They use them in physical therapuetic situations when someone was in an accident of some kind, say they were unable to move for weeks or even months at a time. The muscles that were held stationary for that time period atrophied severely, and is usually extremely stiff.
They can hook up the sim to the muscle, and shock it. This will cause the muscle to contract (to 'workout'). its light intensity, but you cannot injure yourself, but you still get the benefit of exercise. Now when a muscle has been stationary for months, then suddenly has maximum contractions, the muscle will regrow very quickly.
If you have ever been to a chiropractor, you may have been hooked up to one. Its a weird sensation, having your muscle jump up and down and you not being able to control it haha
Behold, I say unto you, many are called, but few are chosen.....