Sebastian Murat pioneered the theory and practice of exhale diving and posted a great deal of information about it on these forums. I have gone back and looked at those threads repeatedly, always find something I had missed.
I can no longer find some of those threads. If they are gone, it represents a great loss of information to all divers. Today there isn't any clear source to explain the theory or practice of exhale (FRC) diving. Below is my layman's attempt to summarize both theory and my kind of practice. My apologies in advance for any errors or omissions.
What is exhale (FRC) diving? Freediving with substantially less than a lung full of air. FRC is a specific definition, the amount of air in the lungs following a passive exhale. It is not a good measure to use in exhale diving because it varies so much among divers and with the conditions the diver is in. In some FRC is nearly the same as empty. In others it is around 40 percent.
Most exhale divers, no matter what they call it, dive with around 50 percent of a lung full, somewhere in the 40-60 percent range. Exhale diving also requires a certain style that uses as little energy as possible during the first half of the dive, allowing time for DR to set in hard, best combined with a breathup that minimizes ventilation and exertion, maximizes relaxation and eliminates hyperventilation.
What it is not:
This will spark some disagreement, but exhales for training purposes are not exhale diving. They are training for depth and warm up for whatever kind of diving you are doing. These exhales are usually full exhales, although not always. They speed up the aclimitization process so longer and deeper dives are more comfortable. They do not take advantage of physics like exhale diving, nor do they(when used with full lung diving) have the long term physiological effects of exhale diving.
Why does it work?
It adapts freediving to take advantage of the physics of the dive and the physiology of the dive response (DR). By doing so, it allows much more efficient use of the O2 available to the diver and reduces the negative impacts of c02. The difference in both is more than enough to make up for lower lung volume, 02 and co2 storage capacity.
How does it work?
Physics: Diving with half a lung full(compared to full lung) implys that the diver is less buoyant at the surface, that there is less buoyancy change as he descends and the the depth of neutral buoyancy is less.
In practice, the exhale diver uses much less energy (and o2) to make the same descent. As an example, full lung divers doing 30 m dives are very buoyant at the surface and should be neutral at 10 meters. They have to swim down hard, fighting buoyancy to reach 10 m and continue kicking lightly to 20 meters on the way to a 30 meter dive. A half lung diver would be neutral about 4 meters, a good surface dive and two soft kicks is all he will exert to reach the same depth. The difference in exertion, 02 consumption and c02 production is large. At depth, because buoyancy change is so much less, both divers will be about the same level of negative buoyancy. The swim up takes roughly the same level of exertion until the divers are above10 meters. From there up the exhale diver will be exerting himself more, but the difference is not great and, so late in the dive, it has little if any effect on o2 concentration in the brain.
Physiology: Blood shift is the primary aspect of DR that makes exhale diving work. Blood shift occurs when the peripheral arteries are contracted while the peripheral veins remain open. The result is minimal blood flow to the skeletal muscles and collection of blood volume in the core, primarily the lung blood vessels. Remaining O2 in that blood and in the lungs is effectively reserved for the brain and heart. At that point, exertion of the skeletal muscles in the legs and arms occurs in a low O2 environment and exertion shifts from aerobic to anaerobic, further conserving O2.
Blood shift is promoted by many factors including rising co2 levels, small lung size, negative pressure in the lungs, low levels of exertion and relaxation. All of these are promoted or more effective during a correctly executed exhale dive. The result is a much faster onset of blood shift and a much stronger shift, conserving 02 for the brain and heart.
The combination of taking advantage of the physics and blood shift can more than make up for the lesser quantity of O2 in the lungs of an exhale diver.
To put it all together, comparing full lung to exhale diving:
In full lung diving, at the onset of the dive there is plenty of O2 and minimal co2 in the lungs, the body doesn't know it needs to conserve o2, Further. most full lung divers breathup is less relaxed,more aggressive and energy intensive than an exhale diver,.so their burn rate of 02 (and co2 production) is higher. Swimming down against full lung buoyancy burns more O2 before the body realizes it needs to conserve and the c02 from that exertion flows easily and quickly back to the core where sensors in the chest feel the surge of co2 and finally begin conservation efforts, blood shift, etc. But its a little late, much(or more) of the extra o2 in the lungs has been burned. Worse, the co2 from that burn has gotten back to the chest sensors, making the diver uncomfortable, cutting the dive short unless the diver is capable of resisting a strong urge to breathe, which puts him at risk of BO because, at this point the full lung diver has less 02 available to the brain than the exhale diver.
Contrast this with a half lung exhale diver who has a very minimal breathup such that the co2 level in his blood is slightly elevated relative to most full lung divers. He is much more relaxed and has spent much less energy breathing up. He dives and descends much more relaxed with much less energy expenditure. Therefore, his rate of c02 production(and 02 burn) is much lower, both at the onset of the dive and thereafter. Increasing pressure (and decreasing lung size) plus the slightly higher starting c02 level aided by greater relaxation kick off blood shift faster and harder, conserving o2 in the core and readying the skeletal muscles to operate in primarily anaerobic mode once the divers begins to exercise. Once the exhale diver begins to exercise, blood shift has reduced the degree to which co2 and other waste products can return to the core, muscles operate at near anaerobic levels, using less o2 even though effort may be higher than the full lung diver as he begins his ascent. Heart rate slows, using less core 02. Co2 level that started out slightly elevated rises much more slowly than in a full lung diver, keeping the exhale diver comfortable for a longer period, extending dive time. O2 conservation allows that extra time to be safe with less chance of a BO.
I can no longer find some of those threads. If they are gone, it represents a great loss of information to all divers. Today there isn't any clear source to explain the theory or practice of exhale (FRC) diving. Below is my layman's attempt to summarize both theory and my kind of practice. My apologies in advance for any errors or omissions.
What is exhale (FRC) diving? Freediving with substantially less than a lung full of air. FRC is a specific definition, the amount of air in the lungs following a passive exhale. It is not a good measure to use in exhale diving because it varies so much among divers and with the conditions the diver is in. In some FRC is nearly the same as empty. In others it is around 40 percent.
Most exhale divers, no matter what they call it, dive with around 50 percent of a lung full, somewhere in the 40-60 percent range. Exhale diving also requires a certain style that uses as little energy as possible during the first half of the dive, allowing time for DR to set in hard, best combined with a breathup that minimizes ventilation and exertion, maximizes relaxation and eliminates hyperventilation.
What it is not:
This will spark some disagreement, but exhales for training purposes are not exhale diving. They are training for depth and warm up for whatever kind of diving you are doing. These exhales are usually full exhales, although not always. They speed up the aclimitization process so longer and deeper dives are more comfortable. They do not take advantage of physics like exhale diving, nor do they(when used with full lung diving) have the long term physiological effects of exhale diving.
Why does it work?
It adapts freediving to take advantage of the physics of the dive and the physiology of the dive response (DR). By doing so, it allows much more efficient use of the O2 available to the diver and reduces the negative impacts of c02. The difference in both is more than enough to make up for lower lung volume, 02 and co2 storage capacity.
How does it work?
Physics: Diving with half a lung full(compared to full lung) implys that the diver is less buoyant at the surface, that there is less buoyancy change as he descends and the the depth of neutral buoyancy is less.
In practice, the exhale diver uses much less energy (and o2) to make the same descent. As an example, full lung divers doing 30 m dives are very buoyant at the surface and should be neutral at 10 meters. They have to swim down hard, fighting buoyancy to reach 10 m and continue kicking lightly to 20 meters on the way to a 30 meter dive. A half lung diver would be neutral about 4 meters, a good surface dive and two soft kicks is all he will exert to reach the same depth. The difference in exertion, 02 consumption and c02 production is large. At depth, because buoyancy change is so much less, both divers will be about the same level of negative buoyancy. The swim up takes roughly the same level of exertion until the divers are above10 meters. From there up the exhale diver will be exerting himself more, but the difference is not great and, so late in the dive, it has little if any effect on o2 concentration in the brain.
Physiology: Blood shift is the primary aspect of DR that makes exhale diving work. Blood shift occurs when the peripheral arteries are contracted while the peripheral veins remain open. The result is minimal blood flow to the skeletal muscles and collection of blood volume in the core, primarily the lung blood vessels. Remaining O2 in that blood and in the lungs is effectively reserved for the brain and heart. At that point, exertion of the skeletal muscles in the legs and arms occurs in a low O2 environment and exertion shifts from aerobic to anaerobic, further conserving O2.
Blood shift is promoted by many factors including rising co2 levels, small lung size, negative pressure in the lungs, low levels of exertion and relaxation. All of these are promoted or more effective during a correctly executed exhale dive. The result is a much faster onset of blood shift and a much stronger shift, conserving 02 for the brain and heart.
The combination of taking advantage of the physics and blood shift can more than make up for the lesser quantity of O2 in the lungs of an exhale diver.
To put it all together, comparing full lung to exhale diving:
In full lung diving, at the onset of the dive there is plenty of O2 and minimal co2 in the lungs, the body doesn't know it needs to conserve o2, Further. most full lung divers breathup is less relaxed,more aggressive and energy intensive than an exhale diver,.so their burn rate of 02 (and co2 production) is higher. Swimming down against full lung buoyancy burns more O2 before the body realizes it needs to conserve and the c02 from that exertion flows easily and quickly back to the core where sensors in the chest feel the surge of co2 and finally begin conservation efforts, blood shift, etc. But its a little late, much(or more) of the extra o2 in the lungs has been burned. Worse, the co2 from that burn has gotten back to the chest sensors, making the diver uncomfortable, cutting the dive short unless the diver is capable of resisting a strong urge to breathe, which puts him at risk of BO because, at this point the full lung diver has less 02 available to the brain than the exhale diver.
Contrast this with a half lung exhale diver who has a very minimal breathup such that the co2 level in his blood is slightly elevated relative to most full lung divers. He is much more relaxed and has spent much less energy breathing up. He dives and descends much more relaxed with much less energy expenditure. Therefore, his rate of c02 production(and 02 burn) is much lower, both at the onset of the dive and thereafter. Increasing pressure (and decreasing lung size) plus the slightly higher starting c02 level aided by greater relaxation kick off blood shift faster and harder, conserving o2 in the core and readying the skeletal muscles to operate in primarily anaerobic mode once the divers begins to exercise. Once the exhale diver begins to exercise, blood shift has reduced the degree to which co2 and other waste products can return to the core, muscles operate at near anaerobic levels, using less o2 even though effort may be higher than the full lung diver as he begins his ascent. Heart rate slows, using less core 02. Co2 level that started out slightly elevated rises much more slowly than in a full lung diver, keeping the exhale diver comfortable for a longer period, extending dive time. O2 conservation allows that extra time to be safe with less chance of a BO.
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