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Now we are around the 80-second mark of this intense hike, but you start to slow down dramatically because of a few factors: your muscles are burning, you have run out of PCr, and you have reached the maximum capacity for glycolysis. At this point, the energy system that helps you keep going is known as oxidative phosphorylation. This system has more oxygen available to it and yields the most ATP, and it can do so for a very long time (it is the central energy system during endurance sports). This process uses two systems inside the mitochondria (energy factories inside cells) to generate ATP: the Krebs cycle (a process that generates most of the ATP during oxidative phosphorylation) and the electron transport chain (a system of rapidly moving electrons that creates kinetic energy to generate ATP). It’s important to note that the two key enzymes that start the Krebs cycle are pyruvate (from glycolysis and amino acids) and acetyl-CoA (from fatty acids). Some supplements that assist in this more aerobic environment are sodium phosphate, cordyceps mushrooms, beet juice, and green tea extract (see ssss1, Supplements for Mountain Athletes).