What are the different bioenergetic systems that fuel muscle?

Muscles require ATP (the energy currency of cells) to contract and produce movement.^[1] Depending on the type, intensity, and duration of exercise, the body will make ATP primarily from one of three energy systems: the creatine-phosphate system (fastest), glycolysis, and fat oxidation (slowest). All three energy systems are always working, but some will work more than others under certain conditions. Through greater activity and training, the efficiency and effectiveness of each of these systems can be increased.