Aerobic training and Maximum Aerobic Threshold.5.
The answer is in your heart rate. Most people like to measure their work out intensity in terms of a generic formula like 220 minus your age. This determines the maximum heart rate and then percentages are worked out from there.
The problem is, 220 - your age isn't a very accurate determinant of your maximum heart rate because it is a number that is generalised from a large population.