Why? Well, because the blood vessels are the conduits that bring those glucose molecules to the cells and these cells essentially uptake the glucose molecules by using these glucose transporters and the glucose is then broken down into ATP and Pyruvate via Glycolysis. So we see that lactic acid fermentation will be especially active in those muscle cells that lie far away from blood vessels. And although lactic acid fermentation will solve the problem of the ATP production temporarily, it's not a very good solution in the long term because it actually causes the deactivation, the inhibition of the process of Glycolysis. Now, we know that over time, if we continue to exercise on a daily basis, we're going to basically build up endurance, we're going to be able to sprint for longer, we're going to be able to run quicker. The question is why?