Winter Open-Water Swimming: Does Cold-Water Habituation Help on Race Day?

For many triathletes, the first open-water swim of the year is the worst one. You wade in, the cold hits your chest and you can't catch your breath. Your heart races and the first few hundred metres are a struggle to breathe, let alone swim. Early-season races often start exactly this way.

Winter open-water swimming has grown hugely, and some triathletes now keep swimming outdoors right through the cold months, partly in the hope that spring race starts will feel easier. This article covers what the cold shock response is, what habituation research shows about reducing it, how long the effect lasts, and what it can't do. It also covers the risks, because cold water is a genuine hazard as well as a training tool.

TriForward's "15.0°C: The New Cold-Water Number" covers race temperature rules and wetsuits, and "Open Water Sighting and Race-Start Anxiety" covers the physiology of the race start. This piece is about the winter: whether regular cold exposure changes the response, and how to do it sensibly.

What the cold shock response is

When cold water hits your skin, especially your chest, cold receptors trigger a powerful reflex:

• A gasp, often followed by rapid, deep breathing you can't easily control.
• A sharp rise in heart rate and blood pressure.
• Difficulty holding your breath, which makes putting your face in the water hard.

The response peaks in the first 30 seconds or so and settles over the first few minutes. It's strongest in colder water and when the skin cools quickly. In open water, an uncontrolled gasp with your face submerged can mean inhaling water, which is why the first minutes are the most dangerous part of a cold swim.

A 2012 review by Shattock and Tipton in the Journal of Physiology proposed a further risk. Cold water on the face and breath-holding trigger the diving response, which slows the heart, at the same time as the cold shock response speeds it up. They suggested that this "autonomic conflict" may cause heart rhythm disturbances, and that some deaths attributed to drowning or hypothermia might be due to it, particularly in people with underlying heart conditions. This is a hypothesis built from existing evidence rather than a tested finding, but it's one reason to enter cold water slowly and keep your face out until your breathing settles.

Habituation: what the studies show

The first habituation study design

A 1998 study by Tipton, Eglin and Golden in the Journal of Physiology tested 15 people: a habituation group of 8 and a control group of 7. Everyone did a 3-minute head-out immersion in stirred water at 10°C, then another three days later.

• The habituation group did six further 3-minute immersions in between, in the same 10°C water.
• Their heart rate, breathing frequency and tidal volume responses fell significantly over the repeated immersions.
• The control group's responses didn't change.

The study was designed to test where habituation happens. The habituation group showed reduced responses even when a different part of the body was immersed, with the same skin cooling as the controls. That suggests the brain learns to respond less, rather than the skin becoming less sensitive. The abstract doesn't give the sex split.

How long does it last?

The most relevant study for triathletes is a 2000 study by Tipton, Mekjavic and Eglin in the European Journal of Applied Physiology. It included 12 healthy men: 8 in a habituation group and 4 controls.

• The habituation group did six 3-minute immersions in 15°C water, after two baseline immersions at 10°C in October.
• In the first 30 seconds of a 10°C immersion, breathing frequency fell from 47 to 24 breaths a minute, ventilation from 72.2 to 31.3 litres a minute and heart rate from 128 to 109 beats a minute.
• At 7 months, the responses were still significantly reduced.
• At 14 months, heart rate was still lower, but breathing frequency and ventilation had largely returned towards their original levels.
• The control group's first-30-second responses didn't change.

Inference: the breathing response roughly halved after only six short immersions, and the effect was still there 7 months later. A triathlete who swims outdoors regularly through winter may still have a smaller cold shock response at a spring or early-summer race. This was 8 men in a tank, though, and nobody has tested it at a race.

Cold showers

Not everyone can swim outdoors in winter. A 2005 study by Eglin and Tipton in the European Journal of Applied Physiology tested whether cold showers could do the same job. 18 people (13 men, 5 women) did two 3-minute immersions in 10°C water four days apart, with six cold showers in between, in one of three patterns.

• 10°C showers on the back, for 3 minutes or in a shorter back-and-front pattern, cut breathing frequency in the first 30 seconds of immersion by 21%, from 54 to 44 breaths a minute.
• Showers at 15°C didn't produce habituation of the breathing response.
• The heart-rate response to 10°C water was largely unchanged by the showers.

The authors concluded that how fast the skin cools is an important factor. Inference: cold showers are a partial substitute for cold-water swims. They need to be genuinely cold, and their effect was smaller than the effect of immersions in the 2000 study.

How cold, and how often?

Putting the three habituation studies side by side gives a few practical clues, though none was designed to answer these questions directly.

• The water needs to be cold enough. Immersions at 15°C produced strong habituation in the 2000 study, but showers at 15°C didn't in the 2005 study. Showers cool less of the skin than immersion, and the authors pointed to the rate of skin cooling as a key factor. Inference: brief immersion in water around 15°C or colder is likely to be enough, but a lukewarm shower isn't.
• A handful of exposures is enough to start. Six 3-minute immersions produced the effects in both the 1998 and 2000 studies. Nobody has tested whether 20 or 50 immersions add much more.
• The effect fades slowly. The 2000 study found reduced responses at 7 months. Its authors also noted that less frequent immersions shortened the duration of the response but didn't reduce its size. Inference: occasional top-up dips through late winter and spring are probably more useful than a big block of cold exposure in December followed by nothing.

Early-season races in cooler climates are often swum in water not far from the temperatures used in these studies, which is why wetsuit rules start to matter, as TriForward's "15.0°C: The New Cold-Water Number" explains. That's one reason the research is relevant to triathletes, even though the participants weren't racing.

The anxiety problem

Race starts aren't calm. A 2013 study by Barwood and colleagues in the European Journal of Applied Physiology tested whether anxiety changes the cold shock response.

• In the first experiment (11 people), participants did two 7-minute immersions in 15°C water. Before one, they were told, falsely, that the water would be 5°C colder. Anxiety raised peak heart rate from 124 to 136 beats a minute and increased ventilation.
• In the second experiment (10 people), participants first habituated with five immersions, then did one immersion with anxiety and one without. With anxiety, the heart-rate response was no different from the original, unhabituated response.

The authors concluded that anxiety increases the cold shock response and reduces the benefit of habituation. The abstract doesn't give the sex split. For triathletes, this matters: a habituated swimmer who is nervous at a crowded race start may lose part of the benefit. TriForward's "Open Water Sighting and Race-Start Anxiety" covers the race-start side in more detail.

What habituation doesn't do

It's easy to overstate this research. Habituation studies measure the first few minutes of immersion. They don't show:

• Faster swimming. No study has tested whether habituated triathletes race faster.
• Protection from hypothermia. The studies measured the initial response, not how quickly you cool over a long swim.
• Protection from swim failure. In longer cold swims, swimming efficiency can deteriorate progressively, as the Knechtle review below describes, and that isn't what habituation studies measured.
• Health benefits. A 2017 review by Tipton and colleagues in Experimental Physiology concluded that the evidence for different claims about cold water immersion varies, and that some remains at the level of anecdote.

A 2020 narrative review by Knechtle and colleagues in the International Journal of Environmental Research and Public Health concluded that winter swimming appears to offer benefits for healthy, experienced people who swim regularly and build up gradually. But for unaccustomed swimmers it carries a risk of death from the initial cold shock or from loss of swimming ability and hypothermia.

Making winter swimming useful and safe

The following suggestions combine the studies above. They're inference for triathletes, not a tested protocol.

1. Get medical advice first if you have any heart, blood pressure or circulation problems.
2. Never swim alone. Swim with others, ideally at a supervised venue, with a bright tow float.
3. Start in autumn and keep going. Habituation developed after only six short immersions in the studies. Regular dips as the water cools are gentler than starting in January.
4. Keep it short. The studies used 3-minute immersions. Habituation is about the first minutes, not long swims. Early winter dips of a few minutes are enough for this purpose.
5. Enter slowly and wait. Walk in, keep your face out and let your breathing settle before you start swimming.
6. Get out before you're very cold. Shivering hard, numb or clumsy hands and confusion are signs to get out.
7. Rewarm gradually. Dry off, dress in warm layers and have a warm drink. You can keep cooling after you get out.
8. Use cold showers as a fallback. They need to be genuinely cold, around 10°C, to have an effect on the breathing response.
9. Practise calm starts. Since anxiety reduces habituation, rehearse entries and the first few minutes at an easy pace.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 46-year-old triathlete with a May half-distance race in a lake usually panics in the first 200 m of early-season races.

• October: she joins a supervised lake group and swims twice a week as the water cools, in a wetsuit, building comfortable time in the water.
• November to February: she keeps going with short dips of a few minutes once or twice a week with friends, entering slowly and getting out before she's very cold. On weeks she can't get to the lake, she ends showers with a minute or so of the coldest setting.
• March and April: she swims in a wetsuit as the water warms, practising calm starts and short efforts after entry.
• Race day: she does a short warm-up swim, enters slowly and settles her breathing before speeding up.

Inference: she may have a smaller gasp at the race start. Whether that makes her faster hasn't been tested, but a calmer first 200 m is a reasonable goal in its own right.

Common mistakes

• Swimming alone. This is the most important rule in cold water.
• Jumping or diving in. Sudden immersion produces the biggest gasp.
• Staying in too long. Habituation comes from the first minutes, not from endurance swims in very cold water.
• Assuming habituation protects against hypothermia. It doesn't.
• Starting cold swimming in midwinter. Building up as the water cools is gentler.
• Driving home before you've warmed up. You may still be cooling.
• Ignoring heart symptoms. Chest pain, palpitations or feeling faint in cold water need medical attention.

Women and older swimmers

The 2000 habituation study included only men, and the 1998 and 2013 abstracts don't report the sex split. Only the shower study included women (5 of 18). Body size and body fat affect how quickly people cool, so smaller and leaner swimmers may cool faster. Older swimmers may have more cardiovascular risk factors. None of the studies tested whether habituation differs by sex or age.

What we don't know

• Whether habituation helps race performance. No study has tested it in triathletes or open-water races.
• How much exposure is needed to keep the effect through a winter, or whether more is better.
• Whether it works in a wetsuit, when much of the body is protected.
• How it differs in women and older athletes.
• How much race anxiety erodes it in real race conditions.

References

• Tipton MJ, Eglin CM, Golden FStC. Habituation of the initial responses to cold water immersion in humans: a central or peripheral mechanism? Journal of Physiology, 1998;512(2):621–628. doi:10.1111/j.1469-7793.1998.621be.x
• Tipton M, Mekjavic I, Eglin C. Permanence of the habituation of the initial responses to cold-water immersion in humans. European Journal of Applied Physiology, 2000;83(1):17–21. doi:10.1007/s004210000255
• Eglin CM, Tipton MJ. Repeated cold showers as a method of habituating humans to the initial responses to cold water immersion. European Journal of Applied Physiology, 2005;93(5–6):624–629. doi:10.1007/s00421-004-1239-6
• Barwood MJ, Corbett J, Green R, Smith T, Tomlin P, Weir-Blankenstein L, Tipton MJ. Acute anxiety increases the magnitude of the cold shock response before and after habituation. European Journal of Applied Physiology, 2013;113(3):681–689. doi:10.1007/s00421-012-2473-y
• Shattock MJ, Tipton MJ. 'Autonomic conflict': a different way to die during cold water immersion? Journal of Physiology, 2012;590(14):3219–3230. doi:10.1113/jphysiol.2012.229864
• Tipton MJ, Collier N, Massey H, Corbett J, Harper M. Cold water immersion: kill or cure? Experimental Physiology, 2017;102(11):1335–1355. doi:10.1113/EP086283
• Knechtle B, Waśkiewicz Z, Sousa CV, Hill L, Nikolaidis PT. Cold water swimming: benefits and risks: a narrative review. International Journal of Environmental Research and Public Health, 2020;17(23):8984. doi:10.3390/ijerph17238984