How to address cardiac drift on long runs

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    I’ve just finished week 10 of Level 2 marathon plan (18 mile long run) and noticed that once again I’ve had significant cardiac drift over the last 7-8 miles… The first 6 miles or so, I maintained Z2 HR (135-149) and pace (10:00 mile)… After that first 6 miles though, heart rate started to climb. Should I accept that cardiac drift is going to occur and maintain my pace, or attempt to slow my pace to maintain a Z2 HR? By the last 3-4 miles my pace was maybe 10:30-11:00 min/mile and my heart rate was bumping up against my lactic threshold…

    This seems to be a consistent issue on long runs… Any advice is greatly appreciated!


    I solved this problem by increasing the duration of my long runs very gradually. Nowadays, even my longest runs with marathon effort are below 5% of aerobic decoupling. It took me 3-4 years of consistent training. I always kept within the appropriate zone, and when my pace got too slow to maintain good form, I knew it was time to go home.

    David Warden

    Phoenix, XFaMan is correct: you’re level of cardiac drift will decrease (but never disappear) through regular Zone 1-2 training. 80/20 training supports this transition because of the significant time spent aerobically training.

    This is another reason why Pace and Power can be more effective than HR in measuring intensity. HR is just an indicator, not an output or outcome, and as a result it becomes less reliable in some circumstances.

    In the meantime, you can continue to use HR and accept that your zones are somewhat “floating” zones after the first hour, and use Pace as a solid secondary measure until you move to Pace or Power as a primary measure.



    David Warden,

    do you mean with

    you can continue to use HR … until you move to Pace or Power as a primary measure.

    that in general, you would recommend to switch to pace or power based training?

    I was assuming that in order to be sure that I am training in Zone 1-2, the HR is the best indicator and if the HR drifts out of the Zone although keeping a constant pace, it means that my training effect is not anymore Zone 1-2. Could you clarify?

    Thinking of it leads me to the question: is my training goal to reach a certain intensity (power/pace) or to stimulate my body for certain changes (like RF trainings will provide a different benefit than intervals). I always thought that it is the functional benefit I am training for.




    Thank you, that was my question as well. I thought HR was the gold standard of effort/input on the 80/20 plan, and so if HR moves out of a specified zone, you had moved away from the plan intent. But honestly, after 90 minutes or so I’m not sure I can slow my pace enough to keep my heart rate in Z2 unless I’m walking!

    Looks like we are getting to the heart of our understanding of 80/20 running! Excited to see what David has to say!


    David Warden

    Looks like we are getting to the heart of our understanding of 80/20 running!

    I see what you did there…

    Let me first include a section from our document Intensity Guidelines for Running (it’s also in the document Intensity Guidelines for Triathlon)…

    There are four ways to measure intensity: pace, heart rate, power, and perceived effort. The testing protocols for all four types are listed below. Each metric has different applications among the three triathlon disciplines. Each metric also has certain advantages and disadvantages. Power is an output, pace is an outcome, and heart rate is an indicator. Let’s use an automobile as an example. Horsepower (power) is the output, and represents actual work performed regardless of terrain, grade, or environmental factors. Your speedometer (pace) indicates the speed, or outcome. Your engine temperature (heart rate) represents how the car is responding to the output and environment. During a hilly ascent, the output (power) might be high, but the outcome (speed) might be low. On a hot day, the engine temperature (heart rate) might be very high even when stopped at a light with almost no output and zero outcome. For this reason, power is considered superior to pace, and pace superior to heart rate to measure intensity. There are some exceptions, such as hills, where HR can be superior to Pace to measure intensity. The recommended best-practice is to use Power or Pace as your primary measure, with HR as a secondary measure.

    Yes, in general you will experience a more accurate intensity measurement using Pace or Power. Clearly, Pace has some disadvantages (hills, wind…) but overall it is superior to HR, and Power has almost no disadvantages compared to Pace and HR, other than device cost.

    It is not correct that drifting out of a zone means the training effect is no longer in that zone. Per the automobile example above, that is simply a how your body is manifesting a response to long intensity.

    If I come up behind you and say “boo!” and get your HR up to 150 for 10 seconds, that does not count as 10 seconds of Zone 3 training. If you are watching a sporting event, and your home team is making a comeback over the final 20 minutes of the game, and you heart is racing on the sofa at 120bpm, that is not a 20-minute Zone 1 workout.

    But, as I’ve said before, I’m not here to bury HR, I’m here to praise it. HR plays an important role as either a primary or secondary measure. If you are using HR successfully, (and many athletes do!) carry on.

    Also remember that Zone training can mitigate this cardiac drift issue. Zone 2 is just that: a zone, a range. If you start your run in mid Zone 2, and it drifts up into high Zone 2, you’re still adhering to the workout.


    is my training goal to reach a certain intensity (power/pace) or to stimulate my body for certain changes

    The answer is yes to both, as both goals are required and support each other. We need to stimulate response (and recovery) in order to gain new speed, and we need to gain new speed to stimulate changes. We believe that 80/20 is the perfect balance that supports both (regardless of which intensity type you use!)


    Edit: A reminder that if you are on an 80/20 HR plan and want to switch to Pace or Power, you can do that for free as part of our Level Guarantee. Just contact us and choose that option when prompted.


    I never had a chance to run with Stryd… but just wonder in this case.. what would it show??? so we have pace: stady, HR gradualy going up, above LTHR at some point.. so probably within next 1-2-3km.. he will walk eventualy anyway …. What would Stryd show in such case.. steady power? changing? up/down?

    David Warden

    Kris, the find of cardiac drift that Phoenix experienced only occurs after about 1.5 hours of running (maybe a bit earlier in less-experienced runners). It’s very unlikely to occur early in a run.

    But, yes, what you would see with Power or Pace (on a relatively flat surface) is power (output) or pace (outcome) remaining steady, while HR continues to climb.

    To be clear, cardiac drift always occurs on a long run, it’s just a matter of how much. After a couple of years of regular aerobic training (which the 80/20 system supports) that drift will start to decline. TrainingPeaks actually has a measurement for this. If you open a run workout that has captured both Pace and HR, you’ll see a value named Pa:Hr. This will be a % between 0 and 10%. A “good” value if less than 5%, and a value over 5% indicates either a) improper pacing for the first half of the run or b) an aerobic system that has not yet had enough total time of easy training.



    True.. very informative…

    Lets try go 1 step further ..So, even though I am big fan of TRI gadgets and toys 😉 but I am not convinced that Stryd will give any additional benefit+ in such case.

    To summarize if our imaginary runner put all the gear on him
    Type of run: long flat
    Pace: steady
    RPE: I assume increasing during the run as HR going up
    HR: 1st half of run ok.. 2nd half going up
    POWER: steady reading as conditions const assumed (wind, surface ect)
    Lactate [mmol/L] ?????

    If I would use the handheld lactate analyzer several times during the run, I suspect that the value would also increase during the run.. correct?

    below reading is increasing with respect to workload (speed). For our runner would it increase for same speed/pace?


    David Warden


    I hope that this thread does not imply that you must run with a Power meter. We think pace and power are better than HR, but HR is just fine. Each intensity type has advantages and disadvantages.

    To answer your direct question… I don’t know. I don’t believe that lactate would continue to build in proportion to HR only. I suspect that if you were to run in Zone 2 at 2 mmol/L for 2 hours, HR would rise from 145 to 152bpm for the exact same pace, but that lactate would remain steady at 2 mmol/L, or at least limited to a very small increase.

    Lactate increase *should* be linked to output, not HR.


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