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.

Don’t fight the water

People sometimes ask me what I think of Total Immersion. A full discussion is beyond the scope of this post, but suffice to say: While I may quarrel with a few of the details, I think it’s general emphasis on “harmony with the water” is quite valid – and its validity increases with swim distance.

T.I. coaches teach their students to not “fight” the water. Beginning swimmers often fight the water (almost by definition), but advanced swimmers aren’t immune. I often catch myself doing this when I’m fatigued and trying to hold a pace slightly beyond my comfort zone. I’ve paid much more attention to not fighting the water since I started doing marathon swims. You might be able to get away with fighting the water in a 50, or even a 200, but in a marathon this is death. A relaxed, efficient stroke is essential.

On days when I’m not feeling so hot, I try to forget about going fast and just focus on relaxing and swimming efficiently. If I’m working out with a team, this may require slight adjustments to sets.

For example, say the coach assigns a descend set – 4×200 descended 1-4. Instead of trying to go faster on each 200, I’ll try to hold the same pace on each one, but with progressively less effort. The only way to hold pace constant while using less effort is to become more efficient. Incidentally, I think these types of sets are useful as a warm-up to a long swim – or during the few days leading up to it.

Increase effortlessness, not effort

I want to expand for a moment on the concept (discussed in the previous post) of increasing effortlessness rather than effort – within a set and over the course of a taper.

In a typical swim taper, in which athletes are preparing for events of 100 or 200m (or at most 1500m), it’s common to gauge the taper’s progress by monitoring pace times in practice. Over the course of a taper, a swimmer’s times on “pace swims” of 50 or 100m will tend to get faster.

In races of more than 30 minutes (~1.5 miles), however, it becomes less important to hit specific pace times than it is to modulate effort. This is especially true of swims 10K and longer (2+ hours).

That’s why, in preparing for tomorrow’s 10K, I’ve focused less on swimming a faster pace, but on how much effort I’m expending to swim a given pace. That’s what I mean when I say: Don’t increase effort (to swim faster), but rather, increase effortlessness (to swim the same speed with less effort).

Pick a pace time – in my case, let’s say 1:15 per 100m. In the middle of my training cycle, it might require 80% effort to swim a set of repeat 100s at this pace – even more with short rest. But by the end of my taper, I should be able to swim this pace relatively “effortlessly” – perhaps 65-70% effort – in other words, the effort I can maintain for the 2+ hours of a 10K.

As I’ve said before, if you can swim effortlessly, the pace will take care of itself.