After approximately four 30-second high-intensity efforts, mouth-breathing is needed due to the metabolic cost becoming too high to sustain nasal breathing.

“There is a place for mouth breathing. However, it's usually, if you need to do a strong exhale, oftentimes you can discard more volume through the mouth, unless you're very trained at nasal breathing. So if you're doing high intensity training, a good way to conceptualize this is to exhale on the max effort, and then to inhale on the less intense part.”

“So nasal breathing is great, but as you increase the intensity of your endurance work, you will need to incorporate the mouth.”

“You're going to try to maintain nasal only as much as you can, but you're going to lose it at some point. You can go through their, Brian and Rob's gear system, and learn more, and then you can kind of see what gear to be in, if you have to go nose in mouth out, or something like that. But I don't really care too much, honestly, in that range.”

“four is just mouthmouth right and this is the case in most sporting applications you're going to be breathing because the nose is restricted right there's only so much space and as we talked about earlier the consequences of not having enough oxygen in or CO2 exhalation if you're restricting that this is going to be problematic”

“and in fact if i mouth breathe i cannot put out any more work and i end up stressing myself further at that point”

“i can do three-minute efforts uh i can do 30-second efforts and i don't need to mouth-breathe at all in 30 second efforts after those 30 seconds efforts after about four of them i then need to start mouth breathing because metabolically the cost is getting so heavy i have to load”

“you instantly shift the carbohydrate burn you just instantly shift it up the moment you start mouth breathing and it becomes more intense”