Let’s get physical

The human body is a fascinating and incredibly complex machine that so many of us take at face value. Running involves countless physiological responses and adaptations that likely never crosses most runners’ minds. One of these is the heart rate drift phenomenon — also known as the cardiovascular drift or the cardiac drift.

A bunch of things happen within your body when you start running. Your sympathetic nervous system goes » Hey, body, wake up! We have got work to do! « which then kickstarts your cardiovascular system and causes your heart rate to increase.

Running requires an increased oxygen level and your blood is the vehicle that carries said oxygen to your cells. Your blood also carries fuel and nutrients and takes away any waste products. In fact, your blood does a lot of amazing things within your body, but for the sake of this post, just know that running requires an increased blood flow, and that the reason for why your heart rate increases during activity, is because your heart starts to work harder in order to provide more oxygen to your body, via the blood.

So, what is this cardiovascular drift then?

Imagine being a few kilometres into a run and having settled into a pace you deem somewhat comfortable. A pace you feel you could keep running at for a long period of time. Your heart rate remains steady... until all of a sudden it spikes! It has increased even though your pace and intensity has not. You are still plodding along at the same speed you did 30 minutes ago, and you have not even reached the hills yet!

Your heart rate is a reaction to work being done, not a measurement of actual work. You might start your run with a pace of, say, 6 minutes per kilometre. During kilometre 1 - 3, your heart rate sits at an average of 135 bpm; by kilometre 4 it might be at 145 bpm; and by kilometre 7, you could be pushing 160 bpm. Even though you have stuck to the same pace from the outset.

Run your easy runs easy.

If you wanted to run 10 kilometres with the intention of maintaining the exact same heart rate throughout, you would have to approach your run differently. If you started out at 6 minutes per kilometre and an average heart rate of 135 bpm, you would likely have to slow down the pace over time in order to keep your heart rate steady.

The amount of beats per minute will naturally increase along the way due to the cardiovascular drift, which is an upward accumulation of the heart rate over time, coupled with a progressive decline in stroke volume and the continued maintenance of cardiac output, occurring while exercise intensity remains constant.

HEART RATE The number of times your heart beats per minute.

STROKE VOLUME The volume of blood pumped from the left ventricle per beat, the amount of blood that leaves your heart to rush to the rest of your body, such as your legs and your lungs, with each heart beat.

CARDIAC OUTPUT Heart rate + stroke volume = cardiac output. The amount of blood the heart pumps through the circulatory system: from your heart, through your body, back to your heart again, in one minute.

Or, in simpler words: the cardiovascular drift is the natural increase of your heart rate during an extended period of cardiovascular exercise.

Your heart keeps you honest

Depending on a number of factors — such as your body temperature going up, external stress, which types of foods you have eaten recently, hydration status and running on technical terrain — it is very possible that your easy pace can become a hard pace due to cardiac drift over the course of a run.

Your heart’s response to exercise is a clear indicator of how hard your body is actually working. Maintaining a steady pace over a longer period of time does not equate to maintaining a steady effort. Slowly but surely, both your body and heart are going to work harder, even if your pace or the intensity never changes.

Constantly running your easy runs too hard can be detrimental to the physiological responses you are looking for. Training via a heart rate effort can not only help you become a more efficient runner, it can help speed up the recovery time between your runs and make future long runs feel a lot easier.

If this scribble has made you just the slightest bit curious on the heart rate training methodology, keep an eye out for my next post, where I will explain how this has changed my perception on a lot of things, not only from a running perspective, and go in depth with what I think are amongst the pros and cons of training this way.