While performing aerobic exercise, your heart rate should stay in a range
between 60% and 80% of your MHR (Maximum Heart Rate). This is your Training Zone.
For best results, aerobic and fat burning, keep your heart rate in the Training
Zone for at least 12 minutes. The longer and more frequently you do this the more
improvement you'll see. An aerobic work out should last at least 12 minutes, not counting
warm up or cool down, only the amount of time your heart rate is within your Training
Why 12 minutes?
This is the amount of time needed for the body to start producing fat burning enzymes. The
idea is that you want to get your body to use fat for energy. After 12 minutes you start
to burn fat at a higher rate. In general, if you train at the higher end of your Training
Zone, you will burn more glycogen, improving your aerobic fitness. Training at the lower
end of your Training Zone for a longer period will allow the body to use your fat reserves
for energy, thus burning fat.