Tag Archives: hypoxia

Study finds elite freediving may cause memory loss

A study that has recently gone to press (Bilaut et al., in press) suggests that elite freediving may cause mild, but persistent short term memory loss. The study subjected elite freedivers (>6 min static breath hold), novice freedivers (>4 min static breath hold) and a control group to specific psychological tests.

The time to complete the tests was positively correlated with freediving static performance. In simple words this suggests that a better breath-hold means a slower brain.

Memory loss: reserved for the elite?

More specifically it was the elite breath hold divers that showed poorer performance on the tests. They took longer to complete the tests and made more mistakes. Novice freedivers and the control group showed normal performance.

The divers had not done any apnea prior to the test, nor had they had blackouts or LMCs in the week prior to the tests. Interestingly, there was no correlation between the total amount of blackouts and LMCs and the freedivers’ performance on the tests.

The authors speculate that the improved static time is not the cause of the poorer performance. Instead the static time is an indicator of the amount of hypoxia that the athletes face during training. More hypoxic intervals over the course of a diving career may lead to “mild, but persistent” memory loss.

The results are in contrast to a previous study by Ridgway and McFarland (2006). This study did not indicate long-term cognitive impairments in freedivers.

  • Billaut, F., Gueit, P., Faure, S., Costalat, G., Lemaître, F., Do elite breath-hold divers suffer from mild short-term memory impairments? (In press) Applied physiology, nutrition and metabolism
  • Ridgway and McFarland (2006). Apnea diving: long-term neurocognitive sequelae of repeated hypoxemia. The clinical neuropsychologist, 20:160-176

Muscle metabolism during breath hold dives

Muscle metabolism in a nutshell (recap of part 1)

This is part two of the ‘muscle metabolism’ article series. In part one (link) we have analysed human muscle metabolism, how muscles are supplied with oxygen, and how they store fuel. We learned that muscles store fuel in the form of ATP (adenosine triphosphate) and CP (creatine phosphate). These fuels are metabolized without using O2 or producing CO2. Muscles are supplied oxygen from the lungs through the bloodstream and through myoglobin. Slow twitch muscle fibers (those that you engage while walking around) contain more myoglobin. However, human muscle contains limited amounts of myoglobin compared to freediving mammals. Fast twitch muscle fibers (those you use during strenuous exercise) contain more high energy phosphates (ATP and CP).

We also analysed the metabolic pathways in muscle. Aerobic glycolysis is slow but is the most efficient metabolic pathway. It generates 34 ATP molecules out of one glucose molecule. Anaerobic glycolysis produces lactate and generates only 2 ATP molecules out of one glucose molecule. Because the reaction occurs faster more energy can be liberated quickly. This process can be dominant for a maximum of 75 – 120 seconds. Anaerobic alactic metabolism uses only high energy phosphates (ATP and CP) present in muscle and can be dominant for about 5 – 15 seconds.

Did you miss part 1 of the muscle metabolism series? Find it here.

Muscle metabolism: concurrent metabolic pathways

A persistent myth in exercise physiology is that all these metabolic pathways are active sequentially. On the contrary, they are actually concurrent. In the figure below you can see the relative contribution of the different metabolic pathways to power output during maximum intensity exercise.

muscle metabolism
Muscle metabolism during maximum power output over time. The ATP-CP (adenosine triphosphate – creatine phosphate) system is depleted first, then AN-G (anaerobic glycolysis) provides the majority of power, followed by A-G and AL (aerobic glycolysis and aerobic lipolysis). The maximum power output decreases over time.

You can see the overlap in time between the different metabolic pathways, but also that the anaerobic systems contribute more at the onset of exercise than at the end. Of course a dive is not a continuous maximum power output. This might be what happens during sub-maximal power output:

muscle metabolism
This may be what happens during sub-maximal power output. Depending on the required power output anaerobic glycolysis will occur or not.

Regardless of the intensity of the exercise, the ATP-CP system is the quickest to respond to a muscles’ energy demand. The ATP-CP system essentially fuels the muscles while the blood flow to the muscle increases as a response to the increased oxygen demand. The increased blood flow allows aerobic metabolic pathways to provide energy. If the energy demand is low the lactic anaerobic system will not be a major contributor.

What happens if we run out of oxygen?

But what happens if oxygen is removed from the equation? How does the body respond?

During a dive you try to conserve as much energy and oxygen as possible. Freedivers work hard only for the first 10 or 20 seconds in order to overcome positive buoyancy, and perhaps a bit longer on very deep dives.

In the figure below is a hypothetical ideal dive and the energy systems that are major contributors to it. The dive consists of three phases, the dive phase, the sink phase and the ascension. During the dive phase you are actively swimming down. Ideally the ATP-CP system is the only contributor to the entire swim to neutral buoyancy. During the sink phase you stop moving and the aerobic system is able to supply enough oxygen to your body for basal metabolic functions. During the ascent most of your energy will be derived from anaerobic and aerobic glycolysis.

muscle metabolsim
In an ideal situation, the ATP-CP system is the only system active during the descent. A combination of aerobic and anaerobic glycolysis provides the energy to surface. As hypoxia becomes more severe, anaerobic glycolysis becomes more important.

A less ideal dive

In a less than ideal situation the lactic anaerobic system may also supply some of the energy required for the descent.  In this case you deplete the ATP-CP system completely prior to reaching neutral buoyancy and anaerobic and perhaps also aerobic glycolysis supply significant amounts of energy. The obvious result is that there will be less energy available for the remainder of the dive. A larger reliance on the anaerobic glycolytic system will also lead to earlier muscle fatigue.  This may occur if you have a very thick suit, not enough weight, or you are not appropriately trained.

If the oxygen supply to the muscles becomes limited, either because of vasoconstriction, hypoxia or both, anaerobic glycolysis supplies the majority of the energy required. The dive reflex has a large impact on the oxygen supply to the muscles. Vasoconstriction limits the supply of oxygen and causes anaerobic glycolysis to start earlier. Anaerobic glycolysis leads to muscle fatigue, which you notice as burning legs on the way back to the surface.

Muscle metabolism in diving animals

The processes that operate in human muscles are similar in freediving animals. However, the muscle composition of species differ. Diving animals also have specific adaptations that help them dive longer and deeper.

Diving animals do long breath hold dives because adaptations. These adaptations include a high hemoglobin concentration, a high blood volume relative to body weight, and abundant myoglobin in the muscles. In addition they have metabolic adaptations that help them dive. Despite these adaptations, the basic metabolic pathways are the same as in humans.

Some animals, such as the Weddel seal, dive on an exhale so that they do not struggle to reach neutral buoyancy. The Weddel seal maintains low levels of aerobic metabolism throughout most dives. These seal have less of a dependency on blood borne oxygen because of the massive amounts of myoglobin in their muscles.

The seal wins in terms of dive duration and depth
The seal wins in terms of dive duration and depth

Other diving animals push to the anaerobic limit on nearly every dive, such as sea lions and penguins. The muscles of Weddel seals are predominantly composed of slow twitch muscle fiber and loaded with myoglobin. The muscles of sea lions and penguins contain more fast twitch muscle fibers.  The difference in muscle composition exists mainly because of foraging and hunting styles.

vivian island freediving
A steller sea lion, loaded with fast twitch muscle fiber.

What should you do? Hit up the gym and train for fast twitch muscles? Or try to be like a Weddel seal?

In essence the diver that emulates a Weddel seal will end up doing the longest and deepest dives. The dive profile of spearfishers is much like that of seals. They make a descent, stay at their target depth for a third of the dive, and then ascend. It is no coincidence that many spearfishers have hit amazing numbers at freediving competitions. And this despite the fact that they never did much training specifically for competitive freediving beforehand

So you tell me in the comments, are you going to hit up the gym, or start spearfishing?

Three studies that you may find interesting:

  • Kooyman, G. L., & Ponganis, P. J. (1998). The physiological basis of diving to depth: birds and mammals. Annual Review of Physiology, 60(1), 19–32. http://doi.org/10.1146/annurev.physiol.60.1.19
  • Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine (Auckland, N.Z.), 31(10), 725–741. http://doi.org/10.2165/00007256-200131100-00003
  • Reed, J. Z., Butler, P. J., & Fedak, M. A. (1994). The Metabolic Characteristics of the Locomotory Muscles of Grey Seals (Halichoerus-Grypus), Harbor Seals (Phoca-Vitulina) and Antarctic Fur Seals (Arctocephalus-Gazella). Journal of Experimental Biology, 194, 33–46.

Muscle metabolism part 1: Muscle fiber types and freediving.




Holistic Freediving by Eric Fattah

About Eric

If you have not heard of Eric Fattah but are interested in the history of competitive freediving, now is the time. In 1998 Eric invented fluid goggles, not realizing that Roland Specker had invented similar goggles in France but never marketed them. In 2001 Eric set the first world record with a monofin in constant weight (- 82 m). He dove to -80.5 m in Vancouver without a wetsuit in waters that are approximately 5 °C (41 Fahrenheit) below the thermocline. Eric dove FRC (Functional Residual Volume: diving on an exhale) for four full years, in an attempt to counter decompression sickness and registers his deepest FRC dive at Vertical Blue to 71.9 m. His experience with decompression sickness led him to implement the first experimental decompression sickness algorithm for freediving in his Liquivision dive computers.

Eric is a world class diver who has invented many techniques, and coached well known freedivers such as Branko Petrovic and William Trubridge. He wrote ‘Holistic Freediving’ in 2012, a book designed for freedivers who want to do targeted exercise to increase their CO2 tolerance, low O2 tolerance, diving reflex, and have specialized (cross-)training programs. The book is phenomenal and contains so many novel approaches to freediving that it is well worth the price tag (US$ 95).

Holistic Freediving by Eric Fattah
Eric Fattah

Holistic Freediving

One part of Eric’s phasic training that you will learn about in Holistic Freediving is ‘foundational training’. This training allows you to become better able to withstand hypercapnia and hypoxia. Even better, it will do so without pushing you to the limit and requiring many days of recovery. Forget max attempts until you have laid the foundation. You will be better able to cope with the deep dives, without having lost many training days because you needed to recover. The cross-trainings described in this book are also novel and very effective. No more Wonka tables or simple static tables. Some of Eric’s dry static tables are done with the help of pure O2 and an oximeter. Other tables incorporate exhale statics and hyperventilation. They are intense, but extremely effective. Within three weeks of doing one cycle of static trainings weekly I managed to do a 3 min 45 breathhold on an exhale. My personal best before that? One minute forty seconds.

The price of the product [95 USD] is proportional to the lifetime of secrets it contains and the extraordinary tribulations I went through to discover them – Eric Fattah

Holistic Freediving by Eric Fattah sample

Mouthfill equalization by Eric Fattah

Ask Eric for a copy below:







Hypoxia and brain function

In this post we are going to take a closer look at how your judgement changes due to hypoxia. Being hypoxic means having too little oxygen to support your body. Hypoxia manifests itself as fatigue, lightheadedness, tunnel vision, altered colour perception, and most importantly, impaired judgement.

How do we recognize hypoxia?

The body has no receptors that tell us we are hypoxic at all. Instead what you feel when you are holding your breath is the increase in CO2. This leads to a buildup of carbonic acid in the blood, and thus increased acidity. If we do not build up any CO2 and have gas in our lungs (any gas), there will be no alarm bells going off. Most freedivers notice the uncomfortable feeling associated with hypercapnia (elevated CO2 levels), but unfortunately have no idea about hypoxia. For obvious reasons, this can be problematic.

Luckily we can have a peek at what happens at low levels of O2 because of pilots’ altitude training. It is revealing, and really, a bit scary:

What do we learn from this? By the time we reach PaO2 = 60%, our judgment is so impaired that we are unable to make any sensible decisions. This carries the implication that as a freediver you need to be on your way to the surface at this point, and hopefully you can complete your surface protocol by force of habit. During a breathhold the drop of oxygen saturation tends to stall for a bit at PaO2 = 70% before dropping further. At this level you should be experiencing tunnel vision and other funny effects, although this will differ for everyone personally.

Lucky breaks at depth

We do get some lucky breaks at depth, thanks to the pressure. Oxygen reacts at higher rates at depth. Because of that, your oxygen saturation is unlikely to drop very low until you come closer to the surface and the pressure decreases. This is the reason that most blackouts occur at, or close to the surface. Let’s say you are at 40 meters and you have 5% total O2 in your lungs, this will react as if you have 5 x 5% =  25% O2 in your lungs because of the pressure. However, if you now go back up and by doing so you drain the lungs to 3% total oxygen at 20 meters, the result of the pressure at this depth will be that the O2 reacts as if you have 9% in your lungs. Oxygen will move back from the blood into the lungs and you are now in the low O2 zone, where you are prone to blacking out (more info on this can be found in this article on shallow water blackout). The point: once you are on your way back up make sure you go back to the surface fast.

Is identifying hypoxia useful?

Knowing this, is it still helpful to know when we are hypoxic? I think so, but you also need to realize when you are going to notice it. This is probably in the last 10 – 20 m of your ascent (depending on how deep you dive). If you have dipped below 70% or 60% PaO2 you should notice this at the surface as some type of lightheadedness or tunnel vision. The depth and duration of that dive should probably be your maximum for the day unless you are still warming up. It will vary daily and between dives, depend on what you have eaten, rest, hydration, and so forth. Doing a 2 minute dive to 30 meters on one day is no guarantee that you can do a 1 minute dive to 20 meters on another day. Even in one dive session you may not always get the same results, so be careful. You can use an oximeter and exhale statics if you want to know what hypoxia feels like. However, note also that in some cases (if your mind wanders at the wrong time?) you may not sense it at all.