Whenever a muscle contracts, it needs energy. This energy is released through a process called respiration. Respiration is the process by which glucoseis converted to energy. (http://en.wikipedia.org/wiki/Cellular_respiration)
The release of this energy is actually the breakdown of a chemical called Adenosine Triphosphate (ATP) During this reaction, two new chemicals are formed: Adenosine Diphosphate (ADP) and phosphate (P).Since there is a limited amount of this in cells, ATP must be rebuilt from these two products. (http://www.s-cool.co.uk/gcse/pe/how-the-body-obtains-its-energy/revise-it/muscle-respiration )
This is known as muscle respiration, and can happen in two ways.