Here is what I do. It might not be scientific enough for pro riders or the Australian Institute of Sport, but it is easy to follow with just a heart rate monitor and produces big improvements.
First, I find my maximum heart rate. This bit hurts, but not for long and doing it about once a year is enough for me. Do a brisk 15 to 20 minute warm-up then go up a long steep hill, speeding up steadily. When my heart is pounding and I am gasping for breath, I go a bit harder. I can pick the difference between "don't want to go harder" and "can't possibly go any harder". I note my heart rate. Last time, it was 187.
Next I calculate 85% (159) and 92% (172) of the maximum heart rate.
Now I am ready for intervals. The three versions I find useful are:
a) 10 min warm up, 5 min gradually increase to 85%, then pedal at high cadence in a speed range that keeps my heart rate in the 85% to 92% range for 20 min. Ease off to about 75% of max heart rate for 10 minutes then do another 20 minutes in the 85% to 92% range. Warm down. After that, I often do another hour at 75% to 80%.
b) Same as a) but grinding big gears at 60 to 65 rpm
c) 4 x 15, i.e. 15 minutes easy, 15 minutes hard, four repetitions, warm down. Sometimes I crack halfway through the last 15 minute session.
Other people in this forum will probably contribute variations on this, but the basic principle is to pick the maximum effort you can sustain for 15 to 20 minutes, repeated 2 or 3 times. If you think the limits should be at xx% instead of 85% or yy% instead of 92%, go for it
