Deception of feelings

A prosthetic leg with biofeedback seemed easier

Elizaveta Ivtushok, N+1

Swiss scientists have found that setting up biofeedback in a prosthetic leg makes it subjectively lighter in weight, which also improves the wearing experience. To do this, they asked a volunteer with a leg amputated above the knee to walk in a prosthesis with and without sensors connected to electrodes in the tibial nerve. After walking with the biofeedback turned on, the prosthesis seemed to be about half a kilo lighter to the participant, and it was also easier for him to perform a task with a cognitive load: to walk and pronounce words in reverse order at the same time. The article was published in the journal Current Biology (Preatoni et al., Lightening the Perceived Prosthesis Weight with Neural Embediment Promoted by Sensory Feedback).

In order for the prosthesis to look as much as possible like a lost limb, it is necessary to set up biofeedback in it – that is, not only to teach a person to control it, but also to feel it. Researchers pay special attention to feedback in prosthetic hands: sensors on them are combined with electrodes that are attached to the remaining limbs or implanted in them so that it is possible to stimulate peripheral nerves and restore – at least partially – the lost sensitivity already in the prosthesis. 

In the development of prosthetic legs, a little less attention is paid to feedback, since the main task in their development is to teach, like an ordinary leg, to walk, bypass obstacles and keep balance. At the same time, the feedback of prosthetic legs still remains important for improving its work and the feelings of the one who wears them. In particular, it could help solve the problem of the subjective severity of the prosthesis: about 70 percent of users of prosthetic legs above the knee complain that the device seems too heavy to them – even though they are usually twice as light as their healthy leg.

Stanisa Raspopovich from the Swiss Higher Technical School of Zurich decided to check it out: the team under his leadership has been developing prosthetic legs above the knee with biofeedback for several years. In particular, they use stimulation of the tibial nerve through implanted electrodes together with pressure sensors in the heel area and accelerometers in the knee area of the prosthesis: this allows the user to feel how he bends the prosthesis and steps on the ground with it.

In 2019, Raspopovich and his colleagues have already shown that biofeedback reduces phantom leg pain, increases walking speed and generally improves the entire experience of wearing a prosthesis. Now they have conducted an experiment with the participation of one user of their prosthesis: he was asked to walk for ten minutes either with the biofeedback in the knee and heel turned on or off.

Before the walk, the participant was seated on the edge of the cot so that both legs – both with a prosthesis and healthy – hung below the knee: they asked not to move the prosthesis so that the biofeedback would not work. Next, the participant was blindfolded with a bandage, and gymnastic weights of different weights were put on the healthy leg and asked to evaluate at what weighting of the healthy leg both seem to be the same in weight. Despite the fact that the prosthesis was lighter than the healthy leg of the participant, he considered the prosthesis and a healthy leg with a weighting weighing two kilograms to be equal in subjective feeling of weight. 

After walking with the feedback on and off, the participant was again put weights on his healthy leg – this time different, weighing from 0.5 to 3.5 kilograms. Scientists found out that after a walk with biofeedback in the prosthesis, the weighting of a healthy leg necessary for weight equalization turned out to be 23 percent lighter – about half a kilo. The participant also managed to increase the walking speed, and his subjective feeling of the prosthesis as a native limb increased by 60.5 percent.

Next, the participant was given an additional task – this time with a cognitive load: while walking, he was asked to say a few words in reverse order. With biofeedback enabled, the participant's speed during the task did not decrease (unlike the condition without feedback), and he also made 20 percent fewer pronunciation errors. 

In general, scientists have once again shown the importance of setting up biofeedback to improve the experience of wearing a prosthetic leg: with sensors turned on in the foot and knee area, the prosthesis seemed easier, and it turned out to be much easier to walk and perform a task with additional cognitive load at the same time.

Restoring the biological feedback of limbs in prostheses is not without limitations, and one of them, as scientists have recently found out, is that the sensory responses of the human body are quite conservative: even with prolonged wearing of a prosthetic arm, the sensitivity of one place cannot be transferred to another.

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