Good point. I don't know that the lungs are technically muscles (probably they aren't...I'm not going to cheat and look it up until later). But for this purpos

Good point. I don't know that the lungs are technically muscles (probably they aren't...I'm not going to cheat and look it up until later). But for this purpose they behave enough like muscles... i.e., they seem to grow stronger. Or maybe it is indeed muscles like the diagram being strengthened. Perhaps someone with a bit more actual physiology knowledge can answer. Edit: Also, I forgot to mention, I do this twice a week and sometimes thrice a week, with off periods of a few months every 5-6 months or so. I've been doing HIIT in periods for 3-4 years since I first read about and I'm a believer. Big change.