When muscles work too hard or too long without sufficient oxygen, pyruvate is converted to lactate, not lactic acid.

“So if you've ever carried a box while moving, or you're carrying heavy groceries to the car, or you're exercising particularly hard and you felt the burn, well, that burning which most people think is lactic acid is actually a process by which pyruvate, which as I said before normally could be converted into ATP if there's oxygen, well, if there's not enough oxygen 'cause that muscle is working too hard or too long, what ends up happening is that a hydrogen molecule comes in there and you get something called lactate. So believe it or not, humans don't make lactic acid. That's another species, we make lactate.”