all species have an average number of heart beats which dictates life expectancy , the faster the heartbeat the shorter the lifespan.
Cardio increases heart rate, the offset is once you are fit you have a lower resting heart rate.
Swings and roundabouts...
that is pretty much exactly the take I have.
A number of contractions before the heart's muscle cells and connective tissues starts detoriorating from not regenerating fast enough and accumulates damage/dead cells and scarr tissue
ofcourse diet and lifestyle, stress, hormones and whatnot elongates or shortens this number of heartbeats.
also when the pulse gets high and your heart contracts harder and faster to supply blood, there will also be a higher demand on the heart and possible damage if you do it too often before you let it recover. It is after all a muscle. A different kind of muscle than skeletal muscle but even so it needs more "rest" to recover aswell. So personally I don't think you should raise your heart rate to the max with like HIT training 2 days in a row, you need to let it rest and recover.
