A few weeks ago I posted about my dismay for those high elevation masks with more and more studies showing how useless (and hindering) they are. It was at that time, my colleague Matt DeBole asked about my thoughts on Intermittent Hypoxic Training. Honestly, I had not researched much of it, and was not a believer in it. Due to it's growth in popularity, I decided to plunge into the research to attempt to prove myself wrong (spoiler: I did).
What is Intermittent Hypoxic Training?
Intermittent Hypoxic Training (IHT) is a type of training where exercise (often running or cycling) is performed in a room or with a machine that has a decreased percentage of oxygen in the environment.
Is it Similar to The Elevation/High-Altitude Training Masks?
Air isn't 100% oxygen, it is a combination of gases, and "normal" air has about 21% oxygen in it.
With that said...No, IHT and elevation masks are distinctly different. IHT involves training in a room or using a mask that has decreased oxygen in the air, but your actual breathing (respiratory rate) is the same. The training mask (photo far left) makes it harder to inhale and exhale (breathing rate increases), but you are still breathing in air that is 21% oxygen. In short...
Training mask: Same oxygen percent as "normal" breathing air (FiO2 = 21%), but increased respiratory rate.
IHT: Decreased oxygen percent than "normal" breathing air (FiO2 < 21%), but same respiratory rate.
Read this post to see why the training masks are ineffective: link
Before we deep dive into the topic, here's some words you may want to know:
- Hypoxic: an environment with less than 21% oxygen in the breathing air
- Normoxic: an environment with 21% oxygen in the breathing air
- VO2 Max: The maximum amount of oxygen the body can use in a given amount of time during exercise. The higher it is, the better "cardio" a person has
- Running Economy: The amount of energy a person uses while running. To measure this, a person's weight, breathing rate, and volume of oxygen consumed during a sub-maximal bout of exercise is calculated. "Good" running economy means a person does not use much energy to run a fast as others who may be out of shape.
- Mitochondria: The terrible word from high school biology class that means tiny things in your cells that produce energy. The more mitochondria you have in certain muscles, the better they can be used for long periods of time. "slow twitch" muscle fibers of distance runners have more mitochondria than "fast twitch" muscle fibers of sprinters.
What Do IHT Companies Claim Their Products Do?
If you were to read a website or brochure, it would really just say "increases performance". What they actually mean is, it may decrease your running/cycling times (make you faster).
Does it Work?
Most studies looked at sprinting and moderate intensity exercises (~70% max HR):
- One study showed at the end of a 6-week program, a hypoxic training group (runners) had an increase of 5% peak power, 5% increase in VO2 max, 35% increase in time to exhaustion. No changes occurred in any of those categories in the normoxic training group doing the exact same sprint program.
- A study showed that running economy improved in the hypoxic group compared to the normoxic group after 6 weeks of training (testing at 6 weeks was performed in a normoxic environment for both groups). Four weeks after both groups did the training, both groups were tested to see if the benefits were maintained. The hypoxic group maintained the benefits over the normoxic group. A third group went through hypoxic training, and was also exposed to hypoxic air during post-training rest; there was no additional benefit in this group.
- One study was a bit contradictory to the studies above and showed that VO2 max and running speed was higher in the normoxic group. It also showed any improvements made in the hypoxic group were not maintained over time.
- Here's a picture of some comparisons of a ton of studies:
It looks as though the biggest findings is that IHT MAY increase VO2 max better than normoxic training.
How Does it Work?
So there's some promising results of these studies, but obviously some conflicting findings. The interesting part is that in the above studies, blood markers were also taken to see why improvements may have happened. Living in high elevation changes the blood markers in a "good" way for cardio. In these IHT studies, they found that there was no significant changes in the blood markers. This makes sense right? Studies found that for significant changes to occur in blood from high elevation living, adaption usually occurs after at least 3 weeks of exposure to high altitudes for 20 hours a day. Whereas IHT is often only performed 1-2 hours 2-5 times a week.
This led to a theory of what may be occurring in the body. They suspect adaptations in muscle are occurring because of a protein called HIF-1α. This protein is unable to be measured in normal blood testing methods used in the studies, however influences how much mitochondria and capillaries are laid down in muscles. They suspect something is happening to that protein causing it to lay down more mitochondria in muscles. More mitochondria means higher VO2 max.
How about for sprints? Sprinting and long-distance running require different energy systems, so the above would not explain sprinting improvement (read more about energy systems here: link). Several studies showed sprinters were able to run longer in a hypoxic environment than their normoxic counterparts. Researchers theorize that training in that low oxygen environment forces arteries in limbs to become larger quicker to get oxygen to those large fast twitch muscle fibers. It's similar to what nitric oxide in pre-workout does...opens up your blood vessels to be larger for increased blood flow. In short, your body senses low oxygen, opens up your capillaries faster to give your muscles more oxygen, which in turn allows your fast twitch muscle fibers to perform better. I don't quite buy this idea, keep reading to see why.
To sum it up:
- Aerobic benefits theory: An increase in mitochondria in muscles
- Anaerobic benefits theory: Increased blood flow -- vasodilation and perfusion
"Wait, hypoxic sprinters outperformed normoxic sprinters? What happened to "live high, train low"?"
To answer this, let's look at 2 studies. Of the studies I read, only 2 compared normoxic vs hypoxic sprint performance within a session:
Study 1: Hypoxic peformed better
Group 1: Sprint as hard as possible on a treadmill in a hypoxic room for 10-seconds 9 times, then sprint as many times as you can for 10 seconds until you can not complete a 10-second sprint anymore.
Group 2: Do the same thing, but in a normoxic room.
Results: Group 1 was able to sprint an average of 13 sprints after the initial 9 sprints, and Group 2 was only able to sprint 9 times after their first 9 sprints.
Study 2: Normoxic performed better
Group 1: Perform an incremental step test (run at increasingly harder speeds each minute until failure)
Group 2: Do the same thing as group 1, but the hypoxic settings were turned on.
Note: Both groups ran in the exact same room, but they did not know whether the room had the hypoxic settings on or off. This is called "blinding" the participants.
Results: Normoxic group had higher VO2 max and higher running speeds.
So, does it contradict "live high, train low"...maybe! Let's get into the issues:
Placebo! Most of the studies admit that their groups were not blinded. In the 2 examples above, the first study did not blind hypoxic group, they knew they were the group getting hypoxic training. They could have been trying extra hard thinking that this super cool technology would enhance their performance. You'd be surprised what the human mind can do. Whereas the second study that supports the "live high, train low" method, had blinded both groups.
For all studies in general, it's hard to compare who had it right and who had it wrong. The variables were all so different:
- some replicated 6,500 feet while others replicated 9,800 feet
- some had people train 2 times a week, others had people train up to 5 times a week
- Some had people train for 1 hour others had them train for 2 hours
- Some had untrained runners and some had elite runners
For those reasons, I think the findings of people performing better in a hypoxic setting may have been a fluke or a problem with the study design. I do believer the gains in VO2 max is legit.
"So...DOES. IT. WORK?! I'm still confused."
Based on current research it seems to be successful in increasing VO2 max more than training in normal air which will allow a person to perform better with long distance running. As for sprints, there is limited evidence to show that it may allow a person to run harder and longer.
"Sweet, what do I actually do with this info now..."
Well first of all understand that this isn't for everyone. This is for those who are elite level athletes looking to shed off a second or 5 on their running times. It's for the elite runners who have to time to fit this in alongside their standard training program. It's also for those elite runners that have sponsorship money to fund the training; it's expensive.
If you did want to do IHT, below are the guidelines that are recommended based on current evidence:
- 70% or higher VO2 max performance
- FiO2 of 16-18%
- 1 to 2 hours a session
- Short rest intervals (1:4 work rest ratio at most)
- 2-5 times a week
So, what are your thoughts? Sound off in the comments. Also, if you are looking for great running training and rehab tips, drop by Matt's website at http://runbetteracademy.com/. He is a highly trained PT that specializes and participates in endurance running.
- Faiss, R. Olivier, G., and Millet, G.P. (2013). Advancing hypoxic training in team sports: from intermittent hypoxic training to repeated sprint training in hypoxia. British Journal of Sports Medicine. 47: 145-150.
- Holliss B.A., Burden, R.J. Jones, A.M., and Pedlar, C.R. (2014). Eight weeks of intermittent hypoxic training improves submaximal physiological variables in highly trained runners. Journal of Strength and Conditioning Research. 28 (8)
- Holliss, B.A., Fulford, J., Vanhatalo, A., Pedlar, C.R., Jones, A.M. (2013). Influence of intermittent hypoxic training on muscle energetics and exercise tolerance. Journal of Applied Physiology. 611-619
- Kilding, A.E., Dobson, B.P., Ikeda, E. (2015). Effects of acutely intermittent hypoxic exposure on running economy and physical performances in basketball players. Journal of Strength and Conditioning Research. 30(7)
- Scott, B.R., Slattery, K.M., and Dascombe, B.J. (2015). Intermittent hypoxic resistance training: Is metabolic stress the key moderator? Medical hypothesis. 84: 145-149
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