Glycolysis

Glycolysis splits a glucose molecule into two pyruvate molecules, producing a small net yield of ATP quickly and without needing oxygen. What happens next depends on oxygen availability: with enough, pyruvate enters the mitochondria for the Krebs cycle; without, it is converted to lactate so the pathway can keep running.

It matters because it is the dominant energy system for efforts lasting roughly ten seconds to two minutes — the 400 metres, a hard set of twenty, the last push up a climb. It is fast but inefficient, extracting only a fraction of the energy that full oxidative breakdown would.

In practice, training this system means repeated hard efforts in that duration range, and the adaptations include better buffering of the hydrogen ions that accumulate alongside lactate. It is also the reason muscle glycogen availability limits high-intensity work in a way it does not limit easy work.