This paper aims to profile the endurance sport: triathlon on its metabolic energy systems requirements and to suggest 2 training strategies that allow the triathletes to meet those requirements and to achieve higher performance.
A triathlon is a multi-sport endurance event consisting of swimming, cycling, and running over different distances. Triathletes compete for fastest overall course completion time, including timed “transitions” between the individual swim, bike, and run components. According to the International Triathlon Union, the main international race distances are Sprint distance (750 m swim, 20 km bike, 5 km run), the Olympic distance (1.5 km swim, 40 km ride, 10 km run), the Long Course (1.9 km swim, 90 km ride, 21.1 km run), and Ultra Distance (3.8 km swim, 180 km ride, and a marathon: 42.2 km run).

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Transition areas are positioned both between the swim and bike segments (T1), and between the bike and run segments (T2), but for shorter distance both transitions are often just one checkpoint These areas are used to store bicycles, performance apparel, and any other accessories essential for preparing and gearing for the next stage of the race. (Wikipedia, 2010)1
The Olympic distance will usually take the very top competitors just under 2 hrs to complete and the average person 2.5-5 hours. The Long Course will usually takes the competitors to complete the race about 5 hours for the very fastest and 7-10 for the average racers. The Ultra distance for the very fastest athletes will finish this race in around 9 hours with the average person coming anywhere between 9.5 to 17 hours.
Adenosine triphosphate (ATP)
All energy originates as light from the sun. Plants convert sunlight into energy through photosynthesis. When human eat animals that have eaten plants or when one eat the plants directly, the energy is transferred to them. The energy is stored as carbohydrates, fats and protein in food. Energy is needed when a human is growing, developing, repairing, transporting of various substances between cells and muscle contraction (exercise) or to do any works.
Adenosine triphosphate (ATP) is the known as the main compound in the human body to power the energy (McArdle WD, Katch FI and Katch VL, 2000)6. An ATP molecule consists of adenosine and 3 (tri) inorganic phosphate groups. When a molecule of ATP is combined with water (hydrolysis), one of the phosphate groups splits away and the energy is released. The molecule of adenosine triphosphate has becomes adenosine diphosphate (ADP) after the splitting.
But the human body keeps only a small amount of the compound (ATP) within cells and it only able to power a few seconds of exercise. The body needs to replace, re-synthesize ATP consistently. To replenish the limited amount of ATP, a chemical reactions to add a phosphate group back to ADP to create ATP will occur, The process that occurs with the presence of oxygen is known as aerobic metabolism, if it occurs without the oxygen it is known as anaerobic metabolism.
Energy sources to replenish ATP
There are a few energy sources available to replenish ATP; they are creatine phosphate, carbohydrate, fat and protein.
Creatine Phosphate is readily available to the human cells and rapidly produce ATP. But, it exists in human cells in a very small amount about 120g. Creatine phosphate together with 100g of ATP, it can produce high energy phosphogens (Sporting Excellence Ltd, 2010)3. .
Carbohydrate from food is taken up by the muscles and liver and then converted to glycogen. Glycogen is then used to form ATP and in the liver it can be converted into glucose and transported to the human muscles by blood.
Fats are stored throughout the body and it is a substantial energy reservoir that can also be used to produce ATP also. But, fat must first be reduced from complex form, triglyceride to the simpler components of glycerol and free fatty acids before it can be used as cellular metabolism. Therefore, the process is too slow for the energy to be release for ATP production when using fats.
Protein can also be used for production of ATP, especially for prolong activity. But, before it can be used, it need to break into amino acids and then convert into glucose.
In summary, the rate at which energy is released from creatine phosphate, carbohydrate, fat and protein body are different and it is determined by different factors. For example one of the factors, if there are larger amounts of 1 type of fuel available, the body will rely on this sources first than others (Sporting Excellence Ltd, 2010)3.
Energy systems to produce ATP
In general, there are 3 separate energy systems which ATP can be produced: these are ATP-PCr system, the Glycolytic system, the Oxidative System. Several numbers of factors will determine which of the energy systems is chosen, example exercise intensity.
ATP-PCr system
In the ATP-PCr system, the main components are ATP and creatine phosphate (phosphocreatine or PCr). PCr is broken down into a phosphate and energy, which then is used to rebuild ATP. ATP is rebuilt by adding a phosphate to ADP (phosphorylation). An enzyme that controls the breakdown of PCr is known as creatine kinase (Wilmore JH and Costill DL, 2005)5. During the first 5 seconds of exercise, the ATP-PCr is relied most predominantly. After that, the ATP concentration will continue to last for another extra 5-8 seconds with some PCr buffering. In total, the ATP-PCr system can sustain an all out exercise for 3-15 seconds and it is also during this time the energy released is at its greatest (Baechle TR and Earle RW, 2000)4. If the exercise continues beyond this period, the body must rely on another energy system to produce ATP.
Glycolytic System
Glycolysis means breakdown (lysis) of glucose and consists of series of enzymatic reactions. Carbohydrates in food, supply the body with glucose and it is stored as glycogen in the muscles and liver for later use.
The end product of glycolysis is pyruvic acid. Pyruvic acid can be channeled through a process called the Krebs cycle or converted into lactic acid. If the final product was lactic acid, the process was known as anaerobic glycolysis (fast glycolysis) and if the final product remained as pyruvate acid, the process is known as aerobic glycolysis (slow glycolysis).
Anaerobic glycolysis (fast glycolysis) produces energy at a greater rate than aerobic glycolysis (slow glycolysis). However, because the end product of fast glycolysis is lactic acid, it will quickly accumulate and caused muscles fatigue (Baechle TR and Earle RW, 2000)4.
As mentioned above, the immediate ATP-PCr system will begin to run out after the initial 15 seconds of exercise, and at the same time this is where the contribution of the fast glycolytic system begins to increase after the initial 10 seconds of exercise. For the next 30 seconds of sustained exercise, the fast glycolytic system will supply the main energy to the body. At the 45 seconds of sustained exercise, there will be a second drop in power output, this is where the fast glycolytic system begin to run out and at the same time the oxidative system will comes in to produce the ATP.
Oxidative system
In the oxidative system, there are four processes to produce ATP:
Slow glycolysis (aerobic glycolysis)
Krebs cycle (citric acid cycle or tricarboxylic acid cycle)
Electron transport chain
Beta oxidation
Slow glycolysis is the same as fast glycolysis that metabolize glucose to form two ATPs. The difference is that the end product pyruvic acid is converted into acetyl coenzyme A instead of lactic acid (Baechle TR and Earle RW, 2000)4.
Upon completing glycolysis, new ATP can be produced by channelling acetyl coenzyme A through the Krebs Cycle. The Krebs cycle is a series of reactions that continues the oxidization of glucose that was started during glycolysis. Acetyl coenzyme A enters the Krebs cycle and is broken down in to carbon dioxide and hydrogen allowing two more ATPs to be formed. However, the hydrogen produced in the Krebs cycle plus the hydrogen produced during glycolysis, left unchecked would cause cells to become too acidic (McArdle WD, Katch FI and Katch VL, 2000)6. So hydrogen combines with two enzymes called NAD and FAD and is transported to the Electron Transport Chain. Hydrogen is carried to the electron transport chain, another series of chemical reactions, and here it combines with oxygen to form water thus preventing acidification. This chain, which requires the presence of oxygen, also results in 34 ATPs being formed (McArdle WD, Katch FI and Katch VL, 2000)6.
Unlike glycolysis, the Krebs cycle and electron transport chain can metabolise both fat and carbohydrate to produce ATP. Lipolysis is the term used to describe the breakdown of fat (triglycerides) into the more basic units of glycerol and free fatty acids. Before these free fatty acids can enter the Krebs cycle they must undergo a process of beta oxidation a series of reactions to further reduce free fatty acids to acetyl coenzyme A and hydrogen. Acetyl coenzyme A can now enter the Krebs cycle and from this point on, fat metabolism follows the same path as carbohydrate metabolism.
The oxidative system can produce ATP through either fat (fatty acids) or carbohydrate (glucose). The key difference is that complete combustion of a fatty acid molecule produces significantly more acetyl coenzyme A and hydrogen (and hence ATP) compared to a glucose molecule. However, because fatty acids consist of more carbon atoms than glucose, they require more oxygen for their combustion. If exercise is intense and the cardiovascular system is unable to supply cells with oxygen quickly enough, carbohydrate must be used to produce ATP instead of fats. Hence, exercise intensity must reduce so that the body can switches to fat as its primary source of fuel.
Protein is thought to make only a small contribution (usually no more 5%) to energy production and is often overlooked. However, amino acids, the building blocks of protein, can be either converted into glucose or into other intermediates used by the Krebs cycle such as acetyl coenzyme A. Protein may make a more significant contribution during very prolonged activity, perhaps as much as 18% of total energy requirements.
In summary, the oxidative system is used mainly during rest and low-intensity exercise. At the start of exercise it takes about 90 seconds for the oxidative system to produce its maximal power output and training can help to make this transition earlier. Beyond this point the Krebs cycle supplies the majority of energy requirements but slow glycolysis still makes a significant contribution. In fact, slow glycolysis is an important metabolic pathway even during events lasting several hours or more.
Training Strategies
The 3 energy systems always work hand in hand with one another. All 3 energy systems make a contribution regardless whether the exercise is short or intense, but usually 1 or 2 will usually be the predominate ones.
In general, the 2 factors of the exercises that affect the energy to be used are intensity and duration of the exercise. As mentioned above, the sport selected for this study is triathlon, triathlon is an endurance sport that consists of swimming, cycling and running of the various long distance. The table below (Foss ML, Keteyian, 1998)7 shows how each of the energy system contributes in order to meet the physical demands for triathlon:
Sport
ATP-PCr & Glycolysis
Glycolysis & Oxidative
Oxidative
Running (distance)
10
20
70
Swimming (distance)
10
20
70
Cycling (distance)
10
20
70
As observed above, triathlon is the sport that uses 70% of oxidative energy system (aerobic), 20% of glycolysis energy system (anerobic) and 10% of ATP-PCr system. Hence, the training strategies that used on triatheletes will focus on increasing aerobic capacity more than anaerobic capacity.
Increase aerobic capacity
The aerobic energy system uses carbohydrate, fats and proteins for resynthesising ATP. The capacity of this energy system can be increased with various intensity (Tempo) runs.
The types of Tempo runs are:
Continuous Tempo – long slow runs at 50 to 70% of maximum heart rate. This places demands on muscle and liver glycogen. The normal response by the system is to enhance muscle and liver glycogen storage capacities and glycolytic activity associated with these processes.
Extensive Tempo – continuous runs at 60 to 80% of maximum heart rate. This places demands on the system to cope with lactate production. Running at this level assists the removal and turnover of lactate and body’s ability to tolerate greater levels of lactate.
Intensive Tempo – continuous runs at 80 to 90% of maximum heart rate. Lactate levels become high as these runs boarder on speed endurance and special endurance. Intensive tempo training lays the base for the development of anaerobic energy systems.
Sessions to develop this energy system:
4 to 6 Ã- 2 to 5 minute runs – 2 to 5 minutes recovery
20 Ã- 200m – 30 seconds recovery
10 Ã- 400m – 60 to 90 seconds recovery
5 to 10 kilometre runs
Increase anaerobic capacity
Even though increase the aerobic capacity is more important than anerobic capacity, triathletes still need to put some training to increase anerobic capacity. Triathletes need to have a well-conditioned anaerobic energy system, or a more sustainable lactic acid system. The limiting factor in triathletes is most-often their anaerobic threshold. If the triathletes have a higher threshold, they are able to maintain a greater pace without dipping too quickly into the aerobic energy systems and if they do start to use the aerobic energy systems their body is better equipped to deal with it for longer and then recover when they are well-conditioned (Endlesshumanpotential, 2007)2..
Sessions to develop this anaerobic energy system:
5 to 8 Ã- 300 metres fast – 45 seconds recovery – until pace significantly slows
150 metre intervals at 400 metre pace – 20 seconds recovery – until pace significantly slows
8 Ã- 300 metres – 3 minutes recovery (lactate recovery training)
Conclusion
In conclusion, Adenosine triphosphate (ATP) is the main compound to power human energy. There are 3 main systems to produce ATP: they are the ATP-PCr system, the Glycolytic system, the Oxidative System. These systems do not work alone, many times depend on the intensity and duration of the activities, more than 1 systems will be working together to power human energy.
As a triathlete, in order to have a better performance, he needs to use training strategies that will increase his aerobic capacity and anaerobic capacity respectively.