The maximum capacity your body has to use oxygen, which gives an assessment of physical fitness.
To calculate exercise intensity find the maximal heart rate target which = 220 minus your age (in years).
Moderate Intensity exercise is 50% (range 40-60%) of VO2 max and the heart rate required to achieve this is 50-70% of the maximal heart rate.
A 40-year-old to achieve moderate intensity exercise, max heart rate should equal 180. (50-70% of max heart rate = 90-126 beats per minute).