A muscle cell can only spend ATP, and it holds barely two seconds’ worth. Everything else — the phosphagen system, glycolysis, oxidative phosphorylation — exists to rebuild that ATP as fast as it is spent. Which system does the rebuilding is not a choice. It is decided by how hard and how long. Move those two dials and watch the handover happen.
Who is making the ATP
The tanks it is drawing on
Set an intensitythen slide time forward and watch the handover
Jump the clock
Body and starting stores
The time axis is logarithmic — one second and four hours have to share a chart, because that is the range over which this handover actually happens.
Illustrative demonstrations of average physiology built from round numbers. None is a training plan, a target, or a recommendation for any particular person.
Energy turnover0kcal/min
This is a model, and models simplify. Real muscle recruits fibre types unevenly, so a “whole body” percentage hides the fact that one motor unit may be almost fully glycolytic while its neighbour idles. Substrate curves come from group averages in laboratory conditions and vary widely between people and within the same person on different days. Nothing here describes what any individual should eat, drink, or do — that is a conversation with a qualified clinician or sports dietitian, not with an equation.