SubscribeIf you’ve known me for a while, you probably know that I used to be a very active technical scuba diver and instructor. I’d dive deep, and long, in dark waters where the shoreline wasn’t visible and hadn’t been for some time, this includes night dives and ice dives. It’s great experience that I bring to my triathlon training. Unlike so many triathletes, I have no fear of swimming where I can’t see the bottom, or even, for that matter, the shoreline.
A lot of triathlon training sites talk about what to do when you get to the site and are preparing for the swim: Things like stretching, doing some warm-up lengths, looking around at landmarks so that you can sight while you’re swimming, landmarking your bike in the holding pen, etc. All good stuff to be sure. But something I haven’t yet seen anywhere, is a discussion of “Mammalian diving reflex”.
Quoted from wikipedia, highlights added by me:
The mammalian diving reflex optimizes respiration which allows mammals to stay underwater for a long time. It is exhibited strongly in aquatic mammals (seals, otters, dolphins, etc.), but exists in a weaker version in other mammals, including humans… Every animal’s diving reflex is triggered specifically by cold water contacting the face – water that is warmer than 21 °C (70 °F) does not cause the reflex, and neither does submersion of body parts other than the face. Also, the reflex is always exhibited more dramatically, and thus can grant longer survival, in young individuals.
Upon initiation of the reflex, three changes happen to the body, in this order:
1 Bradycardia is the first response to submersion. Immediately upon facial contact with cold water, the human heart rate slows down ten to twenty-five percent… Slowing the heart rate lessens the need for bloodstream oxygen, leaving more to be used by other organs.
2 Next, peripheral vasoconstriction sets in… Toes and fingers close off first, then hands and feet, and ultimately arms and legs stop allowing blood circulation, leaving more blood for use by the heart and brain. Human musculature accounts for only 12% of the body’s total oxygen storage, and the body’s muscles tend to suffer cramping during this phase. Aquatic mammals have as much as 25 to 30% of their oxygen storage in muscle, and thus they can keep working long after capillary blood supply is stopped.
:
:When the face is submerged, receptors that are sensitive to cold within the nasal cavity and other areas of the face supplied by cranial nerve V (trigeminal) relay the information to the brain and then innervate cranial nerve X, which is part of the autonomic nervous system. This causes bradycardia and peripheral vasoconstriction. Blood is removed from the limbs and all organs but the heart and the brain, creating a heart-brain circuit and allowing the mammal to conserve oxygen.
In humans, the mammalian diving reflex is not induced when limbs are introduced to cold water. Mild bradycardia is caused by subjects holding their breath without submerging the face within water. When breathing with face submerged this causes a diving reflex which increases proportionally to decreasing water temperature. Activating the diving reflex with cold water can be used to treat supraventricular tachycardia. However the greatest bradycardia effect is induced when the subject is holding breath with face submerged.
In the swim leg of the race, you may experience diving reflex as the feeling of the inability to breathe, gasping for air, the closing of the throat, cramping and panic. Clearly something you want to avoid 🙂 Fortunately, you can prepare yourself for the shock and teach your body to handle it in a manageable way.
When we’re scuba diving in very cold water, we’ll submerse our faces (with our masks off) and force ourselves to breathe through the regulator or a snorkel to get past the bradycardia and autonomic response to hold your breath or breathe quickly and shallowly in response to the cold and dive reflex. This body training will last 30-60 minutes after the “familiarization training” and greatly reduce your body’s negative reactions to the cold water. In much the same way, you should spend a few minutes pre-swim preparing your body for the shock of the water temperature.
Start by standing at a comfortable depth, bend over and with your goggles off put your face in the water and relax. Breathe out with slowly with your face submerged and turn your head as though you’re free-style breathing to inhale. Try to breathe deep and steadily, using your diaphragm, pulling air deep into your belly. Continue breathing in this position until you feel calm and relaxed in your breathing and comfortably accustomed to the cold shock of the water and air.
By acclimatizing yourself to the cold water on your face (including around your eyes), you’ll be much less likely to suffer negative reactions of mammalian dive reflex during the race, especially if you somehow loose your mask during the swim.
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