1. The first anaerobic method of ATP resynthesis comes from a chemical compound called phosphocreatine (PC). Phosphocreatine is an energy-rich compound similar to ATP. It is stored in the muscle. However, we're more concerned with the other anaerobic system -- Lactic Acid.
2. The Lactic Acid System is the second method used to replenish ATP during exercise.

Daryl Conant