It looks like a tattoo, if the nerve is damaged by overheat through lighting, does he still feel anything in his arm?

Furthermore, back to the following issue:

Let assume heart rate is 80 per minutes, if EMS at 70 pulse per minutes, would EMS's 70 frequency interrupt the heart's 80 frequency? since the heart muscle's tense and contract are also controlled by nerve impulse as well.

Since heart and EMS are on different frequency, so from heart's viewpoint, if it is time for muscle to release, but the EMS make muscle to contract, so it may be not enough time for muscle to release before next contract, do it interrupt heart's performance?

Do you have any suggestions?
Thank you very much for any suggestions (^v^)

Surely there's nerve damage, but the "tattoo" you see isn't permanent. It's called the Lichtenberg figure, and it's caused by the rupture of capillaries (tiniest blood vessels) under the skin when the lightning current passes through it.

Now in terms of your other question about the heart, I think a pacemaker works like an EMS by resetting the heart's rhythm. Except the video shows localized stimulation of the abdominal muscles, that won't affect the heart because it's not strong enough or close enough -- tissue acts as an insulator. If you places it on the heart, I'd assume like a pacemaker would stop the heart from beating. It's a very technical question because from my view, if the frequencies are in sync or not, the EMS would alter the heartbeat no matter what.

Do you find any stimulation experiments at the back of the spine instead of abdominal muscles?
Is there any potential harm for this stimulation treatment? Any idea?
Do you have any suggestions?
Thank you very much for any suggestions (^v^)

Yes, there are plenty of studies evaluating the benefits and risk. Let me know if this links helps

https://scholar.google.ca/scholar?q=electro+muscle+stimulation+risk+spine&hl=en&as_sdt=0&as_vis=1&oi=scholart#d=gs_qabs&p=&u=%23p%3DPV9wiyYklbQJ

Thank you very much for suggestions (^v^)