Magnetic muscle implants help amputees feel coordinated prosthetic hand movements

A new study published in Science Advances suggests that vibrating small magnets implanted inside the muscles of an amputated limb can restore a natural sense of coordinated hand movement. These findings provide evidence that the brain perceives movement as synchronized whole-hand actions rather than isolated finger twitches. The research provides a promising path toward advanced prosthetic limbs that allow users to feel what they are doing without relying entirely on their vision.

For people who have undergone an amputation, operating a prosthetic limb often feels mechanical and disconnected. This happens because the surgery separates the muscles from the joints, which disrupts the natural communication between the body and the brain. In a typical body, the nervous system relies on proprioception, which is the subconscious ability to sense where body parts are located in space. A related concept is kinesthesia, which is the specific sensation of body movement in real time.

Kinesthesia is essential for natural motor control. It is lost after amputation, making prostheses harder to use intuitively. When an individual uses a standard prosthetic hand, they send electrical signals by contracting their remaining arm muscles. Because the muscles are no longer attached to their original bones and tendons, the user does not feel the physical movement of the mechanical hand.

To use a standard robotic device, the amputee has to visually watch the mechanical fingers grasp an object to know if their command worked. Scientists have attempted to restore this lost sense of movement in the past with varying levels of success. Muscle vibration can be used to generate perceptions of movement. However, these vibrations typically stimulate both skin and muscle at the same time, which can confuse the brain while using prosthetics.

Previous methods to restore movement sensation often involved highly invasive operations. Surgeons would redirect severed nerves to different muscles or skin areas, allowing patients to feel their missing hand when a robotic device vibrated those newly assigned areas. While these nerve-redirection surgeries are effective, they require extensive anatomical modifications.

A research team led by the Sant’Anna School of Advanced Studies in Italy, in collaboration with Cleveland Clinic in the United States, conducted the research to explore a less invasive alternative. The team tested a new bidirectional interface for hand prostheses called the myokinetic kinesthetic interface. This system uses vibrations generated by small magnets implanted in the residual forearm muscles to restore natural sensations of movement. The interface was integrated with the Mia Hand, a robotic hand developed by the Sant’Anna spin-off company Prensilia.

To test their alternative approach, the scientists recruited a thirty-four-year-old Italian male who had experienced a traumatic amputation of his forearm. The team surgically implanted small permanent magnets coated in a biocompatible plastic into three specific muscles in his remaining arm. In a biological arm, these particular muscles are responsible for bending the wrist, extending the fingers, and moving the thumb. The primary goal was to see if vibrating these magnets could trick the brain into feeling the missing hand moving.

The implant was designed to last six weeks, which the team considered a sufficient period to test the interface between the hand and the brain. During the experiment, the participant placed his arm into a specially designed frame equipped with electromagnetic coils. These coils allowed the researchers to generate localized magnetic fields directly around the arm. By adjusting the electrical current, the scientists could remotely vibrate individual implanted magnets at various speeds without ever touching the participant’s skin.

“The myokinetic kinesthetic interface is unique because it uses a simple, minimally invasive implant to stimulate muscles without touching the skin,” says Federico Masiero, first author of the study and a postdoctoral researcher at the Technical University of Munich. “This approach may be the key to better understanding how human motor control works, but also how to restore movement sensation after amputation.”

The scientists systematically applied vibrations that alternated in their wave patterns, using both smooth and rugged pulses. They tested frequencies ranging broadly from one to one hundred thirty Hertz, while keeping the intensity within comfortable limits to avoid any pain. Following each brief burst of vibration, the participant reported several specific details to the research team. He described the general nature of the sensation, where it felt like it was happening on his limb, and how vivid the feeling was.

The participant was completely new to this type of magnetic stimulation and did not know what to expect. The scientists initially assumed that vibrating a single muscle would cause the participant to feel a single isolated finger moving. Instead, he consistently reported feeling synchronized movements across all the digits of his missing hand. Every time a magnet vibrated, he felt his phantom hand either opening up or closing into a grip.

These sensations felt remarkably natural to the participant. He perceived hand opening and closing with coordinated movements that were very similar to real ones. He reported that his phantom fingers seemed to be moving within normal physical limits. The perceived fingers never crossed through each other or bent into impossible positions, which suggests that the brain maintains a structural map of the hand long after the physical limb is gone.

The researchers documented exact threshold parameters for when the participant felt these sensations most intensely. He experienced the most vivid feelings of movement when the magnets vibrated at an average frequency of eighty-two and a half Hertz, but rarely noticed very low vibration frequencies. When the researchers tested his reaction speed using an adaptive timing sequence, they found he was highly sensitive to the stimulation. He could reliably detect the presence of the vibration in roughly forty milliseconds, which equates to just two to four cycles of the magnetic wave.

The team also tested what would happen if they vibrated multiple magnets at the exact same time at varying intensities. In these scenarios, the participant had a harder time localizing the exact source of the vibration. He did not experience more complex hand postures when multiple magnets were activated simultaneously. Instead, the competing vibrations occasionally made it difficult for him to determine which specific muscle was being stimulated.

To understand if these whole-hand movement sensations were unique to this one individual, the scientists compared his results to past research. By combining data from the world’s only two neural-machine interfaces designed to restore kinesthetic sensation, the researchers found that the brain appears to process this information not as isolated signals, but as coordinated hand grasp movement patterns. The coordinated hand movements felt by the patient appeared similar to those felt by participants with a different kinesthetic feedback system built by researchers at Cleveland Clinic.

The two prosthetic interface systems were structurally different. The one developed at Sant’Anna used implanted magnets and the other at Cleveland Clinic used surgical nerve redirection and robotics. Even so, both kinesthetic interfaces produced similar perceptual results. The induced movement sensations were perceived as coordinated finger movements rather than separate signals.

“The ability to compare independently generated data from two very different interfaces makes these findings especially compelling,” says Paul Marasco, study coordinator at Cleveland Clinic. “It gives us a stronger foundation for designing therapies and devices that work with the nervous system in a more natural way, with the ultimate goal of improving outcomes for patients.”

This shared experience across entirely different types of artificial interfaces provides evidence for how the human brain organizes physical movement. It suggests that the central nervous system groups muscles together to perform actions as a single cohesive unit, a physiological concept known as motor synergies. Motor synergies exist because the brain has to manage an infinite number of possible joint combinations to perform simple everyday tasks. To make things efficient, the brain relies on pre-programmed patterns, like a basic grip or a reaching motion.

Because the brain commands the hand using these coordinated patterns, it appears to process the feeling of movement in the exact same coordinated way. Together, the teams’ findings suggest that the brain may organize movement sensation from the muscles in a more coordinated and more subconscious fashion than previously understood. This provides a natural basis for designing future therapies that align directly with the body’s internal logic.

While the study offers novel insights into human perception, there are some limitations to consider. The most prominent limitation is that this experiment only included a single participant. It is possible that other individuals might experience these magnetic vibrations differently based on their unique biology or the specific nature of their amputation. Extensive future testing with more participants will be required to see if these results apply to a broader population.

“Our solution was implemented as a preliminary demonstrator: the implant was designed to last six weeks, a period we considered sufficient for an initial verification of the interface’s usefulness and effectiveness,” says Christian Cipriani, creator of the interface and study coordinator at the Sant’Anna School of Advanced Studies. “The results were very promising and prompted us to explore a permanent implantable solution, which will allow us to study the interface over much longer periods and with a larger number of participants.”

Additionally, there are potential misinterpretations regarding how the sensation is actually felt by the user. Both research teams observed that some sensations transmitted through their respective interfaces were perceived by the patients without their users being immediately aware of them. The participant noted that the feeling was not tactile in nature, meaning it did not feel like something rubbing against his skin. He sometimes had to concentrate heavily to notice the feeling of movement itself, though his physical ability to detect the underlying vibration was very fast.

This indicates that artificial movement sensation operates quietly in the background of a person’s awareness rather than being a distracting or overwhelming feeling. Moving forward, the team’s next goal is to use prior work reading out the position of implanted magnets to control the prosthesis while simultaneously using superimposed vibration to restore natural sensory perceptions. The longer-term aim is to develop a permanent implant that combines natural grasp sensation with intuitive motor control.

These current projects lay the groundwork for a new generation of more human-like prosthetic devices supported by international funding. This new result paves the way for more intuitive control of prostheses and may also have future applications in stroke rehabilitation, epilepsy, and pain treatment.

The study, “Coordinated hand movement sensation revealed through an implanted magnetic prosthetic kinesthetic interface,” was authored by Federico Masiero, Mattia Gentile, Marta Gherardini, Eliana La Frazia, Charles H. Moore, B. Ülgen Kilic, Valerio Ianniciello, Roberta Reho, Tommaso Mori, Flavia Paggetti, Lorenzo Andreani, Simon A. Whitton, Paul D. Marasco, and Christian Cipriani.