Tick tick tick...

I recently wrote a blog post looking at a recently published paper looking at the effects of endurance sports on cardiac function. Or, more specifically, I was looking at the reporting (and unsurprising overhyping) of this story by the media. What struck me while looking through the comments section of the Daily Mail's take on the story was how many people believe in the idea that we are all allocated a certain number of heartbeats in our lives, and so we shouldn't waste them doing stupid things like running marathons. As in example, this is a quote from Bobby from Inverness:

"You are allocated only so many heartbeats for a lifetime - don't waste them up in silly fruitless 'exercise' - live longer, have a nap"

I suspect that this quote was actually written in jest, but it seems that a lot of people think like this and use it as another excuse not to exercise. The theory is that, since when we exercise our heart beats faster, every hour of exercise is using up 2 or 3 hours of your life that you could be sitting on the sofa doing nothing. Now don't get me wrong - I like to lounge on the sofa as much as the next person. In fact, my second biggest hobby is playing on the XBox; an interesting juxtaposition to my favourite hobby of running. But could I really be reducing my lifespan with all of this running?

First of all, let's look at this theory. It's actually quite an interesting premise, and the evidence for it seems to come from the idea that animals with shorter lifespans have quicker heart rates than those with longer lifespans. So if you take the average lifespan of a species (in minutes) and multiply it by the average heartrate (in beats per minute), you get an estimate for the average number of beats of the heart in a lifetime. And this value seems to be similar amongst different species. I was pretty skeptical, so decided to have a look at a few examples for myself:



Human:
Average heartrate = 60 bpm (asleep), 70 bpm (awake) (Source: Wikipedia)
Average lifespan = 70 years (roughly two thirds awake, one third asleep) (Source: Wikipedia)
Average number of beats =  2,452.80 million beats     

Mouse:
Average heartrate = 480 bpm (asleep), 550 bpm (awake) (Source: Figure 1)
Average lifespan = 3 years (roughly half awake, half asleep) (Source: Wikipedia)
Average number of beats = 812.052 million beats

Cow:
Average heartrate = 60 bpm (asleep), 65 bpm (awake) (Source: Pet Education)
Average lifespan = 22 years (roughly five sixths awake, one sixth asleep) (Source: Department of Animal Nutrition and Management)
Average number of beats = 741.97 million beats

Hummingbird:
Average heartrate = 50 bpm (asleep), 750 bpm (awake) (Source: World of Hummingbirds)
Average lifespan = 7 years (roughly two thirds awake, one third asleep) (Source: World of Hummingbirds)
Average number of beats = 1,900.92 million beats

Of course, these values are based on some random Googling, with some estimation involved (although I have erred on the side of caution to be as fair as possible in each case) so you'll have to give me the benefit of the doubt. But all in all, given the margin of error in these estimates, the values are surprisingly close - in the region of a billion beats per lifetime. I'm actually quite impressed by this, as I thought it was a load of rubbish. But maybe there is some merit to it. Indeed, if you take out the positive (e.g. medical care) and negative (e.g. predators) effects on life expectancy, it perhaps makes sense that your heart may give out after a certain amount of use, similar to a car's engine.

So let's assume for the sake of argument that this theory is true. Am I (and indeed we as runners) wasting my (our) precious billion heart beats by running? I decided to do a little "back of the envelope" calculation to see how the use of my "limited" heartbeats compares to that of a normal person. Now the point here is that, whilst I use up more heart beats while I exercise, my heart rate is generally lower than average. The question is; is this enough to offset the extra work my heart does when I run (and do other things)?

My Weekly Exercise: (heart rates based on a few random measurements with my Garmin)
Total hours in a week = 168
Hours per week of running = 12 (maximum heart rate of ~170 bpm)
Hours per week of cycling = 6 (maximum heart rate of ~140 bpm)
Hours per week of weights = 3 (maximum heart rate of ~120 bpm)
Hours per week of sleep = 49 (resting heart rate of ~48 bpm)
Hours per week awake = 98 (general heart rate of ~55)
Total number of beats per year = 34.26 million beats 

Normal person: (based on above estimates)
Total number of beats per year = 35.04 million beats

So I actually use fewer beats, with 97.8 % of the total number of beats of a "normal" person over the course of a year. Or, another way (thanks to EJ of UltrarunningGuy for pointing this out), this makes my life expectancy 71.6 years, giving me almost 2 years extra to enjoy (which I have probably already wasted on Skyrim...)! And this is being pretty harsh on myself (I don't do all of my running at maximum heart rate for instance), and overly conservative for the "normal" person (I doubt the average human being does nothing in a year to raise their heart rate), so if anything I use up even fewer when compared to normal.

So there we go. Whether or not this theory holds any water (and I'm pleasantly surprised by the results that I found above that suggest that it may do), running ultras does not use up any more heart beats than being a lazy couch potato. If anything, I am generally using up fewer beats in a year, primarily due to the fact that my heart is working more efficiently. Running also has many, many other health advantages, as well as being awesome fun, so I think that I will carry on with my "silly fruitless exercise", thank you very much Bobby from Inverness!

Booyah! Science; it works bitches!

Incidentally, if you're wondering why I chose these animals, I chose the mouse and cow as they are both domesticated and respectively smaller and larger than humans. Also, I have seen the hummingbird touted as proof against this theory due to it's long lifespan and high heart rate, but they fail to take into account the incredibly slow heartbeat as the bird hibernates every day (known as torpor). Learning is fun!

Edit: Added life expectancy calculation following EJ's comment