Direct Phosphorylation( quickest), Anaerobic Respiration, Aerobic Respiration (longer period of time).
Explanation: The energy which can be used by our body for muscle contractions can be generated by muscles cells in three different ways. These includes:
--> Direct Phosphorylation
--> Anaerobic Respiration and
--> Aerobic Respiration.
DIRECT PHOSPHORYLATION: As ATP is used up, it's re-stocked by the small reserve of energy stored as creatine phosphate. Creatine phosphate very rapidly donates its high-energy phosphate to ADP the moment ADP forms, converting it back to ATP. This extra source of ATP is easily mobilized and is very effective as long as it last. It's limited though because the store of creatine phosphate is small. (Normally the supply of creatine phosphate is re-stocked by oxidative metabolism via the ATP produced by the citric acid cycle, but during intense exercise there is not enough time to do this so the stock is burned in 20-25 sec making it the QUICKEST way to supply energy)
ANAEROBIC RESPIRATION: When oxygen levels in the cells are low, glucose molecules undergoes glycolysis to produce 2 molecules of pyruvic acid( ATP). The pyruvic acid is converted anaerobically to lactic acid. The build up of lactic acid causes the pain and cramps of overexercised muscles. It is a fast but an inefficient way of producing ATP for energy use.
AEROBIC RESPIRATION: This occurs only if the oxygen supply is adequate and is the process by which energy is released during prolonged exercise. It utilizes glucose and glycogen. During the glycolysis process, a glucose molecule is converted to 2 molecules of ATP in the cytoplasm of cell. In the presence of sufficient oxygen, the pyruvic acid enters the citric acid cycle and oxidative phosphorylation giving rise to the by products of 36 ATP, carbon dioxide and water. Aerobic respiration is SLOW (takes the longest period) but efficient process.
