The source of energy for our cells, including the energy needed for muscular contraction, is Adenosine Triphosphate (ATP).
Without sufficient amounts of Adenosine Triphosphate, most cells die quickly.
Adenosine Triphosphate is formed when adenosine diphosphate (ADP) and an inorganic phosphate are combined. This process of combining these phosphates requires a large amount of energy. For this reason, approximately 60% of the energy from food is converted to thermal energy during the process of ATP formation and only about 40% of the actual energy from food is converted to ATP.
ATP is the muscle ’s primary energy source, just as gasoline is the energy source for an automobile. How quickly and efficiently a muscle cell produces ATP will be a determining factor of how much work the cell can perform before it fatigues. While there is some stored within a muscle cell, the supply (enough for 2-4 seconds of muscle action). Therefore, muscle cells must be able to produce more Adenosine Triphosphate in order to continue working. Energy is released when an ATP molecule is broken into an ADP molecule and an inorganic phosphate.
Since muscle cells can only store a limited supply of ATP, and they require a constant supply to function, various metabolic pathways must exist to produce it quickly or cell death, not just poor functioning, will occur. It must be regenerated by reactions that add the phosphate back to ADP (the reverse reaction of the image above).
Muscle cells replenish the Adenosine Triphosphate supply using three distinct biochemical pathways. ATP in muscle cells can be formed through a combination of phosphocreatine (PCr) breakdown, glycolysis (glucose or glycogen breakdown), and oxidative phosphorylation (which is inherently aerobic as the oxidation process requires oxygen).
Anaerobic processes do not require oxygen for reactions while aerobic processes utilize oxygen in the processes of breakdown and synthesis. All anaerobic processes occur in the sarcoplasm (the cytoplasm/cell fluid of the muscle cell). All aerobic processes (utilizing the oxidative/aerobic energy system/metabolic pathway) occur in the mitochondria of muscle cells. The intensity and duration of activity will determine which energy system/metabolic pathway is dominant at any particular time.