Training by HR vs Power and cardiac drift | 80/20 Endurance

Training by HR vs Power and cardiac drift

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    So the conventional wisdom in both the cycling and running worlds seems to be that it is better to train by power because cardiac drift often causes you to put out less effort to maintain a given HR and therefore you end up “under-training” in the latter stages of your workout. I’ve also trained under this philosophy. Though I don’t have power when I run, I do try to increase my HR by about 1 bpm per mile or two in my long runs and that seems to keep my pace constant throughout the run.

    But I got to thinking about this. Is there evidence that you *should* be trying to maintain constant power even as your body fatigues? By evidence, I mean research where, say one group tries to hold HR constant another holds pace/power constant and comparing the long term outcomes? Or are we really operating in the realm of theory?

    It seems that fatigue and HR are ways that you body is telling you something and it has always bugged me a bit that the conventional wisdom is that we should ignore that and just push through to maintain a given power. Also, lots of folks who abide by 80/20 and run slow to run fast philosophies seem to suggest that going slightly slower is better than going too fast and it seems that trying to keep power constant is biasing things toward maybe going too hard in the latter stages of the workout. Personally, when I do my upper aerobic foundation runs when I start near my VT1 at the beginning of the run, I find that I usually exceed my VT1 by a few beats near the end of the run to hold my pace constant. I don’t know if this is a big deal or not. Of course, I don’t do near VT1 runs every day as I vary my duration and pace quite a bit across the week but it did get me thinking after I finished my last near VT1 run with a bpm a couple beats above my VT1 threshold heart rate.

    Anecdotally, I don’t know if anyone follows world cup XC mountain biking but Evie Richards has been dominating the women’s side the last year or two and she trains only with heart rate and perceived effort. Doesn’t even know her own FTP. This is just an n=1 example, but then we are talking about a very very high level of competition and small mistakes can cost you big.

    Matt Fitzgerald

    Stephen Seiler addressed this topic in a helpful way in his Endurance Event presentation. It’s worth checking out if you haven’t done so already.

    My personal take is that athletes should never slow down in response to a rise in HR that occurs at a constant speed in endurance workouts. It’s better to avoid getting in this position in the first place by slowing down from the start and by not planning endurance workouts that are too long for the individual athlete’s current fitness.

    That being said, even when endurance workouts are well-planned and -executed, they should sometimes result in a fairly significant degree of cardiac drift, especially in training for longer events. This is really just a sign that the workout has become challenging, as it’s supposed to. “Zone 2” doesn’t necessarily always mean “easy.” It just means “slow.” An athlete should not want all of their endurance workouts to be easy all the way through to the end. Hence, slowing down in response to a rise in HR constitutes a misguided attempt to make a workout that’s supposed to be hard somewhat easier.

    Another way to look at it is this: Do you want to slow down toward the end of your races? No! And who is more likely to slow down toward the end of races: The athlete who practiced slowing down toward the end of all of their endurance workouts or the athlete who practiced holding steady despite mounting fatigue toward the end of all of their endurance workouts? The answer is again obvious.


    As usual, I appreciate your thoughtful responses Matt.

    With regard to your final paragraph. I agree that intuitively, it seems obvious that we should train ourselves to not slow down even if HR is creeping up. But like so many things in endurance training, some things are counter-intuitive (e.g. run slow to run fast). As an example, one of my buddies has been running for a decade and he runs at hard tempo every single run and when I mentioned 80/20 to him, he was dismissive and thought that it “seemed obvious” that if you want to be fast, you need to run fast. I suggested that the science is showing that slower running to develop the aerobic system may eventually mean that his tempo pace might become his high aerobic pace. He just dismissed it. You also make similar points in your book about recreational runners.

    Which leads me to a slight variation in my original question: if we maintained a constant HR rather than a constant pace, would it be possible that our average pace would still increase even if our relative pace within the run will decline? Ultimately, I’m interested in increasing my average pace not making sure that I can run at an even pace during my training. If my overall average pace elevates, why would I care if I can’t hold a constant pace during a marathon if that constant pace ends up being overall slower?

    Matt Fitzgerald

    I’m not sure I’m fully understanding you. It is one of the most well-established facts in all of sports science that positive-split pacing results in a slower average pace and higher finish times in races.


    Sorry about the confusion. I am not saying its wrong because I am currently following the positive-split pacing. I am only asking whether we are doing this out of convention and “intuitive common sense” or whether there is actual evidence to support this.

    My analogy is this: For a long time, most endurance training focused on high intensity because it seemed intuitive obvious that you train fast to race fast. But then Lydiard overturned this conventional wisdom and people adopted mostly slower aerobic paces for roughly 80% of their training.

    Could it be that the positive split approach and using power or pace should be questioned if it potentially leads to over-training? I know my HR can go much higher than my VT1 threshold if I am focused only on maintaining power or pace. So by HR, I spend a lot of time in the grey zone even if my pace or power is correct. I guess I can start at a pace/power that keeps my HR well below VT1 at the start of a session with the expectation that it drifts up to VT1 but that seems arbitrary as well.

    I am not saying it’s wrong. I am asking whether there is evidence to support positive split pacing. Maffetone seems to question power over HR. But that is just his opinion.

    Matt Fitzgerald

    Tons and tons and tons and tons of evidence. For example, in this study of 158,000 recreational runners, male runners completed a marathon 4.5 minutes slower than female runners with equivalent 10K PR’s. Why? The men ran positive splits and the women ran even splits.

    Even-pacing in long workouts does not lead to overtraining. It just doesn’t. You seem to have your mind made up, so go ahead and do what you’re going to do, but in all my years of coaching I haven’t once overtrained an athlete, and whenever I take on a new athlete who tends to fade in long workouts, I consider it a problem that I help them fix, and I will continue to do so.


    Thanks for the evidence Matt. And my mind is not made up. I even said I am doing positive splits. If my mind is made up, I would not be doing positive splits. Questioning it and asking for evidence rather than just taking an opinon on face value is not mind made up. If I was talking to Maffetone, I would also approach it the same way and his view is the opposite as yours.


    Oops sorry Matt. I got the terms mixed up. What I am doing is maintaining an even pace in my runs and not slowing down. I am letting my heart rate increase. Sorry about the confusion.


    I was wondering almost the same, my question is how to deal with cardiac drift when trying to stay in a zone.

    Due to the answers of Matt I learned now that it is not bad to start in your zone but get in a higher zone when time progresses due to cardiac drift.

    I have only one question left, after reading the 80/20 running book I learned that the threshold between zone 3 and zone 4 determines if you are training at low-intensity or high-intensity.

    My question on this is about a training where you run in zone 3, like a tempo run 6 (5 min Zone 1, 5 min Zone 2, 30 min Zone 3, 5 min zone 2, 5 min zone 1).

    I have converted my heart rate zone into pace times. And try to maintain a steady pace which is doable and nice. But after my training I can see that I did like 50% of the 30 minutes in the beginning of zone 4.

    Need some reassurance here that I do the right thing.

    – Am I running too fast and should I make sure to stay in zone 3 otherwise I should see this as a high intensity training? And do that by maintaining a lower steady pace in zone 3?
    – Or it does not matter to get into zone 4 because you maintain your pace and the part in zone 4 is in the lower part so we can consider it a low intensity training?

    @Matt I hope you can clarify how to approach this.

    Matt Fitzgerald

    For precisely this reason, I encourage athletes not to use heart rate as their primary intensity metric in workouts targeting Zone 3 and above. It’s impractical.


    I had to start somewhere, and I did that by determining my heart rate zones via VO2 max test. And by reading your book I learned how to convert those heart rate zones to pace times (chapter 6). I am using those pace time zones now.

    But my Garmin still shows how many time I was in which zone, after the training. Should I ignore that because I am using the pace zones?
    So just maintain my pace a bit below the pace that I noted as a max for zone 3?

    Matt Fitzgerald

    That’s right.


    Thanks, recently started with the half marathon level 2 plan from your book and wanted to be sure I got it right.

    Will probably buy a Stryd in the future and use power as the indicator. But I am living in the Netherlands which is completely flat where I am living so Stryd wont do much in helping with up and downhill, it will help in windy conditions. For now I will stick to pace zones.

    Really appreciate your answers!


    The problem with using Heart Rate alone is that it does not address Heart Stroke Volume. Together they determine blood flow to the muscles to support a given output.

    You can do a deep dive here: Be sure to look at the link to the referenced study.

    I often see the drift at the beginning or towards the end of Zone I and Zone II workouts. If I am using a heart rate effort I will adjust, although I doubt there is a reason as long it doesn’t persist. For pace efforts I will just ignore the drift. I made an exception during a Stride Academy plan, only because I was attempting to learn to control effort by cadence and stride length.

    There “might” be something in the heart rate drift to gauge how you handle fatigue, after all stroke volume falls off for some reason. But this is pure speculation by me.

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