To clarify, I'll give more details and parameters.
This VDOT calculator:
http://www.runbayou.com/jackd.htm
You can find it on other websites as well.
For interval pace says HR: 98-100%
Is true that for short intervals 98-100% HR is not possible to reach at the recommended pace.
But no matter the parameters, is impossible to reach 98-100% HR.
Let's take the default values of the calculator for 5k, 17:43.
In a 5k race, you can reach 100% HR only in the last km, otherwise you'll probably bonk.
The 1000m intervals together take 17:35 to complete, without rest. Basically just after 4 intervals the heart rate could be 98-100%, but only IF you don't rest.
So no matter the parameters, you can't reach 98-100% of heart rate, if you rest.
The only way to keep a higher heart rate would be to jog between the intervals, or start with a longer interval.
In my case (I do 8km in a single workout), I should start with a 3k and continue with the normal workout. I'll try this, sounds fun.