HR is next to useless for short intervals....10 minutes and above maybe.
If you have never used a power meter you will probably disagree...but once you have kept a steady power for even a 5 minute VO2 effort you quickly see how watching you heart rate is pointless.
For a 20 minute interval done steadily for me at threshold the heart rate keeps climbing through out the entire effort...if I rode to heart rate i wouldn't be keeping a steady power level.
