The metabolism-boosting benefits that you enjoy as a result of interval
training are primarily due to the fact that your body, suddenly, needs to
find more energy.
While it was chugging along and supplying your energy needs during your
cardiovascular exercising, it all of a sudden needs to go grab some more
for 30 seconds or a minute; and in that period, it will boost your
metabolism as if it were given a nice, healthy jolt.
As you can see, if you suddenly decided to extend your 30 second or 1
minute sprint into a 20 minute sprint, you simply wouldn’t experience all
of the benefits.
Yes, your body would use more energy if you extend yourself to the higher
range of your aerobic training zone. But your body won’t necessarily get
that jolt that only comes from interval training.
So remember: your goal with interval training is to give your body a
healthy jolt where it suddenly says to itself:
“Whoa! We need more energy here FAST, this person has
increased their heart rate from 180 beats per minute to 190
beats per minute! Let’s go to any available cell, like those fat
cells down at the waist, and break them down via catabolism
so that this person can get the energy that they need!”
Remember (sorry to be repetitive, but this is very important): the whole
point of interval training in this way is to give your body a sudden, limited,
healthy jolt where it needs more energy – quick!