This post was submitted by Luca Malaguti. Luca dove with a 3 mm suit in the cold water of the Pacific for the longest time. In this article he shares some tricks to stay warm.
The Canadian Pacific Northwest is a stunning place for scuba diving, freediving or snorkeling. It offers some of the cleanest waters in the world, fantastic underwater landscapes and unbelievable marine wildlife. The only issue for many people that wish to take part in these activities around Vancouver, the Gulf Islands or Victoria, is that for most of the year the water is quite cold.In the summertime, the Howe Sound can have surface water temperatures that range between 14 to 20 degrees Celsius. However, it’s always cold below the thermocline.
So you want to increase lung volume? There are several ways to do this, but you should probably start with becoming a better breather. That means, you should be able to fill up the entire volume of your lungs with a deep inhale before you actively start to do exercises that will increase lung volume.
Let me show you what I mean.
Active breathing vs. passive breathing
Most people go through life without taking active control of their breathing. The autonomous nervous system takes over the act of breathing if you don’t think about it, which means that you don’t have to think about it…
And that means that most of us never think about breathing.
The body responds to simple stimuli that will increase or decrease the breathing rate. For example, the breathing rate will increase if CO2 in the body increases or if the temperature decreases.
The body does not optimize each breath. If you are relatively untrained your inhales could be short and you may be using less than 75% of your vital capacity for each breath.
Optimizing the respiratory system is something we have to do by consciously using it. In other words, you need to become aware of your breathing, and consciously change the way you breathe.
Get your shoulders out of the way first
But first, let’s become somewhat aware of what our posture does to our breathing. The shoulders are only attached to the rest of the skeleton by the collar bone. Because of that, a substantial weight pushes down on the widest part of the rib cage: the weight of your shoulders and arms combined.
Don’t believe me? Then try this:
Stand up or sit up straight with your arms hanging down the side of your body. Breathe in as deep as you can.
Stand up or sit up straight with your hands on your hips and your elbows out. Breathe in as deep as you can.
If you noticed it was easier to breathe in that second time, you felt what I am talking about.If you did not you are either a blessed freak of nature or you are hardly using your upper intercostal muscles.
Funny side note. Opera singers have their arms wide and in front of them when they sing a strong note for a reason: it opens the chest.
In order to loosen up the shoulders:
Stretch arms out overhead, and in a wide arc let the left arm come underneath the right. Clasp your hands together (as best you can). Hold for one minute and then switch sides. This is part of the yoga ‘eagle pose‘.
Put your hands on your glutes, fingers pointing down look forward and up so that your spine is slightly arced back. Try to gently move the elbows closer to each other behind your back. You should feel a stretch in your front shoulders.
This simple preparation goes a long way. It won’t help you actively increase lung volume yet but it will allow you to get the most out of your breathing later. There are many more ways to open the chest with stretches but let’s move on to the actual breathing.
Three simple breathing exercises
One of the best ways to actively increase your lung volume is by limiting the flow of air through your throat and training yourself to keep that flow constant. Try it:
Keep one hand on your belly, and one on your chest.
Inhale slowly and constrict your throat slightly so that your inhale is audible (another way to inhale audibly is to make a whistling sound with the lips).
Focus on the belly first. Your lower hand should be moving outward.
Keep the sound of your breath constant.
Once it becomes difficult to keep that lower hand moving out, your chest should start to inflate. Try to keep the lower hand outwards as much as you can and continue with inflating the chest. Exhale slowly and repeat.
Alternate nostril breathing
Alternate nostril breathing is a similar exercise but additionally clears your nasal cavities. The exercise is fairly straightforward.
Sit with a straight back and close your right nostril with your right thumb. Inhale fully and slowly through the left nostril.
At the top of your inhale, pause and release your right thumb. Close the left nostril with the ring finger of the same hand.
Exhale fully and slowly through the right nostril, and then inhale fully and slowly. Pause and close the right nostril.
End range of motion exercise
When I was recovering from a rotator cuff injury I was given an exercise to work on the power and strength at the end of my range of motion. It was an easy exercise and I realized it could be done with intercostal muscles as well. I believe that out of the three exercises here this one will give you the fastest results.
Sit with a straight back
Put your hand on your hips (so the shoulders don’t weigh down the ribcage)
Inhale slowly and fully
Once you are at the max inhale, use your diaphragm and intercostals to keep it at the max.
Do not lock the throat or mouth. The air should be able to move freely from your mouth into your lungs, and you need to use your respiratory muscles to keep it in the lungs
Exhale after 10 seconds
repeat 5 times
How long should you exercise? Just remember that five minutes per day beats one hour once per month.
Forget about packing for now
Packing, or over-inflation of the lungs, is a simple tool that can increase lung volume directly and dramatically. However, because it is a passive tool it works against the resistance of the rib cage. The over-pressure in the lungs has caused embolisms resulting in partial paralysis, and a range of other serious injuries in freedivers on land and in the pool. It should be avoided for anything but dives that are well below residual volume. From AIDA’s Facebook announcements:
“Recently several packing related incidents with lung damage and/or transient neurological symptoms have been reported. Keep in mind that lung packing and pack stretching are very advanced techniques that may lead to serious medical conditions and even death. Packing should be performed with care and is something that we do not recommend in the AIDA education program”
Packing alone is not worth your time. Breathing exercises are always good, both to increase lung volume and better breathing overall. Once you are going to push the numbers during very deep dives packing is something you may want to consider. If you are interested in more info, Walter Johnson has a great article on packing on his website Freediving Solutions.
When and how many exercises should I do?
Great question. When considering how much time to put in any exercise remember that consistency and moderation are the long term winners. Five minutes per day will increase lung volume, but one hour on one day per month is not going to do much for you. The best time to do these exercises is right in the morning, so you can reap the benefits all day.
This post goes with a webinar that I gave on data collected with the Moxy Muscle oxygen monitor. In the webinar I test some specific exercises and try to speculate on myoglobin desaturation and training to increase myoglobin stores.
I want to use this post to give some extra thought to some keypoints, and to make the webinar a bit more understandable. But first, keep these things in mind:
The Moxy measures oxygen in the muscles. I put it on my left Vastus Lateralis (quadriceps).
The Moxy gives you one number for total SmO2 (muscle oxygen). This number is a weighted average of myoglobin and hemoglobin.
Everything I tested only relates to me and my (left) quad. The numbers will be different for you. The squats that work for me may not work for you. The apnea walks that don’t work for me may work for you.
On all the graphs in the presentation, the x axis = time in seconds. The left y axis is for heart rate, muscle oxygen and SaO2. The right y axis pertains to Thb and is a measure of blood flow to the muscles.
Ok now you can watch:
I used the Moxy to test a series of exercises. What do these exercises actually do?
For example, the apnea walks that I described here actually did not train my muscles to perform under hypoxia at all. The body has a fantastic set of feedback mechanisms in place to make sure that oxygen is delivered where it is needed, and without the vasoconstriction and blood shift during a dive that oxygen will go right into your muscles. Apnea walks debunked. Sorry everyone.
It didn’t matter whether I did them on an exhale or inhale. I think that apnea walks on an inhale probably don’t work for anyone. On an exhale, they may work for some people, but I doubt it.
What about holding your breath until contractions and then starting exercise? Same thing. The muscles actually never get hypoxic. In fact, even doing this with a more strenuous exercise like a wall sit the muscles never dipped below ~35 %.
Doing a wall sit or even one-leg stand with breath did not help either. You can do a wall sit until failure, but there will be plenty of oxygen in your quads. It’s not a lack of oxygen that causes your muscles to fail, it’s the accumulation of waste.
The only exercise that I found effective was a set of isometric squats with short recovery intervals, done after a forceful exhale. Using these squats and tinkering with the variables (length of recovery, initial static, and squat) I was able to consistently let SmO2 dip below 10%.
Now before you all start doing a 150 kg squat on breath hold, remember that if you do this with too much resistance you might simply be training for fast twitch muscle. If I focus on slow twitch muscle I try to stick to no higher than 30% of my personal max resistance.
How do you know this works? Are you increasing myoglobin in the muscles?
I don’t know if this works. But here is my rationale. In order to get the body to generate hemoglobin (red blood cells) you need to desaturate the blood of oxygen. This is why being at altitude increases your red blood cell count. The body will automatically create more red blood cells once it realizes it does not have enough of them to efficiently bring oxygen to where it is needed.
Along the same lines of logic, we need to desaturate myoglobin of oxygen in order to tell the body to create more of it. This happens naturally on some deeper or longer dives thanks to vasoconstriction and blood shift, but is hard to achieve when cross training.
I can’t promise you that by lowering SmO2 you will cause more myoglobin to be generated. I do think it is a sound hypothesis. Keep SaO2 high, and decrease SmO2. This is what naturally happens in our bodies during a dive, and one of the things to aim for during cross training.
The in-water method of training for myoglobin is called the Foundational Training and described in Eric Fattah’s book Holistic Freediving.
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: 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?
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.
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:
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:
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.
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.
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.
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.
Ferretti, G. (2001). Extreme human breath-hold diving. European Journal of Applied Physiology, 84(4), 254–271. http://doi.org/10.1007/s004210000377
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.
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.
Spearfishing brings with it tons of dangers that we should think about that aren’t covered in the former parts of the ‘risk series’ (shallow water blackout & compromised conditions). First of all, if you are spearfishing you are obviously using a sharp object that can skewer your dive buddy as easily as a fish. Second, struggling with a fish that is more combative than you expect at depth may tire you out and third, you may become entangled in your own line.
Spearfishing risk 1: Being shot
Most of us have seen the gory shots being shared on Facebook or other social media platforms. Spearfishers with spears through their legs, asses, arms, torsos and sometimes even heads. If you are lucky enough to survive the ordeal, you are bound to think about new ways to handle your slingshot skewer.
Never try to film your dive buddy with a camera on your gun
Believe it or not, this happens. Do never point your gun at anyone, not even unloaded. If you make a habit of pointing an unloaded gun at your dive buddies, at some point you’ll make a mistake and it will be loaded. If you have a go-pro on your speargun you are not going to be making movies of your dive buddies, and that is fine. The risk of being shot is lower if you are in a small group (2 or 3), and nearly 0% if you share a speargun. For obvious logistic reasons, sharing a speargun is a bit of a pain in the butt, but that’s better than a spear in the butt. Be careful of new dive buddies, because you can’t be sure of their experience and care with the guns.
Be careful around new dive buddies
If you have boat support, and you are being towed around by rope from reef to reef, make sure that the speargun is with the last person, or if there are more than one, that the spearguns are not armed and are on the boat.
Spearfishing risk 2. Entanglement
When you are spearfishing you are working with lines and sometimes, very long lines. Only if you are shooting small fish that are not likely to take you for a good ride can you get away with just a speargun and a few meters of line. If you are going for the bigger and stronger fish, like tuna and marlin, or big grouper, you will probably have the line of your spear rigged to a buoy with 20 m of line or so. That means you suddenly have a whopping 20+ m of line to get stuck in. Always carry a good dive knife that is easy to access with either hand (I like a knife on my upper leg or the inside or the lower leg so I can always reach it).
Make sure you pick your battle accordingly. If you shoot a fish and it is attached to a buoy, dive to the surface first, and take a few hook and recovery breaths. Your buddy can worry about you, and stay close to the buoy and line, or if you have two buddies, one is your safety, and the other hauls the fish up. If you come up tired after a dive, do not start yanking on the rope right away but give yourself time and do it safely.
If you have ever fought with a big fish, you know that you should be thinking of a float rigged to your speargun, here is one way:
Spearfishing risk 3: Sharks and other big fish
There are two common encounters with big fish while spearfishing. The first is the big fish you spear and which you will need to fight in order to make it back up. The second is the fish that wants your fish. Usually these bull or tiger sharks, but other sharks are often interested too. If you are in great white territory, make sure you wear a shark shield when you’re diving. You might occasionally get your privates shocked, but it’s better than losing a leg. Having a few dead fish attached to your weight belt is perfect shark bait.
If you spear a fish that is too much to handle and you don’t have a float line, you may suddenly find yourself fighting to survive. You don’t want to lose your speargun nor the fish, but fighting a fish at the end of the dive may induce a blackout.
These are the only dangers that are specific to spearfishing, as opposed to freediving. Of course bad conditions can amplify anything that can go wrong (struggling with a big fish? Not a big problem if your buddy can see you, but a really bad situation if they can’t). Read more about those things here:
The GB church is an amazing wreck that was sunk off of the coast of Portland Island. Portland Island can be reached by boat and has a few campsites on it. You can also kayak or in low winds, canoe to the island. The GB church has a mooring buoy attached to the mast. The mooring buoy can be used for pulldowns to the boat. You will be able to see amazing life from 12 m onward. The mast starts at 7 m and is also a great sight.
Dive on high tide, or shortly after high tide. The current is commonly at the surface and will die down around 10 m. You cannot freedive this site at low tide, and there are no reliable current tables for this specific spot.
Address: 2-114 McKenzie Crescent, Southern Gulf Islands, BC V8L, Canada
Disclaimer: this article is not a substitute for safety training, and is not meant to teach you how to dive alone. Always dive with a buddy, and train with a suitable instructor.
What prompted me to write this was an anecdotal shallow water blackout story. Shallow water blackout is the result of insufficient oxygen delivered to the brain. If this happens without a diving buddy it will nearly inevitably lead to death. The chances of a diver surviving with a trained buddy and good safety protocol is close to 100%. The anecdotal evidence of freediving blackouts is nearly always the same: 1) the freediver does not see it coming, 2) it can occur after a dive to any depth, 3) it can occur after a dive of any duration. Shallow water blackouts are common with other breath hold activities too, such as synchronized swimming and underwater hockey and have in some cases led to death. Always remember that if your airways are submerged, you can drown. Two inches or twenty meters of water feel the same to an unconscious person.
Minimizing the risk of a shallow water blackout
Hyperventilation used to be common practice in freediving in order to reduce the urge to breathe during freediving. Hyperventilating has the effect of removing CO2 from the body. Removing CO2 from the body has several effects: 1) it will take you longer to become hypercapnic. This means that it will take you longer to accumulate enough CO2 to feel the urge to breathe in the form of contractions and so on. 2) It makes your blood more alkaline. This has the scary effect that hemoglobin, the red blood cells that transport oxygen, are more prone to hold on to their oxygen. Think of this for a second. A freediver hyperventilates to not feel an urge to breathe, causing them to think everything is ok, while their blood does not deliver oxygen to the necessary tissues as effectively anymore. Sounds like a recipe for shallow water blackout, right? Hyperventilating is therefore a big ‘no’. Some courses do however teach their students to do a specific amount of ‘purging’ breaths, decreasing CO2 while still being safe. Although there is nothing wrong with this method if applied conservatively, it does open the door to abuse. Think of the little voice in your head that says ‘well, I can just purge a little harder and my dive will be more relaxed..’, or ‘If I just do 10 purging breaths instead of 8, it is probably still ok…’. I would personally argue that if you want to create the biggest safety margin, you stay away from any form of hyperventilation, including purging.
exerpt from the description:
“After consulting with some freediving instructors, I have realized that my breathe up wasn’t optimal and that instead of purging (which I thought I was doing), I was hyperventilating on my breathe up.”
Subconscious hyperventilation can be a big problem too. Here it helps to dive in only the best conditions; where you do not need to move at all during your breathe-up. Some studies indicate that exertion may cause hyperventilation, your body prepares anticipates a build up of CO2 and starts preventative measures (see here, and here for some open access material). If a diver is unaware of this, and even worse, adds some conscious hyperventilation or purging breaths into the mix, the risk of suffering shallow water blackout becomes very high even at shallow short dives.
Although I am unsure of the exact mechanisms, there is anecdotal evidence that exhaustion places a diver or swimmer at increased risk of blackout (see here). The data here does not so much apply to freedivers, as the statistical database may not be big enough to find such a correlation. However, all freedivers know that apnea capabilities change from day to day depending on mood, mindset, tiredness and so forth. It is definitely in your best interest to be well rested and in the right mindset before diving. Exhaustion may also occur during the dive session, for example if you need to do a long surface swim, or kayak paddle before diving, or if there is a lot of current or swell at the dive site.
Depth and the low oxygen zone
Partial pressure changes at depth make deep dives more dangerous. Here is a quick recap on how this works: gases react with your body in accordance with their partial pressure, not their absolute concentration. Let’s consider a 30 m dive. At 30 meters, the oxygen in your lungs will react at 4 times the rate (because you are at 4 atmosphere). This means that if you are at 30 meters until you feel like you have 16 % oxygen left, you have an absolute amount of oxygen left of 16% / 4 = 4%. When you ascend the partial pressure will become lower and the chemical balance between the oxygen in your blood and the oxygen in your lungs start to change. In the low O2 zone oxygen will move back from your blood into your lungs. Exactly where this happens depends on a set of variables like the O2 consumption of the diver and the depth. Since most blackouts happen from 0 – 10 m (for the reasons described above) we consider 0 – 10 m the anecdotal low O2 zone. But how do you know what depth you can dive to on a specific day? You don’t, until you hit the limit, which you don’t want to. In a recreational session where you do not have a line or if you dive without a buddy (or with a buddy in compromised conditions such as poor visibility), you will need to take baby steps toward a maximum depth that still contains a large safety margin.
Carbon dioxide accumulation
CO2 accumulation can be as large a risk as a CO2 deficit. The difference is that CO2 accumulation comes from an external CO2 source. There is no confirmed story of a blackout in which CO2 accumulation played an integral role, but because it can potentially cause blackout I have chosen to include it. CO2 accumulation can be caused by standing in traffic, diving next to a powerboat with a faulty engine or even diving next to a big idling boat with the wind in the wrong direction. If you are breathing in exhaust fumes you are likely breathing in air that has a lower O2 concentration and a higher CO2 concentration than you want to. In my case, I was concerned about this when I was diving in the Maldives with only a surface safety and I could occasionally ‘taste’ the garbage burning facility through my snorkel. Too much pre-dive CO2 does the same as an accumulation of CO2 during the dive: it kicks O2 off the red blood cell. This is good when you are diving (generate more CO2 and you release more O2) but pre-dive it means you start with less O2 upon descent. Even worse is that you may not notice it at all.
Entanglement is a huge hazard when freediving without a buddy, or when diving in poor visibility. Of course it does not directly increase your chances of a shallow water blackout, but it does so indirectly. If you can get loose, it will have increased your dive time and commonly increase your heart rate and oxygen consumption. I have been entangled once and did not really enjoy the experience (saved by my diving buddy!). The risk increases when spearfishing around uneven bottoms or in kelp forests. You can partially mitigate the risk by making sure that anything that can get tangled can either be cut (wear a knife) or ditched easily. Make sure that you can ditch your speargun, and can reach your knife easily. I like wearing a knife on my upper leg, so that I can reach it with either hand.
In part 2 we will get into the details of safety protocols, when they work and when they don’t. Let me stress again that this article is not meant to teach you how to dive alone. Instead it is written in order to create an awareness of risk in freediving, and an idea of how to mitigate that risk while freediving. Always dive with a buddy.