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What are O2 Tables?

O2 tables are a training method used by freedivers to increase their maximum breathhold.

There are many different kinds of tables that aim at training different aspects of the breathhold, including CO2 tables which aim at increasing your tolerance to carbon dioxide.

O2 tables specifically target increasing a freediver's tolerance to low levels of oxygen, also known as O2. Oxygen is the essential gas required for the process of cellular respiration, and we get it when we breathe.

Defining O2 Tables

When we use the term O2 Table what we are talking about is one full training session. A session consists of a number of rounds, and each round has two phases - a Rest Phase where you focus on relaxing your body into a calm and cyclical breathing cycle, and a Hold Phase where you hold your breath.

We call the first phase a rest phase because you are typically recovering from your previous breathhold. There are normally 8 rounds in a table, and if you were to write them down on paper your progress would look like a spreadsheet or table, with each line being a 'round'. Hence the word table.

How they Work

In an O2 Table the rest intervals are fixed, while the hold interval increases with each round, usually by 15 seconds. The idea is to increase the breathhold over the course of the table, gradually introducing you to lower levels of oxygen as the hold is increased.

In a standard table, your rest interval will start at 120 seconds, or 2 minutes. This remains the same for every round. Your hold interval may start at 60 seconds, or wherever your feel comfortable, and increase by 15 seconds each round. In this example, after 8 rounds you will be doing a hold of 3 minutes. Increasing the hold interval takes your body slowly into states of depleted oxygen, so that your body will learn to cope with and adapt to these states. As your body gains an awareness of what these low oxygen states feel like, you become more comfortable in them.

Big Fat Disclaimer

Risks Involved

Holding your breath for an extended time can be dangerous. It can lead to lightheadedness, and faintness, as well to states of hypoxia, or low oxygen, or blackout. Blackouts can occur suddenly and without warning. They can be serious, especially if you are alone and without anyone to help you. You should always do any breathing exercises with a friend who also understands the associated risks.

You should never attempt any breathing exercises if you have any underlying medical condition. Before attempting any breathing exercises you should contact your primary medical care provider for their advice.

These breathing techniques are presented as educational, for those people seeking to understand these training methodologies. They are not intended nor recommended as any kind of treatment or therapy.

Breathing exercises should never be attempted in water. The risk of loss of motor control, blackout, and drowning are much greater and more severe in water.

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The Science of O2

Cellular respiration is the process by which a cell takes oxygen and combines it with glucose in a chemical reaction that produces energy and byproducts. This energy produced is adenosine triphosphate, also called ATP, and the byproducts are carbon dioxide, O2, and water. Carbon dioxide is then removed by the body through the lungs by exhalation.

Oxygen is brought into the body through breathing, or respiration. When fresh air comes into the lungs an exchange of gases occurs and oxygen enters the body while carbon dioxide, or CO2, leaves the body. This is called respiration gas exchange.

Oxygen is carried in the blood by a molecule known as Hemoglobin. One hemoglobin molecule can carry up to four oxygen molecules. The presence of carbon dioxide in the blood causes a chemical reaction with water, H2O, which produces bicarbonate, HCO3, and hydrogen ions, H+. These hydrogen ions have the effect of decreasing the pH, making the blood more acidic. During an extended breathhold as carbon dioxide levels increase in the blood and pH falls, hemoglobin's affinity for oxygen decreases. This causes oxygen to disassociate from their hemoglobin-carrier molecules. This is called the oxygen-hemoglobin dissociation curve. The result is that more oxygen becomes available as the level of CO2 in the blood increases.

When the available oxygen in the body is low we enter a state called hypoxia. Hypoxia literally means 'low oxygen', and it is a signal for the body to start moderating processes that consume oxygen. When levels of oxygen become too low, the body will shutoff non-critical systems, such as the brain. This is commonly known as a blackout.

When a blackout occurs, the body still has a bit oxygen left, it is just attempting to conserve what it has until the next breath. If it gets that breath soon, no damage will be done to the brain or other vitals organs. What is important in a blackout is that the body gets fresh oxygen as soon as possible. On land, the body will simply signal for a breath to be taken and then all processes will restart. If the body sends this signal while underwater, then things become more serious.

We can say that hypoxia happens when the available oxygen is low, and there are two ways that this can happen. The obvious way is by too much of the supply of oxygen being depleted. The other way that this could happen is when the oxygen is bound too tightly to the hemoglobin, in which case the hemoglobin will not release it. This can happen through hyperventilation - a process by which levels of CO2 are expunged from the body.

CO2 helps to regulate the availability of oxygen in the blood. In a state with low levels of CO2, such as during or immediately after hyperventilation, the pH of the blood increases. This causes oxygen molecules to bind more readily to hemoglobin, making less oxygen available. Thus, by removing our reserves of CO2 through hyperventilation we actually lock up oxygen in hemoglobin, making it unavailable when we need it the most.

O2 and Freediving

In an extended underwater breathhold O2 levels drop throughout the duration due to the fact that you cannot inhale fresh oxygen. When our O2 levels get too low we enter a state of hypoxia where the body attempts to conserve the remaining available oxygen.

Hypoxia can feel disorienting, but is can also feel very good - like a high. Consciousness starts to fade, a bit similar to a dreamlike state. In later stages of hypoxia the body can become agitated or tense. At some unspecified point the body will decide that enough is enough, and it will shut down the brain to conserve what oxygen remains. This is a blackout.

It is not the purpose of O2 tables to have blackouts, or get close to blackouts.

Rather, the purpsose of O2 tables is to train your body to cope with low levels of oxygen. We are not aiming to go into a hypoxic state. We are attempting to experience decreased levels of oxygen. Decreased levels of oxygen does not necessarily mean hypoxia - it simply means levels of oxygen which are lower than where we are normally used to operating.

We are aiming to develop a relationship to the low oxygen states which happen before hypoxia.

So, if you find yourself becoming hypoxic when you do these tables, back off a bit. Lighten up the hold times. These tables are not about hypoxia - they are just about getting comfortable with a bit less oxygen.

Training Your O2 Awareness

O2 tables are a form of dry training, and the purpose of this app is for dry training. We strongly discourage you from attempting O2 tables in water. Do not ever use this app for the purposes of wet training.

O2 training should always be done while laying in bed face upwards, or on a soft and comfortable surface. As with any breathwork exercises, there is always a risk of being faint or lightheaded afterwards, so take your time to return gently and rise slowly. There is always a risk of a blackout, which can occur without signs or warnings. We recommended to perform these training exercise laying in a face up position - if you do have a blackout, you will fall asleep and in a few moments return naturally.

A blackout occurs when the body's oxygen levels dip too low. In an attempt to conserve oxygen, the body reduces oxygen flow to the brain. Think of it as a low power mode. The body is in an energy conservation state. As soon as oxygen is available it will breathe and wake back up.

Since O2 tables take you into low oxygen states, there is a much greater risk of blackout as compared to CO2 tables. Always follow the above advice to be sure that your practices are done safely.

Why Train O2 Tables

There are many reasons why you might want to train your body to cope with low levels of oxygen. You will gain a better understanding of how your body functions in low oxygen states, and this will pay dividends to your diving. You will gain a greater clarity about how low on oxygen your body is, because you will have been there before in safe and controlled environments. You will also learn to spot signs of hypoxia in others, and as such you will be a better safety diver because you will have a greater understanding of what the other divers are experiencing.

By regularly and safely practicing O2 tables you will learn to relax into the sensations that come with low oxygen levels. This will acclimizate you to many of the sensations and experiences that you may have while freediving, but with the benefit of not needing to be underwater to experience them.

How To Use O2 Tables

The recommended training schedule for O2 tables is no more than 2 times per week, with rest days in between each session. It is best to do them on an empty stomach if possible, or at least 2-3 three hours after eating.

Start gently at first, with a hold interval of no more than half of your maximum breathhold. If in doubt, try a 60 second hold and see how a full session feels. You can gradually increase your hold times as the table becomes easier to complete.

The important part about O2 tables is that you should be able to finish them. It is not necessarily about a table duration being at the limit of what you are capable of, but more about them being challenging enough while still able to be completed. Remember, this is about your body's ability to cope with low oxygen levels gently over time.

Training with Tables

It is not recommended to combine training O2 tables with CO2 tables on the same day. Ideally you train them on different days. O2 tables can be long as well as exhausting, so it is also recommended to do them only once or twice a week, whereas CO2 tables can be done every other day.

And as always, Never Freedive Alone.