There are many studies on muscle stimulation with EMP technology, for example: Safety and Efficacy of a Non-Invasive High-Intensity Focused Electromagnetic Field (HIFEM) Device for Treatment of Urinary Incontinence and Enhancement of Quality of Life by Julene B. Samuels MD, Andrea Pezzella MD, Joseph Berenholz MD, Red Alinsod MD from 2019.
A number of other studies can be found on PubMed®.