Pathways of Energy
There are three energy systems in your body that are activated when you exercise. Which one is activated is based on the type and amount of fuel required to meet the demands of the activity with which you are involved. There are energy systems that are activated for short and intense bursts and others for a more steady release of energy. These metabolic processes are happening inside your muscle cells at a very microscopic level. They are the pathways of energy that are typically referred to as aerobic (with oxygen) or anaerobic (without oxygen). The oxygen we are talking about here is at the cell level. That is important. “Without oxygen” does not mean that a person is not breathing air.
Let’s look at how physical activity activates these energy pathways, of which there are two anaerobic systems, and one aerobic:
1. The ATP-CP system. This provides a short-lived and limited fuel supply. During high-intensity, or sprint events, there is a rapid activation of adenosine triphosphate (ATP) and creatine phosphate (CP). These are two high energy molecules within the muscle cell. CP is used to re-activate ATP through a cycle of metabolic events. What is important to understand is that CP is used up very quickly in the process, in about six to ten seconds. This energy source is beneficial for explosive energy. An example would be throwing a javelin or jumping for a slam dunk.
2. The anaerobic glycolysis system, (also called the lactic acid system because lactic acid is a byproduct of this metabolic energy pathway.) During intense activity, after the ATP-CP system has been used up, glucose is supplied to the muscle cells from stored glycogen. Then more ATP can be used to release energy. This pathway can normally sustain itself for up to two minutes. After that, lactic acid builds up in the cells and the muscles begin to fatigue. You may have heard of this as “lactic acid build up.” If you have ever sprinted at the end of a race you have activated this system.
3. The aerobic system. This is the third system and is designed to supply fuel for longer periods of time. Exercise at low to moderate intensity, approximately seventy percent of maximum heart rate or less, is mainly fueled by your fat stores. During these low intensity exercises, fat releases fatty acids into the blood stream to the cells. Glycogen is also released and glucose is produced. Between these two sources of fuel, exercise can be sustained for long periods of time, depending on conditioning. However, because glycogen stores are used, depletion will eventually happen and fatigue will set in. This result can be postponed by supplying nutrients while training or exercising. Jogging or swimming at a relaxed pace are examples of when this energy pathway is activated.