There are three separate energy systems through which ATP can be produced. These systems do not run independently of each other, rather they all act at the same time to greater or lesser extent depending on the factors at work.
(1) The ATP-PCr System
The ATP-PCr is a very fast acting energy system and unfortunately also lasts a very short time. PCr is broken down releasing a phosphate and energy, which is then used to rebuild ATP through a process called phosphorylation. The enzyme that controls the break down of PCr is called creatine kinase.
The ATP-PCr energy system can operate in the presence or absence of oxygen but because it doesn’t rely on the presence of oxygen it said to be largely anaerobic.
During the first 5 seconds of exercise regardless of intensity, the ATP-PCr is relied on almost exclusively. ATP concentrations last only a few seconds with PCr buffering the drop in ATP for another 5-8 seconds or so. Combined, the ATP-PCr system can sustain all-out exercise for 3-15 seconds and it is during this time that the potential rate for power output is at its greatest.
If activity continues beyond this immediate period, the body must rely on the next energy system to produce ATP, the Glycolytic System:
(2) The Glycolytic System
Glycolysis means the breakdown (lysis) of glucose and consists of a series of enzymatic reactions. Carbohydrates supply the body with glucose, which can be stored as glycogen in the muscles or liver for later use.
The end product of glycolysis is pyruvic acid. Pyruvic acid can then be either funnelled through a process called the Krebs cycle (named after Hans Adolf Krebs, winner of the 1953 Nobel Prize in Medicine) or converted into lactic acid. This process occurs regardless whether oxygen is present.
If the end product of glycolysis is pyruvic acid, the process is called slow glycolysis.
If the end product is lactic acid, the process is called fast glycolysis.
As the name suggests the fast glycolitic system can produce energy at a greater rate than slow glycolysis. However, because the end product of fast glycolysis is lactic acid, it can quickly accumulate and lead to muscle fatigue.
The contribution of the fast glycolytic system decreases rapidly after the initial 10 seconds of exercise. This also coincides with a drop in maximal power output as the immediately available ATP-PCr system begins to run out. By about 30 seconds of sustained activity the majority of energy comes from slow glycolysis.
At 45 seconds of sustained activity there is a second decline in power output (the first decline being after about 10 seconds). Activity beyond this point corresponds with a growing reliance on the next energy system, The Oxidative System, perhaps the most complex of the 3 systems.
