Fuel and fibre types

A muscle stores only enough ATP for a few seconds of contraction, so it has to keep making more.

A small tank

Every cross-bridge cycle and every calcium pump uses ATP, but a muscle fibre holds only enough ATP for a few seconds of contraction. It has to rebuild ATP about as fast as it uses it. Muscle cells do this in three ways, which differ in how fast they work and how long they can keep going.

Creatine phosphate: the first seconds

In a resting muscle, spare ATP hands a phosphate group to a molecule called creatine, making creatine phosphate. When contraction starts, the enzyme creatine kinase moves the phosphate back onto ADP to make ATP again. The reaction is very fast, but the reserve is small and lasts only about 15 seconds of work.

Glycolysis: fast but wasteful

Next comes glycolysis, which breaks down glucose without needing oxygen. The glucose comes from the blood or from glycogen stored in the muscle. Glycolysis is slower than creatine phosphate and not very efficient: each glucose molecule gives a net gain of only two ATP, plus two molecules of pyruvic acid. If oxygen is short, pyruvic acid is turned into lactic acid, which lets glycolysis keep running and may add to fatigue. On its own, glycolysis can power only about a minute of activity, enough for short, intense bursts.

Aerobic respiration: slow and steady

With oxygen, mitochondria break glucose, pyruvic acid and fatty acids down completely to carbon dioxide and water. This aerobic respiration makes about 30 to 32 ATP per glucose molecule, far more than glycolysis, but it is slower and needs a steady supply of oxygen. It provides about 95 percent of the ATP used by muscles at rest or doing moderate work. A protein called myoglobin keeps a small store of oxygen inside the fibres. After hard effort, breathing stays fast for a while, because extra oxygen is needed to restore ATP and creatine phosphate and to deal with the lactic acid. That extra need is called the oxygen debt.

Why muscles tire

Muscle fatigue means a muscle can no longer respond fully to signals from the nervous system. Its causes are not completely understood. Falling ATP, a fall in pH inside the cell, and upset sodium and potassium balance that disturbs calcium release have all been linked to it. The nervous system also shares the work: in long, steady tasks some motor units rest while others are active.

Three kinds of fibre

Skeletal muscle fibres differ in how fast they contract and how they make ATP. Slow oxidative (SO) fibres contract slowly and rely on aerobic respiration. They are thin, packed with mitochondria and capillaries, and coloured red by myoglobin. They make modest tension but keep going for a long time, which suits holding posture. Fast oxidative (FO) fibres contract quickly but still mostly use oxygen, so they make more tension and tire a little sooner; they suit movements like walking. Fast glycolytic (FG) fibres are thick, store a lot of glycogen and rely on glycolysis. They are pale, give fast, powerful contractions and tire quickly. Fast fibres split ATP about twice as fast as slow ones, which is why their cross-bridges cycle faster. Most muscles contain all three kinds, in proportions that fit the job the muscle does.

Turn ATP off in the model to see what running out would mean at the level of the cross-bridges.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.