How do you improve at any sport? You train, train, train.   But how you train is just as important as how much. Armed with the science of exercise physiology, today's olympic-class coaches design sophisticated programs for their athletes.

A muscle is like an engine, and the fuel it runs on is the chemical ATP. Our bodies have three separate systems for creating this fuel however. Exercise physiologists pay close attention to how an athlete's ATP is produced.

The phosphocreatine system gives an athlete lots of ATP very fast, but it's only good for about ten seconds before it needs to recharge. The second system, called glycolysis, provides a more steady supply, but too much glycolysis can make the muscles inefficient. The third system, called aerobic metabolism, burns fats and sugars to make ATP. Aerobic metabolism is best for long term endurance, but it's slow to get going, and, like the other systems, it also runs out of steam.

Olympic trainers take these systems into account. A long- distance runner should use aerobic metabolism for as much of her ATP as possible. To ensure this, she'll be tested on a treadmill to measure her threshold of aerobic metabolism. The trainer then develops a training program which will increase it. On the other hand, a weight lifter should use his phosphocreatine system, which provides maximum ATP in short bursts of time. His work- outs will build up the enzymes of this system, as well as build up his muscles.

That's just one way athletes can get a boost from the science of exercise physiology.