Energy in muscles is stored as:

ATP is resynthesized from ADP by the addition of a phosphate group. Some of the energy for this endothermic reaction is supplied by the breakdown of glucose to CO2 and H2O, but there also exists in muscle another energy-rich phosphate compound that can supply this energy for sho periods. This compound is phosphorylcreatine, which is hydrolyzed to creatine and phosphate groups with the release of considerable energy. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 5.