New research · 4 min read · 2026-10-11
Why your heart rate climbs late in a marathon
In 82,303 marathon runners, those whose heart rate rose least relative to their pace in the last 10 km finished faster.
Late in a marathon, many runners see the same thing on their watch: the heart rate keeps climbing while the pace stays the same or drops. A study of 82,303 marathon runners looked at how much this happens and what it means for your finishing time.
What the researchers did
The researchers compared heart rate and speed early in the race, between 5 and 10 km, with the same numbers between 35 and 40 km. If your heart rate rises relative to your speed, your body is working harder for the same pace. They called this decoupling.
What they found
On average, the ratio between heart rate and speed rose by 16 percent from the start to the end of the race, and it usually began around 25 km. About a third of the runners had little decoupling. They held a higher pace relative to their fitness and finished faster. Men decoupled more and earlier than women on average.
When the researchers added decoupling to their model of marathon times, the prediction got noticeably better. How well you hold up late in a race seems to matter, on top of how fit you are.
What it means for you
Some drift is normal, especially in heat or if you haven't had enough to drink or eat. But a big drift late in your long runs or races tells you something. A related study from the same group found that runners who started their marathon too fast were much more likely to slow down a lot in the second half.
Start your long runs and races at a pace you can hold. On long runs, notice whether your heart rate creeps up in the last part while the pace stays the same, and compare it with similar runs in similar weather.
What the study can't tell you
The study measured drift during the marathon itself, so it explains race results after the fact. It doesn't show whether drift in training predicts how you will race, or which type of training reduces it.
How we use it
PX7 Training follows how your heart rate develops relative to your pace on long, steady runs. Over a season you can see whether you hold up better late in your long runs.
Sources
- Smyth et al. (2022), Decoupling of internal and external workload during a marathon: an analysis of durability in 82,303 recreational runners. Sports Medicine.
- Smyth & Muniz-Pumares (2020), Calculation of critical speed from raw training data in recreational marathon runners. Medicine & Science in Sports & Exercise.
Written by Jørgen Larsen for PX7 Training.