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=Glycogen Usage=
{| class="wikitable"|[[File:Absolute substrate usage against intensity.jpg|none|thumb|400px|The contribution of different energy sources changes<ref name="romijn"/> with exercise intensity. These values were taken after 30 min. of exercise. Note that the total calories available from the blood (free fatty acid and glucose) remains about the same regardless of exercise intensity.]]|[[File:Glycogen usage and intensity.jpg|thumb|400px|Percentage of energy from glycogen plotted against exercise intensity as percentage of [[VO2max|V̇O<sub>2</sub>max]].]]|}
At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.
[[File:Substrate usage over time.jpg|none|thumb|400px|Changes in substrate usage<ref name="romijn"/> over 120 min period at 65% [[VO2max|V̇O<sub>2</sub>max]].]]