There are three energy systems used for muscle contraction. The first is the ATP-PC system. Adenosine Triphosphate (ATP) is used to supply energy that allows muscle fibers to shorten or contract. ATP cleaves a high-energy phosphate bond and using the sliding filament theory produces muscle contraction. Adenosine  triphosphate (ATP) becomes adenosine diphosphate (ADP) and a phoshate group. The PC is phosphocreatine. Phosphocreatine shuttles the phosphate group back to ADP to replenish ATP. This is how the supplement creatine monohydrate works by increasing creatine pools in the muscle fiber which in turn boosts power, adds reps, and increases set duration by extending this energy system.  The ATP-PC system lasts approximately ten seconds at ninety to one hundred five percent of age predicted maximum heart rate (APMHR) and eighty to ninety-five percent of VO2 max. ATP-PC provides a short burst of energy for activities like football, shot putting, and sprinting.
 
Anaerobic glycolysis uses a combination of protein and carbohydrates for energy to produce muscle contraction. Anaerobic glycolysis occurs at eighty to ninety percent of APMHR and seventy to eighty percent of VO2 max. Running a quarter mile would use Anaerobic glycolysis for energy.
 

 

The third energy system used for muscle contraction is aerobic metabolism. Aerobic metabolism uses carbohydrates and fatty-acids as energy sources. This energy system happens at sixty to eighty percent of APMHR and fifty to seventy percent VO2 max. Aerobic metabolism is said to be used by any activity ending with the suffix “ing” such as cycling, walking, running at a certain pace, rowing, swimming, and so on. Discussion of energy systems especially aerobic metabolism with a description of the citric acid or Krebs cycle and electron transport chain can become quite complicated. Please realize is a very basic, simplified description. I hope it was informative. Please visit Tpnbodyperfect.com and view our demonstration video for the best in nutrition software.