I've got an iPhone app that works with a Garmin transmitter to log heart-rate during exercise, and the above is from my last interval-training session. If you're thinking about high-intensity interval training (HIIT), this will give you an idea of the effect on heart-rate.

The first ten minutes are a warm-up. Brisk walking in this case. The spike during this period is from jogging for one lap.

After ten minutes of a general warm-up, I get on the bike for another 3-4 minutes of specific warm-up. I could have combined warm-ups by getting on the bike at the start. I just like to walk for awhile to help make the transition to exercise mode.

I started the sprints at about 14 minutes. The first two were 60s each, the last two 45s each. Rest periods were 3 minutes at the lowest resistance setting on the machine.

A couple of interesting things: Heart-rate increased both during the sprints and the rest periods with each interval, even though the last two sprints were 15s shorter. Also, the last small spike shows what happens when you don't cool-down long enough: my heart-rate went up about 10 bpm when I got off the machine. Not a big deal, but one of the advantages of interval training is that the rest periods "buffer" the heart from too much stress. No point in adding stress unnecessarily.