Originally Posted by JPradun
Sprints up to ~20 seconds use ATP, which is adenosine triphosphate. If you are already depleted, though, you cannot use it. Sprinting in road cycling and crits are mostly about your anaerobic system, which is why you do intervals.
Actually Kuan's got it down. The muscles run off ATP under ALL conditions. It's just a matter of how you produce that ATP
1. Under 10-seconds at 100% effort, you're using phosphocreatine to generate the ATP
2. Longer 10s to 4-5 min anaerobic efforts will use anaerobic fermentation of glucose to create ATP. Builds up H+ and lactic acid.
3. Steady-state aerobic efforts of 5min+ will be using aerobic glycolysis of glucose to create ATP. The fatty-acid spiral is also used to create ATP under areobic conditions.