A discussion on social media about reverse packing showed that the concepts of Active and Passive Residual Volume (which many of you will have been introduced to in our Advanced, Master and Instructor courses) are very useful, especially when considering advanced training techniques that deal with training for deep freediving, flexibility and risk of barotrauma.
Residual Volume (RV)
As it is taught in most freediving course curricula, the term Residual Volume describes
The volume of air remaining in the lungs after a full exhale
... in some variation on the general idea.
It turns out, that this general idea is too general for freedivers who want to think in more detail about their training, their capabilities and their risk of injury, namely of barotrauma of the lungs or trachea.
Most advanced freedivers have heard of or know of mouthfill equalisation, which allows us to equalise our ears, sinuses and any airspace above the trachea when we have descended past a depth where our lungs are compressed to this definition residual volume.
It is also commonly taught, that compressing the lungs past Residual Volume incurs - or greatly increases the risk of - lung barotrauma.
Both of these statements leave a dive where we do use a mouthfill, go beyond RV and come up healthy in a semi-defined limbo: If your lungs were below RV, was it sheer luck that you came up with healthy lungs?
To allow us to have meaningful conversations about this most coveted state of freediving, I propose to use the terms Active Residual Volume and Passive Residual Volume, defined as follows.
Active Residual Volume (RVA)
The term Active Residual Volume describes more explicitly what is generally referred to as Residual Volume in most freedivers' conversations. It describes
The state of the lungs and rib cage, where no more air can be actively exhaled, that is the lung volume can not actively be reduced by the surrounding breathing apparatus.
Passive Residual Volume (RVP)
The term Passive Residual Volume can be used to meaningfully describe
The state where the breathing apparatus can not be further compressed without injury.
Note that in this definition, we allow for outside forces applied to the breathing apparatus!
The Difference Between the Two
When we reach RVA, our rib cage and diaphragm, along with all the other skeletal, fascial and muscular mechanical components may not be able to reduce the gas volume of the lungs actively, but they may still be - and most commonly are - flexible and can be further compressed, for example by outside water pressure.
In dry training, this is easily verifiable when exhaling to RVA and having a training partner, coach or buddy compress the rib cage, for example by manual pressure applied to both sides of the thorax. The Uddiyana Bandha exercise applies a different principle to selectively apply a pressure difference to the diaphragm, utilising mainly the intercostal muscles and ribs to create this pressure difference.
How far our lungs can be compressed this way now is not merely a physiological quantity of flexibility and mobility, but depends greatly on our mindset, relaxation and awareness of our physical, physiological and mental state.
Passive Residual Volume - The Breaking Point
This dependency of RVP on various factors now allows us to understand why lung and trachea barotrauma are such a complex topic. Once a freediver descends below RVP they will - by this definition - suffer an injury, at depth this injury will be caused by the pressure difference - a barotrauma. The extent of the injury can now be viewed as the direct extension of how far below RVP has descended.
Further Reading
For an outstanding, highly related read, we recommend Walter Johnson's excellent article on the Ability Adaptation Squeeze Model that supplies us with more thought experiments and vocabulary to further our learning in the matter.
How do you apply these concepts for yourself? Let us know in the comments below!
