Final answer:
The first source of ATP at the onset of exercise is the ATP pre-stored in muscle cells, immediately followed by the rapid generation of ATP from creatine phosphate, and then by glycolysis.
Step-by-step explanation:
The first source for supplying additional ATP when exercise begins is ATP already available within the muscle cells. This pre-existing ATP is rapidly used up, and then the body starts to generate more ATP from creatine phosphate. After about 15 seconds, creatine phosphate stores are also depleted, and the muscles turn to glycolysis to produce ATP from glucose, a process that produces ATP more slowly but can sustain muscle activity for about 90 seconds.