Essentially, muscles contain two sorts of fiber. They are called simply fast-twitch and slow-twitch fibers. And we have a mixture of them and that’s genetic. But you can, by training, alter the balance of some of the intermediate fibers, make more fast ones or make more slow ones, according to the training you do. So the sprinters have more fast-twitch fibers and concentrate on developing them. Distance runners have more slow-twitch fibers. And obviously I was born with more slow-twitch fibers, but the whole of my training was developing these fibers.
To move oxygen to the muscles is what enables them to release energy to run or anything else. If you are running for 20 seconds there is no time for any oxygen to get from the outside air through your lungs to the muscles. So you’re entirely dependent on what’s called anaerobic breakdown of energy without the presence of air and oxygen. That’s why you feel breathless at the end of it and you just cannot go at that speed for longer than 15 seconds.
These other fibers are very efficient. They contract more slowly but they can go on contracting because the air is provided. The mile requires about 50 percent of the energy to be anaerobic, 50 percent aerobic. So you’ve got a balance between the two, and that’s why it’s a fascinating race. You may see people sprinting at the end of it if they’ve got energy to do so. As the distance increases, the need for anaerobic fibers, fast twitch fibers, gets less and less.