Substrate Utilization During Exercise

Effects of Exercise Intensity

Some clarity of the above graph was lost during the scanning. The top gray portion at the 65% and 85% intensity represents muscle glycogen. No muscle glycogen was utilized at the 25% intensity. Maximal fat utilization occurred at the 65% intensity, and absolute fat utilization decreased at the higher intensity. Rather, muscle glycogen was the primary substrate fueling the exercise.

Effects of Exercise Duration

Carbohydrate utilization decreases and fat utilization increases over time. This is due to decreased muscle glycogen content which can become depleted within 2-4 hr of continuous exercise depending on the intensity. Even though working muscles are taking up and using more glucose, blood [glucose] remains fairly constant because the liver is breaking down more liver glycogen and releasing the glucose into the blood. However, with time, liver glycogen becomes depleted causing reduced blood [glucose] and fatigue.