As a group fitness instructor, one of the things I was taught is that lactic acid is bad. More specifically, it builds up in your muscles when you exercise so hard that your muscles are not getting enough oxygen, causing the muscles to burn and fatigue.
If a recent study is correct, poor old lactic acid has been getting a bum rap all this time. Instead of being a muscle waste product, it may in fact be muscle food. The claim is that lactic acid is what glucose gets turned into before getting absorbed by the muscle cells and converted into energy. Furthermore, it may not even be responsible for muscle soreness, as it was originally thought to be.
If this study holds water, it represents a dramatic shift in our understanding of how muscles work... and in how fitness training manuals will be written.