I forget where I first found this article. Maybe on the nytimes.com site:
Very cool stuff - if you're into lactic acid!

August 23, 2004

The misconception that lactic acid is a harmful waste product began with the work of a Nobel laureate. In 1929, British physiologist Archibald Hill analyzed muscle fibers of frogs and suggested the lactic acid that pooled in the frog muscles after flexing causes fatigue. But researchers now realize the experiment was flawed. Hill studied the frog muscles in isolation, so he couldn't know that within the body, lactic acid is taken up and consumed as a fuel.

But people like Brooks have since shown that lactate from lactic acid is a valuable fuel derived as the body breaks down carbohydrates. Not only do muscles consume it eagerly, the brain and heart also suck it up from the bloodstream to keep systems running. High levels of the stuff can cause some sensation of muscle burn during exercise (thanks to the hydrogen ions that are released when lactic acid is broken down into lactate), but it doesn't stick around long after activity.

And now, new research shows that lactic acid serves as more than a fuel. In a study, appearing in the journal Science, Stephenson and others show lactic acid actually help keeps muscles running when they might otherwise become sluggish. To understand how, it's useful to look at what makes a muscle flex in the first place.

Thomas Fahey, an exercise physiologist at California State University in Chico, explains that muscle actions are triggered by a mechanism known as the sodium-potassium pump. The sodium-potassium pump moves sodium ions out of a cell while pushing potassium ions in. The difference in levels of each kind of ion creates an electrical charge. Muscle cells use this charge to respond to elecrical signals coming from nerves and to contract. For every two potassium ions that are pulled in from outside the cell, three sodium ions are moved to outside the cell.

When a muscle is working hard, potassium ions can start to leak from the muscle cells and accumulate on the outer membrane. This creates an imbalance and stifles muscles from firing. Chloride ions also start to act as a natural braking agent. By testing muscle fibers of rats, Stephenson and colleagues showed the presence of lactic acid counteracts this braking effect and keeps muscles firing.

"It is a very clever trick," said Stephenson, explaining the acid enables "impulses to keep exciting the muscle when they would otherwise fail."

The finding is one more step away from old misperceptions about muscle fatigue and lactic acid and could be used in ways to help athletes prepare their bodies for competition. Stephenson suggests, for example, that doing short sprints during warm-ups for an event could help an athelete's performance by increasing levels of lactic acid in the muscle and preparing the muscle for a buildup of potassium ions.