Why can't runners set their own pace?

I had a conversation with my friend Ben about this. Ben (and his son) was a competitive runner in college:

Me: In a running race, what’s the purpose of a pacesetter?

Ben: The pace setter helps make sure the pack of runners doesn’t get out too fast or slow. This is often used when some runners are in contention to break a record. It takes some of the guesswork and mental stress out of the start of the race. The pacers tend to run about half of the total distance, which is why it is only one lap for this race.

Me: Does the pace setter run at a specific pace? Or just really fast?

Ben: At a predetermined pace, which is communicated to all runners in the race.

Me: That’s what I thought, so, here’s the crux of my question: if the pacesetter knows how to run at a specific pace, why don’t the other runners?

Ben: They do know. But they can get caught up in the competition and lose sight of their pace, causing them to go out too fast or too slow. That would eliminate their chances of setting a record. A pacer isn’t concerned about racing. Only about hitting a predetermined time.

Me: And the pacer can only run that fast because she’s only running half the race?

Ben: Exactly

Why I Looked It Up

I was watching this race on YouTube.

The pacer is very present in that video. She’s the woman running really fast in front of everyone for the first lap. The announcers mention her several times:

1:52: [indecipherable] of Poland is the pacesetter, she will get the front of the group.

2:02: The first 200 meters complete. About 27 high [presumably meaning closer to 28 than 27]. 57 is the instructed first pace here [meaning the pace intended for 400m, which is told to all the runners in advance].

2:29: The pacemaker will bring them through [the 400m mark] in just under 60 seconds.

2:36: Yeah, a little slower than what was prescribed [referring to the pacesetter]. She will now step off the track [you can see the pacesetter run off into the infield of the track, while the actual runners continue]

Ben also mentioned:

The pacer looked back at one point and saw that the runners weren’t where they should be and she slowed a bit.

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