This can be tricky to get one's mind around . 
Studies i have read show that altitude training is only appropriate for the top 1% of athletes overall conditioning. 
Due to the forced inhales It actually adds to chest tension. For example, I use a training mask and my breathing volume lessens. I believe they invite exercise induced asthma. Gasping and breath heaving invites exercise induced asthma for sure. Watch the long distance runners.  It's the nose breathers that last the longest until maybe the last small % of the race.  
Most people think in terms of conditioning. Conditioning is great but not at the expense of volume. The Framingham Study proved that volume (FEV1) is the primary marker for longevity. See the attached article Making Old Age Measure up.
When you stop to think about it, would one rather have a super duper lawnmower engine or a super duper 12 cylinder Ferrari engine? 
The key is to maximize BOTH volume AND conditioning and not one at the expense of the other but to maintain what i call the Optimal Breathing window.  
When people use ANY altitude or training mask approach they tighten up the rib cage and do nothing to release that tension. That is why i included the 12 foot strap in my kit, to release that tension. 

Same tension issue for any breathing resistance trainer including my diaphragm strengthener or any spirometer. They ALL create tension and  NONE do anything to RELEASE  that tension.  They help condition and strengthen the diaphragm but create tension that reduces the optimal breathing window and breathing VOLUME. 
I often show people how they can check in with how their breathing feels now, then do the strapping technique to open up the rib cage and they feel the expansion of breathing ease or to keep it simple MORE THAN BEFORE, then do the breathing resistance trainer, then see if they feel any lessened (LESS THAN BEFORE), breathing volume which some do but that is often tricky to sense. 
So then I have them do the strapping technique again and when they feel the expanded breathing they realize that they had not noticed what the resistance trainer had done to reduce their breathing volume; how the resistance trainer had tightened up their breathing. Boiling a frog comes to mind. 
I strapped a guy at the Huntsman games right before his racewalk. He came back to me and said "that technique was the reason i won. My breathing was so much deeper and easier". 
Watch this quickie about Charlie Boyle. It has some huge insights. 
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