Creatine reduces the oxygen requirement during timed bike sprints

Muscles get energy for contractions by breaking down adenosine triphosphate (ATP) - an important compound that provides cells with most of their energy. Creatine phosphate is an energy-rich chemical that restores the full energy levels of ATP after it has released some of its energy. Creatine phosphate does this by providing an energy-rich phosphate to regenerate ATP.
Creatine monohydrate supplements increase creatine phosphate levels in muscle by 20 percent. Most ATP comes from reactions involving oxygen, but these reactions are slower than the energy-rich reactions involving ATP and creatine phosphate.

Brazilian scientists have shown how creatine phosphate, which is synthesized from creatine monohydrate, could improve performance during high-intensity cycling sprints. Athletes who followed a creatine loading protocol (20 grams per day for 5 days) showed lower oxygen demand during the initial phase of a one kilometer timed sprint.

The body will preferentially use the energy that is available to it. He chose to use more creatine phosphate during the initial phase of the sprint because more of it was available.

(Medicine & Science in Sports & Exercise, 47: 2660 - 2668, 2015)