The Body’s Energy Systems

The ability of an athlete to sustain prolonged bouts of training or competition is based on the body’s ability to generate energy. The human body has the capacity to generate hours of energy for muscular contraction through the use of fat and carbohydrate that is stored within the body. Energy within the human body comes in the form of adenosine triphosphate (ATP). This is considered the universal currency of the body and is consistently used and replenished based on the rate at which an athlete is exercising, the activities the athlete does (or does not do) on a day to day basis, and the amount of energy that is taken in through the consumption of food. In the sport of triathlon, an athlete must become as efficient as possible at using energy for skeletal muscle contraction and train to increase the maximal amount of energy that can be produced.

ATP is a molecule that consists of adenosine and three (tri) inorganic phosphate groups. When a molecule of ATP is combined with water (a process called hydrolysis), the last phosphate group splits away and releases energy. The molecule of adenosine triphosphate now becomes  adenosine  diphosphate  or  AdP. in order to replenish the limited stores of ATP that are present within the muscle cell, chemical reactions occur that add a phosphate group back to AdP to create ATP. This process is called phosphorylation. It is called aerobic metabolism or oxidative phosphorylation if this process occurs in the presence of oxygen and anaerobic metabolism if it takes place without oxygen.

The production of ATP for muscle contraction comes from three energy systems – the immediate energy system, (ATP-CP); the anaerobic glycolysis (nonoxidative) system and the aerobic energy system (oxidative). The first two energy systems are frequently known together as the anaerobic energy system and are able to function without the presence of oxygen. This energy system predominates in exercise bouts lasting up to 4 minutes. The anaerobic energy system can generate large amounts of energy very quickly, but it is limited and provides energy for only a short period of activity. Conversely, the aerobic energy system is slower at generating energy, but it can continue to generate energy for hours so that work can be sustained.


The energy systems are designed to work on a continuum as is depicted in figure 4.1. While one energy system may be predominating, all three work together to provide the energy needed during exercise. Energy production by each of the three systems is dependent on exercise intensity and duration. In Table 4.1, the energy systems are divided based on a time continuum, and the source of energy is provided.  There are three key energy sources that are available and utilized to power the production of ATP. These include creatine phosphate (CP), carbohydrate and fat. In addition, the body at times utilizes lactic acid, a by-product of carbohydrate break down, to produce energy as well as some amount of protein. This is most common when energy reserves are low and exercise is continued. During low to moderate intensity exercise, fat serves as the primary fuel source. As exercise intensity increases, carbohydrate mustincrease its contribution to the energy pool. At the highest levels of exercise intensity, CP is also utilized to provide energy. The use of carbohydrate and fat as a fuel source in relation to exercise intensity is depicted in figure 4.2.

Creatine phosphate is the energy source that is most readily available to the muscle cells and the most rapid to produce ATP. This energy source is responsible for fueling the immediate energy system. As a result, it is the dominant resource for providing energy during initial muscle contraction and bursts of very high intensity, short duration activity (such as sprinting). There is approximately ten seconds worth of energy available in the form of CP at any.

figure-41_medAfter the first ten seconds of exercise or to sustain any burst of high intensity activity, the muscle must call on carbohydrate to supply energy. Anaerobic glycolysis is then the predominant energy system that is being utilized. It is this energy system that dominates during high intensity, moderate duration exercise that lasts between ten seconds and five minutes. At rest, carbohydrate is taken up by the muscles and liver and converted into glycogen. during exercise, glycogen is used to form ATP. it can be converted into glucose in the liver and transported to the muscles via the blood. The use of carbohydrate as a fuel source through anaerobic glycolysis results in the production of lactic acid. As mentioned earlier, this by-product is initially used as a fuel source. As more is produced, it accumulates in the muscle and can inhibit muscle contraction and energy production. As a result, the body recruits a higher level of oxygen to begin facilitating the formation of ATP from carbohydrate through aerobic metabolism after exercising for a few minutes in anaerobic glycolysis. it is this energy system that is known as the long-term energy system and dominates in exercise lasting longer than five minutes in duration.

The availability of glycogen to fuel exercise is limited. The body must therefore become very efficient at learning to utilize fat as a fuel source to generate ATP. fat is stored predominantly as adipose tissue throughout the body and is a substantial energy reservoir. Fat is less accessible for metabolism as it must first be reduced from its complex form, triglyceride, to the simpler components of glycerol and free fatty acids. Fat can supply a vast amount of energy, but its rate of energy release is too slow for very intense activity. An athlete can learn to use fat more effectively and thus increase the amount of energy that is supplied through this fuel source over time. As the aerobic energy system is developed, it increases its ability to produce energy from fat stores. This is predominantly a result of an increase in mitochondria, the power house of the muscle cell. The mitochondria can be viewed as a factory that is able to produce very large amounts of ATP.


The energy systems and the fuel sources that supply them must learn to work on a continuum. The aerobic energy system is the most dominant. During the majority of a race, energy needs to be produced at what is considered moderate exercise intensity. In competition, however, athletes encounter periods of  time that require shorter high intensity pickups. This may include things such as a surge in the pack while on the bike, climbing a hill, swimming against a current or even a sprint to the finish line. This type of high intensity, short duration activities require periods of dominance by the anaerobic energy systems, and the aerobic system must quietly sit back until it is needed once again. in the sport of triathlon, it is important that all of the energy systems become well developed and learn to function in a state of dynamic flux.