Interesting bit of trivia about energy use during exercise, from a new paper in the European Journal of Applied Physiology by researchers in Portugal. We (i.e. endurance-oriented folks) normally think about two different types of energy production: aerobic and anaerobic. Aerobic is what dominates for longer, lower-intensity efforts, where your hearts and lungs are able to keep pace with the oxygen demands of your muscles. Anaerobic comes into play for shorter, higher-intensity efforts -- and we usually associate it with lactate, which is a by-product (as well as a fuel) in the reactions that produce ATP without oxygen.

But there's also a third component -- or rather, another subdivision of anaerobic energy that has nothing to do with lactate (i.e. it's both "alactic" and "anaerobic"): the phosphocreatine system. This is basically your body's "immediate" energy system: your muscles have a small stored supply of phosphocreatine that can be broken down to produce immediate energy. This is the dominant source of energy for short all-out sprints or lifting heavy objects; but once it's gone (on the order of ~10 seconds), it gone until you have a chance (during rest or lower intensity exercise) to replenish it. That means it's not relevant during long endurance exercise -- but it does play a role in middle distance races.

The Portugese study looked at 200-meter swim races (which for the elite athletes studied took an average of 1:52.7, so comparable to an 800-meter running race). Since it's very difficult and invasive to measure phosphocreatine directly (you need repeated biopsies), the goal of the study was to compare two methods of indirectly estimating the amount of energy contributed by this system. One was simply to make some assumptions about how much phosphocreatine is stored in muscle, how much muscle is used in swimming, and assume that the swimmers use up all the phosphocreatine in those muscles during the race.

The other was to measure the oxygen debt of the swimmers after they finish racing -- the assumption in this case is that all your huffing and puffing immediately after a hard sprint is focused on the body's top priority of repleneshing it's phosphocreatine stores; only after that's done does it turn to clearing lactate and so on. Here's a sample graph that shows oxygen consumption before, during, and after a 200-meter all-out swim (measured with a nifty "low hydrodynamic resistance" snorkel and portable gas analyzer):

On the righthand side of the graph, you can see the oxygen consumption gradually declining. They fit the data to a two-part exponential function: there's the "fast" component (the dashed line), which they assume corresponds to the energy being used to replenish phosphocreatine, and the "slow" component, which goes to clearing out lactate and so on.

Anyway, the results: both methods produced pretty similar estimates, of about 32 kJ. In comparison, the anaerobic lactic energy system is estimated to contribute ~ 43 kJ, and the aerobic contribution is ~211 kJ. So the phosphocreatine system contributes about 12% of the energy needed for an all-out 2:00 race -- a good reason to make sure it's not totally depleted by too many long sprints immediately before the race starts. But the overwhelmingly biggest contribution, even for such a short race, is still aerobic. That's why endurance kills.

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