Are those heart rate or pace zones? What are you using as a heart rate monitor? Have you double checked the zones for accuracy in your settings?

Heart rate zones per my Garmin. I haven’t checked them for accuracy but probably should, although this is fairly typical of past races.

If it's a steady paced effort, it makes sense that hr would rise over the course of the race, especially over longer events.

Typically people start 5ks a bit harder, back off in the middle, and finish harder, in which case hr would get higher initially, plateau, and then probably peak again.

So perhaps you're not warming up enough to be able to start as fast as you may otherwise do, perhaps you're not running all out, perhaps your hr zones are inaccurate, etc.

Also, if you're a 45 min 5k runner, I'd expect your hr zones to be a lot lower than a 15 min 5k runner.

So could be a couple of different things.

Assuming you paced it correctly (more or less even speed throughout, not a big speedup or slowdown at the end), this is a sign that your legs lack the muscular endurance to keep up with the required HR at this distance for the effort.

This means that while you 'could' push to zone 5 in the first 0.5 miles of the race, your legs would be cooked, and you'd suffer a dramatic slowdown as expected.

However, since you paced it correctly to have your legs pump out the steadiest effort possible, this meant that your legs were lagging your heart for effort for most of the race until the end (good pacing job to for doing that.) For your current fitness level, your legs are much more of a limiter than your HR. (This will change a lot with training!)

You will find that if you increase the run-specific muscular endurance of your legs by running a lot more training miles per week, you will be able to hit zone 5 much earlier in a 5k. For run specialists, it's normal to be at VO2max Z4-5 right from the getgo, redlining it the whole way.

The reverse also happens as well - if you do a lot of long slow volume but don't work on speed at all, you'll find that your cardio will blow up well before your legs even get significantly taxed. This is my situation - I am never leg-limited in a 5k - my HR goes through the roof in the first 100m, and then it's all about the HR for me from start to finish.

Race distance matters too - obviously you won't be hitting z4-5 until the very end of a marathon just by physiological limits of time able to hold Z4-5, but the constraints are the same. The shorter the race, the easier it is to 'redline HR' the whole race, but you have to have enough leg muscular endurance to hold that HR the whole way.

Sadly, this makes a lot of sense. Thanks.

For an example, 5k is my favorite distance. This was on a rough terrain or it would have been faster, but the HR graph is typical:

https://www.strava.com/activities/1996621201


That doesn't list the Garmin zones, but it was Z1 (0:10), Z2 (0:07), Z3 (0:19), Z4 (0:46), Z5 (18:46). As in, almost immediately getting into Z5 and staying there throughout, slowly increasing after the initial rise.

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The point is, ladies and gentleman, that speed, for lack of a better word, is good. Speed is right, Speed works. Speed clarifies, cuts through, and captures the essence of the evolutionary spirit.

Can you post a link to the workout or even a screenshot showing time/distance/pace/elevation/HR?

Perhaps it is a bad HR monitor that took awhile to 'wake up'. If so, the HR plots are quite distinctive.

Are you sure your zones are right?

Also is it possible you’re too fatigued (peripheral fatigue) to push the heart rate up?

What was your time like compared to other times you’ve done a 5K?

I cannot speak to normal, and I am not a coach. But this is my recent personal experience in a small, local 5K. I won my AG and was the 3rd guy over 18. I was 35% in Zone 3 and 61% in Zone 4. It felt pretty close to all-out. I ran nearly flat power, according to my Stryd.