The discovery may allow scientists to develop drugs that mimic the effects of exercise - which naturally produces muscle needed for endurance.

A team at the University of Texas has discovered a genetic switch that controls muscle action.

Writing in the journal Genes and Development, the researchers said they had identified the molecular pathway in rats that controls the action of a muscle fibre.

Two types of muscle

The human body has two types of muscle, fast twitch, which provides strength, and slow twitch, which provides endurance.

The scientists have discovered a mechanism that induces the muscles to become slow.

This pathway is stimulated naturally by taking exercise such as jogging where endurance is vital, but the new discovery could allow artificial manipulation.

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Lead researcher Dr Sanders Williams said: "We believe that it is possible to design a drug which would have this effect."

Dr Williams's team identified three proteins, known as calcineurin, NFAT and MEF2 that combine to activate muscle genes.

Stimulated by exercise

When a muscle is constantly stimulated by exercise such as jogging, calcium ions build up inside the muscle cell and activate calcineurin.

This in turn causes NFAT to combine with MEF2 to switch on the genes that control the action of new muscle fibres, and ensures that they provide the slow-release energy needed for endurance.

The team is now doing tests to see if this pathway is the same one used in humans and if so, if it could be activated by a drug.

They hope that unused muscle, which is in a state similar to the "fast" twitch state, can be stimulated by this pathway.

No slow muscle

Team member Dr Rhonda Bassel-Duby said: "Patients that are lying in bed or are immobilised don't have slow muscle.

"Our ultimate goal is to be able to design a drug that will convert their fast muscle to slow. They need slow muscle to be able to get out of bed and walk around for an extended period, rather than just one or two minutes."