Why do we get exhausted? Noakes on energy supply and running

Posted by Dave on July 18, 2011 | 6 Comments

Lore of Running

930 pages of science!

This summer I’m reading Tim Noakes’ massive volume Lore of Running, bit by bit, then reporting the critical bits back to you. Last week I covered running potential, discussing whether there are fixed limits to human performance. This week I’m reading Noakes’ chapter on energy systems and running performance.

The basic question in this chapter is this: Why do we get exhausted when we run? Think about it this way. In a 5K race, you don’t run as fast as you can from the start — you hold back a bit so that you can finish the whole run. You might run an 8-minute mile pace. In your first mile, you’re breathing hard, but not completely spent. In the second mile, you struggle a bit and wonder if you’ll be able to keep this up for another mile. In the third mile, you’re hanging on by a thread; you’re nearly completely exhausted — but then you see the finish line, and you actually start running faster. If you were so exhausted, why were you able to speed up as you approached the finish?

If you run a marathon, you’ll certainly run it at a slower pace; let’s say 9-and-a-half-minute-miles. At this pace (assuming you’ve put in the miles to train), your first 13 miles or so are a relative breeze. After all, you’re going much slower than you did in your 5K. But as you approach the 20 mile mark, you start to feel like this isn’t going to be so easy. You breathe harder; your muscles ache. You might even take a few breaks to walk. Yet somehow, 1.5 miles from the finish, you manage to pick up the pace again. For the last half-mile, you’re running your 5K pace, and you sprint across the finish line. What can account for this? Shouldn’t you get more and more fatigued as the race progresses?

These examples are backed up by scientific research: Nearly everyone is able to find an extra reserve of energy at the end of the race, despite feeling exhausted. One study, led by Derek Kay, tested cyclists by asking them to ride on a stationary bike for 1 hour at their best pace. Every 10 minutes, they were asked to go all out for 60 seconds. This graph shows the results:

You’re probably not surprised at the shape of this graph. The first and the last sprints were the fastest, and performance steadily declined for the others. But it provides scientific confirmation that we are somehow able to tap into an extra storehouse of fuel when we know we’re near the end of a workout.

So what, exactly, is the body doing here? Early models of exercise and fatigue had speculated that the muscles were simply “out of fuel,” and that’s why we slow down and eventually stop exercising. But if everyone is able to give an extra effort near the end of a workout, that theory can’t possibly be right. Similarly, some models argued that the buildup of lactic acid in the muscles as a by-product of the energy-releasing process acts as a poison which ultimately prevents us from moving. But once again, the reality of athletes speeding up at the end of races suggests this theory is at least partially wrong.

As Noakes then shows us, the reason these models fail is because they assume muscles are working from a fixed energy source. But in fact the muscles have several sources they can draw on for energy. The most readily-available is the carbohydrate available in the muscles themselves. There is also a store of carbs in the liver that can be distributed to the muscles. Finally, muscles can derive energy from nearby fat stores in the muscles themselves, and also from fat stores in the rest of the body. Those energy stores break down as follows:

Most of the energy our bodies can use to power the muscles is in the form of adipose tissue fat — body fat. The energy in carbohydrate form, which we typically think of as “powering” the muscles, is a relatively small portion of the total energy available. But it’s also the most efficient way we have of converting a physical substance into energy to power the muscles. As we fatigue, we indeed are running low on those most-efficient energy supplies, but we’re not completely out of them. Something in our body doesn’t allow those stores to get reduced to zero, and instead we switch to one of the other sources: First muscle fat and liver carbohydrates, and finally body fat.

But when we get to the end of a race and can see that we’ll be able to rest soon, somehow we can tap some of those “extra” carbs in the muscles and run faster. The problem is, we don’t seem have direct control over how those carbs are used. Wouldn’t it be nice if we knew exactly how much energy was in the tank at any moment: That way you could max out your capacity during the race, finishing exactly as the needle hits “empty.” Instead, what appears to be happening is that an unconscious “central governor” controls these things, ensuring that there is always just a little bit more in reserve in case of a serious emergency. Is there any way to fool that central governor into allowing us to use a little more of the energy than we would have otherwise? Let’s hope so; Noakes has an extensive section called “training the mind” later in the book.

Noakes, T. (2003). Lore of Running (4th ed.). Champaign, IL: Human Kinetics.


6 Responses to “Why do we get exhausted? Noakes on energy supply and running”

  1. Davide
    July 19th, 2011 @ 1:14 pm

    Very interesting post! Very relevant to me at the moment, since I’ve been on a low-carb diet for the past few weeks (trying to cut BF to below 8%). I’m not a runner and I’m not a professional athlete (I row for my university’s club), but I think most of this applies to every kind of athlete that need to pace themselves.

    The feature of a low-carb diet of course is to try and deplete as much as possible the muscle carbohydrates (glycogen) so that they body is forced to use more of the rest, especially body fat. This hasn’t much of an impact on resistance training, and I can in fact maintain more or less my habitual pace when below 85% of my VO2 max; what I’ve found really difficult is interval training at 90-95%.

    The thing that puzzled me is that I was only expecting my pace to be slower than usual, since fat metabolism is less efficient; instead I found that it’s very hard for me to push myself to the limit, and that I feel the need to stop in a rather unpredictable way. For instance, the first few times I stopped in the middle of a sprint because I was feeling completely exhausted, but I wasn’t really out of breath or maxed out in my heart rate, and I was able to recover very quickly, only to fail again when I tried a second time shortly after.

    As I say, this puzzled me a great deal until I read your post: it seems sensible to assume that my “central governor” is really not used to me being in ketosis, and is shutting me off early (or rather, in a sharper way than I was used to). Overriding it is surprisingly hard even for someone who is used to push training to very high rates: I will keep at it and see what happens.

  2. Adrienne Martini
    July 19th, 2011 @ 2:25 pm

    Radiolab did a piece on limits and running that talks about this phenomenon: http://www.radiolab.org/2010/apr/05/

  3. Dave
    July 19th, 2011 @ 3:24 pm

    Davide, I don’t have a reference for you, but I believe there is some evidence that people can adapt to low-carb diets and become more efficient at metabolizing fat during exercise.

    Adrienne, thanks for the link! I’ve heard that podcast and it’s excellent.

  4. Running faster: it’s happening people! | howtorunabitfaster
    December 21st, 2011 @ 6:01 pm

    […] for that. In this serious smart blog we learn about body fuels. I think what Dave is saying is that when the body steps from one fuel source to another things go a bit pear-shaped. Because it’s not a smooth fuel transition, from carbs to body fat, so our fuel stutters and […]

  5. colleen
    March 17th, 2012 @ 11:17 am

    Re: low carb diet–I have reduced my carbs in an attempt to gain more protein and fat. I have noticed the same thing–I stop when I get tired rather than pushing through, and the 90-95% range is my worst. How long do you intend to experiment with this? I’m about ready to shift back to a very low fat, higher carb diet, as this is really annoying me in training.

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