If you can't do 200 push-ups, then you can't buy a machine that has the effect of 200 pushups. And if you CAN do 200 pushups, and you used such a machine, it would feel the same as doing 200 pushups. You'd be just as sore and out of breath. You can't buy anything that lets you work out sitting down, and not getting tired. You already HAVE a device that causes your muscles to contract. It's called a brain.
Actually, they DO have uses. Just try this: Put one on and crank it to maximum. In between pulses, tense your abs as hard as you possibly can. When the machine pulses, you can feel muscles contracting that weren't before, even though you were trying to contract your entire core.
I think they have some use for finishing off after core exercises, post workout. I think there are some muscles deep in there that the brain doesn't really have much conscious control over.
But the people who buy them because they think the machine does the work...if it worked as much as crunches, it would FEEL like doing crunches. If you're not breathing harder, you're not working harder.
