The shorter intervals had a 90% greater oxygen uptake than the longer intervals. This created a larger oxygen debt. Why is oxygen debt important? When you train at an intense level, like sprinting, your body can't supply oxygen at a fast enough rate to fuel the muscles. After the intense effort is completed, your body has to basically repay that "borrowed energy"…it owes oxygen to get those muscles back to their normal state. The more energy your body borrowed during an intense effort the more oxygen it owes…this is called Oxygen Debt. The larger the oxygen debt created by your workout the longer it will take to repay it…with the benefit of more calories burned for a longer period of time after you are done exercising.