Here is a question for all runners:
When you go out for an 8 km training run, how do you know how fast to go? And if I told you to head out for a 16 km training run, would you run at a different speed? And how would you know what that speed should be?
At first glance, these may seem to be a rather obvious questions, but once you begin to give them some serious consideration, they have quite profound implications for how you might think about the physiology of exercise. Here's another example: If you are about to start a 21 km race, the pace you set out at will be quite different compared to the pace you would start at in your weekly 5 km time-trial.
Well, you are thinking, that's obvious. Of course you would start out slower, because if you started both at the same speed, by the time you reached the 5 km mark of the 21 km race, you'd be exhausted. But what I'm really asking is how does your body know how fast you should go? And when does it decide what that speed is?
Perhaps it's easier to answer the second question first. You make the 'decision' of how fast to run (or cycle) within the first few seconds of exercise. By the time you've hit the pavement and reached the first corner, your body has already found its preferred running speed or cycling power output. We know from studies in laboratories that people settle on 'an ideal pace' right at the start, and it only changes marginally during exercise. Now that is nothing remarkable until you consider that all the great scientific minds, all the great exercise physiologist, CANNOT explain why this is the case.
What? You are thinking. Surely they know how your body chooses the speed it wants to go at. Well, the answer is a resounding NO! We know that it happens, because anyone can tell you that your 5 km pace is faster than your 21 km pace, but we don't really understand why.
All the theories we have for exercise physiology tell us that your ability to exercise and the way you regulate performance are the result of what we call 'feedback'. In otherwords, various changes in your body caused by exercise - your lungs, your heart, your muscles, your body temperature - all tell you either that you have to slow down, or that you should speed up. For example, it is often believed that a molecule called lactate forces you to slow down if its levels become too high. The theory is that if you started to quickly, your body's lactate levels would rise and impair your body's normal ability to exercise, and you would be forced to slow down. The same goes for body temperature - if you run too fast, you get too hot and have to slow down. Sounds reasonable enough, But then you remember that we've already described how you know what speed to run at within the first few seconds of exercise. That can't be explained by what we know of exercise physiology, because in these first few seconds, there are no differences! You are not hot, there is no lactate, nothing is wrong! Yet you slow down for a 21 km race, but not a 5 km race! And so if I tell you to run 5 km compared to 21 km, you choose a faster pace even though there is nothing different in your body at the start of exercise.
Of course, previous experience is important, and so you learn, with training, that you need to slow down in order to run 21 km. But that is only part of the answer. The full answer is far more complex than that - what does your body use to judge pace? What signals are taken into account to slow down or speed up?
These are the questions that are at the forefront of exercise physiology research. Much of the work has been driven from the University of Cape Town, where Professor Tim Noakes developed a theory of a Central Governor - an area of the brain that regulates exercise to make sure that we perform optimally without causing damage to the body. The term 'central governor' is perhaps a little outdated, but research continues to be done to answer these important questions. The latest research will look at how the very first seconds of exercise are crucial in determining how much muscle the brain activates. Susan Johnston, an Honours student at the University of Cape Town, will do a study in which cyclists will have to perform time-trials over four different distances - 5 km, 10 km, 40 km and 100 km, and will be measuring how much muscle is active in the first few seconds of the trials. It is expected that the brain will allow more muscle to be activated in the 5 km trials than the 100 km, but that overall, the same pattern will be present. This will show that the brain (or the governor) sets the pace using only the knowledge of the exercise distance, and not the physiological feedback, as we've always thought.
So a simple question does not, in fact, have a simple answer, and hopefully this study will help pave the way to understanding the processes in the future.
More studies are planned, and over the course of the next few weeks, we'll look at some scientific theories for how exercise performance is regulated or controlled.
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Friday, May 18, 2007
Here is a question for all runners: