Freediving Wissen · 06
Why do you feel the urge to breathe when you still have oxygen?
Your urge to breathe tells you that your body wants to breathe, not how much oxygen you have left.
You hold your breath. The first seconds feel comfortable. Then something changes.
You feel the wish to breathe in. Perhaps your diaphragm begins to contract involuntarily.
But does that mean your oxygen is already used up?
Not necessarily.
Your urge to breathe tells you that your body wants to breathe – not how much oxygen you have left.
1. CO₂ – why your body wants to breathe again
While you hold your breath, your body continues to use oxygen (O₂) and to produce carbon dioxide (CO₂).
The oxygen level therefore falls gradually, while the carbon dioxide level rises.
Special receptors register these changes and influence the respiratory centre in the brain.
The rise in CO₂, and the change in acid-base balance that goes with it, are especially important triggers of the drive to breathe.
That is why you can already feel a strong need to breathe while oxygen is still present.
Caution: how much oxygen is actually available cannot be judged reliably from this feeling.
2. The breaking point – when your body wants a say
A breath-hold is often divided into two sections.
First comes the so-called easy-going phase. In this phase the breath can still be held relatively comfortably.
Later the struggle phase begins. The urge to breathe becomes stronger, and involuntary movements of the breathing muscles can appear.
Interestingly, the basic breathing rhythm in the brain continues even during a deliberate breath-hold.
You are not stopping your entire respiratory system. You are temporarily suppressing the breathing movements.
Involuntary breathing movements and the start of the struggle phase are not necessarily the same as the breakpoint.
The breakpoint is the end of the breath-hold, the moment you breathe in again. That ending is influenced by several physiological and psychological factors.
3. Why do the contractions begin?
Contractions are involuntary movements of the breathing muscles, especially the diaphragm.
They arise when the drive to breathe increases during a breath-hold.
But not every freediver experiences them in the same way.
A scientific study with 52 participants showed that such movements were observed in almost everyone. Their strength and frequency often increased as the breath-hold went on.
Especially interesting: the strength of the contractions did not simply match the subjective feeling of breathlessness.
That means two people can experience similar contractions and perceive them differently.
And very important: the first contraction is not an oxygen gauge. Neither its onset nor its number allows a reliable statement about how much oxygen you have left.
4. Why a weak urge to breathe can be dangerous
This is one of the greatest misunderstandings in freediving.
Some people believe they are safe as long as they do not yet feel a strong urge to breathe.
But that is exactly what is not true.
Hyperventilation before an apnea can lower the CO₂ level markedly.
It then takes longer before the rise in carbon dioxide triggers a strong urge to breathe.
Oxygen consumption continues in the meantime.
In the worst case, oxygen can fall so far that unconsciousness occurs before a clear enough urge to breathe is noticed.
This can be especially dangerous during ascent, because the falling surrounding pressure reduces the oxygen partial pressure further.
That is why DAN specifically warns against hyperventilation and recommends breathing normally before freediving.
Less urge to breathe does not automatically mean more safety.
What does this mean for your training?
Research shows that the urge to breathe is a complex interplay of blood gases, the nervous system, the breathing muscles and individual perception.
A few important points follow from that:
A rising CO₂ level contributes substantially to the urge to breathe.
Contractions are normal physiological responses, but they are not a reliable indication of the oxygen store.
The start of the struggle phase is not an invitation to continue the apnea for as long as possible.
Hyperventilation can delay important warning signals and increase the risk of blackout.
Understand your urge to breathe – instead of fighting it
With ApneaReady we want to help you shape your training sessions more deliberately.
Contraction Tracking lets you record the moment of your first contraction and further contractions during a training session.
That way you can compare your own observations across several training sessions.
These records show neither your oxygen level nor your actual safety margin. They are not a basis for continuing an apnea despite increasing discomfort.
In freediving the point is not to ignore warning signals for as long as possible, but to understand your own physiology better.
Your urge to breathe is a signal from your body. But it is not a measuring device for your oxygen.
Safety note
A weak urge to breathe, or contractions that do not appear, is not proof of a sufficient oxygen supply. Never hyperventilate before freediving, and never train apnea in the water alone. This article does not replace qualified freediving training.
References
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Breath-holding and its breakpoint
Experimental Physiology, 91(1), 1–15
The central breathing rhythm continues during a breath-hold; the movements are suppressed rather than the rhythm being switched off. The breakpoint that ends the hold depends on more than one gas, and hyperventilation can prolong a hold.
Read publication ↗ -
Respiratory neuromuscular output during breath holding
Journal of Applied Physiology, 50(2), 435–443
Involuntary contractions appeared in almost all of 52 subjects and often grew stronger and more frequent. Their size did not simply match the feeling of breathlessness. The holds followed carbon-dioxide rebreathing in the laboratory, not freediving in water.
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The ins and outs of breath holding: simple demonstrations of complex respiratory physiology
Advances in Physiology Education, 39(3), 223–231
Breath-hold time depends on practice, psychology, chemoreflexes and lung stretch. The article includes the effect of prior hyperventilation.
Read publication ↗ -
The application of breath-holding in sports: physiological effects, challenges, and future directions
European Journal of Applied Physiology, 125(8), 2049–2065
Review of breath-hold training as a priming strategy in sport, including hypoxic blackout among the risks. It does not establish the contraction counts or the easy-going and struggle phases.
Read publication ↗ -
Hypoxia in breath-hold diving
Breathing is driven largely by carbon dioxide. Hyperventilation does not usefully raise oxygen and can lead to blackout, especially near the end of an ascent.
Read publication ↗
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