Another current read spelled out muscle energy so perfectly and simply that I had to share it with everyone. Now you'll sound super smart and actually know what you're talking about!
"In order to contract, muscles need energy. That energy comes from adenosine triphosphate (ATP), a high energy molecule found in every cell in the body. ATP is the only energy source that muscles can use to power their activity.
"Yet, muscle fibers store only a limited supply of ATP-about 4 to 6 seconds' worth. For muscles to continue working, ATP must be supplied continuously. The most abundant energy source for ATP is glycogen (a starch form of the simple sugar glucose). In the human body, the liver stores glucose by converting it to glycogen. When the body needs energy, the liver is stimulated to change glycogen back into glucose and secrete it into the bloodstream for use by the cells.
"In the cells, glucose combines with oxygen to yield or produce carbon dioxide, water, heat, and ATP. Carbon dioxide, water, and heat are all waste products of this chemical reaction.
*I like this part
"Even though muscle fibers store some oxygen, that oxygen is quickly used up, especially during strenuous exercise. In order to convert glucose into ATP so they can continue working, muscles must receive more oxygen via the blood. That is why respiration or breathing rate increases during physical exertion. In times where work or play activities are exhausting, muscle fibers my literally run out of oxygen. If not enough oxygen is present in muscle fibers, the fibers convert glucose into lactic acid, a chemical waste product. (makes muscles sore)
"Lactic acid is normally carried away from muscles by the blood. It is then transported to the liver, where it is changed back into glucose. In order to do this, however, the liver needs ATP. To produce ATP in the liver, oxygen is once again needed. This is why breathing rate remains high even after vigorous physical activity is stopped. Only after the liver produces the necessary ATP does breathing gradually return to normal."
(Body by Design: From the Digestive System to the Skeleton p. 165)
**This is also why it's important to exercise, albeit gently, the day after you get sore.
glucose + oxygen = carbon dioxide, water, heat & ATP. ATP is energy for muscles.
Glucose + not enough oxygen = lactic acid
Lactic acid + ATP (using oxygen in the liver)= glucose
What a beautiful cycle!