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.

So what happens is that the glucose is reacting and there’s a by-product from that which is called lactic acid. Anaerobic respiration’s good for 100 metre sprints, 200 metre sprints ...