Your everyday muscles movements are made possible by the energy provided from Aerobic Respiration. However, during vigorous exercise your muscle cells may become short of oxygen.
Despite the increased heart and breathing rates, your bloody can simply not supply enough oxygen fast enough. When this happens, muscles obtain energy from Glucose.
But they have to do it without oxygen - This is called Anaerobic Respiration.
Anaerobic Respiration 2
When you have been using your muscles for a long time, they become Fatigued. They stop contracting as efficiently, and are very short of oxygen, this is when they switch to Anaerobic Respiration
However, this is not as effective as Aerobic Respiration. In Anaerobic Respiration the Glucose is not fully broken down. So far less energy is released.
The end products of Anaerobic Respiration are Lactic Acid and Water, instead of CO2 and water produced by Aerobic Respiration
Glucose = Lactic Acid (+Energy)
Anaerobic Respiration - Oxygen Debt
If you've been exercising for a long time, you will still be puffing and panting for a while after you stop. The length of time this is depends on how fit or unfit you are.
This is because the waste Lactic Acid you produce with Anaerobic Respiration presents your body with a problem. You can't breathe it out like you can CO2.
Because of this, it needs to be broken down intopto CO2 and Water. This process requires oxygen, and the amount of oxygen needed is called the Oxygen Debt
Even though the leg muscles have stopped, your heart rate and breathing rate stay high to supply the oxygen needed to repay the debt. The bigger the debt, the longer you'll puff and pant.