During intense or prolonged exercise, muscles can feel like they’re burning the longer and harder someone pushes. This occurs because the muscles produce energy as quickly as possible by dipping into ...

Q. I am a young cyclist trying to race semi-pro in the national mountain-bike series. I need to improve my VO2max and lactic-acid threshold. What sort of training should I do? - Evan, Bay Area, Calif.

Runners have long feared lactic acid. We've always viewed it as something that tires us out during a race and makes us sore afterward. In high school, I can remember my buddy Jeff propping his legs up ...