Scientists at La Trobe University and other institutions found the muscles use acidosis, or the build-up of acid, to avoid fatigue and make sure they keep responding properly and efficiently to nerve signals.
The research appears in the August 20 issue of the journal Science.
Muscle contraction occurs when a network of tiny tubes in muscle fiber -- known as the T-system -- receives electrical signals from nerves to excite movement throughout the muscle.
Chloride ions depress muscle fibers by reducing the excitability of the surface membrane and T-system in this process, but researchers suspect that acidity counters these depressive effects and lowers fatigue.
"It is a very clever trick because rested muscles need the chloride effect normally to prevent them spontaneously contracting," said George Stephenson at La Trobe's Muscle Research Laboratory. "The acidity produced by the strenuous exercise reduces chloride's stabilizing effect."