This is an excerpt from NPTI's Fundamentals of Fitness and Personal Training by Tim Henriques.
Understanding the energy systems becomes crucial because of the need to train in a specific manner to improve in a certain event. First, recognize that all the energy systems are working together all the time. The body does not have a switch that turns one energy system off when another comes on. All the energy systems are working together, but knowing which energy system is contributing primarily to the activity can be useful. The intensity of the exercise is what determines the energy system being used. If a person (over the age of 21, of course) curls a beer for five reps, you might think that the person would be using the phosphagen system because only about five seconds are needed to perform the five reps. If a person curls 50-pound (22.7 kg) dumbbells for five reps, the phosphagen system would most likely be used, but a beer is so light that the person could curl it a hundred times or more, using primarily the oxidative system to power that activity. Don't think about how long the activity was performed; think about how long it could be performed. If intensity is high, you can use duration to separate out the energy systems, but intensity is the key determinant of which energy system is involved.
Going back to the car example, we can view different kinds of cars as representing different energy systems. A drag-racing car, with a huge engine, big tires in the back, and little tires up front, is the phosphagen system. This car can race a quarter mile (.4 km) in six seconds or less, but it has poor endurance and would not be good for any kind of long trip. A sports car is quite fast and performs exceptionally well on a track. It would be able to beat the drag-racing car on a road course type of track, but it would not beat the dragster on the strip. A sports car would represent the fast glycolysis system. An economy car would represent the SG system. It is not as fast as the dragster and does not do as well as a sports car on a track, but it still gets around pretty well and because the SG system uses oxygen, it is much more efficient (burns less gas to accomplish the same thing) than either of the other two cars. A hybrid car that combines an internal combustion engine and one or more electric motors would represent the oxidative system. It can't perform well in either a drag race or a race around a track, and it is even slower than the economy car, but if you were to drive across the country, a hybrid car would need the least amount of gas to make the trip.
An analogy can help you understand how the energy systems work in the body. Picture a lighter, such as a cigarette lighter. On most lighters you can adjust the size of the flame to make it larger or smaller. A lighter is your oxidative system. It is always running, but the more flame you produce, the more fuel you use and the quicker you run out of fuel. Imagine that the lighter is always on (because it represents the oxidative system and we are always breathing while we are alive), so it is always producing energy. The problem is that it doesn't produce much energy. If you need more heat than it can produce even when turned all the way up, you turn to something else. Imagine that you have a blowtorch right next to your lighter. If you want a significant amount of heat, you can turn on the blowtorch. The lighter stays on the whole time, but now it is overshadowed by the blowtorch, which will greatly increase the amount of energy produced. The blowtorch, however, uses up fuel a lot faster than the lighter does, so it doesn't last as long. Again, on most blowtorches you can adjust the amount of heat produced. The blowtorch is the glycolysis system. Finally, if you want a huge blast of heat, you have a flamethrower. This device emits an enormous amount of energy, but its duration is fleeting. When you use the flamethrower, the lighter and blowtorch stay on, but the flamethrower is the main producer of energy. The flamethrower is the phosphagen system. The key thing to remember is that all the energy systems are always in use, but the intensity determines which energy system is dominant.