Cold Shock Response and the Mammalian Diving Reflex in cold water swimming – Positive & Negative Feedback systems

I read a blog recently about cold immersion and cold baths, and cold swimming to a lesser extent. The author was speaking about the positive physical and mental benefits of regular ice baths. Similar benefits to what we as cold water swimmers regularly experience. All well and good. For aspirant Channel swimmers without access to regular cold water swimming, the recommendation for cold showers and baths is old and trusted.

However in his explanation of what was happening in the body the author focused exclusively on the Mammalian Diving Reflex as the primary response of the body when being immersed in cold water and completely ignored or didn’t understand the effect of Cold Shock Response and its place in the equation. (I didn’t save the blog link, sorry.)

That blog wasn’t the only place you see this. If you search on Mammalian Diving Reflex you will see it widely referred as the (only) process  in action when people are immersed or submerged in (cold) water. It’s a classic example of people taking all their knowledge from Wikipedia, because it seems the same Wikipedia core text is used all over the place.

I’ve covered both before and as a cold water swimmer, rather than someone sitting into a cool water bath, and I’ve focused as  much on Cold Shock Response, and the issue of Habituation, the process of getting used to getting into cold water, (not the process of staying in it).

The blog author just cut and pasted a Wikipedia article on Mammalian Diving Reflex, and while the Wikipedia article wasn’t wrong, both it and the blog were incomplete from the perspective of a  cold water swimmer.

So what are each of these and do they interact?

While swimming in Tramore Bay the other day, the water having risen by a degree in two weeks to about 7.5 Celsius, I got thinking about these two responses and how one, Cold Shock Response could be considered a positive feedback system while the Mammalian Diving Reflex could be considered a negative feedback system.

It’s not the first time I’ve wondered about Positive Feedback in a biological sense in open cold water swimming. Previously I considered that the Habituation/Acclimatisation process in cold water swimmers could also be a positive feedback system.

The simple process of improving cold ability

The simple process of improving cold ability

In Systems Theory (and elsewhere) a Positive Feedback System is where a small change causes a further bigger change. Therefore positive feedback is often considered a de-stabilising process. One example might be the international banking system that led to the 2008 collapse: Increased risks led to larger profits which led to larger risks until the system collapsed. However positive feedback can also be a process for change or improvement: If you swim more, you get fitter and able to swim even more, i.e. the training effect. Or the more you get in cold water, the better you get at getting into cold water.

Negative Feedback is often considered a stabilising process, the most common example is a thermostat which regulates heats by switching off when it gets too hot, switching on when it gets too cold: Negative Feedback acts in the opposite direction to the initial impulse.

Cold Shock Response is the bodies response to sudden cold water immersion. It results in varying degrees according to the person’s habituation experience, primarily in elevated heart rate, and elevated stress hormones. It is the elevated heart rate which is dangerous, to lesser extent in the increased chance of cardiac arrest, but more commonly in the chance of aspirating water due to shock and subsequently drowning. Less habituated or experienced swimmers will note an increased heart rate and nervousness even before immersion occurs if they are expecting the cold. I noted some years ago that the first time I ever swam during winter without a wet-suit, I was literally terrified beforehand. Then the initial cold shock drives the heart rate higher. This is a limited example of Positive Feedback, where the initial is destabilised by something (cold) that acts on the input.

The Mammalian Diving Reflex is another innate biological response to immersion. As the name implies all mammals exhibit this, human to weaker extent, but it exists to extend the time that animals can survive while submerged by reducing the need for respiration. This occurs in swimmers through two main biological reactions; decreasing heart rate (brachycardia) and therefore slowing the buildup of carbon dioxide in the body, (as excess carbon dioxide is what cause us to have to breathe); our constant companion, peripheral vaso-constriction, where the capillaries and blood flow in the extremities is restricted to allow more oxygenated blood to be available to the heart and brain.

The Mammalian Diving Reflex is initiated when the fact is submerged, and this is the reason I have previously written many times that you should splash your face before getting in the water, rather than the incorrect but widely cited slashing water down your back.

The Mammalian Diving Reflex is obviously a case of Negative Feedback, where the body reacts in opposition to submersion to protect itself.

So we can see that there is both positive and later negative feedback in operation in cold water swimming, where the negative feedback occurs to stabilise and protect a human through adaptive physiological response. But the initial negative feedback of Cold Shock is very significant and should not be ignores, as so many non-cold water writers seem to do, as it carries its own significant risk factor.

The bottom line though, is that this is another way of saying, that ability in cold water swimming improves with repetition. Habituation improves much more quickly than Acclimatization. In a little as four to five repeats, people become much more comfortable with getting into cold water.

Now get out there!


9 thoughts on “Cold Shock Response and the Mammalian Diving Reflex in cold water swimming – Positive & Negative Feedback systems

  1. Thinking about the effects of the two, I’ve started recent swims with facial immersion. It seems to make it easier to start swimming sooner. Is that psychological or physiological?


    • I think’s that’s personal Simon, as I prefer to start swimming immediately, but King of the Channel Kevin Murphy (more experienced than almost anyone ever) prefers to float and adjust first.


  2. Good point, Laura. I think I’m figuring that into my temperature difference conversion, but who knows. Donal will have to give us the official scoop on that.

    Currently, I’m swimming in a pool that’s very warm (water). If it’s breezy, I can take the warm water better as my arms are cooling with the wind on them on every stroke (freestyle). A trick I learned is to walk in the shallow end on such a day with my arms extended to the side, exposing more skin surface area. This cools me down.


  3. If you are comparing “the feel” of pool to sea, are you factoring in the air temperature? When the air around you is warm the pool feels colder, and also when swimming outside on a cool breezy day your skin gets colder just standing outside, so when you get into the water it feels comparatively warmer than usual.


  4. Hey Donal. I understand that the benefit of splashing water is so that the vagus nerve is alerted (and adapted) that there is cold water coming. And this is important because the vagus nerve plays a role in cardio respiratory mechanisms, including the mammalian diving reflex. So, by splashing water in the face, the mammalian diving reflex is allowed to recover and operate correctly. But my understanding of this reflex is that it allows us to stay in the water longer. So what I don’t understand is the statement that splashing water reduces the chance of aspirating water. I would think that splashing water can have a positive impact in extending our cold water exposure, but I don’t see the link that the vagus nerve and the mammalian diving reflex have with aspirating water. Can you help me out? Thanks a lot.


  5. Excellent post, here, Donal! I had not thought about it from a Positive/Negative Feedback System’s POV.

    And I’m wondering how much Age enters into this positive feedback. I’m finding myself simply not wanting to jump into cold water as the decades roll by. Psychology? Age-seeking comfort? Evolutionary pressure to keep older people around for their wisdom? 🙂


    P.S. Another potential blog post idea (couldn’t find it in your Search): “Temperature Differential: Pool vs. Sea” (or something like that). I’m sure many others have observed that 50C (60F) in a pool is MUCH colder than the same in the sea or ocean. I’ve also attributed it to salt content, but that’s probably too simplistic. Wanna “dive into” that one? 🙂


    • Thanks Harald. The age point is interesting but I wonder how much relates to your long superlative swimming career? Once you’ve achieved so much as you have, the incentive to torture yourself in cold water must wane. I regularly struggle with the period beforehand, indeed see the post I wrote last week on The Worst Three Minutes. However at my local spot, while I’m one of only two regular distance swimmers, every day all year round the polar bears swim, and they are all past retirement age. Certainly it’s the case here in Ireland that the year round daily dippers are almost all older and past retirement age, and they don’t moan about the cold as much as I do. The only day I’ve heard them complain this year was the coldest day of the winter so far.

      I was talking about the fresh/salt on Saturday with a friend when we heading down for a swim, as I only get to go swimming with someone a couple of times a year. Two reasons I’ve never done much with it, is because I rarely swim fresh water outside the pool, and the other is the difficulty in quantifying the difference. As you say, we all believe fresh water feels colder, and the density is the only explanation I’ve seen. I’ll add it to my list of things to think about for a post, and see if I can come up with something.


      • Well, now that you mention it, a lot of Polar Bear or Penguin swimmers (there are Penguin Swims, too!) are in the senior category, but I wonder if that’s because they have more time on their hands. Or nerve endings are dulled? I don’t know.

        Yes, I’m comparing ocean water to pool water primarily. My rule of thumb after swimming for many years off the coast of California is a rough 10-degree (F) differential of “perceived coldness.” As an example, the water off the beach of Los Angeles will usually hit its high mark around 70F in August or so. And it feels like warm bath water to most serious swimmers. But take those same swimmers into a pool with 70-degree water, and it feels downright chilly. Most serious pool swimmers I know prefer pool water temps in the 78-80F range.

        One might think it has something to do with the environment (sunny, gray, raining…), but it’s always sunny and beautiful in Los Angeles so I’m not sure not that’s it.

        Looking forward to your thoughtful ideas about this. FYI: I have never seen anything written on this topic. It’s more like ancient lore passed down through the generations of storytellers. Or lonesome swimmers. 🙂


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