I have several boys who go well over 90% MHR when they aren't close to their VDOT tempo pace. For example, one of my kids just ran a 5:07 1600m. When I enter this time on a VDOT table, tempo pace should be 6:07. The boy's HR is well into the 190s (over 90% MHR) when we do 3x1600m repeats at 6:25-6:30 (1:30-2:00 recovery). Is this normal? This particular boy is 15.
Do you know what his max heart rate is? Are they consistent with all 3 reps? Chances are they can race faster than they can train. You might want to train them at a slightly lower VDot until they build up a little more aerobically.
The vast majority of high school age kids are not well developed aerobically. Their VDOT rating based on their mile PR and their 5k best are often quite different. As a general rule use the following to establish Daniels' workout paces:
F-pace = 800m race pace
R-pace = 1600 m race pace
I - pace = 5k race pace
T-pace = 5k race pace + 20-30 sec/mi
Don't try to tie everything to a single #, especially for developing runners.
this is helpful- thanks