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  • Writer's pictureHarry Chamas

O2 (Hypoxic) Training | Purpose, Mechanism and Benefits

O2 training has long been a staple of freediving training, so much so that it is taught during the 2nd level of most freediving education systems. The purpose of hypoxic training is said to be to train low levels of O2 (tolerance to low O2) and no further information is giving regarding the mechanisms of hypoxic training, or the situation in which a freediver may benefit from hypoxic training. This article is designed to give a more in depth analysis of hypoxic training and hopefully help some freedivers out there to make better decisions regarding their programming decisions.

The first question we need to ask is what do freedivers generally use hypoxic training for? and the 2nd question is does it work?

The classic reason is to train low O2 tolerance and a secondary and less understood reason is to build blood hematocrit, but as you will see, depending on how the training is undertaken, there are other benefits to be had from hypoxic training.


First we will talk about training low O2 tolerance

This is the belief that by constantly exposing the brain to low oxygen levels, the brain will then learn or adapt to handle lower and lower levels of O2 before blackout occurs. This has been taught and believed for many years now, but amazingly there is no scientific backing to this at all. OK there is no scientific backing to a lot of training methods we use in freediving (and it would be scientifically unethical to expose people to severely low levels of O2) but I don't even see any trustworthy empirical evidence to support this either.

So what is happening?

Let's say for argument sake that the fixed saturation that a Bob the freediver can reach before BO (black out) is 50%. Now let's say that Bob's O2 sat reaches 50% after a 100m dynamic. We can deduce he is consuming 1% O2 per meter (in realty the O2 consumption will not be equal at different sections of the dive depending on peripheral vasoconstriction and various other mechanisms).

Bob now undergoes hypoxic training, The amount of total meters covered during this training improves movement efficiency (this is true of any sport but has been seen to be amplified under hypoxic conditions)(1), his repetitions of long dives or diving under hypoxic conditions improves his relaxation - meaning lower heart rate, metabolism and therefore O2 consumption.

Now Bob can do 125m before he BO, that is an improvement of 25%. It could be easy for him to believe he can now tolerate a much lower level of O2 sat, somewhere around 40% sat. But in reality the thing that has changed is the rate of O2 consumption. Due to the previously discussed factors the rate is now 0.8% per meter and this is in fact allowing the longer dive.


What about using hypoxic training to increase hematocrit levels?

Hypoxic training is widely accepted to be of use in the world of sport science(2)(3)(4). The predominant method used is LHTL (live high train low). LHTL is a style of training in which an athlete lives at altitude (2500m-3000m) and is thus exposed to hypoxic conditions (91-90% O2 Saturation), the athlete then descends to lower altitudes in order to train each day, this ensures he/she is still capable of performing with a high enough intensity to not loose conditioning.

How does this work?

Low O2 saturation triggers the release of a protein called EPO (Erythropoietin). EPO is the signal for the bone marrow to increase production of red blood cells. More red blood cells means more O2 carrying capabilities in the blood, and therefore greater potential for physical performance. The advantage to carrying more O2 to freedivers is self evident.

Can we trigger EPO production with breath holds?

Yes(3)(4). EPO has been seen to rise as much as 24% in "elite" level freedivers after a series of max breath holds.

How Hypoxic do we need to be?

To start to get EPO producing benefits, the O2 saturation should drop below 91%, studies have been done using the LHTH (train and live at height) method but at a height that would induce lower levels of hypoxia (+93% sat) and no benefits were found. I have not yet found any studies to indicate that dropping far below 91% is of use. But intuitively I would say that it is. In the study previously mentioned(4) for instance these "elite" breath hold divers were surly experiencing O2 saturation lower than 91%, although this is not mentioned in the report.

How long do we need to be Hypoxic?

The length of time athletes are hypoxic while under LHTL conditions is a huge advantage, they spend around 20 hours per day hypoxic. Studies have been done to test the usefulness of intermittent hypoxic exposure, similar to what we experience while training freediving.

Unfortunately the focus was always on time and not the level of hypoxia, one study showed that the difference between spending 24 secs and 178 secs below 91% O2 saturation on EPO production was 24% to 35%(2). So it is safe to assume that time under hypoxic conditions is of consequence. Interestingly the 24 sec exposure was at 1000m height and the 178 sec exposure was at 2100m, so perhaps we can assume a contributing factor was lower levels of O2?

Unfortunately due to the relatively short half life of EPO (5 hours) and high production time of RBC's it is very difficult to create enough time under hypoxia (while on breath hold) to actually raise your Hematocrit.


Never the less, as usual, freedivers seem to be one step ahead of the science, and there have been instances of freedivers using breath holds to improve their hematocrit.

So here are some tips from me in case you decide to undertake this endeavour.

1. Dive on RV (forced exhale) - Less starting O2 means you will become hypoxic sooner, which is what we want, also CO2 build up will be less so the dive should be less mentally taxing. It also means each dive will be shorter allowing for more repetitions.

2. Dive in DYN or DNF - Studies have shown that there is a difference in the adaptations caused with STA and DYN and that exercising under hypoxia will create a stronger EPO production(2).

3. Supplement - ensure you are getting enough Iron, Vit C, B6, B12 and folic acid to support RBC production.

4. Test Blood - LHTL sustains an increase of hematocrit of 1-1.5% per week. If you are not achieving this, then you may need to ramp up or improve diet.

5. Ramp up - start with 8 hypoxic DYN dives per session (apnea walk/run if no access to pool), 3 times per week. If there is no change in the blood (and diet and supplementation is correct), increase sessions per week, if still no change then add repetitions per session.

6. Use a pulse oxymeter to be sure you are diving in the required saturation (80-60%)

7. Use common sense, don't sprint with lower levels of O2, ramp up slowly, use a oxymeter to feel out your limits, use a proper safety in the pool... don't do it while tight rope walking over a pit of king cobras, this isn't rocket science guys.


Other residual benefits of hypoxic training

Effects on muscle fatigue

Studies have shown that high intensity training under hypoxia has more advantages than medium intensity(9). Results have shown delayed fatigue in comparison to the same training under oxygenated conditions. Possible reasons could be more improved movement efficiency or improved tolerance to acidity, whatever the cause, the importance of any improvements in the anaerobic system for freedivers is self evident.

This method would include -30 sec max intensity sprints while hypoxic, with incomplete recovery until exhaustion.


The traditional approach to hypoxic training involving progressively adding distance while diving on full lungs, gradually builds up confidence with longer dive times, while top level athletes don't need this, it is vital for the beginner/intermediate divers so slowly become accustomed to how it feels to do longer dives.


Con's to hypoxic training


The amount of repetitions per session and per week required to create adaptation is very high, as is the intensity of the exercise. Over a prolonged period of time this high level of intensity can easily lead to overtraining and plateau.


The positive effects of building the blood will start to reduce after a few days, and will have disappeared by the end of a week, This means 5 weeks of hard work will have disappeared after 7 days without hypoxic stimulus.

Cross over

A 5% increase in Blood Hematocrit does not necessarily trance fare over to a 5% increase in performance. There are many dimensions to freediving which can give much bigger benefits to performance (given the same amount of training and effort it would take to increase hematocrit by 5%).



O2 training can increase the maximum potential performance of a freediver given the correct approach and timing.

Given the large amount of time and effort required to create adaptations, I would only recommend a diver use O2 training when he/she has already reached their maximum potential with regards to technique and relaxation (the O2 consumption reducing effects of both far outweighing some extra O2 carrying capabilities within the blood).

The effort required to build the blood and the speed at which these efforts are lost means timing is important. To start 6 month before a competition is risky as a diver will have to maintain hypoxic training without rest for 6 months which will be extremely difficult both physically and logistically. Rather building up 3-6 weeks before a performance will be more likely to be sustained.

I hope this has been of interest to you guys, if there are any science buffs out there I would like to hear your experience or observations with regards to hypoxic training. Take it easy, and dive safe!!!











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