Today's blog is about the effect our body position has on our failure depth (the point you can no longer bring air up to equalise). Of course our body position will also influence other aspects of our diving such as streamlining, trachea squeeze, our direction of movement and lung squeeze. But I will be focusing solely on it's influence on failure depth for today.
The reason it becomes difficult to bring air up at depth is our lungs have been compressed by the weight of the water above us. The deeper you go the smaller your lungs will become. Imagine, on the surface you will have full lungs. At 10m they will be about the same size as when you take a full belly breathe on the surface. At 20m they will be around the size they are after a normal (tidal) exhale on land. At 30m they will be empty or close to empty.
The point where you can no longer bring air up to equalize is known as your failure depth. Our failure depth is not a fixed thing, there are many variables that will affect how deep we can equalise, such as the volume of air within our final breathe, how much our body can compress, how strong our dive reflex is, and our body position. Effectively the failure depth is the point the water has compressed your lungs smaller than you have the ability to compress them using your respiratory muscles.
So to put the theory into context: Let's say Joe has a 6L total lung capacity (a full inhale), and he has a 1.5L residual volume (the amount of air left in the lungs after a full exhale).
0m = 6L lung volume - Joe has full lungs and it's easy to get enough air to equalize
10m = 3L lung volume - Joe's lungs have halved in size but there is still enough air for equalization to be easy
-20m = 2L lung volume - Now remember Joe's Residual Volume is 1.5L, so when trying to bring air up (by contracting his respiratory muscles) he will only have 0.5L available to bring up, and that will only be available using the same amount of tension it takes to do a full exhale on the surface.
-30 = 1.5L lung volume - Now Joe's lungs have been compressed to the point where no matter how much he tries to bring air from his lungs, he will not be able to create enough positive pressure.
An average person should be able to equalize to 30m. If that is difficult then either you are not using Frenzel, you don't take a full breath on the surface, or you are not allowing your body to compress enough to bring air up at depth. If you are sure that you are using Frenzel and breathing a full final breathe, then the reason you can't equalise may be your body position while in the freefall.
Let me explain how the body position will affect your equalization. Like I mentioned earlier, to bring air up to your mouth/ears between -20m and -30m you will have to create a similar amount of pressure as when you do a complete exhale on the surface, not only that but your body must be in the same position as well.
If you think about it, when you try to breathe out your maximum, you will bring your shoulders forward, your neck down and arch your back forward slightly. If you tried to do a full exhale while arching your head, shoulders and back backwards there is no way you will be able to breathe out as much air. The same applies at depth, if your head neck and back are arched backwards you will not be able to compress your lungs enough to use your remaining air for equalisation. Meaning if you have a backwards arch, your failure depth will be much shallower.
When it becomes hard to bring air up to equalize, a novice mistake will be to look down the line to see how far the bottom is. This action is the opposite of what you need to do to get that final equalization. You should allow yourself to become smaller - bring the chin in and allow the shoulders to round forward slightly.
It is worth mentioning that the position which allows you to exhale/compress the furthest is not the best position to freefall in. For example if you fold yourself over double you will be able to empty your lungs further than if you remain upright. But of cause to do this in the freefall will ruin your streamlining and therefore freefall speed. You must find a compromise.
Another thing that may influence your failure depth is if you choose to do the whole dive with your arms above your head. For divers who are very flexible, diving that way may not impact there failure depth, but most of us aren't that flexible and holding our arms up that way will expand our rib cage. This means while the water is compressing our lungs - our muscles are resisting the compression of our rib cage and as a consequence you will reach your failure depth sooner.
If you would like to see if keeping your arms up effects your failure depth and have access to deep water, then you can do a day of diving arms up and a day where you bring them down around 20m to see if you have a consistently different failure depth.
If you don't have easy access to depth then a rudimentary way to see if the above head position will negatively impact your failure depth is to :-
1. Adopt the above head position
2. Exhale as much as you can without compromising the position i.e bending forward (imagine you are freefalling)
3. Let your arms down (while holding the breath out)
4. See if you can exhale more air, again without bending over and compromising the position
5. Now breathe, repeat a few times to verify your results.
If you can exhale any air in step 4 then chances are diving with arms up will cause you to reach your failure depth sooner. Of course this doesn't mean that you can't dive with arms up, but you should understand the compromise you are making.
A big part of freediving is adjusting to the increasing pressure as we descend. You can't fight it, you must submit to it, go with it, think small and as rock climbers say - "don't look down".