Final answer:
As exercise continues, muscle glycogen is initially used as the primary energy source but starts decreasing as the body mobilizes liver glucose and increases fat metabolism. Muscle glycogen is a readily available energy source that provides ATP for exercise but declines as glycogen stores are depleted and other energy sources are utilized.
Step-by-step explanation:
As exercise continues, the amount of muscle glycogen used for energy changes. Initially, muscle glycogen is the main source of glucose, supporting short bursts of exertion through glycolysis and aerobic pathways, like the Krebs cycle and chemiosmosis. However, as exercise persists and glycogen levels in muscles decrease, the body starts to release glucose into the bloodstream from the liver. At the same time, fat metabolism becomes more prevalent to fuel exercise. Therefore, the reliance on muscle glycogen declines, giving way to other sources of energy such as blood glucose and fats.
Aerobic exercise may be powered by both glycogen reserves and fat reserves, with the ratio depending on the intensity and duration of the activity. Muscle glycogen is a readily available energy source and is broken down into glucose 1-phosphate which enters the glycolytic pathway. About 70% of the body's total glycogen is stored in muscles, although the capacity for storage by weight is higher in the liver.
When our bodies have sufficient ATP and when food sources are ample, excess glucose is stored as glycogen in muscles and the liver for future energy needs. Yet, as exercise continues and demands for energy increase, these glycogen stores are mobilized to maintain ATP production, essential for muscle contraction and activity.