It is common wisdom among athletes that lactic acid is the enemy of endurance. The chemical is a byproduct of metabolism during periods of intense exercise, when oxygen supplies cannot keep up with energy output. Its accumulation in work-stressed muscles has long been considered a major cause of achiness, exhaustion and failure.
Now, in a reversal, scientists have found that lactic acid enhances a tired muscle's ability to keep on twitching.
The biochemistry and physiology that propel muscles start with a nerve impulse that delivers the mental demand for action and end with a rush of electrically charged sodium, potassium, chlorine and calcium atoms into and out of cells in synchronized succession.
George Stephenson of La Trobe University in Australia and his colleagues studied those molecular changes in individual rat muscle fibers made to twitch repeatedly and tallied the effects of those changes on the fibers' ability to keep contracting.
They found that the increasing acidity that comes with lactic acid buildup lowered the threshold that a muscle fiber must overcome to contract. A chloride ion-based mechanism that normally keeps resting muscles from contracting was more easily overcome under acid conditions, in seeming recognition that a tired muscle should not have to work so hard to generate the next twitch.