When I was a young runner, lactic acid was blamed for the delayed onset muscle soreness (DOMS) that interfered with my ability to climb onto the school bus with ease the day following a hard practice. Since then, we have learned that post-exercise pain is caused by micro tears created when we tax our muscles, and the former scapegoat, lactic acid, doesn't hang out in muscles after a workout. Lactic acid actually fuels our cells when we push ourselves during high intensity workouts. It is hydrogen ions, a by-product of burning lactic acid as fuel, which creates the heavy legged sensation, burning sensation that accompanies running hard and fast.
You will come across the term "lactate threshold" when reading intense training manuals. This threshold is when the lactic acid begins to accumulate in the blood. And when you push yourself to work past that threshold, you train your body to better deal with the biological by product of using lactic acid as fuel. Increasing your lactate threshold helps you run faster for longer and is considered a good performance predictor for endurance events. Training to increase this threshold involves tempo runs and interval training with long steady paced runs. So now that you know how to increase your lactate threshold, will you?