Final answer:
During the first few seconds of intense exercise, your body produces energy under anaerobic conditions using creatine phosphate and glycogen, which doesn't require oxygen and generates lactic acid as a by-product. As exercise progresses, oxygen supply increases, and aerobic metabolism becomes predominant, particularly during low-intensity activities where fatty acids are the main energy source.
Step-by-step explanation:
Your body produces energy under anaerobic conditions during the first few seconds of intense exercise. This is because the energy demand is high and the usual aerobic pathways cannot supply ATP quickly enough. Instead, your muscles rely on systems such as creatine phosphate which can supply energy for about 8-10 seconds and glycogen, which is used to make ATP more slowly but still sufficiently for activities lasting about 90 seconds, without the need for oxygen. During this anaerobic process, a by-product called lactic acid is produced, which can lead to muscle tiredness and soreness, commonly known as the stitch.
As exercise continues beyond a couple of minutes, your body increases oxygen supply to the muscles, allowing for aerobic respiration to take over, breaking down glucose to produce ATP and sustain muscle activity for longer periods. During low-intensity activities, the primary fuel source transitions to fatty acids through aerobic metabolism, which is very efficient and can continue for an extended time as fat reserves are nearly limitless.