Final answer:
The limiting factor to extremely high duration steady-state exercise is blood glucose levels, which once depleted, causes the exhaustion of energy reserves leading to fatigue.
Step-by-step explanation:
The limiting factor to extremely high duration steady-state exercise is primarily (b) Blood glucose levels. Initially, during increased exertion, muscle glycogen is broken down to produce glucose, which is then used in glycolysis, leading to the production of pyruvate that enters the Krebs cycle and chemiosmosis in the presence of oxygen, resulting in the production of carbon dioxide, water, and energy. As muscle glycogen levels start to decline, the liver releases glucose into the bloodstream and fat metabolism increases to fuel the aerobic pathways. However, there is a limit to how much glucose the liver can release, and once the glucose and glycogen reserves are exhausted, the individual will likely experience fatigue due to the exhaustion of energy reserves. This is why maintaining blood glucose levels is critical for sustained high-duration exercise.