MIT researchers have demonstrated a magnetic muscle-tracking technique that could improve control of robotic prosthetic limbs. Published in Nature Sensors, the study tested three participants with below-knee amputations. Its results suggest that magnetic measurements could provide more accurate and stable control than conventional muscle-signal interfaces.
Known as magnetomicrometry, the technique tracks pairs of magnetic beads implanted in muscle. External sensors measure changes in the distance between the beads as the muscle contracts or stretches, allowing researchers to calculate muscle length and velocity. Each bead measures 3 millimeters in diameter, and its signal can be detected through tissue up to 6 centimeters deep.
All participants had undergone an agonist-antagonist myoneural interface procedure, which reconnects opposing muscle groups within the residual limb. This preserves sensory feedback about limb position and movement, known as proprioception. Participants received magnets in up to four muscles associated with ankle movement.
The researchers compared magnetic tracking with electromyography, which measures electrical activity through electrodes either implanted in muscle or placed on the skin. Participants used muscle movements to direct a cursor toward screen targets rather than operate a prosthetic limb. Electrical activation does not directly reveal muscle mechanics, while skin electrodes can also suffer interference from sweat and neighboring muscles.
The work extends a technique demonstrated in animal studies in 2021 to humans with long-term magnetic implants. Researchers are now conducting a larger study to evaluate control of actual prosthetic limbs using the magnetic measurements. Further testing is needed to establish safety, durability, and performance in everyday activities.
