One reader had suggested using the Karvonen Formula to calculate heart rate ranges which may be a good idea. However, it still doesn’t account for the big fluctuation from the 220 minus age guideline. In fact to use the Karvonen formula you need to know your max. In my mind many older trainees might be afraid to push to higher levels based on the feeling that they are exceeding some theortically safe range? In any case, as you can see below the Karvonen formula actually does a good job of predicting my 90% range. However I know both my max heart rate and my resting heart rate.
Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR example
So for me
180-50 = 130 ( Max HR- Resting HR)
130x.9= 117 ( to calculate 90%)
117+50= 167 ( add in resting to get 90%)
Michael Boyle's Strengthcoach.com Blog 