You get your maximum heart rate (MHR) by subtracting your age from 220. I don’t worry about people going over their MHR because, believe me, they won’t spend a heck of a lot of time there. Plus, you can’t damage a healthy heart by exercising near your max.
What is more important is your resting heart rate -- the number of heart beats in one minute when you’re at complete rest. (It’s best measured just after you wake up in the morning.) For 18- to 25-year-olds, 49 to 55 is in the athletic range, while 56 to 61 is still considered excellent. If it’s above 73, you need to make some lifestyle changes.
But the best indication of your current fitness level -- and the health of your heart -- is your recovery heart rate, or your ability to go from a fast stroke rate to a slow one. Measure your heart rate two minutes after you finish a cardio session. That number is your recovery heart rate. Check it after comparable workouts over time to get a sign of your cardiovascular improvement.
Consider the 20-year-old guy who has a resting heart rate of 64. I'm not trying to get him back to 64 until he wakes up the next morning. But I do want him near his max of 200. As soon as he gets his rate up to around 160 to 180 (doing, say, 60-yard sprints), he keeps it up for a predetermined time and then slows down until it gets down to around 120.
Over a six-week period, his recovery heart rate should improve so that he gets back down to 120 more quickly after working out near his max for that predetermined time. In other words, his heart has become healthier.