Muscles Are Smarter Than You Think: Acidity Helps Prevent Muscle Fatigue

The discovery runs in the face of the previously held belief among physiologists and athletes that acidity, through a build up of lactic acid, is a major cause of muscle fatigue.

Professors George Stephenson and Graham Lamb of La Trobe's Muscle Research Laboratory and Mr Thomas Pedersen and Professor Ole Nielsen from the University of Aarhus, presented their findings to the scientific world in Science, the journal of the American Association for the Advancement of Science, published today (August 20).

Mr Pedersen, a Danish PhD student from the University of Aarhus, came to work for six months in the Muscle Research group to carry out the project with Professors Stephenson and Lamb using a technique developed in their laboratory by which the surface membrane of single muscle fibres--which are half the thickness of a human hair-- is peeled away, without interfering with the ability of the muscle fibre to contract normally to electrical stimulation. This enabled the researchers to change conditions inside the muscle cells and study the effects of acidity on the force response.

'We found that muscles play a "clever trick" in which they use acidosis--the build-up of acid--to help ensure that they keep responding properly to nerve signals and so avoid the fatigue that would otherwise occur,' said Professor Lamb.

Collaboration between La Trobe and Aarhus universities started in 2002 when Professor Nielsen came as a Distinguished Visiting Scholar at La Trobe's Institute of Advanced Study to work for a month in the Muscle Research Laboratory.

Professor Nielsen had recently demonstrated for the first time that acidity could be beneficial to muscle performance, although it was not clear how this occurred.

The La Trobe/Aarhus team identified the underlying mechanism of why acidity is beneficial, discovering that our muscles use a 'clever trick'.

Muscle contraction in a skeletal muscle fibre in response to a nerve impulse is the result of a complex series of events known as excitation-contraction-coupling. There is a network of tiny tubes in muscle fibre known as the T-system that allow electrical signals, set up on the muscle fibre's surface in response to nerve signals, to move deep inside and excite the whole of the fibre.

'Chloride ions play an important role in muscle by dampening the excitability of the surface membrane and T-system, ensuring that they only respond when stimulated by nerve signals and do not become spontaneously excited,' Professor Stephenson said. 'When a muscle is worked hard, potassium ions come out of the fibres and make the membrane less excitable. The acidity generated inside a working muscle helps counter this depressing effect by reducing the influence of chloride, which helps the muscle membranes stay excitable.'

'It is a very clever trick because rested muscles need the chloride effect normally to prevent them spontaneously contracting. The acidity produced by the strenuous exercise reduces chloride's stabilising effect, enabling the impulses to keep exciting the muscle when they would otherwise fail.'

'We have concluded that intracellular acidosis increases the excitability of the T-system, thus counteracting fatigue at a critical step in excitation-contraction-coupling.

'This finding should be of great interest not only to elite athletes and the many laboratories around the world involved in muscle research, but also to all of us because previous orthodoxy was that acidity caused muscle fatigue.'