Now that she’s got your attention if you’re interested in learning more about your muscles continue reading.. For your muscles—in fact, for every cell in your body the source of energy that keeps everything going is called ATP. Adenosine triphosphate (ATP) is the biochemical way to store and use energy. ATP is required for the biochemical reactions involved in any muscular contraction. As the work of the muscle increases, more and more ATP gets consumed and must be replaced in order for the muscle to keep moving. Because ATP is so important, the body has several different systems to create ATP. These systems work together in phases. The interesting thing is that different forms of exercise use different systems, so a sprinter is getting ATP in a completely different way from a marathon runner! ATP comes from three different biochemical systems in the muscle, in this order: Phosphagen system Glycogen-lactic acid system Aerobic respiration So imagine that you start running. Here's what happens: The muscle cells burn off the ATP they have floating around in about 3 seconds. The phosphagen system kicks in and supplies energy for 8 to 10 seconds. This would be the major energy system used by the muscles of a 100-meter sprinter or weight lifter, where rapid acceleration, short-duration exercise occurs. If exercise continues longer, then the glycogen-lactic acid system kicks in. This would be true for short-distance exercises such as a 200 or 400-meter dash or 100-meter swim. Finally, if exercise continues, then aerobic respiration takes over. This would occur in endurance events such as an 800-meter dash, marathon run, rowing, cross-country skiing This is why rest is important between sets(specifically stretching the muscle being worked) because we breakdown fat(fatty acids) to yield ATP. We turn our fat into ATP to use as energy. We breakdown fat mainly during rest. As soon as you start increasing the intensity of movements the breakdown of fat(lipolysis) slows but doesn’t stop. Lipolysis is responsible for resting muscle activity, but its contribution to the overall muscle energy supply will decrease as contraction intensity increases. 👍🏾