There are three main energy systems in the human body:
1) The ATP-CP system provides immediate energy for intense bursts of activity lasting 10 seconds or less and does not require oxygen.
2) The glycolytic system produces energy through the breakdown of glycogen for 1-3 minutes of high intensity exercise and causes the buildup of lactic acid.
3) The aerobic system provides sustained energy for longer duration activities through the breakdown of nutrients like glucose and fatty acids with oxygen and involves chemical reactions in the mitochondria.