
YO peeps, i am at University studying sport science and we have been studying inspiratory muscle trainers like Powerbreathe, incidently one of the senior lecturers is one if the developers of the actual Powerbreathe device itself so we were told exactly how it works. There were many studies carried out on the effect of using one of them and they were all very positive. Improving inspiratory muscle function decreases the work that the rest of the body has to do during exercise, therefore it is possible to improve running ability by improving lung function. You can also "google" it to see the papers etc on it.
I have one myself and have started using it again regularly and i have improved my times quite well. In a lab we done at uni as well, it was no coincidence that i outperformed everyone else in the inspiratory muscle test. (the amount of air i could breathe in, in one single breathe) as i was the only one to own a powerbreathe device.
Hopes this helps
adios
PS - If you go onto powerbreathe.com and go onto "what the experts say" it will explain all there.