There are basically two types of energy systems that the body utilizes, Aerobic and Anaerobic. Each energy system produces Adenosine Triphosphate (ATP), which is used by the muscles to contract.



ATP, or adenosine triphosphate, is a molecule that carries energy within cells. It is often referred to as the "energy currency" of the cell. ATP is composed of three phosphate groups, a ribose sugar, and an adenine base. The energy in ATP is stored in the high-energy phosphate bonds between the phosphate groups.

When cells need energy for various processes such as muscle contraction, active transport across cell membranes, or synthesis of biomolecules, ATP is hydrolyzed to release energy. This process involves breaking the last phosphate bond, resulting in the formation of adenosine diphosphate (ADP) and an inorganic phosphate molecule. The energy released during this reaction is used to power cellular activities.

 


The Aerobic System can utilize carbohydrates, proteins or fat to supply an unlimited amount of ATP as long as oxygen is present. The Aerobic system provides medium to very long duration energy production with low to moderate power (less than 85% of maximum output). The by-product of this system is heat, water and carbon dioxide.

The Anaerobic System can only utilize carbohydrates for ATP production. This system does not use oxygen in the metabolization of its fuel source. The Anaerobic System provides short duration (45 - 70 seconds) and high power. The by-product of the metabolization of glucose (glycolysis) in this system is heat and lactic acid, the cause of muscle soreness immediately after exercise. Muscle soreness 24 to 48 hours after exercise is due to torn muscle fibers and connective tissue. This type of soreness can be reduced by adequate warm-up and cool-down stretching exercises.

Aerobic capacity is the ability of the body to collect and transfer oxygen from the air through the lungs and blood to the working muscles. This is related to cardio-respiratory endurance and is referred to as Maximal Oxygen Consumption or VO2 max. Aerobic Capacity reduces at about 10% per decade after 30 years of age. 

The Anaerobic Threshold is defined as that point where the body can no longer meet the oxygen demand and it's anaerobic metabolism is accelerated. This point varies on an individual basis and is dependent on fitness level. For healthy individuals, this occurs between 50% and 66% of their maximal working capacity. This would be equivalent to running faster than half speed.