Hmmm... well considering that the exertion is primarily anaerobic muscular efforts of short duration, most of the suffering is in your muscles. Any one have any data on the blood-gases during an interval? I suspect that since you usually hit max-HR after you've called it quits, that it's not a concern...
So when some coach says do one minute intervals at 90% does it mean you gotta keep your HR at 90% for a minute, or does it mean (vaguely) the effort should last one minute, and you should reach 90%?
Difference being the length of the effort. The former doesn't count the effort until you get to 90%, the latter counts the effort from the start as long as you get to 90%.
Ask your coach what he intends. Normally, intervals are timed from the start of the interval, not the point at which you reach the target heart rate. This is one reason why some people prefer using a power meter over a heart rate monitor. The power meter does not have the heart rate lag.
The intent of intervals is to stress the muscles. your heart rate is an indication of this stress, but only when it is held at a steady state level. Because of the time lag, heart rate is not a good indicator during a transient condition. You would be better off using your heart rate monitor for longer intervals to learn how it feels to do various exertion levels and use the perceived exertion to gauge your output in short intervals with the heart rate monitor as a cross check near the end of the interval.
Yeah, this is where power-meters really help to get the most from intervals. I suspect the coach was talking about percentage-effort of your maximum muscle-strength. So 90-100% is below all-out sprints and you can hold it for 3-10 minutes. It's pretty much impossible to hold HR constant above LT anyway, once you go above it, HR steadily increases to max if you keep up that level of intensity.
Interesting thing about power-meters is they show the balancing-act between power vs. speed. If you start an interval at say.... 15mph at 95% effort. You'll be generating more power than necessary and will start accelerating. At some point, the power you generate at 95% will balance out aerodynamic drag and you'll stop accelerating.
Could be a confusion with semantics. Intervals are typically used to refer to timed or distance sets done at above LT, basically anaerobically and works the muscles more than the cardiovascular system. Tempo is used to call efforts at LT or slightly below, which works more on the aerobic system. It's not really possible to do a single workout that benefits both as optimally as doing intervals for the muscles and tempo for the cardiovascular system.
Depending upon where a particular rider is in his training and the types of events and performance he wants to achieve, you'd prescribe a certain amount and type of interval and tempo training.
A lot of people get stuck in no-mans-land of training and reach a plateau because they're doing both not enough intensity and not enough aerobic distance.