On that thread, a coach had his athletes doing 400m intervals at tempo pace with very short rest and was wondering what kind of indicator of their fitness that workout might indicate.
My response was that while it might be fine to trot out 400m cruise intervals on a rare occasion (in my opinion, mostly at the end of a season when you are less concerned with increasing adaptation and more concerned with changing paces some to keep your people from getting stagnant while they are winding down for the peak race), it was really far too short of a distance to give any indication of fitness because the distance is so short that you could run it at faster than tempo pace, even with short rest, and still get through the workout. Accordingly, it is just really hard to look at the splits from such a workout and glean anything about fitness.
I used 800m vo2max intervals to illustrate the issue because I suspect that most high school coaches employ those regularly enough that they have seen a similar phenomenon of people being able to run that workout faster than they can race a 5k. Again, the issue with that workout is that it is just short enough to allow you to complete the intervals at a too fast pace yet still be able to gut out the workout.
Interestingly, the slower you are as a 5k runner, that gets less and less true with respect to the 800s workout. If you think it through, that kind of makes sense. Birdwatcher would be out there running 2:15-2:20 per 800. For him, the interval would nearly be over by the time his heart rate came up near his max heart rate (and it might not even make it there until later in the workout, if at all). But my wife would be out there for 3:30 or so per 800, which means she would be getting up near her max heart rate much sooner in each interval and much sooner in the workout. So when you are spending much more of the workout near your limit, it is harder to rely upon speed and grit the way someone who barely comes near their limits can. This is a big reason why I am not a huge advocate of 800m intervals as a vo2max workout for anyone running 18:00 or faster for the 5000m, except early or late in a cycle when you are looking to find your rhythm more than looking for a big bump in vo2max.
Does that make sense?