Lactic acid system
When you exercise beyond the limit of your ATP-PC system the lactic acid system kicks in. This system uses the energy from the breakdown of glucose (sugar) stored in your liver and muscles to resynthesise ATP. You get glucose from eating carbohydrates.
Energy to resynthesise ATP is provided by the breakdown of glucose
This system is also anaerobic as it works without or with very little oxygen. Because of this, lactic acid is produced as a waste product which can lead to fatigue if exercise intensity is not decreased. As there is still insufficient amounts of oxygen present the system it is quite limited and can only supply energy up to about 90 seconds. It is therefore used for activities that are short in duration, for example the 400m sprint, 100m swim, basketball and speed skating.