Question about heart rate paced runs after a swim earlier in the day | 80/20 Endurance

Question about heart rate paced runs after a swim earlier in the day

  • This topic has 5 replies, 2 voices, and was last updated 1 year ago by rjwg2.
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    I’ve been pacing my Z1 & 2 runs by heart rate, having worked it my zones in the manner the book recommended. Furthermore, I’m using a Wahoo Tickr heart rate monitor which is paired with a Garmin Forerunner 935. However, I’ve been finding that when I do an afternoon run a few hours after a lunchtime swim my heart rate reaches the target zone at a much lower pace. For example, my zone 2 heart rate is usually around a 6 m/km pace, whereas just now I was running 7 m/km in the same zone.

    I’m guessing that my heart rate is higher because I am fatigued after the swim. But, I just wanted to ask if in these instances I should pace my Z1/2 runs according to speed zones? I ask because I’m worried that I am not getting a good enough workout at these much lower paces.

    Thanks so much for your help

    David Warden


    One of the most influential pieces of information I ever read was table 0.1 from Wilmore and Costil’s Physiology of Sport and Exercise. It lists how HR varies based in external factors. For example, for a given intensity, here are some examples of how HR can vary:

    70 degrees F 165bpm
    95 degrees F 190
    Humidity 50% 165
    Humidity 90% 175
    Meal 3 hours before exercise 165
    Meal 30 minutes before exercise 175
    8 hours sleep 165
    <6 hours sleep 175
    Exercise at 6am 131
    Exercise at 2pm 139

    In your case, exercising after lunch in the afternoon is easily a 10 point change in bpm. Yes, you may be experiencing some light cardiac drift from your swim, but I bet real money that if you skipped the swim you HR would still be elevated after lunch in the afternoon. Also, how much sleep did you get Monday night vs Thursday night? That’s another 10 point swing. Temperature? HR adjusts by about 1 bmp for each degree F between 70 and 95. There are dozens of factors that influence and change your HR for the exact same pace.

    If you are a long-time reader of this forum, you’ll know that I consistently preach the risks of using HR, and I think you are experiencing those risks.

    So, I recommend you ignore HR, just use Pace (or Power).



    Hi David,

    Thank you so much for this – that’s really helpful. I shall ignore HR for my runs and just go by pace




    Just a follow up question, if HR has these potential risks and should thus be avoided, then how should I pace Zone 2 efforts when I’m out on the bike without a power meter? Should I do it by RPE by trying to talk at a conversational pace?

    Incase this contextual information is helpful, I do zone 4 & 5 interval sessions on a wattbike, and I have a good feel for zone 3.

    Many thanks


    David Warden

    HR is still valuable, but if you have access to Pace or Power it’s not necessary. Most HR running devices are paired with GPS, so almost all runners have access to Pace. Even running power is inexpensive at $200.

    But, entry-level for bike power is $500 minimum, so it’s less common and HR is the right way to go. HR is more reliable than RPE in most athletes, but some athletes can really dial in in and use RPE incredibly reliably.

    The key to using HR (on the bike or the run) is just recognizing those variables in your environment. If you performed a bike LTHR test at 75 degrees at 7am on a Sunday before breakfast, you just have to accomodate for a bike ride Wednesday afternoon after lunch at 85 degrees.

    In fact, many athletes have multiple sets of HR zones that they have tested. Such as:

    – Indoor
    – Outdoor AM under 80 degrees
    – Outdoor PM over 80 degrees.

    Those 3 environments will have about 5 beats between them, so you’re 10 beats higher outdoor PM over 80 degrees than Indoor.

    HR is cheap, ubiquitous, and plug and play. But, you get what you pay for and it still has those drawbacks. It will work as long as you are aware of how HR is influenced by non-exercise factors.



    That’s so helpful, thanks so much


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