Once the body’s ATP-PC stores are used up (~10 secs), it is time for the anaerobic (lactic) energy system to contribute. It is the predominant ATP supplier for activities lasting over ~ 10 seconds, but less than ~ 2 minutes.
The function of this energy system is to breakdown stored glycogen into lactic acid, producing ATP. The anaerobic energy system does not use oxygen, so although it is not as quick to respond as the ATP-PC system, it can still provide energy on fairly quick notice.
A by product of the anaerobic energy system is lactic acid. Being acidic, it lowers the pH of the surrounding cells, which has a detrimental effect on enzymes and other processes. An increase in the concentration of lactic acid is partly responsible for muscle fatigue and the ‘burn’ that you may feel while exercising. Therefore, this energy system cannot be counted on for long term energy creation.
Activities which would predominantly use the anaerobic energy system include a 400 meter sprint, one round of boxing, and a 100m swim. Remember that all energy systems are used for every activity, but based on the intensity and the duration of the activity, there is generally one energy system which is predominantly called upon to provide ATP.