Elitetrack: Sport Training & Conditioning

The “energy systems” refer to the metabolic pathways by which energy is made available for muscular contractions. There are three major “energy systems” or pathways that the body uses to supply energy for working muscles. These pathways are known as the ATP- CP, anaerobic-lactate, and aerobic energy systems.
Before we discuss the exact pathways, it may be helpful to first have an understanding of the fundamental components:

• Adenosine Triphosphate (ATP) is a complex chemical compound that is stored in all cells, particularly muscle cells. ATP is required for the biochemical reactions of muscle contraction to take place. It is comprised of Adenosine bonded to three phosphates. During muscular contraction, ATP is broken down. This results in the release of energy through the breaking of a phosphate bond to the Adenosine. This leaves Adenosine Diphosphate (ADP) and a free phosphate. The greater the demand that is placed on a muscle the faster this breakdown of ATP will be. During intense exercise the ATP that is stored within muscle cells is quickly depleted. For continued muscular contractions to occur, ATP within the muscle must be quickly replenished. This is accomplished by the biochemical processes of the three energy systems.
  
• Creatine phosphate (CP) is a chemical compound stored in muscle that is important for replenishing ATP after the initial stores are exhausted. In this process, creatine phosphate donates its phosphate to ADP to create ATP. In this way, the creatine phosphate serves as an immediate source of high energy phosphate which can be used to replenish ATP. Because of its limited quantity, creatine phosphate only contributes to ATP replenishment for the first few seconds of intense activity.
  
• Lactic acid is a chemical byproduct of pyruvic acid which is produced from the breakdown of glucose.
  
• Glycogen is considered the principal storage form of glucose and is found mainly in the liver and muscles.
  
• Glycolysis is the breakdown of Glycogen which results in the production of ATP.
  
• Aerobic refers to anything that occurs in the presence of oxygen.
  
• Anaerobic refers to anything that occurs in the absence of oxygen.

Now that the preceding terminology has been covered, we can begin to examine the energy systems themselves. Remember that ATP is necessary for all muscular contractions and that during exercise ATP stored within muscle is quickly depleted. The 3 energy systems help to replenish the depleted ATP. There is a direct relationship between the exercise intensity and duration and the energy system which the body uses to supply energy. The following is a detailed description of each of the energy systems.

Anaerobic (ATP-CP) Energy System
This system can be broken down into 2 parts. The first part of this system provides energy for muscular contractions from the ATP that is stored within muscle cells. Remember however that a muscle cell only contains enough ATP for about 4-6 seconds worth of activity. This brings us to the second part of this system. When intramuscular ATP levels are depleted, CP is used to replenish ATP by donating a phospate to return ADP back into ATP. After about 4 to 9 seconds, creatine phosphate stores begin to deplete. When put together, the 2 parts of the ATP-CP system are the major source of energy for about the first 12 seconds of activity and continue to contribute at decreasing rates until about 20 seconds. When physical activity is concluded, ATP and CP stores automatically begin to replenish and can reach near peak levels within minutes. To develop this energy system, sessions involving repitions of 4-12 seconds of high intensity work at near peak velocity are required. Example training sessions to develop this energy system would be:

• 12 x 20m with recovery of 2 minutes recovery between reps
  
• 5 x 60m with 6 minutes recovery between reps
  
• 8 x 30m shuttle runs with 2 minutes recovery between reps

The Anaerobic Lactate System
After about 20 seconds worth of activity, the intramuscular ATP and CP are almost completely depleted and the muscles must then look to muscle glycogen as a means of replenishing ATP stores. This is done through a process called anaerobic glycolysis which results in the production of ATP and lactic acid. Because anaerobic glycolysis involves about 12 chemical reactions to make ATP, it is much slower to supply ATP than the ATP-CP system. Lactic acid can also be used to help replenish ATP stores but this conversion is likewise very slow and excess lactic acid can quickly build up. This build up of lactic acid can have negative effects on the contractile components of muscle. Example training sessions to help develop this energy system would be:

• 5 - 8 x 300m with 5 minutes recovery between reps
  
• 150m intervals at 400m pace with 3 minutes recovery between reps- until pace slows significantly
  
• 6 x 500m with 3 minutes recovery between reps

The Aerobic Energy System
After about 2 minutes of exercise, the aerobic energy system kicks in. This energy system uses carbohydrates, and then fats to make ATP. This system uses oxygen in its chemical reactions to slowly break down glucose in a process called aerobic glycolysis. When muscle, liver, and blood glucose stores get low, fatty acids will then be broken down to produce ATP in an even slower metabolic process. The aerobic energy system produces ATP at the slowest rate of the three systems, but it can continue to supply ATP for several hours or longer. Example training sessions to help develop this energy system would be:

• 4 - 6 * 2 - 5 minute runs with 2 to 5 minutes recovery between intervals
  
• 10 * 400m - 60 to 90 seconds recovery between intervals
  
• 5 to 10 km runs

Summary
The following is a chart summarizing the limits and characteristics of each energy system.

In order to get specific adaptations from each of the energy systems, the coach must scientifically design workouts to challenge each energy system. Anaerobic capacity is limited by the production of lactic acid and the system's ability to tolerate or buffer these acids. Anaerobic lactate training involves high intensity work, such as speed endurance, which stress the lactic acid system. Aerobic training is associated with the capacity to deliver oxygen and the ability to utilize oxygen. In light of that, aerobic training involves both intensity and duration, with the emphasis depending on whether one is attempting to develop aerobic power or endurance. The aerobic energy system is stressed by having athletes engage in continuous, extensive, and intensive tempo runs.