Why is the signal at the wrist so much stronger than on the scalp? · Grey Matter

A scalp EEG sums faint synaptic currents of millions of cortical neurons through skull and scalp, and arrives as tens of microvolts.

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.