Skeletal muscle requires a temporary energy molecule known as ATP to energize the heads of the thick filaments (myosin) during muscle contraction. This supply of ATP is provided in each muscle cell by three metabolic systems.
The first system that provides the ATP needed for muscle contraction is the phosphagen system or CP system or creatine phosphate system. This anaerobic system creates enough ATP for about 15 seconds of initial muscle contraction. It does so by creatine phosphate, located in the muscle cell, releasing its phosphate group to ADP, a less energetic molecule than that which it creates in this process - ATP.
When a muscle is relaxed there will be a greater supply of CP than creatine and of ADP than ATP. The reverse occurs with muscle contraction - creatine has given up its phosphate to ADP and ATP has been formed so fewer ADP molecules are present.
After the initial 15 seconds or so of skeletal muscle contraction, this phosphagen system has been depleted of energy and the glycolytic system is recruited to supply the ATP required for muscle contraction. This process can be anaerobic (without oxygen present) or aerobic (with oxygen present). In either case, 2 molecules of ATP are formed by the breakdown of glucose (hence the name "glycolysis" -"-lysis" means breakdown).
The end product of glycolysis is either lactic acid if there has been no oxygen involved in the breakdown of glucose (anaerobic glycolysis) or pyruvic acid if oxygen has been present in the glucose breakdown (aerobic glycolysis).
Anaerobic glycolysis leads to muscle fatigue or an inability for the muscle to contract. Pyruvic acid from aerobic glycolysis will go into the Krebs cycle for further breakdown to capture even more molecules of ATP. As part of this aerobic process of glucose breakdown, the last "stage" will be the electron transport chain. Creation of 36-38 molecules of ATP will be generated by this aerobic process of glycolysis, Krebs cycle, and electron transport chain.
A person engaged in repetitive sprinting or jumping, examples of anaerobic activities, would rely heavily on the phosphagen and anaerobic glycolytic systems for creation of ATP for muscle contraction. This is the reason why such activities are exhaustive in nature or short-lived in how long we can engage in them before fatiguing.
A person engaged in slower, less intense activity such as jogging can continue activity for longer periods than the sprinter/jumper because they have engaged the aerobic production of ATP through aerobic glycolysis, Krebs cycle, and electron transport chain.
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