Did you slow down significantly during your second minute? This quick experiment displays how your muscles get the energy they need to function. When you were opening and closing the clothespin, your muscles were exercising anaerobically, drawing on their stores of glycogen to produce the adenosine triphosphate (ATP) molecules that supply the energy they needed to contract. As the glycogen ran out, your muscles ran low on ATP and their ability to contract decreased. If you squeeze the clothespin slowly, you’re exercising aerobically. In aerobic exercise, your muscles also draw on oxygen and the glucose and fatty acids carried in by the blood to produce ATP. Your ability to keep exercising aerobically depends on the delivery of oxygen and fuel molecules (glucose and fatty acids) to your muscles. And that depends on your circulation and respiration.