Category Archives: freediving science

Free webinar: Training with a MOXY muscle oxygen sensor

The MOXY is a muscle oxygen sensor that can measure the saturation of hemoglobin and myoglobin of for example your quads.

Do you think your apnea walks are effective?

They might be. You can practice apnea walking with an oximeter, but it is hard to know exactly what is going on in the muscles. For some training you don’t need the oxygen in the muscles to drop, and for other training it needs to drop as much as possible.

Now, the guesswork is over.

Using a MOXY muscle oxygen sensor it is possible to measure the oxygen content in the muscles. You can strap the MOXY to your quad, biceps, or any other muscle to gauge the level of oxygen during exercise.

MOXY oxygen monitor.
The MOXY can be used to measure the oxygen levels in your muscles during exercise. Image taken from here

I have tested a set of exercises, including apnea walking, apnea squats, and bicycle interval training using the MOXY and will be sharing my results during a free webinar. The webinar will start at 12 noon CST, Tuesday December 12th, and run until about 12.45. There will be ample time for questions.

Register for the free webinar now!

Hope to see you there.

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

The surprising truth about your wetsuit, and staying warm while freediving

Staying warm while freediving is an art, especially in colder waters. During a normal 2-hour dive session you can easily lose more than 700 kcal just on heat loss, and if you lose heat too fast, your dive session will be cut short. Here I’ll share some surprising information about the performance of your wetsuit, and what you can do to stay warm in the water.

High heat loss despite a wetsuit

Conductive heat loss is something we can calculate. I am not going to try to bore you too much with the details, so here is the condensed story.

How much heat you lose through conductive heat transfer depends on four things. 1) The temperature difference of your skin and the water around you, 2) the thermal conductivity of your wetsuit, 3) the thickness of your wetsuit, and 4) the surface area of your body.

Wetsuits contain up to 94% nitrogen bubbles. The nitrogen is what prevents heat loss. The neoprene itself is actually quite conductive. Nitrogen compresses just like air, so at depth the suit becomes thinner and the overall heat conductivity increases.

Conductive heat transfer does not include water sloshing through your wetsuit. It is simply the amount of heat you lose if you were to be still, in still water.

Here are some interesting results based on conductive heat transfer alone:

  • A diver in a 3 mm suit and 25ºC (77ºF) water loses more heat than a diver in an 8 mm suit in  8ºC (46ºF) water
  • Heat loss at 20m is 4 – 5 times as high as at the surface
  • Heat loss at 40m is 7 – 8 times as high as at the surface
  • You will commonly lose at least 500 kcal of energy during a 2 hour dive session
  • At 40m depth, you can lose as much heat as a medium sized space heater generates (3000W),  depending on the thickness of your wetsuit
Surprise! You lose more heat through conduction in tropical waters in a 3 mm suit than in frigid water in an 8 mm suit.

Planning your dives so you stay warm

When I initially calculated the difference in heat loss between cold and warm water divers, I thought I made a mistake. How on earth can a diver in 25ºC water with a 3 mm lose as much heat as a diver in 8ºC water with an 8 mm suit? I know from experience that I can spend all day in warm water, and only a couple of hours in cold water.


The answer is that conductive heat transfer is not the full story. If the sun is directly above you (midday on the equator), it will supply the earth surface with >1000 W/m2 of heat. Those 25ºC waters we talked about earlier get much more direct sunlight. In a 3mm suit you only lose ~300 W at the surface,  so the sun is instrumental in keeping you warm.

The sunlight hits regions farther from the equator such as Canada and New Zealand at an angle (see also: solar irradiance). Because of that, the sun supplies much less heat to the freedivers that call these waters home. In these regions, if you want to stay warm you need to plan your dives when the sun is high. That is not difficult in the summer, but in spring and fall you should try to dive in the afternoon, preferably on days without cloud cover. In the winter… may Poseidon be with you.

Wind and evaporation

Part of your body is above the surface during your breathe up. If the apparent air temperature (apparent air temperature takes into account wind chill) is lower than the water temperature, this will cause you to lose heat faster. The increase in heat loss can be as much as 10%, if you dive during a 0ºC day.

More important is the effect of evaporation. When you are above the surface, the water on your suit will evaporate. During evaporation, liquid water extracts heat from its surroundings. Unfortunately its surroundings are you, or rather, your suit.  Evaporating water costs a lot of energy, and how cold you become during a dive might be directly proportional to the rate of evaporation. Wind will increase the rate of evaporation, and so does heat. If you have a smooth skin suit, there will be less water to evaporate than if you have a lined suit.

Diving style and getting in and out of your wetsuit

Diving style is another big part of staying warm. Our muscles are very inefficient, and about 80% of the energy we use is lost as heat. A more active diving style will keep you warm for longer. You will of course sacrifice some performance.

Lastly, what you do before you get in the water will also affect how warm you will be. Make sure to store your wetsuit in a warm(ish) place before a dive session, and try to stay warm while you change. I know one diver that changes in a bathing robe, and transports his wetsuit in a cooler with jugs of hot water. The jugs of hot water do excellent service as a post-dive shower.

In conclusion:

  • Try to dive when the sun is high if you are far from the equator
  • Try to dive in low winds
  • Check the weather forecast and wear a (thicker) suit if there is cloud cover
  • Adjust your diving style to suit the water temperature. Long line diving sessions, deep targets and hangs are for the summertime
  • Stay horizontal at the surface, to allow your entire body to be heated by the sun (if there is some)
  • Try to stay as warm as you can before you get into the water
  • Diving with a smooth skin suit is warmer than diving with a lined suit of the same thickness


Spitting blood after freediving: squeezes and squeeze-like symptoms

You might have come up from a dive and coughed up some foamy pink mucus, or some phlegm with blotches of blood in it. Chances are that you experienced a squeeze. We will discuss the causes for squeezes and squeeze like symptoms in this article. If at some point you were spitting blood after freediving and wondered what was going in, keep on reading.

As usual, remember that I am not a doctor. The symptom of spitting or coughing up blood is called hemoptysis and can have numerous (potentially life-threatening causes). If in doubt, see a doctor.

True squeezes

Squeezes happen when the negative pressure on the lungs or trachea is too much for the tissue to handle. Squeezes commonly do not occur unless you reach residual lung volume. Residual lung volume is reached between 25 and 45 meters for most divers, depending on how big the inhalation was and if the diver packed or not. A squeeze may occur at shallower depth if you have bad posture, or if you have heavy contractions.

Trachea squeeze

The trachea squeeze might be the most common squeeze type. The trachea is the least compliant to pressure changes. It is essentially a tube reinforced with cartilage rings. These cartilage rings don’t stretch very well and keep the trachea open, even if you would rather have it collapse. Blood vessels along the wall of the trachea can rupture if the negative pressure becomes too high.

Phlegm from the trachea looks white, transparent colourless, or transparent yellowish (opaque yellow or green if you have an infection), and will contain blotches or strings of blood if you have a squeezed trachea.

spitting blood after freediving
Find the trachea, lungs and alveoli here.

Lung squeeze

The lungs are soft spongy organs that stretch and compress better than the trachea, but unfortunately they can still get squeezed. If the lungs are subject to negative pressure, yellow fluid can start to leak into the lungs from the alveolar capillaries. Although this is not diagnostic of a squeeze, it does mean that you are descending to depths at which you may be at risk.

If the negative pressure increases, capillaries in the lungs may start to rupture, causing blood to fill the alveoli. This is called a lung squeeze.  If you have suffered a lung squeeze you will cough up pink foamy mucus.

What do you do after a squeeze?

There are no set guidelines for how to deal with squeezes. Most teaching organizations will state you need to be medically cleared for diving by a specialized doctor. Although I agree, my guess is that most divers do not follow this mandate.

Another recommendation that I have seen is as follows:

If your sputum (sputum = what you cough up) contains less than 50% blood, take one week off diving. If your sputum contains more than 50% blood, take two weeks off diving. If you only cough up blood, seek medical attention as soon as possible.

This is a more conservative approach:

If you see less than 25% of blood in your sputum, take one week off diving. If you see 25 – 50% of blood in your sputum, take two weeks off diving. Seek medical attention if:

  • You see more than 50% blood in the sputum
  • You cough up fresh blood more than 12 hours after the dive
  • Other symptoms such as pain or tightness in the chest are severe
  • Any symptoms persist for more than 5 days
  • SaO2 is <95% more than 15 minutes after the dive

Note that the symptoms will probably be gone within a day or two. This does not mean you can go back to diving again. The damaged tissue will likely still be weak and you should give it ample time to heal.

Protecting yourself against squeezes

  • Technique

To avoid getting squeezed, practice your technique at maximum 20 meters depth. Until you can do a dive with perfect posture (including the turn), do not dive deeper.

  • Stretching

Intercostal and diaphragm stretches will help increase the flexibility of the ribcage, and protect yourself against squeezes. In Pre-Dive Preparation, Sara Campbell teaches excellent stretching routines.

  • Gradual adaptation

Your body protects the trachea and lungs by blood shift, an effect of the diving reflex. Your adaptation to depth should be gradual to allow your body to get used to the depth and the required blood shift. In Holistic Freediving, Eric Fattah shares methods of training the dive reflex.

Not all squeezes are squeezes?

From April 2016 onwards, I started getting squeeze like symptoms on dives shallower than 25 m. These dives were well in my comfort zone, my average leisure dive was about 20 – 30 m. I would come up and cough up bloody sputum, indicative of a trachea squeeze. One time, the issue seemed to start 5 minutes after my last dive.

I took a week rest every time it happened and started more serious stretching of the lungs and trachea, to no avail. After I got back to diving, it was only a matter of time before the next ‘squeeze’.

It took me a while to figure out what was going on.

The ‘squeezes’ started after I moved to a busy intersection, with poor air quality. During that time, I had more colds, and often had an aggravated throat. I was more ‘phlegmy’ to start with. After I realized that the ‘squeezes’ started after I moved places, I bought a HEPA (high efficiency particulate absolute) filter and put it in the bedroom.

End of story.

Since I have started using a HEPA filter I have not once had squeeze like symptoms despite diving deeper, I’ve had better sleeps, and less colds. Starting your dive with an aggravated throat greatly increases the chances that you burst a blood vessel in the throat. Poor equalization technique probably increases the risk. Although the symptoms are the same as those of a trachea squeeze, these are not squeezes. They may occur simply when you clear your throat at the surface after a dive.

Immersion pulmonary edema

A phenomenon that results in similar symptoms (to those after squeezes) is called immersion pulmonary edema (IPE). This is a leakage of fluid from the bloodstream into the lungs. It has been reported in triathletes, swimmers, U.S. navy SEALs and scuba divers. You can read more about IPE on the website of DAN (Divers Alert Network). Although IPE may be somehow related to squeezes, the cause is likely different from negative pressure induced edemas.

Please share your experience in the comments.

Breathe up time: the key to relaxing dives

How long should your breathe up really be? When you surface from a dive, your body needs to replenish its oxygen stores and high energy phosphates. The body also needs to get rid of the excess CO2 and other waste products such as lactate. Once that is done, or perhaps at the same time, you need to start relaxing for the following dive. But how long does it take? A minute? Ten minutes? Let’s find out.

This article is not about decompression sickness, which I will cover in another article. 

Oxygenating the blood

For an average Joe with 70 kg body mass, 5 liters of blood and a heart stroke volume of 70 ml, the blood is pumped around completely in 71 strokes (5000 ml of blood divided by 70 ml = 71). Based on the assumptions that 1) his heart rate rises to about 110 immediately after a dive and drops after a minute, and 2) the lungs fully oxygenate all blood that passes by, he should have fully oxygenated blood within 39 seconds.

After 39 seconds, your blood is fully oxygenated.

If you have ever done 2 dives with only a 39 second breathe up in between, you know that this is not the full story. The body is not yet back to steady state within 39 seconds, so let’s continue exploring this topic.

High energy phosphates

You may have read my articles on muscle metabolism and muscle fiber. If you have, and you were not abysmally bored, you may remember the ‘high-energy phosphates’ or ATP-CP. If you did fall asleep on your keyboard while reading here is a one sentence recap: ‘High energy phosphates in muscles can provide energy for about 10-15 seconds of maximal muscle contraction and do so without costing O2 or producing CO2’. But how long does it take to replenish these high energy phosphates after a dive?

Up to 70% of CP is resynthesized in 15 seconds. If you are severely vasoconstricted because of cold or the dive reflex it may take somewhat longer. The remaining 30% will be resynthesized after roughly 2:45 minutes, so you need about 3 minutes in total to replenish high energy phosphates.

After 3 minutes, your high energy phosphates are replenished.

Excess CO2

And what about the stuff we need to get rid of after a dive? How long does it take to vent off all that excess CO2? Available data for the recovery of Steller sea lions suggest long recovery times (large variations between individuals are common). These recovery times are based on VCO2, the exhaled carbon dioxide after a dive.

For a dive of 3 minutes, VCO2 takes approximately 5:30 to recover, and for a dive of 2 minutes we can expect a 4-minute recovery time. The reason CO2 levels take a long time to return to normal is that CO2 is transported throughout the bodies tissues, and it takes time for the CO2 to make its way back into the bloodstream and to the lungs.

breathe up
As usual there is more data available of diving mammals than of humans. This is a grey seal.

Of course, we are not Steller sea lions. Steller sea lions are much larger and adults can weigh over 1000 kg (2200 lbs), and have lung volumes that are proportionally larger than those of humans. But does that mean that they lose CO2 faster or slower than us? My guess is that because of our size we lose CO2 at the same rate or perhaps somewhat faster than Steller sea lions, but I can’t prove it.

The release of CO2? Unknown, but likely less than 4 minutes for a 2-minute dive.


Now I did talk about lactate earlier. As it turns out it takes about an hour for lactate levels to return to normal after heavy exercise (or a dive during which you feel significant leg burn). This recovery time can be reduced to about 50 minutes if you go jog or go for an easy bike ride.

I wouldn’t wait for it if I were you. A better solution would be to not dive to exertion, so you will have a longer dive session. Lactate is only produced in significant quantities once your muscles become hypoxic. You can feel it as leg burn towards the end of your dives.

Lactate? Don’t wait for it…

In conclusion, if you want to make long and comfortable dives your breathe up should be at the very least 3 minutes, so that your high energy phosphates are fully recharged. Better still, take 4 minutes in between dives so that your CO2 levels are back to normal. Of course, during the breathe up, and especially the last 2 minutes you should try to be completely still.

And another method is to not overthink it and just do what feels right… But that would not make for very interesting articles. Let us know what your approach is below!

The terribly wrong image of freediving

A shorter version of ‘the image of freediving’ originally appeared as a Freedive Wire newsletter.

It is a sport that only people with a death wish practice. A sport for adrenaline junkies that like to dance with death. A sport for athletes that do not mind putting their life on the line. This is the image of freediving. But is it correct?

Four out of ten of the results for ‘freediving risk’ on google:

image of freediving

On the web, a bit of danger, adrenaline, and risk sells. These articles will get many reads simply because of their title. But are they true?

People that do not freedive usually see freediving as a sport that attracts lunatics and adrenaline junkies that like nothing more than to put themselves in danger. Freedivers on the other hand, normally use words such as ‘peaceful’, ‘relaxing’, and ‘self-discovery’ to describe the sport. Spearfishers may talk about the thrill of catching a fish on breath hold, but also about the calmness of the underwater world.

This difference is important, because the way that the general public perceives freediving affects for example club insurance policies and where we are able to practice and dive. Why does the difference exist in the first place?

When freediving gets media attention, commonly somebody tries to break a record. Take for example Will Trubridge’s 102 m dive last year. The dive got attention from for example New Zealand Herald and BBC, and was well marketed by his sponsors, Steinlager, Suunto and others. Freediving underneath ice receives a disproportional amount of attention on social media.

Even though we now have strict safety protocols for record attempts, the focus of freediving in the media often lies on the risks of black out and drowning. Sometimes, freediving is described as ‘a dance with death’. I doubt that many divers that do these record attempts truly feel like they are dancing with death.

In fact, Guillaume Nery dived 10 meters deeper than he intended due to a bad mistake by the organizers (AIDA individual depth championship 2015). He stated afterwards ‘I felt like they were playing with my life’. That does not sound like a statement from someone who likes to dance with death. But hey, risk sells.

Despite the image of freediving, most of us do not actually care to dive very deep, to break records, or to dive underneath ice. In contrast, most of us probably like to dive in less than 25 meters of clear warm water, without pushing their limits. But how many people hear about this?

Because the ‘normal’ freediver is hardly ever in the news, people that do not freedive do not know that this part of freediving exist. They believe that we must all want to dive deep, risk blackouts, and live on the edge. How do we change that mindset?

Some freedivers have recently questioned the rules for AIDA competitions. They say there are too many blackouts and this damages the image of the sport. Perhaps that is true. If the general public would never see a blackout, the image of freediving would surely change. However, I believe the responsibility is also partly ours.

“The responsibility to change the image of freediving is ours”

When we talk about freediving, we should aim to let people know that blackouts are not the norm. We don’t go out and expect one too happen. Rather, we train for the worst and dive our best. We mitigate the risk by knowing safety protocols, and minimize the risk by diving within our limits. The approach is similar in ocean kayaking, sport climbing, mountain biking, or any other sport.

Just as a side note: Whistler Mountain Bike park followed 2000 mountain bikers over a 5 month period in 2009. Nearly 50% of them were injured in that time. Of those, 108 injuries threatened life and/or limb. If I google for ‘mountain biking risk’ I see studies, risk assesments, and articles on mitigating risk. No bold statements such as ‘Deadly risk lurks behind the thrill of mountain biking, or ‘The dreadful (and unnatural) toll of mountain biking’.

Yes, blackouts happen. Yes, we train to hold our breaths longer. But that does not mean we disregard personal safety or are all adrenaline junkies.

Freediving buoyancy and its effect on energy expenditure

How much energy do we really expend during a dive? This was a question posed by Connor, after our last post on muscle metabolism (Part 1, Part 2). This article is the result of that question. Perfect freediving buoyancy is nearly a science on its own.

How much energy does a dive cost if you are wearing a wetsuit? How much less energy if you decide to dive on an exhale instead of an inhale? In this post, we will look at freediving buoyancy and how much energy you burn going up and down. The results may surprise you!

Try to imagine you are a perfectly average male (sorry ladies). You are the embodiment of averageness. You are 80 kg, (176 lbs), 1m 75 (5’10”) and have approximately 15% body fat and perfectly standard lungs with 1.2 liters functional capacity, 2.3 liters functional residual volume, 5.8 liters normal full lungs and 8.5 liters packed lung capacity. I realize that there is a small chance that you are not, for whatever unfortunate reason, completely average. That’s ok, even though body composition matters, lung inflation and the thickness of your wetsuit matters even more.

Let’s have a look at what affects freediving buoyancy.

Freediving buoyancy: body composition

Your body composition has a significant effect on your buoyancy. Fat is somewhat buoyant, muscle is somewhat negatively buoyant and bones are very negatively buoyant. Other soft tissues have a density close to that of water. The density of the body without any gas in the lungs (or intestines) is somewhere in between 1.01 – 1.08 g / cm3. This is always heavier than fresh water, and usually heavier than salt water. Our average diver has a density of 1.05 g / cm3.

Freediving buoyancy: the lungs

The lungs are compressible, and this causes their effect on buoyancy to change with depth. There will always be a net positive buoyance from the lungs. This positive buoyancy is great at the surface, but small at depth.

Freediving buoyancy
The body has a specific buoyancy (x-axis) which does not change with depth (y-axis). You can see that the net effect of the lungs is always a positive buoyancy force. However, the magnitude of this force declines drastically with increasing depth. The change in buoyancy is largest close to the surface.

Freediving buoyancy: neoprene

Your wetsuit is made of neoprene. Neoprene without any bubbles in it has a density of approximately 1.3 g/cm3. The wetsuit is made supple and insulating by injecting the neoprene with nitrogen. All these nitrogen bubbles add buoyancy. Neoprene can contain anywhere from 30% to 94% nitrogen. More nitrogen means a stretchier wetsuit that is more insulating at the surface, but less insulating at depth. Here I assume a neoprene nitrogen content of 84%, and a base density of 1.3 g/cm3, leading to a final density of 0.24 g/cm3. These numbers were obtained with some help from friends at Azure Passion. (I am trying to get more detailed specs on a variety of neoprenes, if I succeed, it will be added or linked to here).

Weight, or uncompressible buoyancy?

The last variable, which is the easiest for us to change, is how much weight we carry with us. Some divers also decide to take down uncompressible buoyancy. This basically counts as negative weight. I dive with 600 grams of positive buoyancy (incompressible plastic spheres) in my fin, in addition to 13 pounds of lead on my neck and waist, so that my legs float and I can stay vertical during my descent. But how much weight should we use in total?

freediving buoyancy
Neoprene has an effect on buoyancy that is similar to that of the lungs. Neoprene is always positively buoyant (in a freedivers depth range). Lead of course has a resulting negative buoyancy force, which is constant.

Energy cost of freedives

We should be weighing ourselves so that we can minimize the work we do during a dive. Work is defined in physics as ‘W = F x a, or work equals a force times a distance. If the buoyancy force of your body, Fb = 20 during your entire dive, and you dive to 20 m, you have to expend 20 x 20 = 400 joules (approximately 50 calories) to dive down to 20 meters.

On the graphs in this article, the distance (depth) is plotted on the y-axis, and the force resulting through buoyancy is plotted on the x-axis. The area underneath the curve is the amount of work that a diver has to do to get to a specific depth. This makes intuitive sense, because the higher or lower a buoyancy force is the more work has to be done in order to counter that force. If a diver is very negatively buoyant, it costs a lot of effort to come to the surface. If a diver is very positively buoyant, it will cost a lot of effort to dive down.

In the following graphs you can see a hypothetical 130 m dive, which is close to the current CWT record of 129 m. These curves are for divers with full lungs, and a variable suit. The divers have been perfectly weighted in order to minimize the area under the curve.

freediving buoyancy
Here are a set of hypothetical 130 m dives by our average diver. The suit is variable. If the line is close to x = 0 (indicated with the top arrow), the buoyancy is small and the total energy expenditure is small too. The simple conclusion is that a dive with no suit is much more energy efficient than a dive with an 8 mm suit. A more interesting conclusion is that in order to be weighted properly, divers need to bring positive buoyancy with them, rather than lead weight (which is negatively buoyant). The amount of weight required is indicated on the legend. The minus sign indicates that the weight is negative!

An interesting thing to note is that all divers are neutrally buoyant at half their target depth. This is a recurring finding, for any depth, and holds for any combination of suits, lung fill, body types, and weight.

Another interesting thing to note is that without using incompressible buoyancy,  all divers (except the diver with the 8 mm suit) are too negatively buoyant in order to be diving to 130 m. They will reach neutral buoyancy too early, and hence waste effort on the ascent. For example, our no suit diver requires 1.2 kg of positive buoyancy in order to be perfectly weighted.

A no suit diver expends approximately 822 J (200 cal) on the ascent and descent if perfectly weighted (1.2 kg positive buoyancy). If the diver decides to carry no weight, this increases to 1500 J.

Of  course no freediver in his/her right mind would attempt a 130 m dive in an 8 mm suit. The buoyancy force of a ‘perfectly weighted diver’ in an 8 mm suit at the surface is so high, that it would be close to impossible to dive down to any depth. The diver has to strap on half a ton of weight to make the descent possible. All this weight makes the ascent next to impossible.

Energy cost of 25 m freedives

I know, you don’t dive to 130 m, and I don’t dive to 130 m either. So what do people other than Alexey Molchanov and Guillaume Néry take away from this study? My last recreational dive session averaged 24.5 m, so I will focus on 25 m dives here.

Here is a plot for divers that dive on full lungs with a variety of suits:

freediving buoyancy
The buoyancy force plotted against depth for 25 m dives with full lungs and a variable suit. Note that diving without a suit is much more energy efficient than diving with an 8 mm wetsuit.

Keep in mind that the area in between the curve and the y-axis corresponds to the total energy expended. Because the curve is far away from the axis above neutral buoyancy (12.5 m), this is where we lose most of our energy.

And here is plot for divers with a 3 mm suit, that dive on 1) a forced exhale, 2) a passive exhale, 3) full lungs, and 4) packed lungs:

freediving buoyancy
The buoyancy force plotted against depth for 25m dives with variable lung inflation and a 3 mm wetsuit. Note that diving with packed lungs costs nearly as much energy as a dive with full lungs with an 8 mm wetsuit! FV  implies a forced exhale.

What do we learn from this? Well, the first lesson is one that you probably already know. Your suit should be as thin as possible, because it takes a lot of effort to dive with a thick suit. Second lesson, if you bring down less air your dive will be more energy efficient. Yes, you read that correctly. A forced exhale is much more energy efficient than packed lungs, and the difference is big. Our diver expends less than half of the energy diving after a forced exhale, as opposed to diving after packing. Apart from that, after an exhale, our diver only needs 0.3 Kg to be perfectly weighted, and after packing our diver needs  3.6 Kg!

If you do dive with a thick suit you can offset the extra buoyancy by diving FRC (functional residual capacity, this is diving after a passive exhale). Note that a diver with packed lungs and a 3 mm suit nearly expends as much energy as a diver with an 8 mm suit with full lungs! A diver with an 8 mm suit on FRC expends approximately 476 J on overcoming buoyancy, and a diver with a 3 mm suit and packed lungs expends 465 J on a 25 m dive.

Of course, the significant downside to diving FRC or even RV (residual volume, or diving after a forced exhale) is that you will not carry as much oxygen with you. Some divers feel that the sacrifice is worth it. However, I do not know of any recent records that have been set with an FRC dive, so it appears that oxygen (or perhaps equalization) is a limiting factor for dives to great depth.

Practical tips

  • Aim for neutral buoyancy at half of your target depth. During a dive session in which you dive 20 – 30 m, with 25 m dives on average, aim to be neutral at 12.5 m. A more conservative approach is to be neutral at half of your maximum expected depth (15 m). If you pack, and/or use a thick suit, this is especially important.
  • If you have no problems with lung or trachea squeezes, explore FRC or even RV diving. It may be more comfortable for you. Several threads on the Deeper Blue forums are devoted to FRC diving. RV diving has recently been suggested by Aharon Solomons as a method to acclimatize to depth. Always dive with a buddy and be very careful, because you are missing out on a large oxygen reservoir and are more prone to squeezes.

In a future post I will show you how much energy I burn in an average dive session.


















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.
  • Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine (Auckland, N.Z.), 31(10), 725–741.
  • 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.




Muscle fiber types and freediving

This article is the first in a 3-part series on muscle composition, performance, and failure during breath holding. This article is long and fairly technical. In a rush? Just read everything in bold.

Muscle fiber types and freediving

Muscle fibers use fuel in order to deliver power. This fuel is metabolized via different reactions, called metabolic pathways. There are different types of fuel, different types of muscle fiber, and different metabolic pathways to consider. The purpose of this article is to provide you with an understanding of how muscles perform and metabolize under hypoxic conditions. In light of that will come some speculation on training for freediving. What types of muscle fiber are beneficial for freedivers? What type of fuel do we need in muscles and how do we increase the abundance of that fuel?

Anaerobic and aerobic metabolic pathways

Muscles can perform under either aerobic or anaerobic conditions. Under aerobic conditions, the supply of oxygen to the muscle is sufficient to keep up steady state performance. This includes for example walking and low speed running. Under aerobic conditions, glucose is converted to pyruvate. Pyruvate in turn enters the metabolic pathways called the krebs cycle and oxidative phosphorylation to yield ATP, which is used directly as a fuel in muscle fiber. It is an efficient process that yields 34 ATP molecules per glucose molecule.

If the supply of oxygen is reduced, or the intensity increased, aerobic processes may not be able to deliver enough energy for the work required. This occurs for instance if you are sprinting. The body will require much more oxygen than it can deliver to the muscle. Thankfully, anaerobic processes take over and supply more energy to the muscle, although they can only do so for a short period of time. During anaerobic exercise, the body develops an oxygen debt that needs to be paid later (as evident from increased ventilation after a sprint). Anaerobic high intensity exercise cannot be maintained for more than about 2 minutes. Anaerobic exercise can be alactic and lactic. Alactic anaerobic exercise consumes stored ATP, which is quickly replenished by creatine phosphate. As the name implies, no lactate is produced during alactic anaerobic processes, but they cannot fuel muscles for more than 10 seconds under high intensity exercise. Lactic anaerobic metabolic pathways take over after creatine phosphate and stored ATP is consumed and have the potential to keep you going for another minute or two. Glucose is converted into ATP quickly with lactate as a byproduct. For each glucose molecule only 2 ATP molecules are produced. Because the reaction proceeds much faster than aerobic processing of glucose, more energy can be made available for the muscles for a short duration. Any exercise that takes longer than 2 minutes has a major aerobic component.

Types of muscle fiber

Humans have three types of muscle fiber. They are called slow twitch, fast twitch A and fast twitch B fibers. In some texts these muscle fiber types are referred to as type 1 (slow twitch), type 2A and type 2B. Slow twitch muscle fibers have slow contraction time and a high resistance to fatigue. They rely primarly on oxidative pathways for energy supply (krebs cycle and oxidative phosphorylation). If you are running at a slow pace or walking you are mainly using slow twitch muscle fiber.

Fast twitch B muscle fibers are the opposite of slow twitch muscle fibers. These muscle fibers have a high dependency on fuels and ATP production methods that do not require oxygen. They contain abundant creatine phosphate and enzymes that allow ATP production in the absence of oxygen. These fibers produce high amounts of force, but are sensitive to fatigue. These muscle fibers are recruited for sprinting, powerlifting and other short lived high force activites. Fast twitch A muscle fibers are an intermediate variety between slow twitch and fast twitch B muscle fibers.

Fast twitch muscle fibers are ‘fast’ because of two processes: 1) the rate of calcium release into the muscle cell and 2) the the rate of ATP breakdown. Calcium release is necessary in order to allow contraction of muscle fibers and ATP is necessary in order to ‘reset’ the muscle fiber so it can continue to contract. Hence, the rate of calcium release and the rate of ATP breakdown broadly govern the speed with which individual muscle fibers can contract. As stated in the previous section, anaerobic glycolysis is much faster than aerobic glycolysis, so ATP can be made available faster during the anaerobic conversion of glucose.

Is my muscle aerobic or anaerobic?

By now you may believe that muscles are either aerobic or anaerobic. Of course there is another complication to add to the story here. A muscle fiber will become anaerobic if the supply of oxygen is too low for aerobic ATP production. The supply of oxygen itself is controlled by many variables such as capillary density, hemoglobin concentration in the blood and oxygen saturation. Vasoconstriction, which causes resistance to blood flow, is particularly important in freediving because it restricts the flow of oxygenated blood to the limbs after the onset of the dive reflex. Because of vasoconstriction during a dive, your limbs will be partly cut off from the supply of oxygen and muscles in your legs and arms are more likely to function anaerobic.

Is my muscle fiber aerobic or anaerobic?

Our quadriceps are the most important muscles for propulsion underwater. Just like any other muscle, the quadriceps contains all types of muscle fiber, type 1, 2A and 2B. The proportion of these muscle fiber types will depend partly on genetics and partly on training. While diving, one fiber type may function aerobically and the other anaerobically. For individual muscle fibers this depends on whether they are activated (are anaerobic muscle fibers recruited or not?) and the supply of oxygen.

 Sources of oxygen during breath-hold

At the start of a breath hold, an average human has about 44% of the oxygen stored in their blood, 42% in the lungs, and 14% in the muscles. Oxygen is transported from the lungs, through hemoglobin in the blood, into the muscle where it is consumed. Where does the oxygen in the muscles come from? The muscles of diving animals (birds, mammals and humans) contain a specific oxygen carrying protein called myoglobin. Myoglobin is predominantly found in slow twitch muscle fiber.

Freedivers aiming to reach maximum performance need to increase the oxygen carrying capacity of all three reservoirs: the lungs, the blood and the muscles. Stretching and packing are the ways to carry more oxygen in the lungs. Hemoglobin is the oxygen carrying protein in the blood, and the concentration of hemoglobin can be increased (for example) by doing altitude training. The oxygen store in muscles is myoglobin, an oxygen carrying protein. The concentration of myoglobin can potentially be increased by specific exercise.

How to increase the oxygen stores of muscles

Oxygen stores of muscles can only be increased by periodically desaturating the myoglobin content of muscles. This will induce myoglobin production. Diving mammals with abundant myoglobin are commonly born with low concentrations of myoglobin. Repetitive diving causes the genesis of myoglobin over time. Can humans do the same?

For humans, myoglobin desaturation is difficult to achieve without rigorous training. In order to desaturate myoglobin, muscles need to be contracted while hypoxic (either at the end of a breath hold or during severe vasoconstriction). Eric Fattah and Sebastian Murrat have attempted to developed different training regimes in order to desaturate myoglobin and improve apnea ability. Sebastian Murrat’s training involves apnea walking and FRC dynamics, and Eric Fattah’s technique is called ‘Foundational Freediving’. The Foundational Freediving method is accessible in Eric’s e-book ‘holistic freediving’. Unfortunately, scientific evidence of an increase in myoglobin after using these training techniques has never been published, partly due to the cost of muscle biopsies.

Increasing the anaerobic energy stores of muscles

Fast twitch muscle fibers store ATP and creatine phosphate. These phosphates can readily be used for muscle contraction and do not produce CO2, nor do they consume oxygen. Hence, you can adapt the muscles for breath hold conditions by increasing the abundance of phosphates, rather than by increasing the myoglobin content of muscles. This can be achieved for example by repetitive sprints or high resistance training.

In conclusion

The muscle fiber types that can potentially be recruited for freediving are both slow twitch and fast twitch. Slow twitch muscle fiber requires a supply of oxygen, which can come from the lungs, blood, or muscle fiber itself. The myoglobin concentration of slow twitch muscle fiber can potentially be increased through specific exercise. Fast twitch muscle fiber does not require oxygen and has a high potential for the storage of phosphates. Specific exercise can increase the phosphate content of muscles.

In part 2 we will take a look at different diving animals, muscle composition, and breath hold ability, and in part 3 we will get to the actual training methods. You can find it here.

Selected links:

Selected journal articles:

  • Ferretti, G. (2001). Extreme human breath-hold diving. European Journal of Applied Physiology, 84(4), 254–271.
  • Kanatous, S. B., Davis, R. W., Watson, R., Polasek, L., Williams, T. M., & Mathieu-Costello, O. (2002). Aerobic capacities in the skeletal muscles of Weddell seals: key to longer dive durations? The Journal of Experimental Biology, 205, 3601–3608.

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.