During sustained lower intensity efforts (for example, brisk walking, slow swim) the brain will call upon primarily Type I muscle fibers. Here the intensity is low so epinephrine levels will only be slightly elevated. In a fasting state, working muscle cells will be primarily fueled by fatty acids, with the majority coming from the blood (Figure 11.4 and see Figure 5.4). However, as the intensity of the effort increases so too will epinephrine in the blood and as a result the breakdown of glycogen in working muscle. As this occurs, glucose from glycogen stores starts to become a bigger contributor of fuel. As the intensity level continues to increase, so too will the reliance on glucose. One reason for this is that as the intensity level is increased the brain will support Type I muscle fiber efforts with more and more Type II muscle fibers. Type II muscle fibers tend to use more glucose.
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