Metabolic Energy Systems of Exercise

The following is a brief overview of the possible chemical pathways in the anaerobic and aerobic production of ATP:

ATP-CP System (Anaerobic)

Also known as the Phosphagen system, energy is drawn from the small supply of ATP and CP (creatine phosphate) stored in the muscle which are available for immediate usage. As the amount of phosphagens is limited, this system can only provide enough energy for short, explosive activities up to 10 seconds. Unlike the Lactic Acid System, lactic acid is not produced as a by-product in the ATP-CP system.

Glycolytic / Lactic Acid System (Anaerobic)

ATP is produced through anaerobic glycolysis, a process that involves the breakdown of carbohydrates (ex. glucose) without the presence of oxygen. This is the prevalent system in high speed and high intensity activities such as 800 m sprints and a 200 m swim. Sports characterized by a mix of continuous and explosive movements such as soccer and hockey also employ this system.

The Glycolytic system can provide a fast supply of ATP for duration of one to three minutes. This period is limited by the lactic acid that is produced as a by-product, which impedes the muscle's ability to contract. During this local muscular fatigue, the pace/intensity of the activity must be reduced so that oxygen can be taken into the body more readily to remove the lactic acid.

ATP produced through Anaerobic Glycolysis:

1 glucose molecule = 2 ATP molecules + CO₂ + lactic acid + H₂O + heat

Oxidative System (Aerobic)

This energy involves the breakdown of fats and carbohydrates in aerobic glycolysis and fatty acid oxidation. These processes occur at a microscopic level within the mitochondria of muscle fibres where the chemical reactions of the Kreb’s Cycle and the Electron Transport Chain occur and produce ATP. At the same, unlike the anaerobic systems, the oxidative system requires oxygen and creates ATP at a relatively slower rate. This is compensated by a much greater ATP output that can be sustained for a longer period of time.

The oxidative system is the major ATP supplier for low intensity and continuous activities, recovery from high intensity exercises, and when you are at rest. This system is responsible for nearly 90% of the body’s ATP production.

Aerobic Glycolysis

ATP produced from Aerobic Glycolysis:

1 glucose molecule + O₂ = 38 ATP molecules + CO₂ + H₂O + heat

Fatty Acid Oxidation

ATP produced from aerobic Aerobic Fatty Acid Oxidation:

1 fatty acid molecule + O₂ = 100 ATP + CO2 + H2O + heat

*Since the chemical reaction of fatty acid oxidation requires more oxygen than aerobic glycolysis, it occurs at a slower rate than the latter.

The following charts can be used to supplement the information on the energy systems discussed above:

Energy System		Rate of ATP Production	Capacity of ATP Production
ATP-CP / Phosphagen	Anaerobic	Fast	Low
Gycolysis / Lactic Acid	Anaerobic	Moderately Fast	Moderately Low
Glycolysis	Aerobic	Moderately Slow	Moderately High
Fatty Acid Oxidation	Aerobic	Slow	High

Contribution of Energy Provided by Each Energy System Over Time

Time

ATP-CP

LACTIC ACID

AEROBIC

6 seconds

10 seconds

30 seconds

1 minutes

2 minutes

4 minutes

10 minutes

30 minutes

1 hour

2 hours

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