Posted by Dave on July 18, 2011 | 5 CommentsThis 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.