Tip - What is My Maximum Heart Rate?

Hi Doc! I started training for marathons four years ago and I’m running my fourth this fall. My question is -- how high is too high for a heart rate? I'm almost 52 years old, 5’10” and 186 lbs. I run relatively slow (5:44 marathon last year) and never ran distance before I started four years ago. Most of the time my Garmin shows me at about 160 beats per minute during a work out - a few times it’s inched over 170. I use the Galloway walk-run method and during the walk breaks my heart rate drops to 140 or below.
Tags: Heart Rate