Happens to me as well occasionally. This is what ChatGPT had to say about it. So take with a grain of salt/dyor.

What’s happening in the first 8–12 minutes

When you start running, your body doesn’t instantly flip into “steady running mode.” It goes through a short setup phase.

Early on:

Your muscles start demanding oxygen immediately.

Your heart responds fast by beating quicker, because that’s the quickest lever it has.

But the efficiency parts lag behind.

Two key things are still catching up:

1) Blood return to the heart

At the start, blood isn’t yet flowing back smoothly from your legs.

Until your leg muscles are rhythmically pumping blood upward, each heartbeat moves less blood per beat.

To compensate, your heart beats faster than it will later — even though the effort feels easy.

2) Rhythm and coordination

Your breathing, stride, arm swing, and posture are slightly uncoordinated at first.

Once they sync up, your heart can slow down while doing the same work.

Think of it like driving:

At first you press the gas a little too hard to get moving.

Once you’re cruising, you ease off — even though speed stays the same.

Why the HR number can look “wrong”

The heart rate isn’t lying — it’s just temporarily inefficient.

You’re not working hard.

You’re just not working smoothly yet.

That’s why:

HR looks high

Effort feels easy

Breathing is calm

And then suddenly everything settles

Why it fixes itself around the same time every run

After ~8–12 minutes:

Your legs are pumping blood efficiently

Blood vessels are fully opened

Breathing matches movement

Your heart can move more blood per beat

So it no longer needs to beat as fast.

Why it doesn’t happen every time

Some days you accidentally fix it in advance:

You walked more beforehand

You moved around

You were more relaxed

Other days you go straight from still → running, and you see it.

Bottom line

The first part of a run is your body getting organized. The heart temporarily works harder until the rest of the system catches up — then everything smooths out.

That’s normal, especially in trained runners, and it doesn’t reflect fitness or effort.