You get on the pitch or the court, feeling fit to play. You have done your drills, you have put on your shoes and stretched out your legs. But by half an hour into the game, your legs start to feel heavy, a slight headache begins to creep in, and all of your energy quickly disappears. You may attribute it to your stamina or suppose you simply did not heat up sufficiently. In reality, the culprit is far simpler. You could simply be