Swim efficiency and energy consumption

In the last post I bemoaned the lack of rigorous science about marathon swimming.

Here’s a good example. A few days ago a Facebook friend linked to an intriguing-looking article. Published on a science-y looking website (“Your one-stop resource for longevity, health, exercise, nutrition, and scientific articles all to help you live a longer, fuller life”), the article is authored by marathon swimmer Don Macdonald.

One section seemed of particular interest: “Nutritional Demands of Open Water Endurance Swimming.” An excerpt:

Nutritional endurance demands biochemical changes of your body. The basic calculation for the amount of calories burned while swimming is 2.93 calories per mile, per pound. I weigh 207 pounds, and therefore burn 14,556 calories in a 24-mile swim, (2.93 calories x 24 miles x 207 pounds = 14,556 calories). You must also add 10-15 percent of your burnt calorie total for the energy it takes your body to keep itself warm. In this case, adding another 1,500 calories.

2.93 calories per mile, per pound. Really? How do you figure?

Does it seem likely that calorie burn depends only on distance, and not the time taken to complete the distance? If I swim 10 miles in 4 hours, does a slower swimmer who takes 7 hours burn the same calories as me, despite spending 3 more hours in the water (assuming equal body weight)? Do I burn the same calories in a 1500m warm-up as during a 1500m race?

Actually, there is a school of thought (with some scientific basis) that calorie burn is independent of speed/time in “animal locomotion” generally. As Wikipedia (referencing a 1973 Science paper) explains:

The most common metric of energy use during locomotion is net cost of transport, defined as the calories needed above baseline metabolism to move a given distance, per unit body mass. For aerobic locomotion, most animals have a nearly constant cost of transport - moving a given distance requires the same caloric expenditure, regardless of speed. This constancy is usually accomplished by changes in gait.

The idea is, calories are a measure of work - the work required to move a given body mass a given distance. Hence the common rule-of-thumb in running: 1 calorie (technically, _kilo_calorie) per kilometer, per kilogram. Running at higher speeds burns more calories, but this is counterbalanced by the reduced time taken to complete the distance. More recent evidence has complicated this view – showing differences in calorie burn between walking and running a given distance. For what it’s worth, though, many runners seem to think the rule-of-thumb comes pretty close.

But what about swimming? Is the “net cost of transport” constant, regardless of speed? Does 2.93 calories per mile, per pound make any sense, even as a rule-of-thumb? I’m inclined to say… no. The reason: Efficiency. Humans are very efficient walkers and runners - it’s what we’re evolved to do. An elite runner converts 90% of energy expended into forward motion - but even a recreational runner is about 80% efficient. (I assume Terry Laughlin got these numbers from science, but I’m not going to hunt for it.)

An elite swimmer, however, is only about 9% efficient And a novice swimmer is astoundingly inefficient – T.L. estimates 3%. Humans are pretty terrible swimmers, all considered.

It makes sense that the “net cost of transport” would be fairly constant on land - because humans efficiently convert additional effort into additional speed. In the water, however, most of our efforts are wasted. Water is both dense (compared to air) and unstable (compared to the ground). Even large increases in effort produce relatively small changes in speed. It would seem to follow, then, that the “net cost of transport” in swimming depends very much on speed! Moreover, skilled swimmers are much more efficient than unskilled swimmers - compared to the relatively small differences among runners. Sun Yang’s net cost of transport is less than mine, and my net cost of transport is far less than the average triathlete.

What else is wrong with 2.93 calories per mile, per pound? Let’s plug in some numbers. In the above quote from Don Macdonald, he uses a 24-mile swim as an example. That happens to be the same distance as the Tampa Bay Marathon Swim. Earlier this year I completed this swim in 8 hours, 59 minutes. Flavia Zappa, the last swimmer to finish, came in at 15 hours, 10 minutes (results link). Assume for the moment that we weigh the same.

If calorie burn is only a function of distance, that means Flavia and I each burned 11,251 calories (2.93 * 24 miles * 160 pounds). In Flavia’s case, that produces a not-unreasonable-sounding (but still high) rate of 742 calories per hour. But for me, 11,251 calories equates to 1,252 calories per hour. Not likely.

Isn’t it obvious that an efficient swimmer will burn fewer calories per mile than an inefficient swimmer? To believe a rule-of-thumb like 2.93 calories per mile, per pound, you essentially have to believe that there is no such thing as efficiency in swimming. Anybody who knows anything about swimming, of course, knows that swim speed is mostly about efficiency.

UPDATE 10/31: Karen raises a great point: Energy expenditure during a marathon swim will also depend on conditions (not just water temperature, as Don Macdonald mentions). Swimming through big swells, chop, and whitecaps will burn more calories than swimming across a glassy lake.

The second in a three-part series. See Part 1. and Part 3.

Posted 29 October 2011 in: science Tags: nutrition