20th century Czech runner Emil Zatopek practiced, among other things, interval training with held breath. He could have been training his parasympathetic system

20th century Czech runner Emil Zatopek practiced, among other things, interval training with held breath. He could have been training his parasympathetic system just as well as respiration pathways. It's been my experience in endurance exercise is that if you are inexperienced in it, you overreact to certain signals from your body like rising CO2 or falling O2. After just small effort of a short duration you start gasping for air. Years later, in retrospect, you wonder why you did that. Another adaptation, in high latitude outdoor runners, is the adaptation to inhaling cold, wintry air. The unbearable burning that feels like you're inhaling alcohol somehow goes away. The interesting thing is that it appears to be permanent. Even if you're out of the game for few years, that discomfort doesn't come back. Could be psychological. If you've been there and done that, you dismiss the discomfort signals and don't pay attention to them.