Deep breath in …. probably one of the most common phrases in a yoga class. But what does it actually mean to breathe deeply?

Most yoga teachers and practitioners interpret it as a big breath in. How often have I seen people throw their arms wide, and generally breathe in through the mouth, filling right up into the shoulders. And then comes the inevitable big sigh out through the mouth too. Very often a deep breath a teacher will encourage students to make a sound during a deep breath, further re-enforcing the notion of a deep breath as a big breath.

But is a deep breath a big breath? Understanding the mechanics of breathing helps settle this.

How We Breathe

Unless we are consciously controlling our breath, it moves unconsciously – in other words, we are not aware of it. The mechanism by which this works is as follows:

• Changes in blood chemistry (pH levels) are detected by chemoreceptors in the blood vessels. They relay the message to the brain which sends electrical signals via the nervous system (primarily the phrenic nerve) to the diaphragm to initiate breath.
• The diaphragm is a sheet of muscle that extends across the base of the thoracic cavity from the abdominal cavity. In its resting state, the diaphragm is domed upwards. When it receives the message from the brain to initiate breath, it contracts and flattens (moving between 1 to 10 cm). The lower ribs flare out. This creates a partial vacuum in the thoracic cavity which forces the lungs to expand to fill the void, drawing air in the process.
• At the end of the inhale, the nerves to the diaphragm stop firing and the diaphragm relaxes and returns to a domed position. This compresses the air within the alveoli and causes exhalation.
• At rest or with simple exertion, the diaphragm teams up with the abdominal muscles —specifically the pelvic floor, transversus abdominis, and obliques—which contract gently with exhalation.

Muscles in the chest and neck are not intended to be actively recruited for respiration when we are at rest or engaged in light activity. Slight movement of the lower ribs during inhalation is caused by the downward contraction of the diaphragm.

Breathing, at rest, then is essentially soft, requiring very little effort, and using hardly any energy. In other words, a deep breath is not a big breath at all. It just requires that we use the diaphragm.

The Myth of Deep Breathing

It is a common misconception that a deep breath is a big breath. A deep breath means using the depth of your lungs. And the best way to do this is to ensure that you use your diaphragm as you breathe. You do not need huge gulps of air for gas exchange to take place. You just need a gentle pumping action of the diaphragm.

In fact, “big breathing” may have adverse consequences for our health. Firstly, big breathing engages the accessory breathing muscles in the shoulders and neck, creating tension – and we all have enough tension there already! Secondly, big breathing encourages mouth breathing, and if you want to know why that’s wrong, check out this article. Thirdly, big breathing generally doesn’t involve the use of the diaphragm – over time, this can de-stabilise the core, and weaken the diaphragm. And lastly, big breathing, particularly if it involves exhaling through the mouth, can affect the carbon dioxide levels in the body which, if sustained, can have a profound effect on health – see here.

So, the next time a yoga teacher encourages you to breathe deeply be careful how you react. Deep is diaphragmatic not big.