Final answer:
Muscle cells preferentially use glycogen for initial ATP production during exertion because it's a quickly accessible energy reserve within the cells, supporting sustained ATP production before the body taps into blood glucose and fat reserves.
Step-by-step explanation:
During periods of increased exertion, muscle glycogen is the primary source of glucose because it is readily available within the muscle cells and can be quickly metabolized to produce ATP. Muscle glycogen is broken down into glucose 1-phosphate (G-1-P), which is then converted into glucose-6-phosphate (G-6-P) and enters the glycolytic pathway to produce pyruvate and subsequently ATP through aerobic processes such as the Krebs cycle and Chemiosmosis, if oxygen is present. As the muscle's glycogen levels fall, the liver releases glucose into the bloodstream, and the body starts to increase fat metabolism to fuel aerobic pathways. Glycogen acts as a more immediate energy reserve, allowing ATP production to continue for a longer duration during exercise before resorting to alternative fuel sources such as blood glucose and fats.