Not exactly answering your questions but the idea of using the HR monitor for A+A got me thinking....
You could use HR recovery as an indicator, and when you take longer than say 60/90s to get back down to 120bpm or so it might indicate it was time to stop the session.
I found this passage in Gordon Pirie's book - "Running fast and injury free" - Olympic Long distance runner.
" During an interval session, a given fast stretch should be run at least 10 times, with the interval between runs being determined by the time required for the athlete to recover physiologically. This can be calculated by monitoring the athlete's pulse rate during the recovery interval.
The aim is to run with sufficient speed to stimulate a highest pulse at the end of the fast stretch of 180 beats per minute; that is, 18 beats in 6 seconds.
Recovery at this top end of the heart's effort occurs so rapidly that the best way to count the pulse rate is electronically. Failing this, measure the pulse rate at the wrist, on the left breast, or on the carotid artery (one only!), using the fingertips. An actual 180 maximum heart rate may be indicated by a 17 count in the first 6 seconds, because of the rapid initial drop of heart rate.
The interval should be run at a continuous trot, and with the same rhythm that is used in the fast run; the breathing rhythm should also be identical. This assists greatly in the recovery process. The interval's length is again decided by the heart's behaviour. When the heart rate has fallen to 120 beats per minute - 12 beats in 6 seconds - the recovery is complete and the next fast stretch can be run.
As one might expect, the interval after the first few fast sections will initially be short, and then progressively lengthen to a standard interval as the heart takes on the full workload of the training session.
A typical workout,say 20x200 metres, might see a set of intervals as follows (for a particular athlete at one stage of his development):
No.1 x 200m : 25 secs interval.
No.2 x 200m : 35 secs interval.
No.3 x 200m : 45 secs interval.
No.4 x 200m : 55 secs interval.
The next 14x200m run might require a standard interval of 60 seconds.
As fatigue sets in after this, and the “rest” interval required extends to 65 seconds, stop running!
Progress is indicated by an improvement in the required rest interval (i.e. it gets shorter),and also by an increase in the number of repetitions which can be run before the onset of fatigue. In addition, progress should be accompanied by an ability to run the fast section at a greater speed without breaking the top pulse rule (i.e. keeping the maximum pulse rate below 180 per minute), which should occur with ease, and without extra effort.
The usual times taken to run 100 metres vary from 20 seconds for the beginner down to15 seconds for the highly trained athlete. The equivalent figures for 200 metres are 40and 30 seconds, and for 400 metres 80 and 60 seconds, respectively.
The number of repetitions which can be run varies from 10 up to as many as 40.
Even more can be handled by a world record runner. Before the latter state is reached,however, it will be time to progress to other types of training (described later).
Interestingly, during interval training, most development occurs during the interval; this was the conclusion reached by Waldemar Gerschler and Professor Reindel at theFreiburg Sports Institute after many years of research on thousands of subjects."
