Last week's poll came back, and it lines up with what I'd expect. Most of you either don't train by heart rate at all, or you use max heart rate zones. That makes total sense, it's what shows up on your watch by default, and it's what gets talked about most in the running world. But there's a real accuracy problem hiding inside that default setting, and a study on marathon runners just showed it in action.
ARTICLE BELOW:
The Research
Ten recreational marathon runners came into a lab two weeks before race day for a field test that measured their max heart rate, aerobic capacity, and anaerobic threshold. Then, on actual race day at the Paris Marathon, each runner wore a portable gas exchange mask, the kind that measures real oxygen consumption breath by breath, along with a GPS watch tracking heart rate, for the entire 26.2 miles. Nobody was told to hit a specific heart rate zone or pace. They raced exactly how they normally would, mild weather, low 50s, no rain, and researchers simply recorded what heart rate and true oxygen uptake did together, checking in every 5K, to see whether the two stayed in sync the way we generally assume they will.
What They Found
The research suggests these are the key takeaways
Heart rate stayed remarkably steady from the 5K mark to the finish, sitting right around 88-91% of max heart rate the entire time
Oxygen consumption told a completely different story. It dropped steadily as the race went on, from 81% down to 74% of each runner's max
Those two numbers started the race roughly matched, and by the finish they'd pulled about 19% apart. Heart rate said "steady effort." The body said otherwise
Pace slowed too, but at different points depending on speed. Runners finishing over 4 hours held steady pace until around 15K before fading. Runners finishing under 4 hours held on until around 25K. Heart rate gave no warning either time, it stayed flat straight through both slowdowns
Despite all this, 3 of the 10 runners hit personal bests that day. This drift isn't a sign something went wrong, it's just what happens physiologically over 26 miles
The researchers' conclusion: rate of perceived exertion, how hard the effort actually feels, tracked real physiological strain better than heart rate did. Their specific recommendation was a 13-14 out of 20 on the Borg scale, which lines up with roughly a 5 out of 10 on the effort scale I usually use with athletes, a "somewhat hard" marathon effort, not all-out
🟡 A small study, but a genuinely rigorous one, this is about as close to real race conditions as physiology research gets. Worth noting conditions were mild that day, heat tends to make this kind of drift worse, so a hot race could show an even bigger gap.
What This Means For You
Your body doesn't have a pace that it's trying to hit. It has a physiological state it's trying to hit. There's a biochemistry we're aiming for at different intensities, an aerobic zone, an anaerobic zone, and in perfect conditions, a pace lines up with that zone pretty well. But stress, dehydration, heat, and how tired you already are can all shift where that zone actually lives on any given day, even though the underlying target hasn't moved.
That's exactly what happened in this study. As fatigue built, heart rate couldn't climb any higher, so it plateaued, while the runners' actual output kept sliding. The number on the wrist stopped meaning what it meant at the start of the race.
Here's the two-part fix I use myself and with the athletes I coach.
First, if you're using heart rate zones, consider switching from max heart rate zones to lactate threshold heart rate zones. Max heart rate has two problems: most people are training off an estimated number from a formula, not an actual measurement, and even a perfectly accurate max heart rate doesn't account for the fact that two 40-year-olds with wildly different fitness levels need completely different training intensities. Lactate threshold zones are built around your actual fitness, not your age, which makes them a closer match to what's really happening physiologically. It's not exact, but it lines up with your ventilatory thresholds better than max heart rate zones do.
Second, lean on RPE, especially for a marathon or anything where conditions might not be perfect. For a marathon effort specifically, aim for a 5 out of 10, somewhat hard, sustainable, not a grind. Your watch can't feel the heat, your hydration, or the stress you're carrying into a race. You can.
🏆 How To Apply It
If you want to find your own lactate threshold heart rate, run 30 minutes as far as you can, then take your average heart rate from the last 20 minutes. That number becomes your LTHR
Most watches let you switch your zone settings from "max heart rate percentage" to "LTHR percentage," it's a setting change, not new hardware
For marathon pacing, practice targeting a 5 out of 10 effort in training long runs so it's a number you trust on race day, not something you're guessing at for the first time when it matters most
Late in a long race, don't assume a steady heart rate means steady effort. If it feels harder, it probably is, even if the number in front of you hasn't changed
This applies most in the marathon and beyond, shorter races don't give cardiovascular drift as much time to build up
We are all experiments of one. The goal was never to hit a number. It's to create the right environment for your body to do its job, and meeting yourself where you're actually at on a given day beats chasing where you think you should be.
Hit reply and tell me, have you ever felt like you were falling apart late in a race even though your heart rate looked fine? Or if you need help finding your LTHR, I can help you there too!
-Keep running happy, healthy, and strong
Spencer
Know a runner who would love some more evidence based feedback. Share the link HERE. and don’t forget to remind them they might have to double opt in :)