The age predicted heart rate (220-age) is considered highly inaccurate and is estimated to fit less that 1/3 of the population. There are several other methodologies to determine heart rates and some of the ways can be found in books by the late Edmund Burke (Precision Heart Rate Training), or books by Sally Edwards and Sally Reed (The Heart Rate Monitor Book for outdoor and indoor cyclists). Both not only explain heart rate training, but include several training methodologies.

I'm a Personal Trainer and cyclist and also have been teaching Spinning classes since 1997 and I have taken several classes on heart rate training. You know what my conclusion has been? Use the highest number ever seen as your probably 90% if heart rate max. This is the number you see (after thouroughly warming up), when you are working so hard that you're barely able to breath and feel dangerously close to puking. Most people don't want to work to that level.

Most people will experience high heart rates at the beginning of their workout effort. This is due to the body not being warmed up. The body actually goes through metabolic changes during the warm up that allows the blood flow which is normally centered around your core (where the important organs are), and shifts the blood flow out to the working muscles, allowing those muscles to start extracting the oxegon.

So, when you first start working out it's as though your body is a cold car with all the oil (blood), sitting there in the oil pan. After you start the engine, it takes awhile for the oild to flow to the working part of the engine (the muscles). So working without being thouroughly warmed up means your muscles are not ready to use the oxegon in your blood yet, which forces the muscles to find a different energy path with would be the anaerobic pathways, (without oxegon). That pathway results in the creation of lactic acid .

This may be more than you wanted to know, but in any case, there is a reason we go through a warm up and once your body has made the transition from pre-warm up to post warm up, everything seems easier.

Hope this helped a little. If you get a chance try reading some of the material out there by Sally Edwards & Sally Reed, or just do a Google search on the age predicated heart rates.