Endurance Skating

Training, Racing & LCHF Fueling For Skating And Endurance Sports

Cardiac Drift – Really?

Even if you are not familiar with the term, anyone who does heart rate-based training will be all too familiar with the phenomenon of Cardiac Drift, which according to Competitor Running’s introductory paragraph on the subject, is:

”The natural increase in heart rate that occurs when running with little or no change in pace.


If you look at your HR on a Garmin or Training Peaks chart, you might see something like this:


Rising HR @ constant pace while running

or like this:


Power decoupling while biking

It describes the inability to sustain your running pace or bike power at the same heart rate at the end of a significantly long workout as you could when you began the workout (or, by extension, unable to keep your heart rate from rising for the same pace|power). The longer the workout, the more it noticeable it becomes. It’s a part of the reason why the first mile of a marathon feels piss easy, yet mile 25 feels hellish, although your pace is probably no faster. It reminds us that we are living, breathing humans and not programmed machines.

The orthodox school of thought likes to attribute cardiac drift “to the effect of an increase in core body temperature, resulting in loss of fluid through sweating & dehydration” – conventional wisdom being that as you sweat and lose body water during a workout, your bloody becomes  thicker and more viscous, and so your heart has to beat faster to pump a smaller amount of plasma around your body… hence an increase in heart rate.  

Conveniently, it also fits in nicely with the “drink shitloads of  sports drink” mantra that Gatorade uses to build a sugar-water industry that does far more harm than good.

As you probably gather, I am not a fan of the orthodox school of thought.

What’s Normal, What’s Common?

If you further believe what you read in that same Competitor article:

“exercise research has shown that it is common to see heart rate “drift” upward during an easy run or threshold run, even with no increase in pace or effort—sometimes by as much as 10-20 beats per minute over a 30-minute period.”

Whoa there! 10-20bpm after 30 mins? If that’s you, then you got problemos grandos, and it isn’t anything to do with blood viscosity or hydration. It’s a problem of fitness, and specifically, lipolytic aerobic fitness.

Let’s do the math… say your heart rate drifts up from 140 to 155bpm during steady-pace workout. That’s an 11% increase in heart rate, supposedly due to a decrease in blood plasma. Sorry, but unless your workout took place on on a treadmill while you were donating blood then something isn’t adding up.  Seriously? Who makes this shit up?  Our blood is, like all biological systems, a tightly homeostatically-regulated system, and the idea that plasma volume can vary so wildly just due to a moderate dose of exercise is anathema to the idea of human biology & evolution. when your body blood loses plasma through one process (sweating), it will draw upon another source to balance it out – Tim Noakes has often suggested that this is from excess water held in the intestine.

For sure, Drift happens, I’m just disputing WHY it happens, and therefore what the implications are.

Let’s look at it another way…

Exercise at different intensities works different types of muscle fibres.

First to consider are our slow-twitch type-1 fibres, which are the primary fibres engaged when we do any work – they are fuel at first by fat, but can and do increasingly use glucose as the intensity is ramped up and up.  However, even when they are working hard enough to be significantly glycolytic, they are still aerobic – thus, your metabolism can be either be primarily aerobic-lipolytic or  aerobic-glycolytic at what are all considered “aerobic” heart rates (up to 80% HRmax) by many definitions.

Next up, the type 2 fibers split into two distinct branches: Type 2A and 2B.

2B, also called Fast Glycolytic (FG) fibers are your purely fast-twitch anaerobic fibres. Being anaerobic, they posess much fewer mitochondria, and exist for fast, explosive output.

Type 2A, or Fast Oxidative Glycolytic (FOG) – fibers share characteristics of both Type 1 and type 2B fibers – they are considered “fast-twitch” but also possess significant mitochondria to produce aerobic energy. They sit between the Type 1 and the Type 2B fibres, and begin to engage at/around the point where nearly all the slow twitch type-1 fibres are being utilised – this is often referred to as the Aerobic Threshold in common training parlance. It’s because they are only engaged at this higher level of intensity that they use glucose, hence their name.

So much for the biology, what’s that got to do with Cardiac Drift?

So what happens when you go out for a run that is anything more than a very easy jog is that you will likely be working hard enough to be engaging most of your Type 1 fibers, and – unless you are very attentive – at least a good number of your 2A FOG fibers. We are still “aerobic”, but also significantly glycolytic.

However, 2A fibres are not slow-twitch. They do not exhibit the same degree of resistance to fatigue that the type 1 fibers do therefore tire faster. There’s debate whether this is a peripheral issue (muscle becomes damaged, sends pain signal to brain) or a perception issue (brain knows muscle fuel is running low, generates fatigue as a control mechanism). Regardless, when it happens, the nervous system must begin to steadily recruit more and type 2A fibers, using a limited supply of glucose, to produce the same muscular force required to keep the same output.

More muscle fiber recruitment as you become fatigued = more bloodflow requirement = higher cardiac output requirement.

So, If you are experiencing a significant degree of cardiac drift, it means that whatever intensity you are working at, you are engaging a significant number of those FOG fibers.

Or put another way, you are probably going too hard to be a predominantly lipolytically aerobic effort, and are shifting towards significant glycolysis.

The double-whammy is that when we are less fit, we are both slower AND experience higher drift (ie reduced endurance). If you train at the same heart rate when you are unfit as you do when you are at peak fitness, you are placing a higher metabolic stress on yourself. You can – and should – train harder when you are at your fittest, and conversely train less hard when you are at your least fit.

This highlights one of the problems of trying to be too precise or not being conservative when trying to guess at a physiological threshold such as the Aerobic Threshold or the Lactate Threshold – they are moving targets, and can change by a fair few number of beats on any given day, depending on your level of fitness, restedness & accumulated stress.

It can be hard to accept – when we’re less fit, it feels easier to reach a higher heart rate, and now you’re telling me I got to go even slower?! Yep. But remember, an athlete benefits to a greater degree from a workout when he is less fit than he does when he is at peak fitness – there is little sense in going out and smashing out hard workouts when until you are near peak fitness.

Personal Experience

I bring all this up because I have I begun to prepare my next marathon, now 9 weeks out, and have hit my first real longs run of the training cycle. Although my speed hasn’t progressed as well as I’d hoped in the last year, I can tell that my endurance has improved, and on my most recent MAF Test I’m seeing very little dropoff in pace between the 1st and the 5th km. Correspondingly, I experience very little “drift” on my 60-90 minute runs (as much as I can tell, given the change in terrain). And I’m not even taking any water in on these runs.. I’m not taking in anything on these long runs.


Maffetone talks about the drop-off in pace between the first and the last mile during a MAF Test. Joe Friel refers to it as the degree of decoupling in power output over a long ride.  Others think of it as the degree of cardiac drift experienced…

However you think of it, it all points to the same thing – the state of your aerobic engine, and how well it will hold up to the rigours of prolonged training sessions or races.

The fitter you are – the more fat-burning you can tap into – the less you need to rely on those fast twitch fibers even at higher intensities,  the less pace drop-off, power decoupling, or cardiac drift that you will experience, and the better you will be able to sustain a higher pace or power output at all submaximal intensities.



2 comments on “Cardiac Drift – Really?

  1. Lloyd
    January 12, 2016

    Very interesting, thanks.
    You may want to look at Wim Hof, who can complete full marathons without drinking any water.
    Found you via TJ13. Cheers


    • Van
      January 12, 2016

      Hi, thanks for your comment. Yes, I’m familiar with Wim Hof.. I researched him when looking into cold thermogenesis, though I have to say that he comes across as a little eccentric.
      I have to point out that I’m not purposefully setting out to do these runs on no water – it’s just that at this time of year and at the times of day I do my runs, I rarely feel that I need to take on water. In summer it would certainly be different. I’m usually a little thirsty by the time I finish, but nothing too serious!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


This entry was posted on January 11, 2016 by in Endurance, Science and tagged , , , , , .

Enter your email address to follow this blog and receive notifications of new posts by email.

%d bloggers like this: