When the difference in electric potential between positive and negative charges becomes large enough, a sudden electrical discharge (lightning) will occur. (britannica.com)
The presence of electric potential differences in muscles is the result of motor unit action potentials produced by the central nervous system (CNS). (strengthandconditioningresearch.com)
Although many inferences drawn from electric potential difference within muscles recorded by EMG amplitude are not valid, there are reasons to believe that more confidence can be achieved when assessing tension within a muscle by proxy by recording EMG amplitude. (strengthandconditioningresearch.com)