Low relative HR on a 5k?

A well run 5K would likely average 93-95% max HR over the race and get within a couple of beats of max HR by the end of the race. Also are you running the 5K after a taper or on tired legs? If the later, expect not to reach max HR.

Yes, fair point, I am normally not tapered, but I would still expect to get closer to max than what I do. Other than the marathon, all of the other races are generally untapered as well.

Looking back at my data, the best I have done is 87% of max, this was untapered. I guess I could be expecting too much! But according to all the majority of literature this make my races just a fast tempo from a HR perspective

Maybe try worning on getting your heart rate above 170 for a long as possible in wo's and practice getting more comfortable with your heart rate up that high. A lot of times during wo's we only have our HR uo for brief periods and then we suddenly expect ourselves to perform in 10k's etc where we should have our HR uo pretty high most of the race.

But wouldn't that knacker me out having a high HR in a warm up? Or is that what is expected? I am honestly clueless about this stuff, my WO if I do one is normally a pretty slow jog for a bit and then a couple of very short sprints!

Should I force myself to train more at a high HR? even if I cant mentally hold the reps for that long? ie do fast short 200s just to force it up there?

If you're looking to improve on your 5K time / performance, consider doing a season or so of 5K-specific training, such as following Jack Daniels 5K/10K plan. Doing so will prepare you better (both mentally and physically) for the demands of a 5K.

It would also be a good idea to do a test designed to get a proper gauge of your max HR. One from Advanced Marathoning is to find a hill at least 600m long with 5% grade or so. Run up each 600m rep as hard as you can, and jog back down to recover. Max HR should be at the end of the last rep. Again, make sure you are tapered and fresh before doing this workout!

I have a different issue. Maybe I'm just soft now or something. I never really feel the burn of lactate and I haven't felt sore after a 5k or fatigued in some time. Am I just aerobically out of shape and my muscles can handle it? Maybe I'm just being a b*tch and I can push more.