Thirty motor units, with one fibre each are shown in both the realistic and simplified models above.
In real life, there are several hundred motor groups in the large muscles studied by EMG, and each motor group is made of
several hundred fibres.
The electrodes are able to almost target individual motor groups in the simulation above. With an electrode placed on the
skin, this would be impossible in real life, and the signal would average over very many motor groups.
In the realistic model on this page, the signals are not synchronised,
which is broadly in line with the findings in the following paper: https://journals.physiology.org/doi/full/10.1152/jn.00452.2015
Synchronization of motor unit firings: an epiphenomenon of firing rate characteristics not common inputs
Joshua C. Kline and Carlo J. De Luca (2016)
In both models, the moderate activation of the muscles involves only a subset of the motor units / muscle fibres.
In the realistic model, at moderate activation, the firing rate is decreased for some motor units.
This reflects the findings shown in the following:
Neuroscience. 2nd edition. Purves D, Augustine GJ, Fitzpatrick D, et al. (2001).
(After Monster and Chan (1977))
Distance between electrodes: https://pubmed.ncbi.nlm.nih.gov/22169134/ (Inter-electrode spacing of surface EMG sensors: reduction of crosstalk contamination during voluntary contractions) by Carlo J De Luca et al.