Energy systems
Evidence: strong
Muscle resupplies energy through three systems that run at once and blend by intensity and duration. Distance running is overwhelmingly aerobic, which is why aerobic development dominates training.
Every muscle contraction is powered by ATP, and the body holds only a few seconds’ worth, so it must be resupplied continuously. Three systems do this, and the key idea is that they operate simultaneously and blend smoothly by how hard and how long the effort is, rather than switching on and off (Baker et al. 2010):
- The phosphocreatine (ATP-PCr) system. Immediate and very high-power, but tiny: it fuels all-out efforts of a few seconds, such as a sprint start, then is spent.
- Anaerobic glycolysis. Breaks down carbohydrate without oxygen for fast ATP, dominating maximal efforts of roughly 20 seconds to a couple of minutes. It produces lactate, which, contrary to the old story, is a fuel the body reuses, not a waste product or the cause of fatigue.
- Oxidative phosphorylation (the aerobic system). Burns carbohydrate and fat with oxygen in the mitochondria. It is comparatively slow to ramp but effectively unlimited in capacity, and it powers everything sustained.
Why distance running is an aerobic event
The longer the effort, the more the aerobic system dominates. A 100 m sprint is fuelled mostly by the phosphocreatine and glycolytic systems; an 800 m is already aerobic-dominant at around 60 to 65%; by 1500 m the aerobic contribution is roughly 77 to 84%, by 5 km around 90%, and from 10 km to the marathon it is upward of 97 to 99% (Spencer & Gastin 2001; Baker et al. 2010). Distance running is, energetically, almost entirely an aerobic event.
This single fact shapes training. It is why the bulk of work targets the aerobic system through volume and the adaptations it drives in muscle, and why the anaerobic systems matter mainly at the sharp end: a final kick, or the shorter track events where glycolysis still contributes meaningfully. It is also why buffering aids and beta-alanine, which help the glycolytic, acidosis-limited efforts of one to a few minutes, do little for the marathon.