Makers of the Training Mask, (also known as elevation or altitude masks), claim that an athlete can reap the benefits of working out at altitude by wearing a mask while they train. But what does the research say? Do training masks really work? Or is it just a gimmick?
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Altitude Training

I recently took a short vacation with my wife to Flagstaff, AZ. It's a really beautiful place, and just happens to sit at over 7,000 feet elevation.
My second night there, I decided to do some interval training on the hotel treadmill. After two rounds, I felt noticeably more fatigued than normal and was breathing much heavier than I usually would be.
This was because at over 7,000 feet elevation, the air is thinner and there is less oxygen available. Spending prolonged time working out in this environment would cause the body to adapt to the limited oxygen supply by producing more red blood cells, which transport oxygen to working muscles.
How Does Altitude Training Improve Your Performance?
It is well established by research that in order to accumulate any physiological benefit from training at altitude, an athlete must spend at least 2 weeks in that environment.
The limited oxygen at high altitude stimulates adaptations in the body, such as an increase in red blood cell production, which improves the body's ability to transport and use oxygen. Training at elevation has real, measurable physiological benefits for an athlete. But does wearing a training mask work in the same way?
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Research on Training Masks
Studies done to test the effectiveness of the training mask show clearly that wearing the mask does NOT simulate training at altitude.
6 week studies have shown no physiological benefits of wearing the mask vs. a control group in trained athletes. No change in red blood cell production was observed, nor was lung function improved.

The group training with the mask did, however show one important improvement.
The Group training with the elevation mask showed a 15% improvement in Power at ventilatory threshold. The ventilatory threshold is basically the point at which anaerobic processes are beginning to take over, and lactate is beginning to accumulate faster than it can be cleared out.
Improving power at this threshold would certainly be an advantage. However it remains unclear exactly why the mask induces this response.
Researchers believe that the mask likely acts as a resistance trainer for the respiratory muscles, as wearing the mask forces you to work harder to pull air into your lungs. So, in this regard, the mask may provide some benefit.

A Better Approach to Use:

If you're looking to improve your cardio-respiratory abilities, and overall endurance.. I have a better approach. It's simple. You don't need to go into the gym wearing a mask. You simply need to go through your workouts while breathing only through your nose.
It's harder than it sounds, but regularly training while breathing only through the nose would yield far better results than wearing a training mask.
This concept is broken down thoroughly in a really interesting book called "The Oxygen Advantage" by Patrick Mckeown. In the book, Mckeown breaks down how we're supposed to be breathing vs. how most of us breathe. He makes a strong case that learning to breathe properly can help lose weight, have more energy and perform better in all areas of life.
The book is really great and well worth reading. I'll post the link here if you want to check it out.
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Conclusion
So, do training masks work? The answer is, "not really."
Studies show that training masks do not offer the benefits that they claim to. They do not simulate training at altitude, but may act by strengthening the muscles used during breathing, thus stimulating some improvements in performance.
For most people in the gym who aren't even close to their natural conditioning potential, the mask is completely unnecessary and unlikely to provide much benefit.
Learning to breathe properly, intelligently programming your training, and spending some workout sessions breathing only through the nose should pay dividends to your conditioning, all without spending a dime.
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If you still want to check out the training mask, read reviews, etc.. then I'll post that link as well right here.