Your body uses different fuels, known as substrates, for the different intensity and length of
exercises. For very high intensity
exercises lasting for less than two minutes, such as a heavy weightlifting or an all-out sprint, your body generates energy without oxygen, drawing on muscle stores of creatine phosphate and glycogen, the storage form of glucose. For lower-intensity exercise of longer duration like walking, running or cycling, your body uses oxygen to burn glycogen and fat.