Freediving is one of the fastest growing watersports today which has obvious links to the SCUBA industry, but it's not just as simple as holding your breath and swimming down as far as you can. There are about nine separate disciplines from static apnea, face-down on the surface timed, to No-Limits Apnea where divers can use any means including weighted sleds and inflatable bags to get far down quickly and return to the surface safely. Herbert Nitsch is the current, AIDA recognised, World Record Holder for a 214m No-Limits dive in Greece in 2007.
Freediving has been practiced for centuries all over the world in fresh water and around the coast and is still practiced today. Freediving would have started as a resource gathering method for food and other unreachable items that the sea produced. Since then it has progressed and technology has grown with it to push boundaries further to go deeper and stay underwater longer.
The main problem, that we all know, is the increase in pressure as you descend. The air spaces in your body reduce in size as you go down and need equalising, the bigger the air space, the bigger the change in volume. To equalise these air spaces you need to add volume by transferring gas from your lungs, or other air spaces, to equalise the pressure differential. The main air spaces that are affected are your face mask, sinuses and the middle ear. Because of this low volume masks are popular to reduce the amount of equalisation that is required. To equalise the sinuses some divers use the wet equalisation method by allowing water to flood the sinuses so there is no air space to equalise.
The increase in pressure can squeeze the blood out of your extremities and into your chest, called blood shift. This shift forces your blood supply around the lungs which, if put under enough pressure, will keep the pressure inside the chest high enough to allow the diver to go deeper without their chest collapsing. The problem comes when the pressure continues and blood can enter the lungs and create a pulmonary edema.
The changes in pressure affects the partial pressures of the gas in your system that can fool your body which I'll talk about later in Shallow and Deep Water Blackout
. The increase of pressure compresses the air in your lungs and increases the concentration or partial pressure of each of the gases.
Mammalian Diving Reflex
Humans have an evolutionary ace up our sleeves when it comes to breath hold diving; whenever our face comes into contact with cold water our body prepares itself to hold your breath for an extended period. These changes are exhibited strongly in marine mammals like seals and dolphins but it still exists in our programming. Infants up to the age of about six months will naturally hold their breath and move their limbs in a swimming motion when submerged, please don't test this theory out for obvious reasons though, but tend to lose the ability when they start to learn to walk.
Acting as a survival technique, when submerged, your body will slow your heart rate down to 10-25% of normal to lessen the need for Oxygen rich blood circulating around non-essential areas. Studies have shown that a seal's heart rate can drop from 125bpm to about 10, allowing them to stay submerged for longer periods. After your heart rate has dropped your blood vessels start to constrict in the extremities with capillaries restricting blood-flow to the toes and fingers before progressing to the hands and feet over time.
With the increase in pressure blood is pushed from the extremities to your chest, with all of your blood retreating from your limbs your muscles are very dependent on stored Oxygen in your muscles. The protein Myoglobin, similar to Haemoglobin in your blood, is responsible for storing Oxygen in your muscles to be used when needed. Marine mammals have a large amount of Myoglobin in their muscles, storing about 25-30% of their Oxygen supply in their muscles which gives their muscles a very dark colouration compared to terrestrial mammals. Humans have much lower amounts of Myoglobin in their muscles and can only store about 12% of the bodies Oxygen storage in the muscles.
Even with the Mammalian Diving Reflex freedivers are very susceptible to cramp in their muscle groups because of the limited Oxygen storage in their bodies.
Shallow Water Blackout
Some freedivers hyperventilate to flush as much Carbon Dioxide out of their systems as possible, but because of certain physiological reasons this can be quite dangerous. Your brain uses the level of Carbon Dioxide in your bloodstream to tell you when to breathe; high levels of Carbon Dioxide cause your body to want to breathe. The brain doesn't monitor the amount of Oxygen in your system which can cause issues just on the surface but this can be exacerbated when depth is added to the equation.
Shallow Water Blackout occurs when Oxygen levels drop below acceptable levels in the body before Carbon Dioxide levels trigger the urge to breathe, this can occur just under the surface or seconds after surfacing, it happens quietly and the diver does not feel it coming. Because of this backup and safety divers are always needed and it is required for record attempts to count the diver must remain on the surface unaided after their dive, if they pass out or the safety diver has to help then the attempt does not count.
Deep Water Blackout
The changes in pressure as you descend and ascend exacerbate the shallow water blackout effect. As you descent, as when SCUBA diving, the partial pressure of the gas in your lungs and blood stream increase meaning that more is absorbed into your tissues, so at 10m the concentration doubles which is fine. The problem comes when you metabolise some of that Oxygen and then ascend; at 10m your body thinks it has plenty of Oxygen but in the few seconds it takes to ascend to the surface the amount of Oxygen in your system has halved. If the new partial pressure is below hypoxic levels the brain will simply switch off.
The increase in partial pressure of Nitrogen has a narcotic effect; impairing judgement and concentration. This effect can be seen in one of Tanya Streeter's record attempts where she forgets a simple part of her lift bag routine. Because of this all deep dive attempts have multiple safety divers to ensure safety at all times.
Freediving looks incredibly simple with very little equipment and beautiful images but there's a lot more to it than most people think. The average person at rest can hold their breath for about 30 seconds... I'll wait while your try... but with even simple training you can extend that beyond 2 minutes. But before you start practicing in the pool by yourself just remember that there's a lot going on inside your body that you can't feel and unconsciousness in and around water is very dangerous. It is always important to receive proper training before you undergo any serious freediving to keep you and your buddy safe.