Each row is a putative motor unit source separated from the 3-channel sEMG by the same math used for HD-sEMG decomposition (delay extension → whitening → fastICA → peak/K-means → silhouette filtering).
True motor-unit decomposition needs a high-density electrode grid (64+ channels). With only 3 channels the spatial resolution is far lower, so these are separated sources — not verified physiological motor units. The grey line is the total firing rate.
Details: docs/adr/0003-spike-trains-view-design.md