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Exhale (FRC) Adaptation

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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laminar said:
I would have thought that in general, a hydrated person would be better off that a dehydrated one.


First, Congratuations on your new Canadian DNF record! I would love to hear all about it.

Hydration is not always a good thing. Overhydration can lead to seizures or, at least, to the lowering of the seizure threshold by loss of electrolytes. Sambas are a form of seizures and share some of the same mechanisms. We know the relation between sambas and blackouts. I believe that overhydration explains some of competition failures. While not germane to lung squeeze, overhydration has its downside. For what it's worth.

Hydration may be a relative term, if we consider the duration of immersion and the prompt vasoconstrictive effects of the dive response, especially in cold water, as you may experience in Canada.

With a restricted vascular tree (dive response), increased peripheral resistance and a relativeley elevated plasma volume, pulmonary capillaries may become prematurely overdistended resulting in pulmonary oedema even at very shallow depths. For example, whilst chatting to Davide Carrera last year, here in Townsville, we discussed the issue of hydration/overhydration, as I understand he may have had this problem in the past.This effect is not, however, just restricted to breath-hold divers, but swimmers also. Its my opinion that underhydration followed by immersion/submersion may better increase plasma viscosity, reducing convectivive efficiency, and thus conserve O2 just that little bit better. Of course, it would have detrimental effects in say, warm water under exercise conditions whilst breathing.

Anyone wishing for the relevant references can send me a personal email. Please give me a day or so as I'm flat out changing those nappies.....again!

The real conundrum is that yes, due to the nature of the relationship between pressure and depth, depth acheivment will increase exponentialy

The relationship between absolute pressure and volume assuming the lungs act like a balloon, and temperature is constant is inverse or hyperbolic, not exponential.


As pressure increases, the volume decreases.
i got my first chance to do some proper FRC diving over the weekend. i've done many such dives before wearing wetsuits+weight to max40m. this time i was at the SETT tank in Gosport which is 34C and apparently 28.1m deep (not 30m as many often claim!). obviously i didn't need a wetsuit or any weight in that temperature and so it was very nice to be able to sink properly from the surface and glide down with minimal buoyancy change. perhaps not surprisingly, there's a really big difference between doing FRC in 34C and doing it in cool/cold water! the warm water helps to keep your body relaxed and therefore make it easier to equalise the pressure.

the other great thing i discovered was how much difference warm water and no-suit makes to your ability to perform BTV. normally i struggle to do BTV head up to any depth and can never do it head down. but in the SETT, i found i was able to do BTV FRC dives all the way to the bottom quite consistently. this really surprised me!! but it was a nice surprise! obviously the 34C water temperature helps - it's really nice and comfortable. the other thing that helps is not having to wear a suit means there is less pressure/constriction around the throat and jaw. i think that extra freedom helps you to open up the tubes.
I was going to give a longwinded answer, but I am tired. It is because the relationship V=K/p that the Specific Value ([Actual acheivable depth] - [Initial depth resulting in ultimate lung compression]) would grow exponentially over time. Assuming linear growth capacity, if in the first month you gained 10 meters, in the next month you would gain 20, and then 40.

Think of it this way:

@ 10 meters the lungs are half the volume they were at the surface,
but it is not until 30 meters that they are 1/4 what they were at the surface,
and not until 70 meters that they are 1/8.
I think you are talking about the rate of change of volme with depth. Depth is directly proportional to pressure. As pressure increases linearly, the volume of gas in the balloon model changes inversely. The rate of change of volume varies according to an inverse quadratic law.

dV/dP=-1K.P^(-2)= -1K./P^2

The rate of change of volume starts off large then quickly becomes smaller.

Theoretically, at a fraction of a millimetre below the surface of the water, the rate of change is infinite, at a large distance below the surface, the rate of change appproaches zero.

When you say exponential you are talking about a mathematical relationship in the form:

y=A.exp^x or y=A.b^x. Where y and x are variables, A and b are constants. Exponential growth and decay occur naturally in many aspects of nature, but not in this context. Volume doesn't 'decay' exponentially with depth.

What does all this have to do with diving. Not a lot probably. I'd have trouble making 20 m so my understanding doesn't help much. However, I spent many years learning this stuff, so its nice to use it occasionally. It is mildly annoying seeing people use the wrong terminology. I was a mathematics teacher in a past life.

As you go deeper, the change in lung volume diminishes, so that in theory it should get easier for the lungs to adapt to the depth change. I think that is what we're trying to say.

(Better add the caveat that the relationship V=K/P used above assumes zero air pressure at the water air interface, which is wrong but for our purposes, the trends are the same. Don't want to upset the engineers and physicists.)

So that is the long winded reply. I saved you the trouble. :wave
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I hold that my statement is accurate...

[Volume @ Depth] =
[Volume @ Surface] *
( [1 atmosphere] / [Pressure in atmospheres] )
For [Volume @ Depth] > [Reserve Volume]


So for Volumes greater than reserve [Lung Strain] = 0
for Volumes less than reserve
[Lung Strain] = ([Reserve Volume] / [Theoretical Internal Volume]) - 1

So let's plug some data in

Lets say you hit reserve on FRV @ 10m @ 2atm.

You dive for weeks pushing further and further as you become comfortable ,and now you find that you can dive to 30m. 30m = 4atm, which means that your training has resulted in a capacity to dive to
[Lung Strain] = 1


Assuming training results in a liner progression, additional training over the same course of time will result in a capacity to dive to
[Lung Strain] = 3, which corresponds with 70m

And iterating once again, we get
[Lung Strain] = 7 corresponding with 150m

At some point surely things get non-linear, but for initial conditions this is certainly what I would call an exponential relationship

[Lung Strain] = 2^t - 1
[Acheivable depth] = [Reserve Volume Depth] + [Intial Training Capacity] * (2 ^ t)

and it demonstrates that initial progress will be amazingly slow until one passes t=1

You who say otherwise are misunderstanding the statement.

Furthermore, I was the one asking. I experienced no progress when practicing FRV dives. I formulated my statement based on the statements of others in this thread. And I do beleive that their analysis of the situation was accurate. In retrospect, It certainly explains my results.
It will get easier, if you remember that intrathoracic pressure changes are largely non-linear with depth (Boyle's Law) so that great distension and engorgement is required in the first 10-30m after which volume changes are relatively small with increasing depth. If you were to put it in perspective, a 30m exhale dive, for example, is equivalent to an 80m inhale dive, pressure wise. Therefore, a 30m exhale need not be an extreme dive and should be quite manageable with practice. Even Micki, my partner, was puling off 10m exhale after just 4 days parctice and her friend was doing 15m with no previous experience or practice and with apparently no discomfort.
Hi Jason,

I am too tired to review your math, but practically speaking, I would say that a mathematical relationship has less relevance to a person trying to progress to depths where no strain is felt or injury caused because so many factors can instantaneously change the effective volume and pressures acting on your tissues.

If you did an FRC dive to 20m with ease and then forced an equalization or had a contraction, then it's a whole different ball game. If you are too cold, sinking too fast and anxious, over or underhydrated (thanks Sebastien), descending too quickly, too hot or too insulated from the water (a wetsuit), or simply if you haven't done the exercise enough times or gradually enough, then progress and safety will be hindered (in my opinion and experience).

The last time I really practiced FRC diving on a regular basis was some time ago and even though it was only twice a week and in 8C water and with a 5mm suit on and in the dark and at the end of a long day at work -- all non ideal conditions for this kind of delicate practice, in about four weeks, I could dive to 35m in FRC with no discomfort (mouth fill equalizing) and 16m on a full exhale with no discomfort. Each dive session involved 6 or 7 repetitions, in which I was weighted to sink slowlyfrom the surface. In the beginning, the first dive of the session was made between 12-15m with some discomfort. At the end of the training cycle, the first dive of the day was 20-25m with no discomfort and then gradually repeating or progressing downward slightly. My training partner (efattah) did manage a full exhale to 30m, but felt that it was too much stress, even after the training cycle.

If I could do it over again, I would do it with no suit, at least every second day, in warmer water (16-20C), and in daylight :duh , although the sparkling biolumescene at depth was cool. And I would continue the training for as long a period as possible. I would also probably not progress by too much within a single session and concentrate more on a slow effortless descent and noting the changes in my body and periphery.

Quite honestly, I think while there may be other contributing factors to my own experience with lung barotrauma, I think the overwhelming reason for it is going too deep too fast. Many local freedivers here in Vancouver have experienced lung barotrauma, though fortunately none were as severe as I experienced. And almost all were in some competitive program.

I think Sebastien's contributions to freediving will be more meaningful in the long term as a philosophy that truly does bring us closer to whales, rather than humans pretending to be aquatic creatures without carefully learning how to actually act like one.

Hi Glen,

Thanks! The DYNF record was an accident, really. In November I had done two-three weeks of 25m repeats to refine my technique, sort of as something to do for fun. I haven't trained in the three months. Didn't get in the water at all in the weeks leading up to the competition. On Sunday, I finally relaxed enough to enjoy myself and came up feeling really good.

Hope to see you at May regionals!


It seems we are talking cross purposes here. Sorry.

I personally find the psychological factor makes a huge difference to the success of an FRC dive. If I am relaxed the dive goes much better than if tense or anxious. If anxious, I get contractions much earlier, feel sick in the stomach, and get much quicker lactic acid build up than if relaxed.

On a slightly different vein, I'd be interested in others experience on the turn around point for FRC. How do you assess when you need to start making the ascent. Is it contractions? A feeling of breathlessness? In a pool, I'd estimate that I could go anywhere between 20 and 60 seconds before feeling getting contractions. Is this a reliable guide to the turnaround point?
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Hi Decca,

As far as turn around point, I would say that it has to be carefully relearned. It is totally different than on inhale dives. That's not quite accurate to say either, because on inhale dives, I think it is very difficult to judge when to turn around if you are focused on achieving a certain depth, and not carefully monitoring your body. The high pressures of 02 make this even harder that most would admit. Trial and error seems to be the most common approach, and so when I started FRC diving, I started from the top (0m) and slowly worked my way down again.

One thing I noticed right away, is that narcosis was significantly reduced on FRC vs. the same depth on an inhale and even more vs. inhale + packing.


One of the reasons the dive goes/feels much better is because the urge to breathe is postponed. If CO2 cannnot be buffered by storing it in muscle tissue, as with a stress-induced peripheral vasoconstriction, for example, then the PaO2 (in the blood) will rise very quickly; similar effects can also result by descending vey quickly through the water column without first hyperventilating and/or warming-up the muscles. This can result in a very strong urge to breathe referred to as the "uglies".

For serial diving, where it is the aim to have a rapid turn-over of dives, thus maximizing underwater time and reducing fatigue, it is important to maintain patency of blood flow to the working muscles, e.g., locomotors. Enhanced blood flow to such muscles will help buffer the CO2 but will, however, also lead to a much more rapid usage of O2 stores. So, although being relaxed is conducive to efficient serial diving by preventing excessive build-up of anaerobic waste products, the urge to breathe and prolonging recovery duration, it will also limit dive duration. This is why, when serial diving it is crucially important not to seek to set any records and to ascend when the urge to breathe first arises. The problem is that many divers , even world record holders still mistakenly believe that their best performances occur when they're relaxed and warmed-up.

On the other hand, although cold-water and stress (e.g., no warm-up) will result in an earlier urge to breathe than when relaxed and warm, PaO2 is maintained higher for longer. Unfortunately, however, this means that muscles must rely on a limited blood flow (=HbO2) and intramuscular O2 stores(=MbO2) for efficient functioning. Because O2 stores are severly limited to working muscles under these conditions, they will have to predominantly rely on anaerobic (without O2) energy stores -> muscle fatigue, long surface recoveries. Although this is not a cost-effective way to undertake serial dives, it is by far the best and most intelligent way to set personal bests. Fortunately, the fatigue is only localized and does not result in a quickly developed and excessive urge to breathe, since the build-up of CO2 in this instance is largely due to vital organ metabolism as opposed to muscle metabolism. Remembering of course that in this instance the urge to breathe is centrally (where blood flow exists) instead of peripherally (where the muscles reside) located.

From experience, doing literally thousands of controlled trials over the years, I suggest that divers go dynamic (=ascend) when the urge to breathe results. Typically, that ends up being a 1:1 (descend:ascend) ratio. A 60" free-fall, for example, may only mean reaching a depth of 35-40m. Of course, with descends taking longer to effect than ascents, as they are undertaken passively, it means that one only needs to ascend, perhaps for as little as 40". That being the case, the 1:1 ratio is a conservative one, especially if depth is achived before the urge to breathe arises.

Hi Sebastien, thanks for all the fascinating info!

When you say "I suggest that divers go dynamic (=ascend) when the urge to breathe results", do you mean descending until contractions start? Don't you think getting contractions at or near max depth will increase the risk of lung squeeze?

Also, so far I have thought that it is a good idea to "warm up the lungs" too, i.e. prepare them for the deep dive by doing empty-lung dives etc. It seems you think this is also unneccesary?

I can only speak of my own experiences:
I use to believe that I had to warm-up the lungs before a deep exhale. I now understand that this, at least for me, is now unnecessary. It seems that if there is enough stress (perceived stress) redistribution of venous blood volume into the lungs can happen quite quickly. For example, I recall doing doing exhales to RV, to 20m in 6-7" with +30kg with absolutely no pressure sensation or warm-up. I also recall doing 60m dives at FRC with a 15kg ballast as my first warm-up prior to deep 'no-limits' dives. That was, however, not always the case. Clearly, something has changed morphologically.

I understand your concern about contractions causing potentially excessive negative intrathoracic pressure at already large -ve pressures. I belived this has been described with inhales by some Turkish researchers. Although some divers assume because they can cover a certain distance in the pool this might somehow equate to their performance in the sea. One has to consider pressure adaptation. I recall one diver last year or the year before aiming to establish a record vertically, only to have a lung a squeeze at 60 or so metres after packing. So, one might be able to hold breath in the pool for say, 60" before going dynamic, but does that mean that diver should descend for 60" in the ocean without acclimatization/adaptation....probably not and I wouldn't recommend it. Indeed, most divers who first try exhales feel excessive pressure at between 8-15m. I guess one has to do the time and build-up the dives. There's no easy fix nor short-cut. That's life!

So, when I suggest descending until contractions begin I am referring to someone whose laid the foundation not someone who can swim far but can't handle the physical pressure

To answer your question though: "Don't you think getting contractions at or near max. depth will increase the risk of lung squeeze?". For the ill-prepared ... probably, yes. For those diving to a depth at which they are acclimatized/adapted ... probably not. One has to be vigilant, however, about factors (previously discussed in this thread, I believe) that may predispose to to lung squeezes, e.g., hypervolemia.

Hope this may clarify thing a little more.


Thanks for your response. Reading it helps to reinforce some of the things you taught in the course.


Given the differences that can occur in the perceived need for air, and actual need, it seems to me experience is an important factor in determining what is safe.
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Went spearfishing today for five hours. Dive depth varied, with maximum around 16 m. All diving was done using FRC. I measured a dive to 14 m which I did comfortably with little sense of lung squeeze.

Getting the weight just right was important. Initially I was a bit bouyant and had to kick down for the first five metres rather than sink.

FRC diving was not so good if you wanted to get down to a fish quickly in shallower water.

I typically finned for a minute, had a minute or two rest, then dove for around a minute. The water was very calm, so finning was easy. Letting water in through the hood was important on these warmer days. Maybe should leave the hood off altogether.

(Shot some nice fish as well.)

Thanks for your reply, it definitely clarified things for me. I agree that pressure adaptation is too often overlooked, especially in places like Finland which are not ideal for all-year round depth training. Do you have any experience with using pool dives of 3-5m depth at less than RV as a training tool? Right now that is pretty much the only thing I can do here for pressure adaptation. Definitely not the same as actual deep dives, but I'm sure they are better than nothing.

This is the first time I have heard that drinking too much water and hypervolemia could predispose one to lung squeeze. In fact I've alvays thought the opposite and have drank a LOT before diving, and until now have thought it could only be beneficial.


Not doing a warm up for a max dive does indeed make sense to me as the blood is not redistributed to the peripherals and vice versa for repeat serial diving/spearfishing. I am just curious how does one overcome the Uglies as you refered to them, is it a simple case of slowing the descent?

Also I have been doing FRC static/dynamic training in a 25m pool typically waiting around 40 seconds then swimming the 25 metres waiting 1 minute then repeating, what would be the best way to increase on this method as we are beginning to find it easier and easier each time, Does the wait need to be equal to the total dive. After doing that for a session we wait for about 5 - 10 minutes then do a max static and max swim, should we do this first up when we get in the pool? The pool typically is around 29 - 31 degrees probably a bit too warm for peak perfomance.

Regarding the adaptation to depth, how do the lungs actually change so that you feel nothing? I understand all of the presure volume differences mentioned but I am assuming something must happen to the lung tissue so that you no longer feel strain, e.g does it strengthen or become more elastic? My appologies if this was mentioned in the thread already I may have missed it.

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