If you've ever seen a doctor in a movie take someone's heart rate just as they've collapsed and someone yells "Is there a Doctor in the house?!" you may have wondered why the physician always grabs their writs and looks at this watch as he calls for complete silence. From an outsider's perspective it looks like he's trying to see if the heart beats are in-time with the second hand as it ticks forwards.
With this assumption it's easy to assume that a good heart-rate is monotonous, with the same space in between the beats. But in reality, a healthy heart beats at odd intervals, almost totally contrary to a 'well oiled' machine.
Think about your heart in this scenario
You've just finally managed to escape from a predator that's been tailing you for an hour (hey, it could happen… but let's hope it never does). You've got to someone quiet and safe, and all you can heart is your heart beating so steadily that it fills up your entire attention.
Ba-bump. Ba-bump. Ba-bump.
You notice it's strangely regular for having been in a such a stressful situation. Why is it so regular you ask?
Turns out, this is a defining factor of body stress and an indicator that your body is still in flight or flight mode. Flight or flight mode is basically what it says on the tin. It's a response that is linked to the sympathetic nervous system, similar to the gas gas pedal on a car. When you get stressed or need to run (or choose to run!), gas flows into the engine to allow us to go faster. That's our heart (the engine) pumping blood around to allow for more nutrients to feed our muscles.
When the body realizes that it has to drive for a while or at top speeds, your heart goes on 'cruise-control' and starts pumping those vital nutrients around your body in a uniform way, so you can race faster, for longer — hence the monotonous heart beating.
This obviously a good thing for a short while, your body wants you to escape that predator or win that race or just keep doing those jumping jacks because it's good fun. But eventually you're going to run out of steam and this is where the parasympathetic nervous system has to come into play.
Heart rate variability is measured by calculating the time between R spikes on an ECG trace
And it's not the same for everyone — state of mind, altitude, temperature, body position, hormones, drugs, age and gender all play a role in affecting HRV, so when someone says 'this is a good heart rate speed', it may be for them… but not necessarily for you.
So what's your heart rate variability like?
Just sit there (maybe like you're doing now) and place your index and middle finger over the wrist of you other hand. Clasp down using your thumb to apply some pressure. Start counting your pulse out loud, you may begin to notice it's totally uneven (if not that's okay, go to the next step anyway).
Next, stand up and jumping 10 times (its okay to smile as you do this) and then take your pulse again.
Notice how steady it is? Now you know why your heart beats at different speeds.