The ability of skeletal muscle to re-synthesize adenosine triphosphate (ATP) during exercise and recovery after exercise ultimately depends on the diet of the cross country runner. Today we will focus on the 3 most important “fuels” for successful distance running.
Fat and carbohydrate (CHO) provide the majority of the metabolic fuel for ATP re-synthesis, with protein and ketone bodies able to contribute small amounts in certain situations. Both fat and CHO are stored in intramuscular regions of skeletal muscle and in adipose tissue in the case of fat. Both CHO and fat can also be stored in the liver and to a small extent in the blood.
Generally, signals inherent to the muscle dominate the regulation of intramuscular fuel use, with some help from extramuscular signals. The mobilization of extramuscular fuels into the blood for delivery to the working muscle is controlled by various physiological systems, including the neural, endocrine, and cardiovascular systems.
The ability of skeletal muscle to regulate the uptake and use of these extramuscular fuels is an important aspect of skeletal muscle metabolism. The ability of skeletal muscles to improve regulation and signaling has been shown to improve through specific endurance training stimuli.
Most cross country training sessions involve a combination of fat and CHO metabolism to provide the required ATP through aerobic metabolism (oxidative phosphorylation) in the mitochondria. The three inputs required for oxidative phosphorylation in the electron transport chain (stage of aerobic respiration resulting in the bulk of re-synthesized ATP molecules) are oxygen, free adenosine diphosphate (ADP), and inorganic phosphate (P).
* Training Resource: Peaking Workouts for Cross Country Runners
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Because of the near-equilibrium nature of the oxidative phosporylation reactions, the substrates and products of the previous reactions (glycolysis) determine the end-rate of ATP production. Therefore, oxidative phosphorylation does not regulate ATP production reactions, but the pathways that supply their substrates do.
Because fuel substrates; not the aerobic respiration reactions limit ATP production, then the difference lies in the fuels molecules. The maximal rate of ATP synthesis from fat molecules can provide only enough ATP to sustain exercise at 60% to 70% of VO2 max, depending on the aerobic fitness of the individual. This range is called the aerobic threshold (AT). On the other hand, synthesis from CHO can sustain aerobic exercise up to 100% of VO2 max in untrained or trained individuals.
The anaerobic production (substrate phosphorylation) of ATP is required when oxidative phosphorylation cannot provide all of the required ATP. This occurs during the transition from rest to exercise (start), during an increase in power output (surge or kick), and at power outputs above that required for exercise at 100% VO2 max.
The main sources of anaerobic ATP are the degradation of phosphocreatine (PCr) and the production of lactate within the glycolytic pathway (glycolysis). A major advantage of these two anaerobic pathways acting together is the high rate of ATP production as compared to that of the aerobic pathways (6 times faster). These systems allow cross country runners to engage in quickening pace strategies and burst activities during the race.
CHO is a fuel for both aerobic and anaerobic provision. Aerobic use of CHO yields 38 to 39 mmol ATP for each mmol of glucose (glycogen) produced oxidatively. Anaerobic use of CHO yields much less energy, producing only 3 mmol ATP for each mmol of glucose derived from muscle glycogen. However, the rate of ATP provision is about twofold faster when derived anaerobically (glyocolsis) over aerobic oxidative respiration involving carbohydrate and even more over free fatty acids (Figure 1).
* Coaching Resource: Advanced Topics Symposium in Cross Country
Successful cross country racing involves the use all three fuel substrates.
In addition CHO is used both aerobically and anaerobically in the energy systems because of the demand. However, the aerobic use of CHO is the means and fuel for more than 85% of the ATP molecules needed to complete a cross country race at the highest level for any individual.
Plan the training time accordingly.