Why is this important: Traditional prosthetics are getting better, but they don’t deliver the natural, fluid stride most people take for granted. They rely on sensors and robotic programs to “simulate” a normal gait, which is far from perfect. Now, researchers at MIT and Brigham and Women’s Hospital have developed an innovative solution that puts control back in the hands of the wearer—the brain.

The study, published last month in Nature Medicine, details a pioneering surgical technique called agonist-antagonist myoneural interface (AMI), a new approach to amputation that aims to preserve the neural and muscle connections needed for smooth limb control.

In effect, the AMI reconnects the prosthetic limbs to the muscles of the residual limb so that the two can continue to “communicate” with each other and transmit that vital positional sensation to the brain. These muscle signals are processed by a robotic controller that determines how far the prosthetic ankle joint needs to be bent, while calculating the torque and power output required.

The team tested this interface on seven AMI patients who had powered prosthetic legs. The results were quite astonishing: The AMI patients moved at normal speeds, automatically adapted to slopes and obstacles, and even performed more complex movements like pointing the prosthetic toes upward while climbing stairs.

Lead researcher Hugh Herr called the study “the first prosthetic study in history” to show complete neural modulation of a leg. Here, the nervous system alone drives a natural biological gait, independent of any robotic control algorithms. Essentially, the AMI tricks the brain into thinking the prosthesis is just another biological limb under its direct control.

How did such limited neural input enable such a wide range of movements? According to graduate student Lenny Song, “a small increase in neural feedback from your amputated limb can restore significant bionic neural controllability, to the point where you allow people to directly neurally control walking speed, adapt to different terrain, and avoid obstacles.”

The researchers compared the AMI group to seven people who had undergone traditional amputations and used the same powered prosthetic legs. The AMI patients outperformed them in every way: faster walking speed, smoother movements, and better coordination between the prosthetic and intact limbs. They could even push off the ground with normal force.

AMI patients also saw reduced pain, muscle atrophy, and other lingering problems associated with traditional amputations. Although their limbs received only about 20 percent of the normal neural input, that was enough for the brain’s hidden talents for biomimetic movement to take over.

Of course, the actual IAM procedure remains a complex operation. But Herr’s ultimate vision is to “rebuild human bodies” by merging biological systems with mind-controlled bionics.