Energy Pathways

D. Matthews and E. Fox, in their revolutionary book, "The Physiological Basis of Physical Education and Athletics", divided the running requirements of various sports into the following "energy pathways": ATP-CP and LA, LA-02, and 02.

* ATP - Adenosine Triphosphate: a complex chemical compound formed with the energy released from food and stored in all cells, particularly muscles. Only from the energy released by the breakdown of this compound can the cells perform work. The breakdown of ATP produces energy and ADP.
* CP - Creatine Phosphate: a chemical compound stored in muscle, which when broken down aids in the manufacture of ATP. The combination of ADP and CP produces ATP.
* LA - Lactic acid: a fatiguing metabolite of the lactic acid system resulting from the incomplete breakdown of glucose. However Noakes in South Africa has discovered that although excessive lactate production is part of the extreme fatigue process, it is the protons produced at the same time that restrict further performance
* O2 means aerobic running in which ATP is manufactured from food mainly sugar and fat. This system produces ATP copiously and is the prime energy source during endurance activities

These energy pathways are time duration restricted. In other words, once a certain time elapses that specific pathway is no longer used. There is some controversy about these limitations but the consensus is:
Duration Classification Energy Supplied By
1 to 4 seconds Anaerobic ATP (in muscles)
4 to 10 seconds Anaerobic ATP + CP
10 to 45 seconds Anaerobic ATP + CP + Muscle glycogen
45 to 120 seconds Anaerobic, Lactic Muscle glycogen
120 to 240 seconds Aerobic + Anaerobic Muscle glycogen + lactic acid
240 to 600 seconds Aerobic Muscle glycogen + fatty acids

The result of muscle contraction produces ADP which when coupled with CP regenerates ATP. CP is stored in the muscles. Actively contracting muscles obtain ATP from glucose stored in the blood stream and the breakdown of glycogen stored in the muscles. Exercise for longer periods requires the complete oxidation of carbohydrates or free fatty acids in the mitochondria. The carbohydrate store will last approximately 90 minutes and the free fatty store will last several days.

All three energy systems contribute at the start of exercise but the contribution depends upon the individual, the effort applied or on the rate at which energy is used. The following graph depicts how the energy systems contribute to the manufacture of ATP over time when exercising at 100% effort. The thresholds (T) indicate the point at which the energy system is exhausted - training will improve the thresholds times.

Energy Systems