Skeletal muscle fibers have the unique ability to switch between rest and contraction states, using different sources of ATP for energy. The contraction cycle and Ca2+ transport back into the sarcoplasmic reticulum for relaxation require significant ATP. However, the ATP reserves in muscle fibers are limited and can only sustain contractions for a few seconds. Additional ATP production becomes necessary for prolonged contractions. As a result, muscle fibers generate ATP through various sources, including creatine phosphate, anaerobic glycolysis, and aerobic respiration.
Creatine phosphate
While creatine phosphate is exclusive to muscle fibers, anaerobic glycolysis and aerobic respiration are used by all cells to produce ATP. During periods of low energy demand, creatine phosphate is formed by transferring a phosphate group from excess ATP produced by muscle fibers. This acts as a quick source of ATP during muscle contractions. While other energy-generating mechanisms take longer, creatine phosphate and ATP stores provide enough energy for maximal muscle contraction to last approximately 15 seconds.
When creatine phosphate reserves are depleted due to muscle activity, glycogen reserves are used as the next source of ATP. It breaks down glucose into two molecules of pyruvate, and two molecules of net ATP are produced in the cytosol. It can provide energy only until the reserves last, which is around 2 minutes of maximal muscle activity.
Aerobic respiration
Muscle tissue obtains oxygen from the blood and myoglobin within muscle fibers. Under conditions of sufficient oxygen, aerobic respiration occurs in the mitochondria as the pyruvate undergoes a series of reactions that generate ATP, carbon dioxide, water, and heat. Aerobic respiration supplies sufficient ATP during rest or light to moderate exercise, utilizing nutrients like pyruvate, fatty acids, and amino acids. It is the primary ATP source for activities lasting several minutes to an hour or more.
Anaerobic glycolysis
However, in case of insufficient oxygen supply, the pyruvate from glycolysis is converted to lactic acid, yielding NAD+ in addition to the two molecules of ATP produced by previous steps of glycolysis. This process, called lactic acid fermentation or anaerobic glycolysis, is an extension of glycolysis. Lactic acid is released into the blood and can be converted back to glucose by the liver.
