Surface electromyography · Grey Matter

Surface electromyography (sEMG) records the electrical activity of muscles through electrodes on the skin, and since a muscle fires only when its motor neurons command it, a band around the wrist can read the motor commands for the fingers without touching the brain.

Surface electromyography. Surface electromyography (sEMG) records the electrical activity of muscles through electrodes on the skin, and since a muscle fires only when its motor neurons command it, a band around the wrist can read the motor commands for the fingers without touching the brain.

Each motor neuron in the spinal cord drives a group of muscle fibres, a motor unit, and each of its spikes fires them all together. Electrodes over the muscle pick up the sum of these motor unit potentials, from a few microvolts to about a millivolt, with useful content up to about 500 Hz once skin and fat have filtered it. The command comes from the motor cortex, and the muscle amplifies it into a signal far larger than an EEG sees on the scalp. In 2025 Patrick Kaifosh, Thomas Reardon and colleagues at Meta's Reality Labs reported a dry wristband with 16 bipolar channels sampled at 2 kHz, whose decoders, trained on data from thousands of people, worked on new users with no calibration: 0.66 targets acquired per second when steering a cursor, 0.88 gestures detected per second and handwriting at 20.9 words per minute. A product descended from it, the Meta Neural Band, went on sale with the Ray-Ban Display glasses in the United States on 30 September 2025.

It needs the path from brain to muscle. Amputees use it to drive myoelectric prostheses, but where the spinal cord or the motor nerves no longer carry commands, nothing reaches the skin and only a brain-computer interface can read the intention.

It trades bandwidth for safety. A wristband needs no surgery and carries none of its risks, while implanted speech decoders reached 62 and 78 words per minute in 2023.

It varies between people and sessions. Electrode position, skin and anatomy change the signal and neighbouring muscles mix in each channel, which the 2025 decoders overcame with the size of their training set.

Where the motor path is intact, the muscle is the brain's best amplifier.

For people whose nerves still reach their muscles, a wristband competes with an implant on everything except the ceiling.

Questions: When is a wristband enough, and when is a brain implant needed? A wristband reads motor commands where they arrive in the forearm muscles, so it works whenever the path from the motor cortex through the spinal cord and the nerves to the muscles still carries them. For those users it needs no surgery and already handles cursor control, gestures and handwriting at about 21 words per minute. When that path is cut, as in a high spinal cord injury or advanced motor neuron disease, the commands never reach the skin, and the only place left to read them is the brain itself. That is why implanted interfaces are tested first in people with paralysis, where they also reach higher rates, such as speech decoded at 62 words per minute in 2023. Why is the signal at the wrist so much stronger than on the scalp? A scalp EEG sums faint synaptic currents of millions of cortical neurons through skull and scalp, and arrives as tens of microvolts. At the wrist each spike of a single motor neuron fires every fibre of its motor unit at once, and muscle fibres produce large action potentials close under the skin, so surface electromyography sees from a few microvolts up to about a millivolt. The muscle works as a biological amplifier for the command, and that margin is what lets a dry band decode individual finger gestures that scalp recordings resolve only poorly.