First, let me say that I am not trying to postulate a universal theory of everything. I am merely trying to get us to a model which is valid to the extent that it is useful and suggests productive avenues of improvement in our thought processes, training, and tactics. I will be making some generalizations, as that is the nature of models, but rest assured, I am aware that no model can possibly account for all the variables that present themselves on even one dive. Still, I feel that this model will be useful, because I think it brings a bit of a useful focus to the recent events that have traumatized all of us.
“Ability” as it relates to a particular dive broadly covers the skills that to some extent at least, can be learned. These skills might include such things as diet, preparation, breathe-up, duck-dive, breath-hold, relaxation, stroke, streamline position, equalization, turn, surfacing, surface protocol, and psychological factors. Certainly, this list is not all inclusive, as there are other skills involved, but I think you get the idea here - you go to school, you learn the skills. Also, I do understand that breath-hold seems to be a difficult fit here, but in the end, as you look at the yin and the yang of it, I think it makes somewhat more sense. The reason for that, I think, is that all the other skills relate so integrally to the breath-hold that it is impossible to separate it from the pack.
“Adaptation” and in particular here, adaptation as it relates to the chest, lungs, and diaphragm, is the the body's ability to successfully cope with the demands of apnea and depth through all of the mechanisms of the dive response. For the most part, adaptation cannot be learned, it has to be trained. You get some purely by virtue of being born a mammal, the rest you get through endless stretching, long training, and a gradual approach to depth.
“Lung Squeeze”  is something we are all familiar with, but for our purposes here, allow me to define two types of lung squeeze, each on opposite ends of the spectrum of lung squeezes.
“Depth-induced squeeze” first assumes perfect technique throughout the dive, with no mistakes, except that the freediver passes the limits of his own adaptation to depth. The diver comes up squeezed because the pulmonary capillaries just could not handle the stresses and pressure they experienced. The stresses that cause this squeeze are by definition not self-inflicted.
“Self-induced squeeze” assumes good adaptation to the attempted depth, but the freediver does something or some combination of things during the dive that causes the squeeze. In this, I am thinking in general of certain movements, tactics, techniques, and stresses that might cause a squeeze.
“Depth-induced threshold” would be the depth below which a squeeze would be inevitable. It is mainly a function of three things - depth, form, and discipline. It assumes perfect form within the discipline involved. Because the motions used are different in each discipline, the stresses induced within the lungs are also different.
The depth-induced threshold is therefore NOT the same for every discipline. The reason an understanding of this concept matters is that it REQUIRES that a gradual approach to depth be repeated for each discipline.
Physical changes over time that affect the chest, diaphragm, or pulmonary capillary flexibility would impact the depth-induced limit. An improvement in the flexibility of the chest, diaphragm, or pulmonary capillaries might increase the threshold, while disease or injury to them would probably reduce it.
This threshold is not a number you could ever know with any degree of certainty, but it is still a useful concept when you think of it as “about” a particular depth or as increasing or decreasing for some reason or another.
“Effective lung depth” is the depth your pulmonary capillaries feel relative to that they would feel on a perfect dive starting on a full lung and assuming your current body. Assuming full lung and perfect technique, your effective lung depth would always be identical to your actual depth.
Once deviating from that full lung or perfect technique, things that might increase effective lung depth would be any motion or muscle movement during the dive that increases the total volume of the chest or reduces gas volume in the lungs, anything that increases blood pressure relative to the air pressure in the lungs, anything that stresses the blood vessels directly (similar to twisting an inflated garden hose), or any combination of the above.
More specifically, reverse packing, stretching your arms overhead, pushing your diaphragm down, starting the dive with less than a full lung, and stress of any kind are some of the things that could increase effective lung depth. FRC and empty lung dives would drastically increase effective lung depth. If on a full lung you normally hit residual volume at 30 meters, a 10 meter empty lung dive would send your effective lung depth to as much as 70 meters. Extreme care should always be exercised on FRC and empty lung dives.
On the other hand, packing might have a beneficial effect by reducing effective lung depth before the dive even starts. Remember though that we are discussing packing only as it relates to effective lung depth. Packing has bad effects too, so the risks must be considered carefully before doing it, and even then, it should be done with extreme care.
An understanding of effective lung depth is absolutely vital because it is what matters to the capillaries, not actual depth, so any time your effective lung depth passes your depth-induced threshold, a squeeze is by definition guaranteed. Put another way, any time you get squeezed, it is because your effective lung depth exceeded your depth-induced threshold.
Applying the Definitions
There are two things, other than maybe the sea bottom, that place ultimate limits on a particular dive - ability and adaptation.
“Adaptation-limitation” - I could have all the skills in the world, and be able to hold my breath for 20 minutes, but only be able to dive 50 meters due to the limits of my adaptation. You all know me – I do a beautiful dive all the way down to 50 meters, come up, smile, execute a perfect surface protocol, and then proceed to spit blood. I am sure there are many other freedivers like this. We look good in the water, but seem to get a lot of squeezes. We might call ourselves adaptation-limited freedivers.
All the adaptation-limited freediver has to do is dive to his or her abilities to get squeezed.
“Ability-limitation” - On the other hand, my capillaries might stretch like rubber, but I have crappy form, weak legs, a crappy turn, and on a good day, cannot hold my breath 4 minutes. It is a struggle for me every time to get down to depth and back. You see me in every competition. I whack the judge with my fins on the duck, perform what might be generously described as a “stroke” while I descend as far as I can. I get in trouble with my ears at 30 meters, turn ugly, and struggle back to the surface to demonstrate the only perfect thing in my performance - a perfect blackout. I might refer to myself as an ability-limited freediver.
An ability-limited freediver cannot get to the depth-induced threshold on abilities alone, so in order to get squeezed, this person has to do something during the dive that would cause effective lung depth to pass the depth-induced threshold.
“Limitation reversal” is a rapid shift from ability-limitation to adaptation-limitation. When all of a sudden your skills will take you past the depth-induced threshold, there is much danger, so caution is appropriate. Two things could precipitate a reversal. Either some kind of impairment to adaptation or improvement in ability would do it. Adaptation could be immediately and negatively affected by any chest or lung injury or illness. Squeezes and serious lung diseases such as pneumonia could cause this. Skills could improve in any number of ways. Even something as simple as an equalization workshop or monofin course could be enough to do it.
However this reversal happens, it is a very serious situation, and not something that can be objectively measured. That is why a gradual approach to depth is so important as your skills are improving. It is also why it is so important to properly handle squeezes. The rules of the game can change without you even knowing it. For squeezes, use the squeeze-rest-heal strategy, and even then, a gradual return to depth is appropriate.
It seems to me that, in terms of training, the biggest thing we can do to prevent squeezes is to train in such a way that the only thing that gets squeezed is the difference between effective and actual lung depth. The goal of course, is for effective lung depth to ALWAYS be equal to actual lung depth. This affects everybody. A better handle on this concept will help keep both the adaptation-limited and ability-limited freediver from causing self-induced squeezes. This is important especially when things go wrong in a dive, but also even when everything seems to go right. Why would anyone want to dive to a depth of 50, 75, 100, or 120 meters, turn to ascend at the bottom plate, and through bad technique proceed to send their lungs down another 25 meters?
Case Study – Nicholas Mevoli
Using the concepts of ability-limitation, adaptation-limitation, depth-induced squeeze, self-induced squeeze, depth-induced threshold, and effective lung depth, let's take a look at the special case we all are concerned about now. Hopefully we will find more than just blackouts, ruptured eardrums, early turns, and squeezes. Hopefully, we will discover ways to better train, ways to better strategize our dives, and ways to be safer as a sport.
Admittedly, our information here is very thin. I have almost no data at all on his training. It is a limitation I accept because I have to, and because I think we can still draw some useful conclusions without it. Here is his competition record :
Deja Blue III 2012-05-04
CWT AP86 RP86
CWT AP91 RP91
FIM AP88 RP88 Deep BO at 25 meters, squeeze
2012 World Championships
CWT AP61 RP61
Vertical Blue 2012
CWT AP95 RP92 Pull, ruptured eardrum
Caribbean Cup 2013
CWT AP92 RP92
CWT AP96 RP96
CWT AP100 RP100
CNF AP56 RP56
FIM AP75 RP57 penalty
FIM AP81 RP81
2013 Depth World Championships
CWT AP98 RP55 DQ
CNF AP65 RP65
FIM AP91 RP80 penalty
Deja Blue IV 2013-10-05
CWT AP90 RP90
CNF AP65 RP65
FIM AP75 RP75
Vertical Blue 2013
Nov 10 CNF AP72 RP60 Early
Nov 11 CNF AP72 RP69 DQ Pull
Nov 15 FIM AP95 RP95 DQ Assist, upper respiratory squeeze, ears
Nov 17 CNF AP72 RP72 DQ BO
Looking back at his performances, we see a number of skill-related issues, early turns, blackouts, ruptured eardrums and such, all initially pointing to him being an ability-limited freediver.
Not so fast though, because we also see three recorded squeezes, which imply a possible adaptation-limitation:
DEJA Blue III, May 2012 - 88 meter FIM attempt - deep blackout and squeeze
Vertical Blue 2013 - 95 meter FIM attempt - required assistance, got squeezed (perhaps a minor one), and had ear issues
Vertical Blue 2013 (two days later) - 72 meter CNF attempt - stopped twice on the descent, probably due to equalization problems, and came up with a big squeeze
He may have had other squeezes during his career, but these are the ones I am aware of. Interestingly, of his 5 DQs before his final dive, 2 ended up in squeezes and one other ended up in a ruptured eardrum.
Two questions present themselves, and perhaps tighten our focus a bit:
First, is there something common to all of his squeezes that we might use? Yes, there certainly is. In every case, the squeeze is accompanied by a skill issue - a blackout, ear trouble, assistance, stopping twice on the descent. It is at least possible, and maybe even probable, that his squeezes were all self-induced. This would seem to lend weight to the assertion that he was an ability-limited freediver.
Second, why do we not have any reports of squeezes on any of his white carded performances? To answer that, I ask another question - what would his history look like if he was adaptation-limited? I submit that if he was adaptation-limited at all, we would see squeezes on at least some of his successful performances, yet we do not - so again, probably ability-limited, not adaptation-limited.
One could be forgiven for concluding that on max dives, Nicholas was primarily an ability-limited guy. This is not meant at all as a criticism or insult. It is simply one way of looking at things. Many elite freedivers fall into this category. Being very well adapted, they are limited only to the extent that they can further hone their skills.
So, why should we care about this?
I think that having an idea whether I was constrained by ability or adaptation might be quite useful information. I am sure we can think of ways that knowledge would productively affect our actions and thought processes.
If we and our coaches take an honest look back at our own histories and conclude that we are most limited by our adaptation, a lot could change. We might modify our training to focus more on frequent deep dives to develop adaptation. In terms of competition performances, we would have it pretty easy, only having to adjust our expectations and AP a bit in order to reflect our renewed grasp on reality and reduce our risk significantly. We prevent squeeze to the maximum extent possible by restricting our AP to safe levels.
The good news for us is that we can do all of this before we even get wet. We are safer already, and more confident in a good dive, as our skills will normally get us to the AP relatively easily without squeezing. Trying to do personal bests during a competition is exactly the wrong way to go for us.
If, on the other hand, we conclude that we are ability-limited, then we have bigger problems, I think. Our troubles are not necessarily caused by our APs. To the extent that we have issues, they are largely self-inflicted during the dive itself. Our abilities are by definition stretched to the max on our dives. Many of the important decisions we need to make along with the actions that might cause squeezes all happen at depth, where judgement and coordination might be impaired. The consequences for us here are just as severe as if we were adaptation-limited without the benefit of being able to make the major fix on the surface.
We might show rapid improvement and do many PBs over a short period as our skill levels improve, but a gradual approach to depth is still vital for us - we would not want to stumble into a limitation-reversal. On any particular dive, we focus on reducing the risk of squeeze with better technique and decision-making.
In our preparation and training, hard work on technique is important for us, because our squeezes are almost always self-induced. The good news for us is that what can be self-induced can also be self-corrected. Put simply, we need to unlearn the squeeze-making skill.
Once we get into the water, strategy can also help us to be safer.
Turn for Tomorrow
A few years ago in Athens, Tom Sietas was doing a static world record attempt. He did a 10:12, a new world record, proceeded to get touched by his safety diver, and promptly got disqualified. Rather than letting it get to him and hold him back, he took it in stride, came back a few days later, and repeated the exact same time for a new world record.
Of course, deep dives are different, but the idea is the same. Many of us are goal oriented, so we tend to think of an early turn as a failure, even if we come up perfectly afterward, but perhaps there is a better way to look at it:
For a freediver who is ability-limited, and particularly if prone to things like blackouts and self-induced squeezes, an early turn should not be thought of as a surrender or a failure, but should instead be considered a strategy – a strategy for getting to the surface safely and coming back another day.
This is a strategy that can be coached, “When you get into trouble, rather than continuing on and making worse problems for yourself, turn immediately, go safely to the surface, and come back tomorrow. There is always another day and another dive.”
Why not give this strategy a name - the Turn-for-Tomorrow (TFT) strategy.
On any normal day, the TFT strategy would have probably been more than adequate to get Nick turned and up safely, and had it been a normal day, that is how I would have coached him. He would probably have turned somewhere in the 63 meter range and ascended normally to the surface, and likely been ok, on a normal day.
Problem is, this was not a normal day, so with the benefit of hindsight, and given what we have discussed in this article, what would we do if we could go back in time and re-coach the situation?
I probably would have coached him to not dive at all.
He had suffered a squeeze two days before. While reportedly minor, there is still a lot we do not know about squeezes. In my mind, that squeeze would have possibly compromised his adaptation and reduced his depth-induced threshold. That could take Nicholas, normally an ability-limited guy, and make him adaptation-limited. At the very least, it could significantly change his depth-induced threshold and bring it closer to his AP.
Once we make that determination, we must face two facts. The first is that his adaptation is now a total unknown. It is fundamentally impossible to prevent a depth-induced squeeze, even under perfect conditions, if you don't have at least a clue as to where the depth-induced threshold sits. The second is that all of the strategies we had for preventing his self-induced squeezes are of marginal use now when he is possibly in depth-induced territory. Knowing all of this, how could we ever have come up with a safe AP?
We could not possibly have known what depth was safe from an adaptation perspective. There was no time for a gradual approach to depth. There was no strategy, tactic, or technique he might have used as a normally ability-limited guy that would have kept him safe during that dive. The squeeze-rest-heal strategy was the only way to go.
The Perfect Storm
Two things combined to create a perfect storm on Nick's dive:
A recent squeeze perhaps caused a limitation reversal by compromising his adaptation and reducing his depth-induced threshold. At best, the squeeze would have pushed his depth-induced threshold nearer to his AP. In any case, he was closer to a squeeze before he even got in the water.
Bad technique showed up in the last ten meters before the bottom plate. The combination of struggling to equalize, stopping, going head up, drifting, and then stopping again probably increased his effective lung depth. Other things not visible in the video, like reverse packing, getting less than his normal full breath before the dive, or even an earlier than normal onset of contractions could have further increased effective lung depth. And by bad technique here, I refer to anything other than a perfect freefall to the bottom plate.
One of those effects might have been survivable, but both together? As shown below, once effective lung depth passed the depth-induced threshold, a squeeze was inevitable.
The ONLY way he could possibly have prevented a squeeze once he got in the water would have been an early turn when he first stopped, and even that is no guarantee. In truth, he should never have done a limiting dive on potentially damaged lungs.
A big question in this case, and one that will probably be forever unanswered, is how much the squeeze during his FIM affected his last dive. I have always taken the position that it had some effect, and that effect was negative. I think that, at the very least, the FIM squeeze caused damage that sent his depth-induced threshold shallower. How much shallower, who knows? For sure we need to change the way we deal with squeezes. The good news here is that at least at one subsequent competition, the organizers and athletes took action on their own to deal with the issue of prior squeezes. It was a good start, but at some point, AIDA will need to address it as well and figure out a fair and effective way of dealing with the problem.
The worst case effect of the previous squeeze, a limitation reversal, is shown below. In this case, the only effective solution would have been to stay out of the water.
Guillaume Nery's Accident at the 2015 World Championship
I am almost at a loss for words to even address this. For sure it is a demonstration of some of the concepts of this article. Either Guillaume was ability limited and 129 meters was as far as he could go, or he was adaptation limited, knew it, and selected his AP accordingly. Sending someone past their AP is beyond dangerous for obvious reasons.
In the first case,
You would think we had learned something by now about sending people beyond their abilities. Again, this is not meant as an insult to Guillaume at all - just a way of looking at it. Whenever you send anyone beyond their abilities, or their abilities suddenly improve (say an equalization workshop), you run the risk of them exceeding their adaptation limit. Guillaume's abilities would have gotten him probably to about 129 meters. Sending him past that put him in territory he had not explored yet because he was not yet able. Guillaume went to 139 meters and somewhere along that path exceeded his depth-induced threshold. A blackout was practically guaranteed. A squeeze was inevitable.
In the second case,
Sending Guillaume past what he may have known was his adaptation limit defeated every bit of planning, forethought, and safety that Guillaume had built into his performance and the AP he selected. A squeeze was inevitable.
The judges and organizers made a huge mistake in this case, and we were extremely lucky we did not get Guillaume killed in the process.
So what do we know now that we did not know before we opened this article? We have talked about the concept of self-induced squeezes, and disconnected them a bit from depth. We have introduced the concepts of ability-limitation, adaptation-limitation, depth-induced threshold, and effective lung depth. We have used those concepts to look at Nick's record and final dive, and also to examine our own diving. With the insight we have gained, we have then introduced some specific strategies and mindsets that will help us be safer freedivers.
There is a certain obvious benefit to hindsight here, but I think the model we have here is still useful in that it proposes strategy rather than surrender, cause rather than blame, and a degree of clarity rather than uncertainty.
These concepts and strategies are valid to the extent that they ultimately help us to coach, dive, judge, and compete safer. Hopefully they will help in some small way to prevent another tragedy from occurring.
Note: See reference  for a summary of the autopsy reports on Nicholas Mevoli.
Walter L Johnson
First published on December 26, 2013
Revised January 2, 4, and 6 of 2014; June 6, 14, and 15 of 2014; April 14, 2015, December 5, 2015, February 21, 2016, May 31, 2016
This document may be freely distributed and used in whole or in part in training materials and for training purposes without citation. When any part of this document is used in published printed materials or electronic media, proper citation shall be included as would be considered proper and normal for such works.
 Peter Lindholm and Claes EG Lundgren, The physiology and pathophysiology of human breath-hold diving, Journal of Applied Physiology, vol. 106, no. 1, pp. 284-292, 1 January 2009.
 AIDA Ranking List, AIDA International.
 Dr. Per Vestin, Official Summary of the Autopsy Reports Following the Death of Nicholas Mevoli, AIDA International, December 2015
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