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Note: This doesn't seem to be a warm-up issue, see the updates at the bottom.

I've recently incorporated 80:20 training into my routine and try to keep within heart rate (reserve) zone 2 for a lot of my runs. I've noticed that often my pace improves throughout the run, getting progressively faster whilst maintaining the same heart rate. Here is a particularly clear example, where my first km was 6:52/km pace, but the last was 6:07/km. On these runs, I am generally trying to run as fast as I can whilst keeping to zone 2 (I occasionally drift briefly in to the bottom of zone 3) as that is more enjoyable. I don't perceive an increase in effort through the run. I was using a chest strap heart monitor, so I think the data is fairly reliable.

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I don't think this is particularly problematic, just curious as my intuition would be that my pace would drop as I get more tired?

Thought I'd add some running dynamics data in case that was useful:

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It looks like my stride length is increasing, as the vertical ratio (the ratio of upward and forward motion per stride) decreases, which suggests I am running more efficiently, but that is not saying much as I can't run faster with a constant heart rate without running more efficiently. My cadence is also increasing, but only very slightly.

Update:

The only answer so far cannot be right. Here is my 15k run this morning (this time on the track). Again HR recorded by chest strap, so should be reliable. My pace improved from 7:38/km at the start to 6:43/km at the end. I really can't be still warming up after running/jogging for an hour and 45 minutes! enter image description here (I had gels at 5k and 10k, which explains the lack of improvement in pace at those points - I lack the coordination to deal with the gels without it affecting my running - it takes me most of that km to consume them.)

Update on the Update:

I ran a 10k in zone 2 this morning without a warm-up (was short of time - not something I generally do), and this time I started out faster, slowed down a bit and then sped up again in the last 4k.

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If the usual gradual speed up was because of not being sufficiently warmed up, running without a warm up ought to exacerbate the gradual increase in pace, rather than slowing down in the first few km. Only one run, of course.

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According to Dr. Phil Maffetone, it likely means you haven't warmed up enough.

During any MAF Test, your times should always get slower with successive repetitions: the first mile should always be the fastest, and the last should be the slowest. If that’s not the case, it usually means you haven’t warmed up enough.

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  • I don't see how that is the case, surely after running the first 5k I ought to be fully warmed up for the second 5k? FWIW I had walked briskly about 1km to the place I was running and then performed dynamic stretching and lunges etc. before starting the run, so my heart rate was fairly elevated already (115 bpm, but I can walk for an hour at a normal pace with a HR of 70-80 bpm).
    – Dikran Marsupial
    Commented Dec 3, 2023 at 16:45
  • +1 for the suggestion though. The MAF test seems to be designed to be undertaken at the maximum aerobic heart rate though, which would be the top of zone 3. I checked my last two zone three runs and the first 10km was at a pretty constant pace in both; for my last zone 4 run, the pace decreased through the run, but not by very much.
    – Dikran Marsupial
    Commented Dec 3, 2023 at 17:13
  • Your MAF HR correlates pretty closely to the top of Karvonen zone 2. If that's the top of zone 3 for you, then it could be your zones are wrong. Consequently, this could mean you're running far too slowly. At a recovery run pace, you're not fatiguing the same
    – Ten O'Four
    Commented Dec 3, 2023 at 17:38
  • Also, how long you need to warm up depends on your body. For some people, a 5-10 minute progressive warm up is too much. For others, 20 minutes isn't enough.
    – Ten O'Four
    Commented Dec 3, 2023 at 17:40
  • The top of Karvonen zone 2 doesn't sound like "max aerobic level" for me - I have no problem running for a couple of hours at the top of zone 3. From what I have read, Karvonen zone 3 is considered an aerobic zone. There seems to be a lot of variability in these zones, but the Karvonen system works for me, the results fit much better with the usual textual descriptions (e.g. zone 2 - can hold a conversation) fitness.stackexchange.com/questions/46297/…
    – Dikran Marsupial
    Commented Dec 3, 2023 at 20:44

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