The Energy Systems of the Body
Filed under: Exercise Science
No presentation on training would be complete without a discussion about energy systems. Any athlete serious about their performance should have a basic understanding of energy systems and how they apply to training. This important information will give the athlete a competitive advantage and increase their chances for favorable outcomes.
ATP
The immediate source of energy for muscular contraction is the high energy phosphate compound adenosine triphosphate (ATP). ATP is the body's carrier of energy. When we eat foods, the energy from the chemical breakdown of these products is not directly available for energy. This energy is transferred and stored as potential energy. When required, this stored energy is delivered through ATP to the working muscle. ATP provides the body with the necessary energy requirements.
When ATP splits chemically and releases one of its three phosphate molecules (tri-phosphate), energy is created and adenosine diphosphate (ADP) is formed. ADP needs to bond with another phosphate molecule to restructure and reform ATP. The muscle cells, however, have a limited amount of available ATP. Muscle cells can produce ATP by any one or a combination of three metabolic pathways. The purpose of the energy systems is to regenerate ADP to ATP and/or supplying the body with enough ATP to meet energy demands. The three metabolic pathways are:
ATP-PC (Creatine Phosphate) System
Lactic Acid or Gylcolysis System
Aerobic System
ATP-PC System
This energy system provides the energy for muscular contraction during short-term (5 seconds), high intensity (all-out) exercise. Energy for ATP regeneration is supplied quickly by chemical energy transfer from another high energy phosphate compound, creatine phosphate (CP). Muscle cells have a CP concentration 4 times greater than ATP. Therefore, CP acts as a reservoir of high energy phosphate use to restructure the ATP molecule.
CP <–> C + P = Energy
ATP + H2O <–> ADP + P + Energy
The ATP-PC system is very dynamic because it provides immediate energy for these very intense efforts. However, the drawback of this system is that this ATP production is very limited and an athlete can only sustain these all-out attempts for a very short period of time (approx. 5 seconds).
In a world-record 100m sprint (9.84s or 27.1 mph) the runner cannot maintain maximum speed throughout the run. During the last few seconds, the runner actually slows with the winner often slowing the least. In this situation, the quality of intramuscular, high energy phosphate significantly affects performance. 1
ATP is what pays the bills. The quality and quantity of stored high energy phosphate molecules influences a person's ability to generate energy. An athlete's genetic predisposition toward higher ATP and CP stores may play a role in an athlete's performance. In addition, specific training in this energy system will increase an athlete's phosphate stores and increase short-term, high intensity efforts.
Lactic Acid / Glycolysis System
This system provides energy for high intensity exercise lasting slightly longer (1-3 minutes) than the ATP-CP system. In this system, oxygen is not needed for the formation of energy and is termed anaerobic metabolism. The phosphate molecule needed to regenerate ATP from ADP comes mainly from the breakdown of glucose and stored glycogen and results in the formation of a byproduct called lactic acid. An accumulation of lactic acid will eventually seep into the bloodstream and form lactate and cause a rise of hydrogen ions which will result in an acidosis. The point at which lactic acid production exceeds lactic acid removal is called lactate threshold. The resulting acidosis is often felt as a "burning" or a "heaviness" in the working muscle. Eventually with rising lactate levels, ventilation will increase and gas exchange will be adversely affected causing the athlete to slow down and reduce exercise intensity.
Specific training in this energy system will eventually lead to favorable training adaptations. Increasing lactate threshold is an important physiologic marker to gauge increased fitness. Chronic training at or beyond one's lactate threshold using interval-type exercise sessions will decrease production of lactic acid for a given exercise intensity, will increase lactic acid clearance from the working muscle and also will allow a greater tolerance to lactate accumulation.
Aerobic Metabolism
This energy system is the most efficient system for producing the most ATP molecules. For exercise lasting longer than 3 minutes, the majority of energy comes from this metabolic pathway. Energy originates within the mitochondria from stored glycogen and fat and needs oxygen in the process to convert fuel to ATP. The slower you go, the more you rely on fat for energy and the glycogen stores are spared. As the pace increases, there is a gradual shift away from fat and carbohydrates become the preferred fuel. If effort continues to increase, oxygen delivery will eventually be unable to keep pace with demand and energy requirements will become anaerobic.
Under aerobic conditions, the rate of lactic acid formation in the working muscle is equal to its removal, resulting in no net lactic acid accumulation. Adaptation within the muscle cells from prolonged aerobic training allows a high rate of lactate clearance. A training affect will demonstrate an increased work intensity before lactic acid production exceeds removal.
Training necessary to improve this energy system is exercise below your lactate threshold level. The objective of this type of training is to gradually increase training duration and to build a good aerobic base before more strenuous exercise is attempted. In order to maximize your potential, adequate time should be spent preparing your aerobic capacity. (See Base Training)
Conclusion
The benefit from understanding energy systems enables the athlete or coach to design a training program to meet the specific energy needs of the event. Prior to developing a training plan, breakdown the event and look at the specific components (demands) and requirements. Determine your energy needs and how much time should be devoted to training each system. Remember, specificity of training is the most important training principle. Training relative to the event will increase your chances for a favorable outcome.
1. McArdle, WD, Katch, FI, Katch, VL. Exercise Physiology. 5th ed Baltimore, MD; Lippincott Williams & Wilkins, 2001.
2. Burke, ER, Serious Cycling. 2nd ed Champaign, IL; Human Kinetics, 2002.
3. Friel, J. The Cyclist Training Bible. 3rd ed Boulder, CO; Velo Press, 2003.