14 Sep 2007
Muscles must instead draw more heavily upon their limited supply of glucose. When this happens, glucose is spent rapidly. As a result, fragments of glucose molecules accumulate in the muscle tissue and cause fatigue. This is why, if you exercise intensely, you may have to stop or slow down to "catch your breath" (replenish your oxygen supply). By slowing back down your body will once again rely upon aerobic metabolism.
Therefore, exercising with too much intensity will inhibit the body's ability to burn fat. Keep intensity in check by engaging in moderate, low-intensity aerobic activity. This includes activities where energy demands do not exceed the available oxygen, and fat can supply much of the energy, permitting glycogen to be conserved.