Melanie,
When you're racing, what is your level of intensity throughout? About what percent of your Max HR are you performing at and able to sustain?

I am assuming we are discussing Xterra races, or races of that length and shorter.... given that is pretty much all I am racing these days. I would like to stress that the following is info about ME. Heart rate data is totally dependent on YOUR physiology. It is unlikely that most people are going to have heart rates anywhere near mine. It is also the result of 15 years of training that my racing heart rates are close to my maximum. I would like to stress the entertainment value of the following post.

Ok, the intensity level only changes relative to the elevation of the race. At elevations higher than 5000 feet, I am unable to recover from anaerobic efforts. Therefore, I tend to race just at or under threshold for those races. This means the relative effort level on a scale of 1-12 is about an 8 or a 9 for most of the race. I believe this is one of the reasons I do not race as well at altitude.

Below this elevation, I race at an effort level anywhere from 10 to 15 on a scale of 1-12. I will constantly go above threshold and above my percieved maximum to either ride a section, pass someone, attack, or generally race aggressively. I think this is why Hawaii is such a good course for me.

How this relates to heart rate depends on the temperature and the elevation. I will consistently reach somewhere between 202 and 208 bpm on a Lactate Threshold (Maximum Aerobic Power) Step test (starting at 100W, go up 25W per three minutes until failure). So lets just assume my maximum racing heart rate is 202 to be conservative. I have raced a NORBA in Phoenix where my average heart rate for 90 minutes of mountain bike racing was 200bpm on an admittedly flat, hard, power course where you would pedal consistently hard. This corresponds to 99% of max heart rate and was entirely due to the fact it was 97 degrees that day.

Compare that with my heart rate average in Tahoe this past year. It was cold (started in the low 60s) and the race started at 6800 feet. There my average was 183 bpm, a heart rate that corresponds to LOW zone 3.

In general, I will race at around 195bpm in summer temperatures at most races when I am fit. Anywhere from 90-95% of my maximum heart rate is a race where I have reached race fitness and am starting to perform well. EXTREMELY good races are on the 95% side of the spectrum as I am starting to feel really good. I usually start the year much lower than that. In fact, Temecula last year my average heart rate was only 185 bpm on a hot day, which is an indication of some fatigue and lack of fitness.

Hope that helps. Obviously, this is not a guide for anyone else, just some personal info in case you are wondering.

Whew, that's some intense racing. Thanks for the reply, pretty interesting.

To continue further on this topic, what is your maximum HR is _base_d upon?
A test (if so, what) or age-_base_d?

Max heart rate as evidenced by physiological testing, specifically a max watts step test.