual limb is better suited to interact with a neurally-controlled prosthesis. In addition to increasing functionality, the AMI was designed with the primary goal of enabling adoption of a prosthetic limb as part of a patient's physical identity (known as "embodiment").

Early results have been remarkable. Patients with below-knee AMI amputation are better able to control an experimental prosthetic leg, compared to people who had their legs amputated in the traditional way. In addition, the AMI patients show increased evidence of embodiment. They identify with the device, and describe feeling as though it is part of them, part of self.

Where We're Going

True embodiment of robotic devices has the potential to fundamentally alter humankind's relationship with the built world. Throughout history, humans have excelled as tool builders. We innovate in ways that allow us to design and augment the world around us. However, tools for augmentation are typically external to our body identity; there is a clean line drawn between smart phone and self. As we advance our ability to integrate synthetic systems with physical identity, humanity will have the capacity to sculpt that very identity, rather than just the world in which it exists.

For this potential to be realized, we will need to let go of our reservations about surgery for augmentation. In reality, this shift has already begun. Consider the approximately 17.5 million surgical and minimally invasive cosmetic procedures performed in the United States in 2017 alone. Many of these represent patients with no demonstrated medical need, who have opted to undergo a surgical procedure for the sole purpose of synthetically enhancing their body. The ethical basis for such a procedure is built on the individual perception that the benefits of that procedure outweigh its costs.

At present, it seems absurd that amputation would ever reach this point. However, as robotic technology improves and becomes more integrated with self, the balance of cost and benefit will shift, lending a new perspective on what now seems like an unfathomable decision to electively amputate a healthy limb. When this barrier is crossed, we will collide head-on with the question of whether it is acceptable for a person to "upgrade" such an essential part of their body.

At a societal level, the potential benefits of physical augmentation are far-reaching. The world of robotic limb augmentation will be a world of experienced surgeons whose hands are perfectly steady, firefighters whose legs allow them to kick through walls, and athletes who never again have to worry about injury. It will be a world in which a teenage boy and his grandmother embark together on a four-hour sprint through the woods, for the sheer joy of it. It will be a world in which the human experience is fundamentally enriched, because our bodies, which play such a defining role in that experience, are truly malleable.

This is not to say that such societal benefits stand without potential costs. One justifiable concern is the misuse of augmentative technologies. We are all quite familiar with the proverbial supervillain whose nervous system has been fused to that of an all-powerful robot.

The world of robotic limb augmentation will be a world of experienced surgeons whose hands are perfectly steady.

In reality, misuse is likely to be both subtler and more insidious than this. As with all new technology, careful legislation will be necessary to work against those who would hijack physical augmentations for violent or oppressive purposes. It will also be important to ensure broad access to these technologies, to protect against further socioeconomic stratification. This particular issue is helped by the tendency of the cost of a technology to scale inversely with market size. It is my hope that when robotic augmentations are as ubiquitous as cell phones, the technology will serve to equalize, rather than to stratify.

In our future bodies, when we as a society decide that the benefits of augmentation outweigh the costs, it will no longer matter whether the base materials that make us up are biological or synthetic. When our AMI patients are connected to their experimental prosthesis, it is irrelevant to them that the leg is made of metal and carbon fiber; to them, it is simply their leg. After our first patient wore the experimental prosthesis for the first time, he sent me an email that provides a look at the immense possibility the future holds:

What transpired is still slowly sinking in. I keep trying to describe the sensation to people. Then this morning my daughter asked me if I felt like a cyborg. The answer was, "No, I felt like I had a foot."

Tyler Clites
Tyler Clites builds human cyborgs. After graduating from Harvard in 2014 with a B.S. in Biomedical and Mechanical Engineering, Tyler earned his PhD in 2018 from the Harvard/MIT program in Health Sciences and Technology. As a postdoc in the Biomechatronics Group (Professor Hugh Herr, MIT Media Lab), his current research focuses on the development of novel techniques for limb amputation surgery, with the goal of improving the neural and mechanical interfaces between persons with amputation and their prosthetic devices. He is pursuing a career in academia, where he hopes to explore applications in which surgical and mechatronic design can be leveraged together in new bionic systems for physical rehabilitation and human augmentation.
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