A new bionic leg allows amputees to imitate natural walking

In 1982, when he was only 17 years old and was climbing in the Huntington Ravine (New Hampshire), Hugh Herr was surprised by a blizzard that kept him lost for three nights in an inhospitable place, at temperatures that were close to 30 degrees below zero. He was rescued alive, but the consequences of the extreme cold to which he was subjected forced both of his legs to be amputated.

After the accident, the biophysicist decided to dedicate his career to developing technologically advanced prostheses, a goal he has more than achieved. Considered a world leader in bionics, he has received numerous awards for his work, such as the Princess of Asturias Award for Scientific and Technical Research, which he won in 2016 “for having designed the first prostheses that manage to emulate human locomotion, allowing him to overcome disabilities such as those he himself has,” as noted by the jury.

This Monday, the magazine Nature Medicine highlights a new advance by the American scientist. It is a neuroprosthetic interface which allows a bionic leg to respond completely to the user’s nervous system. “No previous study has been able to demonstrate this level of brain control,” Herr said at a press presentation of the prototype. “It is the nervous system that controls the movement,” he added.

The interface allows you to restore the proprioception of the individual, the natural ability that any person has to feel the position and movement of the different parts of their body. “The sensation is as if you had a normal leg. It feels natural, it moves naturally,” added the Massachusetts Institute of Technology (MIT) researcher, whose team has carried out a trial with 14 people, all of them with a leg amputation. below the knee on one of his legs.

Half of them received the device developed by Herr, while the rest had a more conventional prosthesis. And the results showed palpable benefits of the new approach, called ME and that surgically connects pairs of agonist-antagonist muscles (necessary for correct walking) with external electrodes that can be removed and are capable of ‘reading’ and sending information about the position and movement of the limb to the spinal cord.

According to the researchers, it is difficult for people with a leg amputated below the knee to regain normal walking despite the growing technological sophistication of the prostheses that have been developed in recent years. This is because when we move a leg, a large number of muscles are put into operation, which act in agonist-antagonist pairs, and while they act they send all kinds of proprioceptive signals to the nervous system, informing it, for example, how is executing the movement or where the leg is. After an amputation, that dynamic is lost, a process that attempts to restore the device designed by Herr.

When measuring the effectiveness of the bionic prosthesis, the researchers found that walking speed was 41% greater in people wearing the device. In fact, the way they walked was similar to that of a person without an amputation, they point out. The usefulness of the device was tested not only on smooth floors, but on stairs, ramps or terrain with obstaclesthey point out.

For Eduardo Rocon, researcher at the CSIC’s Center for Automation and Robotics, “it is about a very interesting research which allows progress in a very popular field, the integration of robotics and neurology“.

The great contribution of the work, he points out, is that it allows control by the brain and at the same time that the brain receives information through implants “These are placed on the residual muscles resulting from the imputation. The fact that the person can feel what is happening helps them to control it better,” says the researcher, who points out that when we walk we adapt our gait to the characteristics of the ground or the needs of the moment. For example, running on asphalt is not the same as running on sand.

Rocon highlights that improvement in proprioception The device achieves this through partial stimulation of the residual muscles, which shows that with only partial information, the brain is able to extrapolate that data and act accordingly.

“In this work, a lower limb prosthesis has been developed that allows acquiring information about the execution of movement and activating nerves with a proprioceptive function that provide information to the spinal cord and, subsequently, to the brain. By incorporating proprioceptive information, the nervous system The central nervous system has a greater capacity to regulate the movement that is intended to be performed with the prosthesis. Specifically, it achieves that the biomechanics when walking have an execution of movements more similar to people without damage. To this end, sensors are incorporated that collect information from the prosthesis. pressure produced by supporting and lifting the prosthesis from the ground,” says Juan de los Reyes Aguilar, head of the Experimental Neurophysiology Group in the Research Unit of the National Hospital for Paraplegics of Toledo, in statements to SMC Spain. “In addition, electromyography of muscle contraction is obtained in preserved leg regions, above the level of amputation. Both types of information are transformed into electric shocks that serve to activate the proprioceptive nerves of the leg (located between the knee and the amputation site). The result is that an ‘active prosthesis’ is achieved, since it provides proprioceptive information to the central nervous system to achieve better regulation of movement,” he says.

De los Reyes adds that, “although prostheses for people with amputations have existed for a long time, the models used until now lacked the capacity to transmit proprioceptive information, therefore, the authors call them ‘passive prostheses’. The work shows that the implantation of active prostheses in people with amputation achieves the recovery of biomechanics close to that of natural movement (which the authors call ‘biomimetic’). In addition, they had greater walking speed than people with prostheses without proprioceptive sensors. Also, by having more proprioceptive information, people who used the active prosthesis showed greater capacity to act when faced with unexpected obstacles or the need to change the movement from walking to climbing stairs.”

“Although the result represents an improvement in the functions lost by people with an amputated lower limb,” continues the expert, “the mechanism that must be implemented is more voluminous, heavier and more complex than that of a prosthesis without sensors. It is possible that the excessively bulky appearance of the active prosthesis may create rejection in its use by some people, being the classic prostheses easier to wear and more integrated in the anatomy of the person, allowing to dress normally, etc. The authors discuss this point and consider that perhaps it could be a limit the widespread use of active prostheses. Therefore, one of the foreseeable improvements in the design of future active prostheses will require the miniaturization of technology; “will be able to help reduce the weight and volume of the entire mechanism required to implant the sensors and transmit proprioceptive information.”