For your muscles -- in fact, for every cell in your body -- the source of energy that keeps everything going is called ATP. Adenosine triphosphate (ATP) is the biochemical way to store and use energy.
The entire reaction that turns ATP into energy is a bit complicated, but here is a good summary:
- Chemically, ATP is an adenine nucleotide bound to three phosphates.
- There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions.
- When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule.
- In some instances, the second phosphate group can also be broken to form adenosine monophosphate (AMP).
- When the cell has excess energy, it stores this energy by forming ATP from ADP and phosphate.
ATP is required for the biochemical reactions involved in any muscle contraction. As the work of the muscle increases, more and more ATP gets consumed and must be replaced in order for the muscle to keep moving.
Because ATP is so important, the body has several different systems to create ATP. These systems work together in phases. The interesting thing is that different forms of exercise use different systems, so a sprinter is getting ATP in a completely different way from a marathon runner!
ATP comes from three different biochemical systems in the muscle, in this order:
- phosphagen system
- glycogen-lactic acid system
- aerobic respiration
Now, let's look at each one in detail.