Why your heart rate in zone 2 sometimes tells you almost nothing

If you run easy and pick up the pace by ten seconds per kilometre, your heart rate often stays the same. If you slow down again, it does not drop either. Your heart rate monitor is usually fine. In the easy range your heart rate simply tells you little.

In short

When you run easy, your heart rate responds little to your pace.
Just read a zone as a range, not as a hard line.
When your heart rate stands still, steer by pace or feel.
Start at the bottom of your zone.

1.What happens in your body

When you run easy, your heart rate responds little to your pace. If you go a bit faster, your body handles it at almost the same heart rate.

That holds up to your aerobic threshold. That is the point where easy running turns into steady running. Below that threshold your heart rate stays flat. Above it, your heart rate rises much faster with every pick-up in pace.

Your heart rate is also slow. After you speed up, it takes one to two minutes to catch up. If you change pace a few times during an endurance run, what you see on your watch is mostly that delay.

During a long run your heart rate also creeps up slowly, even if you keep the same pace. This is called cardiac drift. It comes from heat, fluid loss and fatigue. On a long easy run it is simply part of it.

That is why your heart rate is not a good signal to steer by when you run easy.

2.So why is that boundary there at all

Zones make "easy" and "hard" measurable. They help you keep your easy days truly easy. If you run those too hard, you are too tired for the days that matter.

Your zones are calculated from one number, usually your threshold heart rate or your maximum heart rate. If that number is a few beats off, all your zone boundaries shift with it.

Even when your zones are set right, one beat of difference means nothing. A heart rate monitor itself already varies by a few beats, a chest strap a little and a wrist sensor more.

So zones are still useful. Just read a zone as a range, not as a hard line.

3.What to steer by instead

When your heart rate stands still, steer by pace or feel.

Your breathing is a good guide. If you can talk in full sentences, you are running easy.

Look at the whole run, not every minute. One kilometre a few beats above your boundary says little. What counts is whether the whole run was easy. A good test is whether you could repeat it tomorrow.

Also watch how your heart rate moves at the same pace. If it stays level for an hour, you ran easy enough. If it climbs in the second half while your pace stays the same, you ran too hard. That is true even if you stayed under your boundary.

That link between pace and heart rate is called aerobic efficiency, and it says more than your heart rate at one moment.

Start at the bottom of your zone. Your heart rate will still rise on its own during the run, and that leaves room for it.

4.What Trimetra does with this

Trimetra calculates your heart rate zones per sport. It uses your resting heart rate, your threshold heart rate and your maximum heart rate. Your threshold heart rate is set separately for running and for cycling.

So running and cycling each get their own boundaries.

The upper edge of your easy zone can also sit on your aerobic threshold. Trimetra estimates that threshold from your own easy workouts. You get it as a proposal, and you can see where the number comes from.

If one of your values clearly changes, you get a proposal to update it. You take it or leave it. The app never changes your settings without you knowing.

Frequently asked questions

Is a flat heart rate a sign that I am fit?

Often, yes. A trained body handles small changes in pace without your heart rate having to rise. It can also be fatigue. When you are very tired, your heart rate sometimes will not go up, even when you run harder. So also look at your resting heart rate, your sleep and how you feel.

Should I just raise my zone boundary then?

Not after one run. If you often run comfortably just above your boundary while your heart rate stays flat, your aerobic threshold may be higher than your zones now show. You only see that after several workouts, not by feel.

Is a wrist sensor good enough for endurance runs?

For easy running, usually yes. A wrist sensor is less reliable during quick changes of pace and when you run hard. A chest strap is better there. Wear the wrist sensor tight and not right on the bone of your wrist.

Which matters more: staying in the zone or holding the pace?

On an easy day the point is to stay easy. Whether you watch heart rate, pace or feel for that matters less. If your heart rate stands still, use the signal that does move.

In the app

ProfileYour zonesYour heart rate zones are in your profile per sport, and running and cycling each have their own boundaries.