62 upvotes and 3 downvotes for a completely incorrect answer with ridiculous logic! LOL people must have stopped reading books.
1) It is simply false that 800 and 1000m is not long enough to reach max heart rate.
Most credible sources claim you can reach your max heart rate in 60-90 seconds!
S. Canaday (Whom I thought was Rojo because of the moranity) and almost every other poster go against all textbooks/running books I have read. Both standard sports physiology and books by Lydiard and Daniels say that they recommend something like an all out 2 minute effort to test your max heart rate (followed by one more after a longer break - if you couldn't run as far the second time, then you know, that you hit your max the first time)
yes you can keep your vo2 max for about 8 minutes and "that means 3k is the race where you'll most likely hit mhr"?? err no. that means that it is the race where you'll run most of the race around vo2 max , but perhaps you run the first 6 laps at 90-95 % of mhr and are too tired to really hit your max hr in the last 1,5.
2) Well-trained athletes can maintain mhr for 2-4 minutes
that 800, 1000 and 1500 is more anaerobic than 3 and 5k is exactly the reason that they are more likely to be the races where you hit your max hr.
Think about it like this: The higher the speed you are running, the faster hr will increase. 90 seconds is enough to get it as high as your body demands, so if you run 1 k in 2.30 then you'll probably hit mhr in 90 seconds and your body will tell your hr to increase really hard the last minute because of the pace, but it just can't because it is already maxed out.
TLDR: You can hit max heart rate in less than 2 minutes and running anaerobically really tells your hr to go up => 800 to 1500m more likely than longer stuff to hit max hr.
For the record one should theoretically hit max hr in any race from 800m to 10k at some point if you go all out, and it should not be too difficult as it is possible to hold for a little while