The answer is glucose, as a simple form of carbohydrates.
After we digest food, our body absorbs the glucose digested from carbohydrates. Then, through the circulatory system, the blood with a high glucose concentration is travelled to working muscles.
To generate energy, the muscles undergo aerobic respiration, or anaerobic depending on the supply of oxygen. We turn glucose to energy. Atp is a form of energy our body uses.