The primary energy systems used during exercise are, aerobic lipolysis, aerobic glycolysis, anaerobic glycolysis, and stored adenosine triphosphate coupled with creatine phosphate (ATP/CP).  It is interesting to note that the systems which produce the higher amounts of ATP are usually the ones that produce it at the slowest rates.  As a result, the body is constantly in a state of compromise, and individual energy systems are rarely used in isolation.  Instead, the body will often favor one system instead of using it at the exclusion of all the others. 

Aerobic Lipolysis

Aerobic lipolysis burns fat in the Krebs cycle.

Stored fat is the largest reserve of energy, so even an extremely lean person usually has enough stored fat calories to walk over 100 miles.

Aerobic Lipolysis is the slowest producer of ATP so it is most useful for extremely low energy activities such as slow jogging, walking, breathing, talking, etc.

It is the primary source of energy used to remake ATP from ADP, as well as to recycle spent fuel back into glucose between sets of intense exercise.

Aerobic Glycolysis

Aerobic glycolysis is the second largest producer of usable energy for exercise in the human body, but produces ATP at a noticeably higher rate than aerobic lipolysis.

If someone runs out of usable glycogen during an endurance event this is called “hitting the wall”, and results in a significant drop in the rate of energy output.

Aerobic glycolysis begins by breaking down glucose into pyruvate to produce some ATP, after which the pyruvate is oxidized in the Krebs cycle to produce even more ATP.

The rate of ATP production is limited by the VO₂ Max.

Aerobic glycolysis is useful for moderate rates of energy output for moderate time periods such as in running a 2 mile race.

Anaerobic Glycolysis

Anaerobic glycolysis is not limited by oxygen uptake and produces ATP at about 100 times as fast as aerobic glycolysis.

It produces much less ATP per molecule of glucose and is therefore much less energy efficient.

It also begins with the breaking down of glucose into pyruvate to produce ATP.

When energy is needed at rates that are greater than what can be supplied by aerobic glycolysis and lipolysis, much of the pyruvate undergoes fermentation to produce ATP without the need for oxygen.

One of the end products of anaerobic glycolysis is lactic acid which causes a burning sensation in the muscles, and inhibits further glycolysis.

This inhibition of further glycolysis reduces the energy output during the exercise.

The lactic acid eventually goes from the muscles into the blood and to the liver where it is converted back to pyruvate and then back into glucose in a process called gluconeogenesis via the Cori Cycle.

Anaerobic glycolysis is usually used in high repetition exercises where the athlete feels the muscles “burning”.

Adenosine Triphosphate/Creatine Phosphate (ATP/CP)

Stored ATP reserves are the quickest ways to have ATP available for energy output.

However the extremely high rates of energy output is offset by the short supply which lasts only seconds.

In many cases, creatine phosphate (CP) will donate a phosphate group to adenosine diphosphate (ADP) to change it back into adenosine triphosphate (ATP).

This increases the amount of ATP available after which the creatine becomes creatinine, which is excreted in the urine.

Creatine phosphate is created in the body from amino acids and a phosphate group. However, Creatine that is consumed in either natural form (i.e. beef or salmon) or in supplemental form, gets converted into creatine phosphate when it enters the body, which in turn increases the energy output available via the ATP/CP system.