Open Water Swimming

An interesting blog in general, The Science of Sport, maintained by two athletes and sports scientists, explores a number of topics and questions related to sports physiology and science (see, for example, their ongoing series exploring the increasingly exotic technical suits, and the question of whether these confer an unfair advantage to the swimmers who wear them). For OW swimmers, of interest is their post on cold water exposure.

Note that water conducts heat about 25 times faster/more efficiently than air, and if the water is moving, or you are moving in the water (or both) that rate can as much as double.

There’s a whole set of involuntary physiological responses our bodies muster when exposed to cold, which the SofS authors detail. Of course, if you’re swimming hard, that helps warm you up, but at the same time (oh irony) increases even more the rate at which you lose heat to the water.

When at rest your muscle tissue actually acts as in insulator. This changes when you exercise because now you are pumping lots of blood to the working muscles, and it is the blood that transports heat around the body. Therefore when you start to swim in cold water you send more blood to the muscles, and all this does is increase your heat losses as now the blood—-and the heat it contains—-is close to the surface of the body and the cold-water. Since water conducts heat very well, the heat from your body readily moves to the water. . .and the consequence of this is a decrease in core temperature even though you are producing some heat with your muscle contractions.

It’s also interesting that cold water can significantly affect your swimming. As the SofS authors note: “a good swimmer in warm water will be an average swimmer in the cold. “

That’s a point that has implications not only for performance in early-season or cold water swims, but obviously for safety as well. Hypothermia can sneak up on you, and the colder you get, the less you might recognize how cold you are, because mental confusion is one of hypothermia’s symptoms.

What defines “cold” water? To some degree that seems to be a highly personal definition. Lynne Cox, author of Swimming to Antarctica, has trained herself to swim in the coldest waters on Earth, but she’s also ideally built for that kind of swimming (see video clips from her Antarctic swim here).

USMS Swimmer magazine had a piece in the Nov/Dec 2007 issue about acclimating to cold water swimming. Mentioned in the article were two San Francisco swim clubs that regularly swim in the Bay, where temps range from below 50 to low 60s year ’round.

When I swam the 1 mile “James River Splash” last summer (part of the James River Adventure Games–sign up for the 2008 Splash at SportsBackers.org), the water temp. in the James River was right around 70 degrees F, a temperature that plenty of open water swimmers would consider too warm. I completed the swim just fine, but when I got out, despite layering up in fleece, I was still feeling noticeably cold a half-hour later.

There’s a phenomenon known as “afterdrop”–it’s a particular hazard in cases of serious hypothermia–in which, as a cold person is warmed up, the body’s core temperature may actually decrease. What happens, if I can summarize, is that when exposed to cold, the body responds by restricting blood flow to the skin and the extremities. When your cold exposure ends (as when I got out of the river at the end of the swim), circulation increases again to the skin and extremities, which now may be several degrees colder (or in serious hypothermia, significantly colder) than your core (my fingers and toes went numb during that swim, demonstrating my body’s perhaps too efficient vasoconstriction response. ) The result is that the blood is cooled as it circulates to these colder parts of the body, and that colder blood returns to your core, lowering your core temperature. So just removing yourself from the cold environment does not mean you will warm up immediately, and in fact, because of afterdrop, the warming up period can sometimes be the most dangerous for someone who is severely hypothermic; as the colder blood circulates back to the heart, it can provoke irregular cardiac rythyms.

In the “Warm Up Swim” Lynne Cox video you can see how violently she is shivering after getting out of 40-ish-degree water, even though wrapped in blankets. In a different context, I experienced this when I was underdressed on a bike ride this winter. When we got to the end of the ride, I did feel cold, and my hands were almost completely useless–I couldn’t squeeze the brakes. But it wasn’t until we’d been indoors for several minutes that I began to shiver violently.

So for me, despite (or, as pointed out above, in part because of) the fact that I was exercising vigorously, swimming for about 30 minutes in 70-degree water resulted in a small but noticeable drop in my body temperature (involuntary shivering can begin when core temperature has dropped less than two degrees F). Some of those San Francisco swimmers mentioned in the Swimmer article, however, can manage to swim up to two hours in sub-50-degree waters. Again, physiology may be a factor–your body size does affect how well and how long you can retain body heat. But in addition, Tom Keller, President of one of the San Francisco clubs mentioned in the Swimmer story, says that new swimmers are encouraged “to acclimate themselves to the cold, beginning with a wetsuit and working their way to swimming without the protection of neoprene.” Also, “Most swim through the gradually declining temperatures of the fall season so they can be ready for their winter swims.”

Perhaps it’s time for a James River Polar Bears club?