Final answer:
The initial energy sources for exercise are creatine phosphate, which is used for about 8-10 seconds, and muscle glycogen through glycolysis, lasting for about 1.3-1.6 minutes before aerobic respiration takes over.
Step-by-step explanation:
The initial energy source for the first few moments of exercise is creatine phosphate, which supplies energy very quickly but only lasts for about 8-10 seconds. Subsequently, muscle glycogen is broken down to produce glucose, which is used to create ATP through a process called glycolysis.
This glycogen system can supply energy for approximately 1.3-1.6 minutes without the need for oxygen, which is particularly important as it takes some time for the cardiovascular system to respond to the demand for increased oxygen supply to the muscles.
As exercise continues past two minutes, oxygen starts to reach the muscles, allowing aerobic respiration to kick in and provide a more sustained energy supply.