Final answer:
The amount of ATP stored in muscle cells fuels activity for only about 3 seconds, after which the body needs additional mechanisms to regenerate ATP for muscle activity.
Step-by-step explanation:
The amount of ATP stored in a muscle cell will fuel the muscle's activity for no more than 3 seconds. Muscle cells have multiple mechanisms to regenerate ATP after it is quickly depleted. The first line of defence is creatine phosphate, which can supply ATP for muscle contraction but only lasts about 8-10 seconds. Once depleted, the body relies on glycolysis to generate ATP from glycogen, which can last for approximately 90 seconds and is used in anaerobic conditions such as during high-intensity exercise. Following these immediate systems, aerobic respiration, which is the most sustainable source, takes over to produce ATP using oxygen, but this process takes time to ramp up. The initial burst of energy from the ATP stored in muscles is very short-lived, necessitating these different pathways for continued muscle contraction.