In the first few seconds Your muscles start using adenosine
triphosphate (ATP), energy molecules your body makes from food.
That burst of power you feel? It's ATP converting into another
high-powered molecule, adenosine diphosphate (ADP). Muscle
cells—expert recyclers—will turn ADP back into ATP after the
initial surge. In the first 90 seconds In order to unleash more
ATP, your cells break down glycogen, a form of glucose fuel
stored in your muscles. Cells also pull glucose directly from
your blood (one reason exercise is helpful in fending off high
blood sugar). Your body gobbles more glucose, and your muscles
release lactic acid—also known as the burn in the age-old
workout mantra "feel the burn"—which signals the brain that
you're under physical stress. In the next few minutes Your heart
starts beating faster and directing blood toward your muscles
and away from functions you don't need at the moment, such as
digestion. To make the best use of glucose, your muscle cells
require an influx of oxygen. Cue hea