Plenty of research has shown that interval training, alternating high and low intensity work, can really rev up your metabolism and help with fitness. Interval training includes three Target Heart Rate (THR) ranges:

Low Intensity Range 60-70% of maximum (Range 1)
Medium Intensity Range 71-80% of maximum (Range 2)
High Intensity Range 81% and up (Range 3)

To determine your THR ranges, it is best to know your Resting Heart Rate (RHR), which is your heart rate in beats per minute when you first wake up in the morning without an alarm. You can get a good enough estimate by taking your pulse at a time when you feel relaxed. Plug your RHR into this equation:

(220 – Your Age) – RHR = X

The equation of a 38 year old person with a RHR of 64 beats per minute looks like this:

(220 – 38) – 64 = 118

To determine your THR for each range, multiply X (118 in our example) by the range percentage and then add back in your RHR.

Range 1
118 x 60% = 70.8 + 64 = 134.8
118 x 70% = 82.6 + 64 = 146.6

Rounding up, our example person has a low-intensity THR of 135-147 beats per minute.

Determining the other two ranges is just as easy and just as important if you are doing interval training. The higher-intensity ranges help you burn more calories over all and help build cardiovascular fitness, while the low-intensity range allows you to build endurance and recover from the more intense work. Alternating back and forth between the ranges is a great way to add variation to your workout!