Hold Your Breath Part 6
How Long Can You Really Hold Your Breath-Part6?
The Freedive-Earth Maximum Breath-Hold Calculator
Just a quick reminder before we get stuck in, especially if you're new to freediving. Don't forget the most important rules: Go slow, and Never, Ever Freedive alone. If you haven't already, start by downloading the calculator here. (No excel software? Open the file online here). You'll get an excel file which, when you open it up, you'll see has space for you to put in details of your height, weight, Vital Capacity, True or Estimated Residual Volume, some information about your muscularity and altitude training. Enter this information in the appropriate blank multi-coloured boxes on the left hand side. Don't put anything in any of the other boxes because you'll muck up all the sophisticated mathematics used in programming the model. Or something like that. If you have a look around, you'll see that at the top of the sheet is a purple box labelled maximum breath-hold time and that's the bottom line, really, the number it would take you 5 blogs worth of donkey work to calculate for yourself! You'll also see lots of other boxes on the page which you may or may not be interested in:
Light Blue Colour
These values are all mentioned to a greater or lesser extent in the other blog articles in this series. Those to do with lung volumes you'll find explained in blog 2 (coming soon!), those that refer to fluid volumes or percentages you'll find in blog 3 (watch this space!) and those that relate to muscle mass you'll find explained in blog 4 (not far in the future).
Things like BMI, Muscle Mass and Lung Volumes you might find interesting to look at for themselves, because they're things that have an impact on training and freedive fitness. You can play around with the parameters you enter to see how these things are affected.
Oxygen Carrying Capacities The values in red boxes show oxygen carrying capacities for your various different body compartments. This allows you to see how much oxygen is stored in your lungs, bound to haemoglobin or myoglobin, or dissolved in the blood plasma. You might find it interesting to compare these values and to play around with how your breath-hold might be affected by, say, packing a few more times, or putting on 2kg of muscle. The red box in the middle, just above the maximum breath-hold time, shows your Total Oxygen Carrying Capacity, that is, the total amount of oxygen you can hold in your body given the parameters you've set.
Light Yellow Boxes
Rationalisations The boxes in the bottom left, labelled rationalisations are the method the calculator uses to work out the most appropriate value for things like body water percent and muscle mass based on the values you enter for height, weight and muscularity. The final values are an average of the numbers suggested by the model in relation to what you entered. Black Box Blackout Limit Just next to the red box for total oxygen carrying capacity you'll see a small black box containing an estimated value for the amount of oxygen remaining in the body at the moment of blackout. This is calculated by a linear extrapolation of the amount of time taken for the PaO2 to drop from 11Kpa to 3Kpa. It's subtracted from the total oxygen carrying capacity to calculate the final breath-hold time.
Limitations of the Model
There are a few key aspects of the static breath-hold that this model doesn't take into account. The first and most important of these is the effect of the Mammalian Dive Reflex (MDR) on your Basal Metabolic Rate. The model assumes a minimum oxygen consumption of 250ml/min based on the accepted value of the BMR in physiology text books. There is very little (none that I could find) information out there about the BMR in relation to diving in humans so rather than guess, I've left it with the known value. The reflex will act differently and at different times for each individual anyway. It also doesn't include any meditation techniques that you might use to further reduce your oxygen consumption. Where possible in the text I've given suggestions for how to calculate things like body water content and residual volume using accurate measurements rather than tables of 'average' values but there is inevitably going to be some inaccuracy. In order to make the model simpler, it uses average values (according to sex and body type) for muscle mass and body water calculations on the assumption that not many people are going to be bothered to measure their own values for this. Even where accurate measurements are taken, most of the values (muscle mass for example) are still inferred from other measurements, rather than measured directly, so there are two more sources of inaccuracy: a) in the average values used and b) in any numbers that you might measure for yourself.
The model contains no reference to the fact that more muscle, as well as providing more oxygen through larger amounts of myoglobin, also carries a cost in the form of higher oxygen consumption.This is the fact that has created the controversy over whether it's better for freedivers to have more or less muscle. In my view, the sheer size of the volume of oxygen stored, even in relatively small amounts of muscle makes it advantageous to do some gym work, not to mention the benefit of greater strength for the depth and dynamic disciplines. For static, however, the problem is going to be in keeping all that extra muscle completely relaxed so that the actual metabolic rate is as close to BMR as possible. The BMR value used implies complete physical and mental relaxation. This is clearly very difficult to achieve! In addition to all of the above, each person will have a slightly different distribution of oxygen throughout the body, and there will always be body types and physiologies that don't fit the model.
Finally, the value for blackout limit (see above) is calculated using an extrapolation of the amount of time taken for the PaO2 to drop from the initial value down to 3Kpa, where blackout will occur. 11Kpa is the accepted upper limit of normal for a normal individual breathing air, but techniques like hyperventilation and packing will probably have an effect on a) the initial PaO2, b) the trajectory of the decrease in PaO2 during the breath-hold and c) the total oxygen carried at blackout as a direct consequence of the Bohr effect. Both of these things may cause the model to under or over-estimate the blackout limit. It's probably best to view the numbers in this model as relating to a theoretical maximum, and a way to play around with the different aspects of physiology that contribute to your breath-hold to help you to direct your training. However you use it, we hope you find it interesting, and maybe even useful.
If you've got any feedback or questions for us at all, any special expertise to bring to bear, or any suggestions at all as to how the model can be improved, we'd love to hear from you. Drop us an email here, comment on this post or write in one of our forums.