Regular computers measure bicycle statistics; heart rate monitors measure your pulse. While this seems like a very marginal option, strictly for professional athletes, it actually tells more than the current speed of the bicycle. I am familiar with the heart rate monitor made by Polar. It consists of a chest belt and a display unit that can be installed on the handlebars or worn at the wrist. The connection is wireless.
The heart rate is surprisingly variable. It can jump up and down rapidly as exertion changes. It can jump from 90 to 180 or vice versa in much less than a minute - and it takes longer after a long ride. Watching the heart rate allows the rider to keep his pulse in a safe zone and avoid extremes. For example, I avoid exceeding 160 beats per minute in flat terrain because I wouldn't be able to keep it up for an entire ride, while exceeding 190 is not unusual for short bursts of speed. Just rolling along at 120 or 130 is relaxing. On a long tour I start with heart rates around 150, and after a week I barely exceed 110 even uphill. I monitor my heart rate much more closely now than my speed because it lets me plan my rides better.
Of course I am talking about 100+ km rides. There is not much point in wearing a heart rate monitor when riding to the grocery store around the corner. But I do recommend them to long-distance riders. On a recent 300km ride on a single day I adjusted my speed to my heart rate, starting with 120 bpm. I had to revise this later until my target rate was 150 bpm near the end of the ride. I have done long rides before but never felt so confident that I could finish the ride safely.
I recently (5/2000) got a new Polar M52 heart rate monitor that also derives fitness information, calory measurements, and training schedules from heart beat variability. It is a wonderful toy. I can't verify the calory estimates but they seem plausible. The fitness test seems dubious though - I have improved from 52 to 57 within two weeks of moderately hard riding, much faster than the manual claims is possible, with an intervening result of 44. (The numbers must be looked up in a table.) A friend got 72, way off the scale.
It has two annoying flaws, in addition to a chaotic user interface: training cannot be stopped and resumed for a pause because stop mode quickly times out and aborts the measurement, and although the belt-to-computer signals are encoded the code lock is inevitably lost and is pretty much impossible to reacquire, which makes the computer pick up signals from other people. Occasionally the heart rate display is wildly inaccurate for short times. When the chest belt battery is near the end of its life, you get absurd readings like 220 beats (my heart never goes much above 200 even in absolutely extreme situations). You can exchange the battery in the display unit, but the chest belt must be sent in; it's sealed. It's not very practical to cut open the belt to replace the battery; the battery is soldered to the board and the belt is difficult to re-seal (see picture to the right). Polar now offers a belt with a replaceable battery.
See also an article on "GPS Satellite Navigation"
