Swim efficiency and energy consumption

In the last post I bemoaned the lack of credible science about marathon swimming. One is reminded of the William Goldman quote about the movie industry: Nobody knows anything.

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, kilocalorie) 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.

13 thoughts on “Swim efficiency and energy consumption”

  1. Another factor of efficiency that occurs to me is cardiovascular fitness. Specificity of training tells us that a more highly trained muscle group uses fuel and oxygen more efficiently – you can do more work with less calories. Therefore you use less calories per time/distance, in spite of speed, than the 15 hour finisher because your more highly trained muscles are doing the work more efficiently than the 15 hour finisher, who I assume lacks in fitness as much as stroke efficiency.

  2. I’m not sure you can assume that the 15 hour finisher is less fit than the 9 hour finisher.
    I assume these numbers all change with age as well

    1. If fitness is defined generally, as VO2 max , then you’re right, you can’t assume Flavia is less fit than me. For all I know, she might be a champion marathon runner! It sounded like Jim was talking more about swim-specific fitness (“specificity of training”). I have to think such a thing exists, because my swim training never seems to have any effect on how much I huff and puff when I go hiking!

  3. Evan, didn’t you measure your calorie consumption for the Tampa Bay Marathon Swim by weighing yourself before and after and calculating your calorie intake during the swim? That seemed like a pretty good way to estimate it. To be more accurate, you’d have to weigh the food and liquid intake and measure urine output (which would necessitate getting out of the water).

    It would be interesting to do the simple version of the test for all participants in a marathon swim. Or even a 10K Postal Swim.

    1. Katie, good memory! I estimated (after waiting for my hydration to return to baseline) that I lost about a pound of fat during TBMS. It was far from a precise measurement, but seems to make sense: 1 lb fat = 3500-cal energy deficit + 2800 calories consumed = 6300 calories total energy expenditure. Divide by 9 hours = 700 cal/hr. The original post is here.

  4. I agree that “fitness” doesn’t really tell us very much here…also, what finishing time can tell us about swimming efficiency / fitness depends on which swim it is. In the English Channel, for example, this can reflect conditions as much as individual ability. But fundamentally, I agree that the treatment of calorie burn solely as a function of distance doesn’t make sense, since this is to confuse efficiency with intensity. By definition, the concept of efficiency (as opposed to intensity) presumes differential ratios between energy input and performance – a factor which is written out by attempts to map calorie burn directly onto distance. The more interesting question, then, for sports science is perhaps how to calibrate the relationship between swimming intensity and efficiency (especially when efficiency presumably relates not only to technique, but also to physiological processes such as metabolism).

    I’m reading all this with great interest as a slower marathon swimmer who is working hard to be a faster one, and finding the prospect of correcting my stroke inefficiencies far more palatable than the idea of working at a higher intensity than my habitual happy plod (probably Zone 2 in HRM terms).

    1. Karen, thanks for the insightful comment. Your point about conditions is well taken – and further reveals the absurdity of estimating energy expenditure solely as a function of distance & body weight.

      “How to calibrate the relationship between swimming intensity and efficiency” — This is, indeed, one of the primary challenges for any marathoner! Every swimmer becomes less efficient with greater speed, because of exponentially increasing drag. In terms of metabolic efficiency, the anaerobic threshold would seem to be the thing to pay attention to – i.e., always staying below it in a marathon swim, but at the same time raising it as far as possible through lots of training just above and just below it.

      I’ve been reading with interest your new push to refine your stroke technique, and am excited to see how it goes. I think this is absolutely the right order of priorities. Small technique improvements can lead to enormous speed improvements… who knows what you’re capable of!

  5. I read in that huge tome by Maglischo “Swimming Fastest” that Olympic swimmers spend 91% of their energy pushing water out of their way, and 9% of their energy going forward. That jives with Toussaint, Knops, De Groot, and Hollander.

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