Heartbeat sweet spot

The lifespans of cars are often measured in years and miles. The lifespans of humans are measured in years alone. Might there exist a miles-equivalent for humans? One such contender is number of heartbeats.

Contrast the cases of (1) a sedentary person who doesn't exercise, (2) a fit person who exercises quite a bit, and (3) an elite athlete who exercises because it's his full-time job to do so. As a lazy blogger (and a philosophical one to boot), I'm going to make up some numbers to illustrate a point. This is way easier than actual research.

Let's say the sedentary person has a resting heart rate of 70 bpm. With 1,440 minutes in a day, this heart rate equates to 100,800 heartbeats in a 24-hour period. (1440 min ⋅ 70 bpm = 100,800 heartbeats.) Because the person is sedentary, we (generously) assume that the heart rate remains resting all throughout the day, though I suspect that climbing a flight of stairs would register quite a blip in the ol' heart rate monitor for this guy. But let's keep things simple.

What about the fit person? Let's assume he has a resting heart rate of 50 bpm but spends two hours each day exercising with an average, much elevated heart rate of 140 bpm. You may notice that the difference between 140 bpm and 70 bpm is bigger than the difference between 70 bpm and 50 bpm but that 22 hours is bigger than 2 hours. So does the fit person's heart beat more or fewer times than the sedentary person?

((22 hr ⋅ 60 min/hr} ⋅ 50 bpm) + ((2 hr ⋅ 60 min/hr) ⋅ 140 bpm) = 82,800 heartbeats

It turns out that the 22 hours of a lowered resting heart rate greatly outweighs the 2 hours of an elevated, stressed heart rate, and our fit person is racking up “miles” at nearly a 20% slower rate than our sedentary person. Good for him.

Finally, what about our elite, über athlete? Let's assume this elite individual has a resting heart rate of 40 bpm and spends an average of 6 hours a day at an average heart rate of 160 bpm. How many total heartbeats does this person accumulate in one day?

((18 hr ⋅ 60 min/hr) ⋅ 40 bpm) + ((6 hr ⋅ 60 min/hr}) ⋅ 160 bpm) = 100,800 heartbeats

The first thing you may realize is that our elite athlete's heart beats the same number of times as our sedentary person's heart and that the numbers I'm making up are probably not entirely random. That's what happens when bloggers make points. However, I think these numbers are probably somewhat close to the truth.

It seems common sense that too much exercise can burn a person out, like how a brightly burning candle burns quicker, and that perhaps part of the root of the issue lies with failing to minimize one's heartbeats. I used this concept as an excuse to make the following chart to illustrate visually how I imagine a sort of sweet spot in terms of how much exercise a person should undertake.
You will notice that there are no numbers and no units anywhere on the graph. The exact numbers and units, after all, constitute the big question, a question which probably varies from person to person. However, I'm going to stick with my previous assertion that an average of 1½ hours per day—about 10 hours per week—serves as a good heuristic.