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Bethany Brookshire

The future of the robotic leg

A perspective published this week shows just how dazzlingly far we’ve come, and how far we have yet to go

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I went to high school and college with a guy who had only one leg. He usually used crutches to get around. He was very deft on them and could move as fast as a human could run (in recent years, he’s become a triathlete and body builder). He had a prosthetic leg that he could attach, but he never wore it. It turned out he had good reasons why.

After my high school graduation, a whole group of us were camping in national parks across the United States for an environmental awareness–themed trip. It was the trip of a lifetime, and we did all the hiking we could. The guy with one leg hiked just fine, and often had better balance on one leg and two crutches than some of us had on two feet. But about halfway through the trip, he broke a crutch. I forget how it happened. It must have been something major; they build those metal crutches to last. But now, to get around, he had to use his prosthetic leg.

You could tell it totally sucked. With each step he’d have to swing the whole side of his body forward, heaving the leg along. When it hit the ground again, it could bend, the ankle could flex, and he could lean into it for balance. But it was clearly uncomfortable, slow and took a lot of effort to use. Thankfully, he got a new crutch mailed ahead, and in a few days was back up to speed.

It’s been more than 10 years since I last saw that prosthetic leg. In a perspective in the Nov. 6 Science Translational Medicine, a special issue devoted to robotics and neuroprosthetics, Michael Goldfarb and colleagues from the Department of Mechanical Engineering in Nashville, Tenn., discussed the current issues with leg prosthetics, including many of the issues I saw my classmate struggle with, and the promise and challenges of new robotic limbs.

Prosthetics and mobility devices have come a long way, from the foot blades that elite runners use to exoskeletons. They now come with their own motors that run the pistons that stand in for muscles. These 21st century devices have sensors that can detect the angle of the prosthetic knee and the pressure on the artificial foot. They can detect how fast the prosthetic is moving. All of this information is fed into a controller that acts as a central nervous system for the limb, coordinating all the information so the limb moves smoothly through space.

Here we run into a problem. The limb moves smoothly through space, yes. But what is it moving in coordination with? As Goldfarb and colleagues point out, previous prosthetic legs and limbs have been passive. They can move and bend, but they rely on the wearer to propel them forward, to “sling” the prosthetic as they move the rest of their leg. This takes a lot of effort, and I remember watching my classmate twist most of his body to sling his prosthetic. It was clearly uncomfortable, but it has a hidden advantage. The limb will always end up moving in coordination with the rest of the person’s body movements.

When you have a robotic limb, this becomes more complicated. It can act and react. But it needs to be able to do that along with you. One of the biggest challenges that Goldfarb and colleagues pointed out was how to integrate the robotic limb with the movements of the rest of the body. To do this, the limb will have to be able to figure out what you want it to do (something that isn’t even a result of conscious thought in most people), and then do it, adjusting the pressure and motion to match what you are already doing. When you take a step, it needs to step, too. But it can’t do it at the same time! It has to be offset, just enough, to keep you moving forward and not falling on your face. And if you think walking is fun, wait ‘til you try stairs.

How do you coordinate a prosthetic limb? There are several possibilities. You could record activity from nearby muscles to predict the desired movement. You can record from surrounding nerves to pick up motor impulses from the brain. You could even implant electrodes directly into the motor cortex of the brain itself. These methods of coordinating the brain and the prosthesis are in development, often with stunning results. You only have to observe the triumph of a paralyzed woman as she picks up a bottle of coffee and takes a sip to believe that the future is now.

Other improvements are also in the works. For example, one of the more irritating and difficult parts of a prosthetic limb is the effort required to move it. When you are dealing with a prosthetic leg, putting it down on the ground is only half the work. You also have to lift it, with no help from your ankle. The authors of the perspective article describe new developments to help with this, especially the “powered push-off” of the foot, to make the biomechanics easier as you walk. This will also help with some of the long-term problems associated with prosthetics. When people work with prosthetic limbs for a long time, they end up compensating for the deficiencies of the prosthetic with other joints. This means many people end up with joint problems after long term use. Improving the biomechanics of the limbs would decrease the amount of compensation that people have to do, decreasing the stress on other joints.

Scientists are also working to help with balance. People with prosthetic legs fall as frequently as elderly people do. If your prosthetic scuffs the ground, say, and trips you, you have very little chance to catch yourself before you fall. The automatic correcting of your leg isn’t there. Improving the biomechanics with things like powered push-off could help staying upright easier by reducing scuffing. Not to mention the improvements if we could fully coordinate the movements with the rest of the body.

The strides that we’ve made in developing robotic limbs are amazing from many angles. The progress in pressure sensing, automatic movement and balance all reveal just how beautiful our limbs are. When we see the improvements made in robotic limbs, we begin to understand how complex everyday movements are, how many tiny movements and sensory experiences go into each one. How each large and tiny muscle has its opposite, how the things we sense about the slope of the ground or the rocks under our feet cause us to make lightning changes to keep us upright and moving smoothly.

The developments that we have made in prosthetics highlight how much we now understand about the way we move. They also showcase just how many technological breakthroughs we’ve made in sensors, motors and pumps. How sensitive we can make things and how tough we can make them as well. From microchips that can be implanted into the brain, to replacement limbs and joints that run smoothly for years, even inside the human body. Each robotic prosthesis is a masterwork of design, biomechanical understanding, neuroscience, technology and more.

Many more improvements are already on the way. Some have been implemented in prosthetic arms, for example, and robotic leg prosthetics are coming on to the market. As the field develops, I hope that someday everyone who needs one will be able to use prosthetics with completely natural movements. My old classmate was plenty fast on his crutches, and he might well say he doesn’t need or want the new robotic limbs. But for those who do, it will be wonderful to have a limb that acts and reacts, that can meet you, stride for stride. We’ve come a long way, but we’ve still got further to go.

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