Can anyone link some good articles on these? I've been doing some research and would like opinions and possibly some reliable sources about the science involved.
They have their uses, but in NO WAY simulate altitude. They are good for training the body to cope when being forced to work harder to function when the body is fatigued, and they force the diaphragm to work harder, but they have not one single thing in common with training at altitude except that its harder to breathe.
I agree with BodyBlaster. it doesnt replicate high altitude training, it wont increase red blood cells and it wont increase mitochondria. The only thing it does is strengthen breathing muscles like the diaphram.