– Apologies to those email subscribers who saw this earlier in the week and are seeing it now again.

For this post, let’s take all the usual pre-swim and first three minutes stuff for granted.

You’re in the water. The water is cold. And you are cold. We start there.

The water is about 10° degrees and you’ve been in for say 30 minutes (without a wetsuit). I’m picking 10° Celsius for a very specific reason, that most people will agree it’s either cool or cold, but many people will still start to stretch out their swimming times at that temperature. So 10°C and 30 minutes is a compromise. It may require colder water and longer times for some, and shorter times and warmer water for others.

The day is overcast and a little breezy. Therefore you are receiving no external heat input from the sun or the ambient temperature.

You have The Claw and can no longer touch your little finger to your index finger. You still have 500 metres to get back to your exit point and you feel you need to warm up. So you swim harder. You metabolise more ATP, some of which provides the energy to propel your arms and legs faster, and some goes to produce excess heat, which helps warms you slightly. But your limbs are really cold and it can’t warm you enough.

Luckily , you’ve been swimming in cold water regularly and you have built-up some brown fat (Brown Adipose Tissue, aka BAT)  on your shoulders and lower back. BAT doesn’t develop all over like ordinary fat but in those specific parts of the body. You don’t realise it, but the brown fat has also been burning calories specifically to provide heat for you, as unlike ordinary white fat, BAT is metabolically active. Ordinary white fat provides energy by being an ATP store and of course it also insulates. But BAT along with some insulation, burns ATP to produce heat, the blood flows through the BAT and warms up. This is known as NST, Non-shivering Thermogenesis. NST is not sudden, it starts when you are exposed to cold and the BAT is insulating and protecting you immediately, and also providing heat.

Maybe you’ve forgotten your watch, or made a distance mis-calculation or the tide is stronger than you realised, and result is the exit is still 1000 metres away. And while the BAT is useful it also consumes your energy reserves more quickly.

A kilometre is a long way when you are really cold. Now you can no longer touch your little finger to your index finger. Your fingers are fully spread. You can’t swim at the same rate you were and start to slow down. And then you get a little shudder. And then another. And then you are shivering in the water.

Shivering is the body’s last attempt to warm itself. Like with any exercise, like with all your swimming, not just at speed, your body is burning more ATP again in a desperate attempt to warm you up. Like all your metabolic processes this is an ancient evolutionary step, from the eons  before we had mastered clothes and heat, even before we’d shed our fur, cold nights on the African plains. And here are, us stupid swimming apes, voluntarily shedding our learned advantages and protections and stepping into a lethal environment where we no longer have a natural protection. And all we have left to protect us from death by hypothermia is a desperate last little biological process. A biological process that evolved … FOR LAND. Not for water where direct conduction of heat away from the body is 30 times the convection heat-loss rate of air. This isn’t the normal shivering we experience during Afterdrop, because then it is helpful for rewarming.

In water, shivering is dangerous and accelerates remaining energy and heat loss.

Shivering will not heat you in water. It will not protect you. Every experienced open water swimmer will tell you, that once shivering in the water develops you are in real danger. (And that’s excluding the fact that you were already in danger merely by being in the cold water to begin). This on our scale, where cold water itself is low down.

If shivering starts, get out of the water.

If you have started shivering you should have already been out of the water. If it happens when you are in the water, you need to get out immediately. If you are crew and your swimmer starts shivering forget about stroke counts or cognitive tests or anything else, and pull them out immediately.

Cold water swimming ability comes from experience. It’s not a talent. It is a skill. Skills can be learned. Part of the skill is developing the knowledge and experience to avoid swimming until you start shivering while still in the water. The learning process for some people (like myself) is facilitated by knowledge.