Runners and other athletes have long been told their muscles ache because they're full of lactic acid. But new research questions this locker-room wisdom.
Scientists have discovered that lactic acid actually helps muscles keep firing when the main pathways tire out.
Misconceptions about lactic acid began with a 1929 experiment by Nobel laureate Archibald Hill. Hill observed that a buildup of lactic acid - a byproduct of anaerobic respiration - correlated with a decline in muscle performance in isolated frog muscle.
But the process of muscle excitation is complicated, involving the movement of different ions in a cascade of intermediate steps. It's a bit like a miniature Rube Goldberg machine. The trouble with the 75-year-old experiment was that it observed the effects of acid only on the final steps in the sequence.
"[Hill's experiment was] done on muscle fibers that were not electrically stimulated and were simply exposed to heavily buffered calcium solutions, which means that the major part of the muscle's activation process was bypassed," Thomas H. Pedersen, from the University of Aarhus in Denmark, told LiveScience.
Pedersen and his colleagues have revisited the consequences of increased acidity in muscles. The scientists worked with rat muscle fibers that were specially prepared to allow stimulation at multiple points along the excitation chain.
"We start the muscle activation at the electrical stimulus, which is how the nerve activates the fibers, and this is the step in which we find the protective effect of acidosis," Pedersen said.
This lactic acid protection postpones the onset of fatigue in muscle's that are repeatedly activated. As described in a recent article in Science, Pedersen and his collaborators demonstrated that the presence of lactic acid reduced the threshold for spontaneous firing, making it easier for depleted muscles to keep on going.
The details are complicated, but basically, in an intense workout, potassium ions accumulate outside of working muscles, making it harder for sodium ions to propagate the electrical signal. Lactic acid counters this fatigue by interfering with the flow of chlorine ions - effectively lowering the amount of sodium current necessary for muscle activation.
"So the muscles play a clever trick in regulating the chlorine movements when the sodium system becomes depressed - exactly when needed," Petersen explained.
The implication seems to be that a little lactic acid will enhance performance.
"If athletes are engaged with very intensive exercise such as a 100-meter sprint, the warm up could include a few sprints to prepare the muscles for the upcoming potassium load," Pedersen said. "In fact, sprinters already do this."
But what about the ache, coach? Pedersen said that lactic acid "probably still affects nerve endings in an active muscle and causes pain - just a signal that the muscle is working."