Energy: producing and using fuel

The Energy pillar answers one question: where does the energy for a muscle contraction come from? Two mechanisms and two videos describe ATP, phosphocreatine and the glycogen stored in muscle.

The Energy pillar answers the question: where does the energy that lets a muscle contract come from?

Every contraction consumes the same molecule, ATP. The immediate ATP reserve in muscle is very small: the body rebuilds it continuously, through several routes that differ in both speed and staying power. Understanding those routes means understanding why a sprint and a long walk do not tire you in the same way.

What this pillar explains

Why ATP is the only fuel the muscle fibre can use directly.
The role of phosphocreatine as a stopgap store for very short efforts.
What glycogen is, where it is stored and when it becomes the dominant source.
Why storing glycogen brings water with it, and what that changes on a scale.
What "running out of energy" means at cell level, and what the phrase covers day to day.
What becomes of the carbohydrate and the fat you eat, from the gut to the store and on to muscle.

How to work through the pillar

Six written pages, two videos, one recommended reading order.

1.
ATP and phosphocreatine describes what fuels the shortest and most intense efforts.
2.
The aerobic system takes over as soon as an effort lasts, and never stops.
3.
Muscle glycogen and water describes the muscle's carbohydrate store and weight swings across a few days.
4.
What happens to dietary carbohydrate works upstream: what goes on between the plate and that store.
5.
What happens to dietary fat does the same for the second fuel, whose route and store follow different rules.
6.
Fatigue closes the pillar on what "not being able to go on" really covers.

The videos animate the same mechanisms. They are there to make a cycle visible that is hard to picture from text alone, and they always point back to the matching written page, which carries the sources and the limits.

This pillar reads well straight after the Body pillar: once a muscle has been identified, the next question is what it consumes in order to contract. It also prepares the Recovery pillar, since rebuilding stores is part of returning to balance after an effort.

Note

Three routes, not three switches

Energy pathways are often presented as systems that hand over to one another in turn. It is a convenient teaching simplification, but an inaccurate one: they all run at the same time, and what changes with the intensity and duration of the effort is their relative contribution.

The pages in the pillar flag this kind of simplification explicitly, with the matching level of certainty.

What this pillar does not do

These pages describe a general physiology. They calculate no individual requirement, recommend no carbohydrate intake and propose no eating plan. Questions of applied nutrition belong to Shapier, and medical situations — diabetes, metabolic disorders, ongoing treatment — belong to a healthcare professional.

A worrying sign during effort, such as faintness, chest pain or unusual breathlessness, warrants medical advice and is not covered anywhere on this site.

The pages in the pillar

Put it into practice in Shapier

Follow the Energy and recovery pathway

The pathway that organises this content on the practical side is followed in Shapier; Body Lab only explains where the energy for a contraction comes from.

Follow the Energy and recovery pathway

Body Lab explains; Shapier lets you act and track.

The whole Energy pillar

ATP and phosphocreatine
Where does the energy for the very first seconds of a maximal effort come from? This page describes the energy currency of muscle, the buffering role of phosphocreatine, and why the energy systems do not take turns the way they are usually described.
With a 3D scene
The aerobic system
The pathway that supplies most of the energy as soon as an effort lasts. It burns carbohydrate and fat with oxygen, produces a great deal but slowly, and never switches off.
With a 3D scene
Muscle glycogen and water
Why can the number on the scales change within two days without any change in fat mass? This page explains what muscle glycogen is, its role as a fuel and the water that accompanies it inside the muscle.
With a 3D scene
What happens to dietary carbohydrate
A carbohydrate you eat is taken apart into simple sugars, absorbed, then sorted. Some is burnt at once, some stored in the liver, some in the muscle — and those two stores do not do the same job at all.
With a 3D scene
What happens to dietary fat
Fat does not dissolve in water: its route out of the intestine resembles that of no other nutrient. It joins an enormous store, and its share during effort follows a curve that rises and then falls.
Described without a scene
Fatigue
It is not one thing. The drop in force and the sense of effort do not follow the same timeline, do not come from the same place, and are not measured the same way.
With a 3D scene
The ATP–phosphocreatine system — the video
An animation that follows one ATP molecule through a short, intense effort and then through the rest that follows: it is spent, recharged by phosphocreatine, and backed up by the other pathways. The page carries the same content in text, with its sources and limits.
With a 3D scene
Glycogen and weight swings — the video
An animation that explains why the number on a scale moves by several hundred grams from one day to the next: muscle glycogen is stored along with water, and that water changes in quantity far faster than tissue does.
With a 3D scene

Available offline

Offline download is available in the mobile app.

Trust and method

Author

equipe-editoriale-shapier

Editorial review

Thanh Chau

Scientific review

Pending

Published on

August 2, 2026

Reviewed on August 2, 2026

Next review due August 2, 2027

Limits of this page

This entry page describes how the pillar is organised; the energy mechanisms, their sources and their limits are set out on the detailed pages.

Educational content. Body Lab does not diagnose and does not replace professional advice.

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