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Mechanism
Intermediate
9 min read

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.
3D scene
ATP–phosphocreatine cycle
The scene shows the loop that rebuilds ATP. The aerobic pathway is what feeds that loop continuously, without interruption. Without the 3D the idea is unchanged: ATP is not stored but recycled, and the aerobic system is what keeps the recycling running.
Open in the explorer

What fuels an effort that lasts?

The pages on ATP and phosphocreatine and on muscle glycogen describe stores that empty fast. Past a few tens of seconds, most of the energy comes from elsewhere: from a slower pathway whose capacity is on another scale entirely.
Key point
The answer in three sentences
The aerobic system makes ATP by oxidising carbohydrate and fat inside the mitochondria, using oxygen. It yields far more ATP per molecule of fuel than the fast pathways, but it yields it more slowly.
It runs continuously, including at rest and during a sprint. What changes with intensity is not whether it is switched on, but its share of the total and which fuel it favours.

The mitochondrion, the workshop of the aerobic pathway

Mitochondrion
A compartment inside the cell where fuels are oxidised in the presence of oxygen to rebuild ATP. A muscle fibre contains hundreds of them, and their number rises with endurance training.
the cell's power plant
The mechanism has three steps. Fuels — sugars and fats — are first cut into two-carbon fragments. Those fragments enter a cycle of reactions that strips electrons from them. Those electrons then travel down a chain of carriers, and it is that journey that rebuilds ATP. Oxygen comes in right at the end, as the recipient of the electrons: without it the chain jams and everything upstream stops.
Oxidation
A reaction that removes electrons from a molecule. In muscle, oxidising a fuel means recovering its energy in usable form, gradually turning it into carbon dioxide and water.
cellular combustion

Two fuels, a gradual shift

The aerobic system burns carbohydrate and fat, in a proportion that changes with the intensity of the effort.

What each fuel provides and what limits it

Criterion
Fat
Carbohydrate
Store available
Very large
Limited, a few thousand kilojoules
Rate of supply
Slow
Faster
Oxygen needed
Higher per unit of energy
Lower
Intensity where its share dominates
Low to moderate
Moderate to high
At low intensity, fat supplies a large part of the energy. As intensity rises, the share of carbohydrate grows until it dominates. The reason is mechanical: making ATP from fat takes more oxygen and more steps, which caps the rate. Carbohydrate is quicker to mobilise.
Certainty level · Established
The aerobic system becomes the dominant source of ATP as soon as an effort passes a few tens of seconds.
Reviews of energy system interaction converge on this point, drawing on gas exchange and biopsies during maximal ergometer efforts. The exact timing of the shift varies with the estimation method, the intensity and the protocol used, and published values differ between research groups.
Gastin PB (2001) · Baker JS, McCormick MC, Robergs RA (2010)

Timeline of a prolonged effort

What fuels a continuous effort, minute by minute

  • 1
    Start
    0 s
    ATP and phosphocreatine stores cover the moment while breathing and cardiac output climb.
  • 2
    Ramping up
    30 s – 3 min
    The aerobic pathway takes over; oxygen delivery catches up with demand.
  • 3
    Steady state
    3 min – 1 h
    Carbohydrate and fat are oxidised side by side, in a proportion set by intensity.
  • 4
    Depletion
    beyond
    Muscle glycogen falls, the share of fat rises and the pace becomes harder to hold.
Certainty level · Probable
Carbohydrate availability limits performance in prolonged efforts of sustained intensity.
Reviews on carbohydrate for training and competition describe a drop in pace associated with glycogen depletion. The size of the effect depends on duration, intensity, food intake before the effort and training status; it is not seen in the same way in short or low-intensity efforts.
Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011) · Jeukendrup AE (2010)

Seeing the aerobic pathway in the energy cycle

ATP–phosphocreatine cycle

The scene shows the loop that rebuilds ATP. The aerobic pathway is what feeds that loop continuously, without interruption. Without the 3D the idea is unchanged: ATP is not stored but recycled, and the aerobic system is what keeps the recycling running.
Current step
1. A loop, not a line
Cellular energy runs in a closed loop: ATP is spent, then rebuilt. A muscle stores almost no ATP in advance; it recycles it continuously.
Scene description
Circular diagram: a pale ring carries three stations linked by arrows running anticlockwise. At the top, ATP is drawn as a large adenosine sphere followed by three small phosphate spheres; at the bottom left, ADP carries only two of them and a detached phosphate floats alongside; at the bottom right, phosphocreatine hands over its phosphate and leaves a creatine sphere behind. To the right of the circle, six discs stacked on a spindle represent the phosphocreatine store: the four thick discs at the bottom are still available, the two thin discs at the top have already been used, and an arrow links the store to the resynthesis station. A small marker travels around the ring to follow the cycle. The volumes are symbolic: neither the shape nor the size of the molecules is depicted.
Visible structures
  • ATP
    Adenosine triphosphate carries three phosphate groups. It is the form of energy a muscle fibre uses directly in order to contract.
  • ADP and free phosphate
    When ATP gives up a phosphate, ADP and a free phosphate are left behind and the energy of the bond becomes available. The cell then has to rebuild ATP.
  • Phosphocreatine
    Phosphocreatine stored in the muscle hands its phosphate to ADP, which rebuilds ATP almost immediately. What remains is creatine.
  • Phosphocreatine store
    The stack stands for a limited store: the solid discs are still available, the pale ones have already been used. It is rebuilt during recovery.
  • Creatine
    Once its phosphate has been handed over, creatine is what remains. It is phosphorylated again when the effort stops and energy becomes available.
Guided steps
  • 1/5
    Cellular energy runs in a closed loop: ATP is spent, then rebuilt. A muscle stores almost no ATP in advance; it recycles it continuously.
  • 2/5
    ATP carries three phosphates, shown here as three small spheres in a row. It is the only form of energy the contractile proteins can use directly.
  • 3/5
    By releasing its third phosphate, ATP becomes ADP and frees the energy that powers contraction. The detached phosphate stays available inside the cell.
  • 4/5
    Phosphocreatine transfers its phosphate to ADP: ATP is rebuilt almost instantly, without oxygen. It is the fastest route a muscle has.
  • 5/5
    The stack of discs stands for the phosphocreatine store: it supports a very short, very intense effort, then runs down. It is rebuilt during recovery, once the other pathways take over.
Model licence · Shapier — Propriétaire — usage interne ShapierLab

Three common misreadings

Caution
What the word aerobic does not mean
That the pathways take turns. They all run at once. What changes is their relative contribution, not whether they are switched on.
That there is an intensity at which you burn fat. The share of fat is higher at low intensity, but the total energy spent is lower. The two ideas are often conflated, and this page prescribes no way of training.
That the aerobic system only matters for endurance sports. It is what rebuilds phosphocreatine between two sets, and so what governs recovery within a strength session.

What this changes for the other pages

The aerobic system ties the Energy pillar together. It consumes the muscle glycogen described elsewhere, it rebuilds the phosphocreatine that explosive effort depends on, and its working depends on the water and blood volume covered in hydration and performance.

Sources

Main sources

  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
  • Baker JS, McCormick MC, Robergs RA (2010). Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
  • Jeukendrup AE (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care.
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Check my understanding

When does the aerobic system switch on?
Why is talk of a fat-burning intensity misleading?
What does the aerobic system do during a strength session?
Choose an answer

Read next

  • 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
  • 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
  • Hydration and performance
    Water is not a side note to effort. It carries the blood, carries heat away and accompanies glycogen storage: three roles that explain what losing water changes.
    With a 3D scene

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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
  • The share of carbohydrate and fat depends on intensity, duration, recent food intake and training: no single value holds for everyone.
  • Measurements rely on gas exchange and biopsies taken in laboratories, on small numbers of people, under conditions more controlled than real training.
  • This page describes a general mechanism. It assesses no individual's fitness and proposes no diet.
Sources
  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
  • Baker JS, McCormick MC, Robergs RA (2010). Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
  • Jeukendrup AE (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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