During physical exertion, the body uses three energy systems: the Anaerobic Phosphagen System, Glycolytic System and the Aerobic System. Depending on the activity, the body uses one energy system more than the others.
Understanding of the 3 energy systems, gives a person valuable information about which system is being used during a given activity. Once an individual has an idea of the activity and the corresponding energy system being used, they can then work on improving the function of that system. This information is also valuable to a personal trainer or coach when guiding clients though workouts.
When a person engages in short intense activities, this creates a high demand for ATP. ATP, also know as Adenosine triphosphate, is considered by scientists to be the energy unit of life. ATP is stored in our cells, and provides us the energy for almost all our activities. In a short burst, a large amount of power is produced by our muscles. Our body is able to do this by storing creatine phosphate in our muscles. During this process, the phosphate is converted from ADP to ATP. For this small 10 second window, no carbohydrate or fat is used. This system fatigue is rapid, because of the limited amount of stored creatine phosphate in our muscles.
Glycolysis, literally means the breaking down of glucose. This system uses carbohydrates in the form of blood glucose or muscle glycogen. These carbohydrates are then broken down into glucose. “For every molecule of glucose broken down to pyruvate through glycolysis, two molecules of usable ATP are produced. Thus, very little energy is produced through this pathway, but the trade-off is that you get the energy quickly.” (Karp 2012) Intense activities lasting from 30 seconds to around 2 minutes use Glycolysis as the main energy system. If the effort does not cease at about 2 minutes, this is when Acidosis kicks in. Lactic acidosis occurs when the body produces too much lactic acid and cannot metabolize it fast enough. Lactic acidosis causes muscles to lose their ability to contract properly, and intensity ultimately decreases.
The Aerobic system occurs in the mitochondria of the cell and is known for being a long term energy system. When physical activity lasts more than 2 to 3 minutes the Aerobic system becomes the dominant energy system. This occurs after the phosphagen and anaerobic systems have fatigued. During this time the body uses oxygen to produce the necessary ATP, because carbohydrates and fats are only used in the presence of oxygen.
One take away from this article is that all three of these systems work to meet the energy needs of the body during physical activity. These systems do not simply shut off and on, but rather dominate at different times, depending on intensity and duration of the activity.
Need more in depth information on Energy Systems during exercise? Check out: Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3005844/