I'm sure I read somewhere ages ago that animals have a rough maximum lifetime amount of heartbeats but not humans for some reason, but I've always raised an eyebrow at the amount of sports people who seem to die pretty young. I may just be imagining this or I don't register the athletes who live until they're 100 or whatever so this post reminded me and I did a quick google and came up with this;
If you exercise, you will develop athlete's bradycardia (lower resting heart rate) and will "use up" your allotment of beats more slowly, thereby adding years to your life. If you exercise too much, however, the extra beats you use will exceed the number you save by having a lower resting heart rate. You will die sooner. At least that's the hypothesis, if you believe it.
The scientists who proposed this hypothesis performed some very complicated mathematics to determine that the optimal amount of daily exercise time to gain the maximum number of years is 22 minutes, resulting in a 6% increase in years of life. The break even point is 97 minutes. This means that if you exercise more than 97 minutes a day, you are actually taking away years of your life. By the way, this hypothesis has not been proven
http://www.ultracycling.com/training/ultra_health.html