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Your body needs ATP, but how much do you already have in your body, and how does the process of creating more ATP work? Even though ATP is crucial, the body has only enough stored to perform a few seconds of maximal exercise.

For any maximal effort lasting longer than that, the body must regenerate ATP. There are a few systems in the body that can do this, each creating ATP at different rates and some kicking in depending on whether oxygen is available to the system or not. The three systems the body uses to transfer energy and regenerate ATP are:

•The ATP-PCr system (anaerobic)

•The glycolytic pathway (anaerobic)

•The oxidative phosphorylative pathway (aerobic)

To understand the three energy systems, imagine hiking up a steep grade. Suppose that you want to race your friends up it, so you begin to sprint. You feel fresh and have tons of energy. The energy you get for the first 8 to 12 seconds comes from the ATP-PCr system, which is anaerobic (without oxygen). It is the fastest available system you have. PCr is phosphocreatine, and when this bond is broken down by creatine kinase, the energy released is used to regenerate ATP.