The body is basicaly operated on and controlled by electricity.
Food and Oxygen are metabolized into energy, which is transmitted down the nerves by a change in the disposition of electrolytes, resulting in an incredibly weak electrical current.
That current is sufficient to cause the muscles to contract, which is why 5 milliamperes will override your body's own electricity and make you hold onto a live wire. That's also why the bad guys on TV go down 'right now' when hit with a Taser.
The amount of energy in any medical device is tiny when compared to the units that we are used to for measuring power, so they use a Joule, which is a very small unit for energy.
You can pick up the tiny impulses and run a sensitive chart recorder...called an EKG; or you can whack the body big-time with enough power to seize or control muscles...that's what a defibrillator does.
Generally when using the machine, the Doctor starts with a setting that is just high enough to get the desired result, and increases it in small increments if needed.
