Hold your breath-part4
How Long Can You Really Hold Your Breath-Part4?
Part 4 - Muscle Power
All the posts in this series relate to the Freedive-Earth Maximum Breath-hold Calculator which is available here for you to download for free. Once again, remember that any breath-holding activity in water is potentially dangerous. Never, Ever Freedive Alone. Amazing though it may seem, deep diving mammals, in particular seals and whales, usually dive on an exhale because that's the only way they can avoid getting bent by the transfer of excess nitrogen from the lungs into their tissues. The oxygen stored in their muscles is what allows them to dive for long periods without needing a reservoir of oxygen in their lungs. The pigment responsible for this amazing feat is myoglobin, which is also present in human muscle, though in much lower amounts. Although it's puny in comparison to whale myoglobin, human myoglobin still stores a large amount of oxygen, similar, in fact, to the amount stored by haemoglobin. Spaan (1991) quotes 1.34ml/g for myoglobin compared to 1.36ml/g for haemoglobin. It's relatively abundant, too: Laboratory studies on cadavers as well as living people find concentrations as high as 2.2g/100g in skeletal muscle (Nemeth & Lowry 1984), (Åkeson et al 2009). Current thinking is that it's used in human physiology as a relay protein: It has a very high affinity for oxygen, even at low partial pressures and so will only give up oxygen to tissues when they really need it. For this reason it works really well as a temporary oxygen store, close to hard-working tissue: It readily takes up oxygen from haemoglobin and dissociates rapidly, giving it up quickly to tissues that become very hypoxic.
Calculating Lean Muscle Mass
Whether it's a relic of an aquatic past for us, or more to do with the need to run long distances in search of food, myoglobin is certainly useful as a source of oxygen during a long breath-hold. In a second we'll work out the oxygen carrying capacity of our example diver but first we need to know roughly how much muscle she has. Like most things in physiology, the muscle mass of a person (and much more so the myoglobin content) depends on a large number of things.
The 'average' man has something like 42% muscle and the 'average' woman 36%, but lots of weight training is clearly going to make a difference to both of them, as is a lot of sitting on the sofa eating doritos. Lean muscle mass is difficult to work out because, although body fat percentage can be worked out using skin calipers, the remaining weight of muscle is still difficult to separate from the weight of bone. If you want a more accurate number, you might want to get one of those fancy weigh scales that does it for you with a small electrical current. Alternatively you could take an (educated) guess. The table below has some suggested values for different body types Body Type Muscle Mass (Men) % Muscle Mass (Women) % Obese (BMI 30+) 32 23 Overweight (BMI 25-30) 34 25 Normal (BMI 20-25) 36 27 Athletic (BMI 20-25 and regular training) 41 32 Muscular (BMI 25-30 and regular training) 45 36 Skinny (BMI<20) 32 23 Without measuring directly, it's impossible to say for certain how much muscle our diver has but let's say she's been training hard, and has a BMI of 22. That puts her at 32% lean muscle mass. We know from the previous example that her weight is 60kg, so her muscle mass is 0.32 x 60 = 19.2kg. Of this, we know that at least 2.2g/100g contains myoglobin. If she's been at altitude for a prolonged period of time, it might be more than this (Nepalis, for example, have as much as 16x more myoglobin in their muscles than people who live at sea level.) But there's relatively little evidence that short periods of time at altitude make any difference to levels of myoglobin. So let's take the figure of 2.2g/100g. It's a conservative estimate anyway because the authors of the quoted study point out that all the samples they took came from people who had died at the extremes of old age and disease. A young fit person is likely to have more to go on. 2.2g/100g is the same as 22g/kg so all we need to do is multiply by her muscle mass to get a value: Myoglobin Content = 22 x 19.2 = 422.4g The oxygen carrying capacity of this myoglobin (if you remember the value from earlier) is 1.34ml/g, so the total oxygen carrying capacity of her muscles is something like 422.4 x 1.34 = 566.02ml. Not bad!
To my mind this illustrates the unequivocal advantage of building muscle for freediving. We'll see in the next section that it's not quite that simple, but for me, this solves a debate about whether it's better to be skinny, or to get in the gym and lift some weights. If our diver's body type were different, for example, with a muscle mass of only 23% that equates to a myoglobin content of 60 x 0.23 x 22 = 303.6g, with an oxygen carrying capacity of just 303.6 x 1.34 = 406.82ml. By staying out of the gym, our diver would have lost out on 159.20mls of oxygen, or about 40 seconds of breathhold, not to mention the added benefit she'd get from weighing a bit more too, (and so having much more blood) considering that she probably doesn't have much fat to lose. Finally, we're coming to the end of our journey. The last stage is to put together all 3 compartments to find out how long our diver can really hold her breath. I'm certainly holding mine.