The mammalian dive reflex (MDR) is part of our evolutionary heritage that all humans possess to some extent. In extreme situations such as near drowning it helps by increasing our survival time, but normally in most people it is a reflex that lies dormant. Though dormant it, like most reflexes, can be honed and developed.
Though the MDR is not used by most people on a daily basis, such an important evolutionary development is unlikely to be random. There is a strong argument that at one point in our history the survival of the species was helped greatly by our ability to dive in the worlds oceans. But for now what interests us is that this evolutionary reflex can be strengthened with the right training techniques.
Until relatively recently the more dramatic effects of MDR in humans were unknown, with doctors forecasting certain death for anyone breath-hold diving to a depth that would pressurise the lungs to less than residual volume. Nowadays MDR in humans has been studied and the main processes that allow free-divers to reach depths below 200 metres are somewhat better understood. Bradycardia, vasoconstriction of the extremities and preferential blood shift are the three main processes that allow a human to survive such incredible depths. Contraction of the spleen also helps by providing more 02 rich blood for use by the vital organs. Together these processes constitute an evolutionary muscle that, through disuse, has become somewhat weak in the average person.
One exception are breath-hold divers, whose diving reflex is significantly more effective than the average persons. It has been observed that some high level free-divers can cause vasoconstriction (and not just bradycardia) by simply performing surface static apnea. It is clear the reflex can be developed through repetitive training. The world records speak for themselves.
There are various methods to strengthen the MDR. Without doubt the most effective is actually free-diving, within safe limits, to progressively deeper depths where the blood shift will come into play. Repetitive deep diving increases the efficiency of the dive reflex, causing increased bradycardia and a more effective blood shift. It also develops the flexibility of the diaphragm, lung tissue and thoracic cavity.
Obviously it is not always possible to train in such depths. To counter the lack of depth a very simple technique is used. Instead of diving with a full inhaled breath, the last breath is exhaled to varying degrees so that the diver is diving to pressure with less than full lungs. This technique reduces the starting lung volume which means that residual volume is reached before it would normally be by full lung diving. For example if a person makes a full exhalation before submersion, then residual volume can be reached in only a few metres of depth. Exhale diving carries its own risks; there is an increased chance of lung squeeze due to the increased pressure and lower levels of CO2 mean less warning time. In the case of a black out, the victim will often be negatively buoyant, so they will sink and there is increased chance of water inhalation. These factors mean that exhale diving should considered an advanced training technique and only practiced under expert guidance.
Variations of less than full lung diving are used both as training itself and as warm-up practice for deeper dives. In this way divers get their juices flowing (literally) before actually going deep. For example; a diver planning on a full lung dive to 40 metres might practice a half lung dive to around 15 metres, followed by another half lung to around 23 metres. In this way the body is prepared for the last depth by effectively re-creating the lung pressure at a lesser depth.
Another technique is Negative Pressure Static Apnea (NPSA). This is used sometimes as a warm up technique before doing the exhale dives. Simply put, the diver exhales passively and descends to a metre and a half in depth. There he waits for the first two or three abdominal contractions and then ascends to the surface where he inhales, exhales and inhales once again. After passively exhaling the second breath he descends again to a metre and a half. This can be practiced for around four or five minutes, just as you jump in the water to start the days diving.
Over the last few years it has become clear that some people regardless of training have a stronger MDR than others. Genetic disposition or natural ability aside, these techniques, learnt under expert guidance can the help everyone, to some degree, re-discover their ‘aquatic heritage’.