This breakdown of ATP stimulates the contraction of the muscle. During exercise, the energy system being used to produce ATP depends on how long you have been exercising and at what intensity.
Lactic acid is toxic in large amounts, and therefore produces fatigue.
This system provides energy for a high - intensity activity of 30 seconds to 2 minutes duration. Lactic acid will generally be reduced to normal levels within 30 minutes of recovery, depending on the type of recovery undertaken - active or passive recovery. However the CP is exhausted after a relatively short period of time. This system provides energy for only a 1 to 10 second duration but occurs rapidly because it requires only one anaerobic chemical reaction. These energy stores are replenished after 2 -7 minutes with continual oxygen supply to the muscles.