Adenosine triphosphate (ATP) is the source of energy for all muscle contractions. Energy is released when ATP is broken into ADP+Pi (adenosine diphosphate and phosphate group). Maintaining the availability of ATP for muscle contraction is the limiting factor, since ATP is not stored in large amounts in skeletal muscle. Viable sources of ATP come from both anaerobic (does not require O2) and aerobic (requires O2) means. The primary energy source for a given activity will primarily depend on the intensity of muscle contractions.
The two main anaerobic sources of ATP are from Phosphocreatine (PCr) and Anaerobic Glycolysis. Intramuscular PCr stores are used for rapid high intensity contractions but are depleted in less than 30 seconds and take several minutes to replenish. For example, PCr provides the majority of the energy for a 100 m sprint. Additionally, the ability to perform repeated bouts of near maximal effort is largely dependent on PCr stores. Augmenting PCr stores by Creatine Supplementation can increase the amount of work that can be performed in repeated bouts of high intensity exercise.
Anaerobic Glycolysis refers to the breakdown of glucose (glycolysis) to pyruvate, which in the absence of O2, is converted to lactic acid. In muscle fibers, glucose is made available through the breakdown of muscle glycogen stores. Anaerobic glycolysis is not limited by the availability of glycogen; instead, the accumulation of lactic acid and other metabolites is the limiting factor. High intensity exercise with a duration of 1-3 minutes (e.g. 800 M race) will rely primarily on anaerobic glycolysis, resulting in a large accumulation of lactic acid.
Aerobic glycolysis occurs when O2 is available to breakdown pyruvate, which yields ATP through chemical reactions that occur in the Krebs Cycle and the Electron Transport System . As in anaerobic metabolism, glucose may be obtained from stored glycogen. Glycogen stores are plentiful, and therefore glycogen depletion is only a concern for athletes who are continuously exercising for more than 90 minutes or intermittent exercise over substantially longer periods of time. For example, it is not uncommon for endurance athletes to become glycogen depleted. In marathon races this is referred to as "hitting the wall". In order to reduce the chances of depleting glycogen reserves during a contest, athletes often "carbo load " prior to the event. This involves manipulating the carbohydrate content of one's diet in order to maximize glycogen stores.
The most abundant energy source available to the muscle fiber is fat. The breakdown of fat to yield ATP is referred to as lipolysis. While the supply of fatty acids is essentially unlimited, the rate at which lipolysis occurs is the limiting factor in obtaining ATP. Lipolysis is responsible for resting muscle activity, but its contribution to the overall muscle energy supply will decrease as contraction intensity increases. For example, glycogen depletion occurs when the rate of lipolysis cannot meet the energy demand of the exercise, and the reliance on glycolysis expends the available glycogen stores. Once glycogen depletion occurs, exercise intensity will be reduced dramatically. However, a small decrease in intensity (e.g. slowing the pace) earlier in the exercise bout would spare glycogen sufficiently to avoid depletion. In turn, the importance of facilitating lipolysis during endurance events cannot be overemphasized.
Energy Systems Versus Running Speed
Based on world record times, humans can maintain maximum sprinting speed for approximately 200 m. The average speeds for the 100 m and 200 m world records are similar (21.6 mph and 22.4 mph, respectively). However, with increasing distances, average speeds decline. The average speed for the marathon world record is 12.1 mph, which is 55% of the world record sprinting speed. This is remarkable since the marathon is more than 200 times the length of a 200 m race.
Although natural selection plays a crucial role in elite sprinting and marathon performance, the energy systems also must be highly trained and exercise-specific to be successful. For example, the energy needed to maintain an average sprinting speed of 22 mph for 200 m or less and an average running speed of 12.1 mph for the marathon are acquired by two very different systems (the predominant energy systems required for running at different speeds are shown in the first figure).
The primary energy source for sprinting distances up to 400 m is PCr. From 400 m to 1,500 m, anaerobic glycolysis is the primary energy source. For distances longer than 1,500 m, athletes rely primarily on aerobic metabolism.
The rate of glycogen and fat utilization will vary according to the relative running speed. Although the rate of glycogen utilization is low while running a marathon, the duration of the event increases the possibility of depleting glycogen stores. In contrast, the rate of glycogen utilization is substantially higher during a 5,000 m run, but glycogen depletion is not a concern because of the short duration of the event.
Maximum maintainable speed drops by approximately 7 mph as running distance increases from 200 m to 1500 m (about 1 mile). However, as the distance increases from 1 mile to 26 miles, maximum maintainable speed only drops an additional 3.5 mph. On average, a healthy, fit, non-elite, male athlete can be expected to sprint at an average speed of 16-18 mph for 100-200 m and approximately 6-8 mph for a marathon.