Although opinion varies, many doctors caution that exercising at a level higher than 85 percent of a person’s maximum heart rate increases the risk of heart attack. For men, maximum heart rate is calculated by subtracting your age from 220. For me, the equation reads 220 - 56 = 164. Eighty-five percent—my red-flag number—is 139. Throughout big-game season, I found myself battling to keep my heart rate in the safe zone, especially while hunting elk. Oct. 25 was a typical logbook entry. Fifteen minutes after starting up the west face of Garnet Mountain, I had already exceeded my 85 percent maximum by eight heartbeats.