I tried to do a quick google and all I got was
The formula:
220-age= MAX heart rate (220-20=200 MHR)
60%-80% of this max:
200 x .6 = 120 beats per min, 200 x .8 = 160 beats per min.
Therefore the TARGET HEART RATE range for a 20-year old, working at 60-80% (up to 85% for very fit
people) of his/her max heart rate is 120-160 beats/min.
http://www.***********.com/community/ask_the_experts.asp?q=7