Question: If i ran 1500m, why do my legs burn?

Claire Lee answered on 27 Jun 2013:

When you run (or do any exercise for that matter) your body needs to turn stored glucose in its cells into energy.

The best way of doing this is by a process called aerobic respiration – your body uses oxygen to break down the glucose and gets energy, carbon dioxide, and water.

But when you just start running you aren’t breathing deep enough to get enough oxygen for your cells to keep up. Also, after you’ve been running for a while and you get tired, your body can’t keep up with the oxygen it needs.

In that case it does something called anaerobic respiration – basically, without oxygen. In this case the glucose breaks down into (less) energy, carbon dioxide, and ethanol.

The ethanol and CO2 then group together to form something called lactic acid, and when you get a buildup of lactic acid, that’s what gives you the burn!

(I’m rather impressed that I remembered all that from high school biology!)