Lactic acid build-up causes a burning sensation in your muscles during intense exercise. Lactic acid is not responsible for sore muscles the day after exercise, contrary to popular belief.

“Lactic acid is seen by many as the gentler, more tolerated alternative to glycolic acid due to its slightly bigger molecule size.” It also works effectively on body skin. “In recent ...

So you been sprinting, Lily? Just a bit of cramp? So that's the lactic acid building up. When an athlete does too much or runs too vigorously, not enough oxygen can be delivered to the muscles.