Swimming the channel

How can someone swim the channel, four times in a row?
23 September 2019

Interview with 

Christof Schwiening


Ocean and an island


Sarah Thomas set a new world record this week when she became the first person to swim the English Channel four times over, non-stop. She was in water at 18 degrees Celsius and swimming constantly for fifty-four hours. Phil Sansom spoke to Christof Schwiening from the University of Cambridge, about how she accomplished it...

Phil - Just after dawn on Tuesday morning, Sarah Thomas staggered onto Shakespeare Beach in Dover and became the first person ever to swim the Channel four times non-stop.

Sarah is an American long-distance swimmer, 37 years old, and a survivour of breast cancer. Now, the year after she’s finished her treatment, she’s set a world record.

How did she do it? How did she bounce back from cancer therapy to spend more than two days in nail-bitingly cold water, swimming what was supposed to be the length of three marathons, but thanks to strong tides ended up being closer to five?

Well, it’s partly thanks to physiology.

Sarah’s trained for a number of challenges, the first of which is energy. She kept her calories up by drinking from a protein recovery drink every half an hour. But to use those calories most efficiently, she needs a certain type of muscle, called type 1 or slow twitch muscle. It’s red muscle, the kind that gives you dark meat from a turkey. Unlike type two fast-twitch muscle, slow-twitch muscle is really good for endurance, because it’s really good at using oxygen. It has a high blood supply, it has little molecules called myoglobin that take the oxygen out of the blood, and it has lots of mitochondria for turning that oxygen into energy. Perfect for long-distance exercise. And with training you can turn more of your muscle fibres into type one.

Phil - The second challenge is the cold. Christof Schwiening is a physiologist from Cambridge University who was pretty impressed with this feat.

Christof - If I were to fall into the English Channel,  I wouldn’t survive for more than about 1 hour or so. So, how is it that Sarah managed to survive for 54 hours in water close to 18 degrees C?  Well, what Sarah has managed to do is minimize the extent and consequences of the hypothermia she will have suffered during the swim.

Phil - She didn’t even wear a wetsuit - only a cap, goggles and a swimsuit! She had to have trained her body to avoid losing heat wherever possible.

Christof - The major adaptation that minimizes heat loss is simply keeping warm blood away from the cold water – that occurs through the restriction of blood flow to the skin and the presence of a relatively thick layer of subcutaneous fat.

Both of these are not unique to Sarah, but what Sarah seems unusually good at is maintaining a continuous high heat output even when her core body temperature has fallen to a level where most of us would be a shivering wreck. Part of that ability is her aerobic fitness – with an ability to continuously metabolize fuel.

But, she must also have managed to ‘train-out' her shivering response – shivering is counter-productive when swimming - and this is a well-known adaptation in people who are routinely exposed to the cold.

Phil - All this meant that she coped surprisingly well with the cold! She said the worst part may have been the salt water, drying out her mouth and throat. She even got stung by a jellyfish. And through all of it, she kept going.

Christof - So, I suspect that Sarah – physiologically  – is not that unusual. Sarah’s real strength comes from her ability to focus her mental effort on continuing to put one arm in front of another when even the most simple mental task becomes almost impossible. That requires practice and stamina that few are able to muster.


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