Breathe Harder?

I have seen devices that supposedly help strengthen your lungs by breathing into what looks somewhat like an asthma inhaler. This is promoted as improving your lung capacity to improve exercise abilities. Is this a useful device or another waste of plastic?

I do not have specific information, but from what I understand the only thing they improve is your strength at blowing (chest muscles and so on). What restrains you during excercise is your lungs’ ability to absorb the oxygen and cope with irritation, not so much their physical ability to breathe in and out. If you want to increase lung capacity, I would think you’d want a device that forces air into your lungs and stretches them out. For diving or whatnot.