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 ATP = Adenosine Triphosphate CP = Creatine Phosphate ATP/CP : This cycle usually lasts the first one to three seconds during heavy exercise such as in resistance training. The Adenosine Molecule contains stores of ATP in cells. During exercise the cell releases one of the phosphate groups from the ATP cell, with this release energy is created. Through this conversion the ATP cell has become an ADP cell with the removal of one of the three phosphate groups.  The ADP cell will immediately be restored back into ATP if creatine is present. Creatine is made from the liver and kidneys which provides phosphate created by three amino acids; glycine, arginine and methionine. 
Glycolysis (why we feel the burn)
Glucose is used in this cycle to provide continuous energy through glycolysis. One of the two types is non-oxidative glycolysis where oxygen is not present. Non-oxidative glycolysis is used during exercises lasting more than four seconds and up to 60 seconds. As the glucose molecule breaks down it produces two ATP molecules which provides immediate energy to the muscle plus two other lactate molecules. The lactate molecules are removed from the cell and transported back into the bloodstream and liver, then it’s converted into glucose so it can return to the muscle. In resistance training, this energy cycle is limited due to the build up of lactate, this is why we feel a burning sensation when performing those last painful reps of a set. Krebs Cycle Endurance activities heavily rely on the Krebs cycle. Exercising at intensities low enough to allow the use of oxygen is dependent on a constant supply of glucose to drive the Krebs cycle. This cycle is present usually when you perform below your 85 percent MHR. The Krebs cycle is a circular wheel in the mitochondria into which acetyl coa enters where carbon dioxide and water exists. Several energy products (ATP, NADH, FADH) are sent through the lining of the mitochondria where they run through a electron transport chain, producing 3 ATP and 2 ATP. [|] |