Heart rate inquiry?

I am 57 years old and weigh 297 lbs. I walk daily. According to the American Heart Assn. website, I need to get the rate within a range of 80 to 143 to get "aerobic" exercise.   But my normal resting heart rate is 51 bpm.  My doctor even asked me if this rate was always this low. Does anyone know if there is any significance to having a low resting heart rate, or if it affects the aerobic rate?

Check with your cardio but some of the BP meds, sotalol to name one, are weight based and with you losing weight like you are you may be inadverntly taking too much of the stuff.  If you are it results in your heart rate being held lower than it should be. 

I always thought a low resting heart rate means an efficient cardio system. Prior to surgery my resting heart rate was always in the 85 to 95 range when I went to the doctors office. Now it's in the low 50's. My doctor says it is because of all the running and a good cardio system.

pan head

I also have a low resting BPM in the low 50's.  I took Coreg for 3 to 4 years, I don't take it any longer.  I had a BPM of 80 or more before the Coreg, after the Coreg low 50's.  I have been off of Coreg since Feb 2007.  I ask the Dr. about it and he just looked at me like I was crazzzzzzy.  So I can't really give you an answer, just the story.