Once the initial anaerobic energy phase (ATP-CP system) has passed (after 8-10 seconds), your bodies revert to using carbohydrate stores in the form of glucose by breaking down glycogen for energy...this is known as the Lactic acid anaerobic energy system.
The breakdown of glucose yields phosphate which is used to replenish Adenosine Diphosphate (ADP), returning it to Adenosine Triphosphate (ATP) which is used to produce energy.
But isn’t Lactic Acid bad?? This by- product has been given a lot of stick over the years; it was blamed for the ‘burn’ and fatigue experienced during training...when in fact, it was providing energy all along! The real culprit is Hydrogen! This is produced in the muscle during exertion when oxygen is in short supply, which increases the acidity of the blood causing that characteristic burn you feel as you fatigue.
Consider consuming a protein and carb blend such as XL Nutrition Xtra protein and carbs to feed your glycogen stores to stave off fatigue.
Opt for an isotonic beverage such as High 5 Energy Source 2:1 Fructose.
Also go for a beta-alanine or carnosine supplement...this has been seen to buffer (soak up) excess hydrogen ions helping you train more effectively, and for longer! These supplements also support the metabolism of fat and weight loss!