#42 Transcript | Erika Schagatay - The Science of Freediving

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Donny: Ok, Erika, welcome to The Freedive Cafe podcast. It’s my great pleasure to finally get a real scientist on the show, someone who’s done extensive research into our passion of freediving and perhaps we can finally shed some light on some of these topics, because usually we clumsily try to articulate our thoughts about these things in an uncertain or speculative way, especially those of us who are instructors. So I hope that once we’ve covered some of these topics the listeners will be able to discuss and educate others with a bit more confidence.


Also I’d like to say that many of the questions that I will ask you today have been gathered from the members of the excellent Freediving Science Facebook page, so many thanks to those guys, especially Ioannis Aliazis. Ioannis collected these questions into one place and emailed them to me so thanks for that.

OK, before we jump through the rabbit hole, let’s just find out a little bit about you Erika. Where are you from? Where are you now? And how did you end up there?

Erika: I was born in Holland, I grew up in North Sweden and I lived for a long time in South Sweden and now I live in the middle of Sweden where I work at the Mid Sweden University.

Donny: Ah Interesting, so you’re originally from the Netherlands? Do you still have some contact there? Do you have family or visit there?

Erika: Yeah, my grandparents are unfortunately gone now but I have a cousin there that I visit sometimes.

Donny: And are you a freediver yourself?

Erika: Yeah, I wouldn’t say I’m a good freediver but I have done courses, I’m a three star AIDA freediver, I’m a Swedish Sports Federation instructor also. So I enjoy very much freediving and I’ve done so since I was 12 or 13.

Donny: And what inspired you to choose freediving as your main research interest?

Erika: Actually it was when I travelled in Asia and I saw that there were people that were really, really, good freedivers and because I was also a freediver I wanted to know what they knew that made them such good freedivers and if I could learn from them.

Donny: And which people are you referring to exactly?


Erika: The Bajau people in Indonesia, for instance, but also other populations.

Donny: You actually visited them in Indonesia?

Erika: Yes, in the late 80s when I was a biology student I wanted to know who were the best divers in the world. So I went to Indonesia because I read in old travel stories that there were people who could breath-hold for a very long time.

Donny: And did you learn something from them?

Erika: Oh yes. I’ve been back many times since then. I came back a week ago from the last visit and I’m learning new things every time.

Donny: OK, let’s start with some of the basics. Specifically, I have a couple of questions about things were taught about on our first freediving courses. So I’ve got this buddy and also a Patreon supporter, he’s in Guam, Farron Taijeron, and he would like to know, does ‘hook’ breathing, or recovery breathing as some call it, really prevent blackout, and if so, in what way?

Erika: This is a nice question to address to me right now because four days ago we submitted the first scientific study of hook breathing. And we can see that in people with a proneness to squeeze, people that recover a little bit more slowly that the average, it really, really, helps.

Donny: And in what way does it help?

Erika: It helps to speed up recovery so that you don’t have so much desaturation before you are recovered. And we think the mechanism is because it counteracts pulmonary edema. It’s a very new study and it’s going to be probably published in one or two months, we hope.

Donny: But even, let’s say for not a very deep dive, let’s say it’s a 30 metre dive for somebody who’s comfortable with 40 metre dives, do you still recommend doing hook breathing or recovery breathing under all circumstances?

Erika: I don’t see that it hurts. It doesn’t have, as far as we can see, any negative effects, but as we could see, we did studies at only 30 metres, because for ethical reasons we cannot do maximal dives, but for all these people there were a few that had a proneness to slow recovery and for those people, hook breathing really helped. So probably when you increase the pressure in your lungs you kind of exit all the fluid that is collected in the lungs when you have a mild squeeze and then your oxygen uptake starts very much faster.

Donny: Super interesting, so it actually has a protective quality with regards to lung barotrauma then, as well?

Erika: Yes.

Donny: Which is something that we’re not taught in our courses but is going to be something that would be very interesting to find out more about.
Is hyperventilation really the number one cause of blackouts?

Erika: Well, it’s also a good question because it depends, of course. It depends who you refer to as the diver and it depends on what you do as a diver.

Donny: Under what circumstances could hyperventilation lead to a blackout?

Erika: In every circumstance it will delay the feeling of an urge to breath caused by the build up of CO2 in your blood. So, in many situations, the warning system is put out of function. But if you are a trained freediver, you might have an upgraded sensitivity to hypoxia, so then it will take over the signalling that you have to breathe. So mainly, it will be a bigger problem in the untrained diver, in the physically untrained diver.

Donny: Now AIDA, to take just one example, is encouraging no kind of breath manipulation before a dive, just completely normal, relaxed tidal breathing. Even the term ‘breathe up’ is not to be used anymore. To what extent do you think this is justified and are there really dangerous ways to breathe up, and are there specific techniques that can better prepare us for a dive that are safer?

Erika: I’m sure there are breathing techniques that can prepare a very skilled and trained diver, competition diver, for a better attempt. But for beginners and medium performers in competitions I think it’s probably true that you should avoid it. But it’s complicated, it depends on a lot of things.

Donny: What would it depend on? Which different factors are playing a role here?

Erika: If you breathe up or if you hyperventilate slightly, of course you have a little bit more oxygen in your body which is a good thing. And if your chemoreceptors are sensitive to hypoxia you might end the dive in time anyway if you’re a trained diver.

And this is not the case in an untrained diver, where CO2 builds up to respond to the urge to breathe. And also you have the situation that the hypercapnia that builds up if you don’t hyperventilate has an effect on your vessel status, on the circulation, especially in your brain. So it can, in some situations also be good to be hypercapnic. Basically, hypercapnia will make your vessels in your brain, which is the most important organ to be supported with a good blood supply, the vessels in your brain open. So, if you hyperventilate a lot, they might have a vasoconstriction, that is the shrinkage of the vessels.

Donny: Right, so oxygenated blood would be prevented from getting to the brain maintaining consciousness.

Erika: Yes. And it was very interesting when it started to be people that produced very good in competitions without any hyperventilation, or without any warm up.

Donny: Right, and that’s pretty much becoming more standard these days. That’s the generally accepted method these days, right?

Erika: Yes, because when I started studying freediving in competition everyone was hyperventilating like crazy and then suddenly some people were producing really, really good diving times without this. And probably this is the more natural way of diving for humans. I see this also in professional divers in Japan and in Indonesia. There they never hyperventilate. It’s probably because our bodies are quite well designed for diving if you keep it in the natural conditions.

Donny: Right, there’s no need for us to go to extra special lengths to manipulate our breathing because we’re ready, evolutionarily, to jump into the water and hold our breaths?

Erika: Yeah, at least when it comes to freediving for food or for maybe spearfishing repeatedly. Of course, extreme dives of competitions are different. Well, there are pros and cons.

Donny: And that leads us nicely into the next topic which is mammalian dive response, or reflex. Which term is correct, by the way? I’ve heard that ‘reflex’ is technically incorrect, or does it not really matter?

Erika: Well, ‘response’ is what biologists use because it is a collection of reflexes, it´s several reflexes that work together to conserve oxygen. So I call it ‘the diving response’.

Donny: Right, so ‘the diving response’. Now a lot of listeners will be quite new to freediving and as yet may be unfamiliar with all the facets of the diving response. Could you first briefly outline what the diving response actually is?

Erika: The diving response is something that happens when you hold your breath and it’s enhanced when you put your head under water. It’s a system of reflexes acting together to save oxygen for the main organs, the brain and the heart that are sensitive to lack of oxygen. So you rid your peripheral parts of blood flow in favour of your central organs, so to speak. And it’s called the diving response because it was discovered in seals, and seals are very good divers, many species can breath-hold for up to an hour, and this can only be done because oxygen stores are conserved very dramatically. And people also have this response. I did my thesis on the diving response, showing how it worked and also the consequences, and it’s quite clear that humans also have an oxygen conserving effect when they express the diving response.

Donny: So we can say that the diving response is a number of physiological changes that happen in response to breath holding, water immersion, hydrostatic pressure, that enable us to dive deeper for longer.

Erika: Well, it enables us to dive for a longer time because it constricts the vessels to the organs that can sustain hypoxia, it’s kind of a system where you postpone your oxygen consumption until after the dive. If you want to do a really long dive you have to pay after, because you cannot make a short recovery and dive again. For instance, Bajau people, they try to minimize the duration to the limit of their aerobic dive time so that they can recover really quickly and go down again.

Donny: Yeah, I was reading one of your studies that you sent me and you can read that the Bajau have very short dive times but then very short recovery times between the dives so they can end up diving for half the time or more than half the time that they’re actually in the water, which can be several hours in the day.

Erika: Yes

Donny: Based on your research, what can the freediver do to most effectively activate this diving response? Are there any techniques or tricks that we can use to deepen it’s effects? Can it be trained? Can we get better and better dive response over time?

Erika: Yes, we can easily train the diving response. Even an untrained person can train the diving response in one to two weeks to really prolong dive duration. So it’s really plastic. Much more than other responses that we have that facilitate diving. And all you need to do is to hold your breath every day. We tested a training program where you do maximal breath-holds five times spaced by two minutes of rest every day. Only that. And you can postpone the breaking point by up to 30-40% by doing so.

But it depends on your level, of course. If you’re a beginner you have more to gain. If you’re already a trained diver, of course you cannot prolong the dive durations by 40% by a week of training. It’s of course dependent on where you start.

Donny: But that will be very interesting for beginner freedivers to hear, and something quite concrete. One to two weeks. Five maximal breath-holds a day with two minutes rest. Doesn’t consume too much time and you think that we’ll see results from that?

Erika: And not in water because if you are in you are on your couch you don’t need a safety.

Donny: Right, yes, of course. All right, one more thing on that line is then….would you recommend that over CO2 tables, traditional CO2 tables that we’re used to, that we get taught on our courses?

Erika: Well, if you need to produce very long breath-holds you should also do CO2 tables I think. So competition divers would need to do CO2 tables. It depends on why you dive, of course.

Donny: So perhaps in the beginning someone should focus on their maximal breath-holds and later on work on their CO2 tolerance, is that what you would say?

Erika: Yes, I would say that. Talking about beginners, I would say in that order.

Donny: I understand that you’ve done a lot of research into the spleen and the spleen effect. This is something that I know absolutely nothing about, it’s something that we sort of mention in passing about freediving. I’d really like to understand more. I know you’ve researched it, what can you tell us about that?

Erika: Well, the spleen is like a big sack of red blood cells, and you need them to store oxygen, and it was discovered originally in seals that they have a huge spleen, and when they dive they eject these red cells into circulation so that they can oxygenate the body better. And we discovered in 2001 that humans have this ability, too. It’s written in the text books that your spleen is a small organ that has absolutely no function. We found that it wasn’t like that…

Donny: That doesn’t make much sense…

Erika: No, it has some immunological functions but it wasn’t considered very important, and you can survive without your spleen. But the question is, can you be a good diver without your spleen? And we tested the spleen function and the spleen size in very many world-class divers and we found that the biggest spleens were found in the best divers, so we connected it to diving ability. And later we did several studies on the function of the spleen and it’s quite clear that it helps you to dive.

Donny: So what happens in the spleen that helps the diver to dive deeper? It’s a function of the dive response, right?

Erika: It’s actually not connected to the diving response. I know it’s taught that it’s part of the diving response but it’s actually quite separate because it is initiated by completely different mechanisms and it needs a little bit more time to kick in. And it doesn’t reverse between dives, so there are three differences. The spleen contains red cells and when you have a spell of hypoxia you eject them into your circulation. So they are not stored in the spleen anymore. Your spleen shrinks and pushes out the red cells and you can use them to store oxygen in your blood, and when you store more oxygen in your blood you can breath hold for a longer time.

Donny: Right, so, the spleen releases these oxygen rich blood cells in response to hypoxia. Is there a certain, average oxygen saturation where that starts to occur? Does it happen gradually and increase over time or does it happen as one event?

Erika: Usually it happens across the first three dives if you do a dive series, so when people were using warm-up dives it was quite clear that that’s when you empty your spleen and that keeps for up to ten minutes after their warm-up dive, for your maximum dive. But one thing that I’m curious about is that if people that are really experienced divers can make this happen by just mental preparation. I don’t know the answer but this is a study I want to do. Because it seems that people that use no warm-up, they can produce very good diving anyway. So we are a little bit confused about this. I think that in people that are really trained divers, it could possibly be that they can make the spleen shrink just by thinking that they should dive.

Donny: Right, that’s absolutely fascinating. So on these big no-warm-up dives they are maybe, perhaps subconsciously, they’re already emptying the spleen in the most efficient way.

Erika: Yes. Ten percent of your red blood cell storage can be in your spleen when you are resting, and you really want to recruit that when you’re diving.

Donny: Absolutely. I remember from reading the study you sent me about that, there was one tiny little detail which I remember which was that there is some evidence that the spleen can actually regrow, after it’s been removed?

Erika: Yes. The spleen can increase in size, actually by hypoxic training. This is something new, we haven’t published it yet, so don’t tell anyone….

Donny: OK…..too late….[laughing]

Erika: [laughing] I don’t think the physiologists listen to this. But anyway, the spleen is very plastic and if you remove part of your spleen it will grow back. And there are also people with multiple spleens, and this is really very interesting, I have seen that in several good divers. So if you’re curious about your spleen situation go to the hospital and ask them to check if you have several spleens.

Donny: If you have more than one spleen, yeah.

Erika: We have seen people with three good spleens.

Donny: That’s incredible, wow. So there’s a strong training response. The spleen is actually enlarging in size in response to hypoxic training, which is ….

Erika: Yes. Long term hypoxic training. It’s not as quick as the diving response. The diving response you can increase by one to two weeks of training, and the spleen training needs for sure a couple of months, up to a year.

Donny: But that again is another what seems like a really clear indicator that our bodies are very well adapted to hypoxic conditions, to diving, by nature.

Erika: Yeah. The Bajau people in Indonesia, they have really large spleens.

Donny: So they have a certain percentage, on average, larger than the regular non-diver?

Erika: Yes.

Donny: Well, that’s right, there was a study that was around the media actually, not too long ago, that their spleens are quite a lot bigger than the regular persons.

Erika: Yes. They say that they found that it was completely genetically achieved, but I’m not quite confident that this is the whole answer, because also the best divers among them have bigger spleens than the other ones. I’m not as confident as they are that this is completely genetic.

Donny: OK, let’s talk a little about packing. What are the advantages and disadvantages of packing?

Erika: We did the first study on packing that was published in 1998 and by then it was quite unknown what were the mechanisms, and so on. And we showed that you can have a packing black out, which is already known about among freedivers. And the consequences were, of course, that you could get more oxygen into your body. And there are different effects if you want to produce a very long breath-hold or a very deep breath-hold, because if you’re producing a long breath-hold you have more oxygen to use and you also have a bigger volume where you can put your CO2. If you do deep diving, your maximal volume at the surface will also determine when your residual volume is surpassed when you go down, and you can have that happen at a deeper depth if you pack before you go.

Donny: So, if you have a diver with a certain lung capacity, whos let’s say their lung capacity is 6 litres and their typical residual volume is 25%, so they reach residual volume at one and a half litres, they can pack several litres on top of that and still reach a lung volume that is equivalent to the 1.5 litres, but that will occur at a deeper depth.

Erika: Yes. The you have the reverse packing that you can train your residual volume to shrink, and that’s a different story, you know.

Donny: So, this question comes up a few times, and it’s something that I’ve thought about myself. So the majority of elite freedivers are using packing for their big dives. Sometimes we see divers packing for up to half a minute or even more. I know guys who pack for more than half a minute, and what I wonder sometimes is, if you’re packing for that long, of course CO2 is already building up in the system because there’s no exhalation. Could it be that the CO2 build-up outweighs the benefit of the extra O2 that they’re taking down with them, and it might be a bit of a waste of time?

Erika: It’s funny that you say that because this is what I think every time when I see people do long time packings. Of course it does, because you need oxygen, you’re kind of craving the same oxygen that you are storing.

Donny: If you do a forty second pack then you’re essentially doing a forty second breath-hold before you do your dive, right?

Erika: I haven’t studied this specifically but pure logic says that if you’re packing for a very long time you’re also losing time. But also the excessive packing may cause over-stretching of the lungs and lung rupture. This is quite clear, also from experience, from case studies that people who pack their lungs to rupture, and this is of course very damaging. So if you pack, you shouldn’t pack excessively, you should pack to the extent where you have still some elasticity in your lungs.

Donny: Let me just ask you quickly, why do packing blackouts occur?

Erika: Because when you pack your lungs you increase your internal pressure in your thorax and your blood cannot enter back to the heart in your veins. And the heart is kind of, not filling up and you cannot sustain your blood pressure, especially the blood pressure going to your head, where you need the oxygen. So you have a packing blackout and drop down because the blood pressure is too low in your head.

Donny: Let’s talk about squeezes and lung injuries now, but since we were just talking about packing, I’ll lead into that with this question that came up when I was talking with another guest a couple of weeks ago, an as yet unreleased episode, and he was talking about how lung over-expansion injuries can occur if one has packed and then squeezes. I guess the fluid that enters the air space of the lungs has now reduced the gas volume there so with the expansion of the lungs on ascent that could cause some over-expansion injury… is that really the case?

Erika: I don´t know, I would have to speculate. We haven’t looked at it specifically but also again logic would say that if you have fluid in your lungs that has entered during the dive, packing would lead to even more over-expansion.

Donny: So logically, it would make sense that it could lead to an over-expansion injury.

Erika: It could. And also from interviewing divers, and we have done several surveys with packing and so on, it seems that it would increase the risk of this kind of injury.

Donny: What is a squeeze, basically, and what research have you done in this area?

Erika: We have published a few studies on squeeze and basically they can be divided into two types; in the lungs or in the airways. The first one is… parenchymal squeeze – I don’t know how to pronounce it – the first one is an overall swelling of the lung tissue which hinders the gas exchange between the blood and the air and the other one is rupture of a vessel that puts fluid inside the lung.

Donny: Right, so there’s even a type of squeeze that does not involve fluid entering into the gas space?

Erika: Yeah, this is also discussed because it’s not really exactly known, it differs a bit about how you explain it. It could either be interstitial fluid or it could be leakage into the alveoli, also.

Donny: Right.

Erika: It’s not hundred percent known and probably both of them occur, but one is a widespread problem that is called pulmonary edema. Edema means swelling. The other one is rupture of a vessel that brings fluid into your lung spaces.

Donny: Are squeezes an inevitable aspect of deep freediving for all divers?

Erika: Interesting question, hopefully not, but I think everyone will eventually reach their limit, and the sea is very deep. I’ve also studied high altitude physiology and luckily the highest mountain is only 8,848 metres high and most people can’t,

but many people can do this after very serious training, but of course if people try to get deeper and deeper everyone will eventually squeeze I would believe, this is a guess, because I don’t know. But also, seals and whales have this problem.

Donny: So this also occurs in diving mammals?

Erika: Yes.

Donny: That’s very interesting.

Erika: So it’s a matter of your limits. And people are not as deep divers as seals and whales. We are for sure among the shallow divers in the mammalian world. I did a paper together with a specialist in mammalian physiology, diving mammal physiology. His name is Andreas Fahlman and he studied the most deep diving species of seals and whales, and people are often compared with the most extreme divers and then we end up in the terrestrial range. But when we compare ourselves to semiaquatic mammals like beavers and otters, and several species actually of seals, we are quite typical divers.

Donny: Now, we’re talking about, so eventually a squeeze will probably occur when someone reaches a limit that will depend on their personal physiology, but is it possible that smaller squeezes or lung injuries are happening much more often even at the recreational level that we’re not aware of, that could still be compounded over time to lead to a bigger problem.

Erika: This is a very important question, and I must say we don’t know yet. I would say, if people dive in the ranges where we see the professional divers of Asia moving, regularly down to about 15 metres, as a routine or maximum 20 metresthey can go much deeper but they don’t dive every day deeper than that – then I don’t think it’s a problem. But as soon as you want to extend that and to approach your residual volume, yeah it could be the case.

Donny: What would be the best way, if there is any, of evaluating the seriousness of a squeeze. And how long do you think one should really rest after dive, I mean after a squeeze?

Erika: We think oxygen uptake measurements could reveal if you have a squeeze. Not a very mild squeeze, because you have an over capacity of your lungs when you’re breathing on the land, on land. And from normal healing processes I would speculate that you need to rest a week or more. But this is speculation I haven’t studied it specifically and this is really based on other things. I think it is important to rest because we know for sure that repeated serious squeezes can be lethal, and you know what I mean.

Donny: Yeah, yep. I’ve heard that some research has show that genetics may play a role in how likely someone is to squeeze. Can you tell us something about that?

Erika: Well, it’s not my field with genetics but it’s quiet clear that some people squeeze easily and some people do not squeeze easily. And also with lung diseases, completely without any involvement with diving, people’s lungs are quite different. And there is a study showing that there is a genetic component also to the dive induced squeeze, and this is quite interesting. It’s like, among us there are very good divers and not so deep divers, by design. And if we could identify that, of course it would be not so nice for the people that really want to do competition diving if they find out they are a non-diver, but maybe it would help to prevent squeezes.

Donny: So what you’re saying is, as yet we still don’t understand what the differences are on a physical level, we can’t identify what quality of the lungs is leading to this difference?

Erika: Well it has to be researched and applied, and I together with this mammalian diving specialist Andreas Fahlman, I did a big application to the Swedish research council for funding for this but we just missed it by a few points, you know, it’s really hard to get good scientific money for these things. I wanted to do exactly that.

Donny: Well, I hope you can someday. Alright, let’s talk a little bit about nutrition, and diet for apnea. So I’ve been told that the reason freedivers feel utterly weak and exhausted after deep freedives or many freedives in one day is that a greater than normal amount of free radical damage is taking place during apnea. Is that true, and if so why does that happen anaerobically so much, what is happening on a physiological level?

Erika: Well, it’s again something that I would rather not speculate in, it’s suggested, of course you can do a lot with your diet to try to scavenge free-radicals but it’s not been investigated, as far as I know, in the context of freediving, at all. I haven´t done anything on it. It’s a good question though.

Donny: Well hopefully we’ll get some answers one day, in the meantime…

Erika: You know, I’m in this business because there are so many unanswered questions. There are a few answers, I’ve done this for thirty years but I just have a few answers, haha.

Donny: Predicting that the next question will get a similar answer then, because it’s regarding dietary nitrate, which seems to have an oxygen conserving effect for breath-holders. Do you know much about that, is that the case?

Erika: Yeah, we’ve done several studies on that.

Donny: Right.

Erika: This is something we discovered. So we published the first paper on that, where we showed that if you eat beetroot juice you can prolong your breath-holding time. And I know a lot of people have also studied beetroot juice shots before competition and it’s quite amazing and there are several mechanisms, known ones, and maybe some unknown ones, that are responsible.

Donny: Right, so this is just dietary nitrate that you could get in high quantities in something like beetroot juice.

Erika: Yes.

Donny: And you can confirm that that’s beneficial for the freediver?

Erika: Yeah, and it’s beneficial even if you just take a shot two and a half hours before your main performance.

Donny: As little as a shot?

Erika: Yes, 70ml of this kind of, enriched….it’s a completely biological product but it’s concentrated. It’s a concentrated beetroot juice. We have shown in a study, that we have done, that beetroot juice helps you hold your breath a longer time. We haven’t done it on deep diving, but of course it helps if you have a margin to your limit in breath-holding duration then also.

Donny: So is there an amount of dietary nitrate that would be recommended or just generally beetroot juice would be the best option?

Erika: Yeah, and you don’t need to drink that terribly much. We used 70ml shots, that is just half a milk glass of beetroot juice, every time you´re going to dive. And it also lowers your blood pressure, so don’t drink too much.

Donny: Are there other plants which, or foods that contain also high levels of nitrate that you could also consider instead of beetroot?

Erika: Yes, there’s another one, and we´ve discussed doing studies on it, and its also spinach. But if you want to drink spinach juice every day, you would be a very good freediver but I don’t know people that would like to do that.

Donny: Hey, I blend spinach nearly every day, so I’m happy with that.

Erika: Alright! Spinach is good, also other green leafy vegetables like rucola salad and all of these leafy greens that you can eat.

Donny: Well, I eat fresh green leafy vegetables every day and my freediving’s still not that great so maybe I need to up the dose.

Erika: Probably, you have reached your limit when it comes to diet. Then you should try fasting.

Donny: Right, so what can you tell us about fasting, because I’ve heard…I read the studies that you sent me so we have, when we’re fasting as the body is starting to utilize fat more for energy than stored glycogen, then there’s an advantage in that there’s less CO2 being produced but at the same time, more oxygen is also being used, is that the case?

Erika: No, less oxygen is used.

Donny: Less oxygen is used? But more CO2 is produced?

Erika: Yep. Of course your body goes into a kind of saving mode, you would reduce your metabolism, your overall metabolism. Do you follow?

Because you can’t feed your body a lot, you into like, saver mode, and then it’s much easier to conserve oxygen and also your diving response can kick in much more powerfully, because you don’t have any food to digest. So your system is empty and you don’t have to put any blood in your intestines.

Donny: So, does it then follow that you would recommend, if someone was diving in the morning, for example, diving fasted, completely fasted?

Erika: I wouldn’t recommend anything. People can decide what they make of this but a lot of divers, they prefer not to eat something before their maximal static apnea, for instance. And some people, also before a dynamic. And even the Japanese Ama before going to the first diving shift in the morning…they say they don’t normally eat because it will make them feel ill or something. But I think it’s a normal condition, to dive in a moderate way, without adding too much energy, because when you eat, your metabolic rate increases. But I’m not sure if it’s good to recommend it for dynamic apnea and deep diving, because if you want to produce a maximal dive you probably need the energy for the muscles. We don’t know yet, I would say.

Donny: I’m just trying to think about my own experience because I usually fast for the morning anyway. I found that eating something in the mornings before I dive was better for me because my legs didn’t get so tired when I was ascending from a deeper dive, especially when I’m teaching and doing lots of diving. So I guess there’s a balance to found there somewhere…

Erika: Yes.

Donny: So I got a question from Nicolas Winkler, who´s a patron of the show, and he’s a freediver but he’s also a trimix post-circuit rebreather diver, so the question that’s nagging him is, would CO2 tolerance from freediving training mask the onset of symptoms from a CO2 hit on a rebreather? Or provide an advantage in dealing with an emergency like that? Now, I know nothing about this subject….

Erika: Well, of course if his chemoreceptors are down-tuned from the freediving training they will be so also when he’s using trimix. And the consequences of this he would know better but, yes, there would probably be…the effect would be still there when he is using trimix. I would say. This is a speculation, but this is what I can think of concerning the problem.

Donny: And he has a second part of that question as well which I’m definitely interested in, and that is, does frequent…because he is frequently breathing high partial pressures of oxygen-rich gas mixtures in technical diving…would that counteract the O2 training that he does for freediving.

Erika: Maybe.

Donny: That is a good question, isn’t it?

Erika: Yeah. It’s kind of an interesting question. It’s not sure that both ends of the spectrum have the same exact mechanisms but, yeah, it’s hard to speculate actually.

Donny: I’ve got a question from Ahmed Youssef. He has a question about the impact of freediving and breath-hold training on neurocognitive functions, he’s a medical student and came across some information that apnea may be detrimental to neurocognitive functions. He also aspires to be an elite level freediver someday but of course his neurocognitive functions are extremely important to him. Do you have any information or thoughts about that?

Erika: Well, I think that there is….if you mean long term effects? General effects?

Donny: Yeah, long-term effects.

Erika: Yup. There is up to date no studies showing long term detrimental effects but it hasn’t been studied enough, I would say. That goes for the maximal apnea performance. For sure if you do repeated moderate breath-hold diving like the professional divers, this seems to be not at all harmful. This is just again my, more my feeling than my scientific proof because I haven’t proven it. But if you want to do really serious maximal dives it has to still be investigated, if it causes a detrimental function. But in the short duration, of course after a diving shift or a maximal dive you are a little bit stupid but you are also effected in the same way if you have run a marathon or if you had done something else to your maximum ability.

Donny: Basically, there’s not enough research….?

Erika: No. There are some studies on cognitive function showing that after a diving shift you are a little bit stupid but you recover it really quickly. And it is yet to come , the long term studies if you can reduce your cognitive function by being hypoxic a lot over the years.

Donny: That would be hard to study as well wouldn’t it because you would have to take somebody and study them, their neurocognitive functions, assess them and then follow them for a long period of time while accounting for all the other variables that could be affecting their brain function over the years.

Erika: Yeah, but I think there is a short cut. You could also study freedivers that had competed in advanced diving for a long time and compare them with beginners

and compare them with other people, because then you would have an idea, because I don’t think people start diving because they’re stupid.

Donny: Right, good point.

Erika: That would be the only other explanation if there was a difference. But some of the smartest people I know are quite advanced freedivers so I don’t think they’re becoming stupider as freedivers. But I think some people, it could be that some people get injuries and we have to study that over the years.

Donny: Let’s talk about the limits of human diving. So the current world records as they stand now, we’ve got 130m constant weight, 102 no fins, 300 dynamic in the pool. Do you think we’re approaching the limits of what is humanly possible? Let me expand that question by putting it like this …. do you think that the current knowledge that top athletes have about training methodology is very good and they’re really on the point edge of what is possible, or is still so much to know and learn that we can reasonably imagine a time when the limits can be pushed way beyond what we have now?

Erika: Yeah, this is kind of the main question I’m working at. I think people have not reached the human limits but maybe people have reached their personal limits. Because this is still a very unusual sport, and given that there is a genetic component, most people with talent haven’t tried, even, yet. So if this becomes a very common sport like football or running or something like that, that’s when we know.

Donny: Right, there’s just not enough people doing it at the moment.

Erika: No, no, but I also think there are mechanisms that we don’t know yet that could be used to develop freediving and another aspect is of course that there are safety aspects that we haven’t fully used yet, that should be put in place before people do this. This is my main concern. That just as these frequent squeezes and some of the uses of packing and so on, I think it’s really not necessary to make freediving better, it’s probably counteracting that.

I would like to add, I think our bodies are very well prepared for moderate-effort, for moderate freediving, moderate depth, moderate duration, moderate swimming distance, but with the competition sport everyone is doing their maximum and a little bit more and this might be harmful. But this is true for other sports as well.

Donny: You said that you recently got major funding to do more research. What is next in line for you? What are your plans for research in the future?

Erika: Well we’ve got a major grant to investigate how to avoid blackout in freediving. That is mainly referring to recreational freediving and also advanced spearfishing, because people still die as you know in freediving accidents because they have hypoxia. So we have to try to discover a system for warning them better if not by the body signals, we’ll have them warned by another artificial system. This is a very designated programme. I received this funding because we have done a lot of work in freediving safety before and we want to look further into how to avoid drownings in freedivers, I would say. We received this funding to study further the safety of freediving to avoid that people die mainly from hypoxia, both in recreational freediving and in competition diving and professional diving, and I am using the funding to employ a post-doctoral student to add that to my group, so that we can study it more.

Donny: Erika, I’m going to ask you one last question. Two last questions, there’s a traditional question that we’ll get to in a second, but, what do you think is the most fascinating thing that you’ve discovered about freediving in your time doing research?

Erika: The most fascinating thing of all our research put together is that the human body is quiet well adapted to freediving.

Donny: Right, so anyone who doubts that we;re really meant to be doing this crazy thing, we actually are quite good at it by nature.

Erika: Yeah, and the competition diving is something very specific. Many populations they dive for a living and it might be one of the forces that made us human. I think probably freediving made us human. I’m writing a book about it but it’s not finished yet.

Donny: Ahh, I look forward to reading that one.

Erika: But you know, there is this book “Born to Run”…

Donny: McDougall…

Erika: I´m writing another one, “Born to Dive”.

Donny: Ok, let’s leave it there, for this time and perhaps we can pick it up later on and expand on some of these amazingly interesting topics that we’ve been talking about. Before I let you go this time I’m not going to let you get away without asking for a book recommendation because I always ask my guests if they have a book or an author that they could recommend. Do you have something that you could recommend, Erika?

Erika: The first one that comes to my mind is The Heart of The World, which is a biography or a travel story, but it is documented stuff, yeah, it’s a guy who is trying to discover the Central Asian big waterfall that connects a river in Tibet with Brahmaputra, I believe. So he’s travelling with a bunch of people, this is a real geographer and Tibetan scientist.

Donny: It’s Ian A. Baker…A Journey to Tibet’s Lost Paradise.

Erika: It’s absolutely one of my favourite books. I’ve read it twice. I recommended it to a bunch of people and a few of them completely finished it, so its about struggling in the mud, among the leeches to find a way to the Shangrila, haha. –

-The real Shangrila!

Donny: That’s brilliant, thank you for that recommendation. That sounds like the perfect thing for those long road trips between dive sites and long rest days between training sessions that our listeners are looking for.

Erika: It’s quite amazing actually. It’s about struggling and believing and trying to understand things. So this is the first one that comes up in my mind without thinking of it … I probably should have thought of it before.

Donny: OK, Erika that’s all I’m going to ask you tonight. It was fascinating, thank you so much. So Erika, I’ll let you enjoy the rest of your afternoon, evening there in Sweden, thank you so much for talking the time to talk to me. It’s been an absolute pleasure. Let’s continue our conversation somewhere down the line, hopefully in the not too distant future. Yeah, thank you.

Erika: Good luck with the hurricane and everything in Taiwan!

Donny: I think we missed the worst of it. It swung south to The Philippines, so we got lucky and they didn’t get so lucky but, so it goes.

Erika: I know a lot of people in The Philippines, I hope they’re OK.

Donny: Yeah, it can be pretty bad so let’s hope that they get through as unscathed as possible. Thank you so much, enjoy the rest of your night.

Erika: Yeah, you too.

Donny: OK, thank you, bye bye.

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