Many cyclists estimate the workload demands of their training and racing by measuring heart rate, and heart rate is also used to set the intensity of various training sessions and to gauge whether intensities established in races are OK (i.e., sustainable). This is a potential problem, because heart rate is responsive to a number of different factors in addition to cycling workload. For example, "cardiovascular drift" can cause heart rate to increase during cycling, even when intensity remains constant (1 & 2). POWER

A cyclist's psychological state can also cause heart rate during exercise to move hither and yon, and even a cyclist's position on the bike, caffeine intake, and sleep patterns can produce changes in ticker tempo (3). As a result, many cyclists have become increasingly interested in utilizing power output as the key monitor of training quality and race performance (Greg Lemond got the power ball rolling in the early 1990s when he first began to use power as a key training variable). Using power instead of heart rate appears to make sense, because the mechanical power output produced by cyclist to move a bike forward is the key variable which determines the actual demands placed on the cyclist during exertion (4); it is a much-better indicator of total physical stress than the simple beating of the heart.

Fortunately, power output can be measured directly on the bike with the use of a "mobile crank dynamometer" (5). Two power monitors seem to be dominating the cycling marketplace right now - the SRM and the Power Tap. Although their costs are considerable, these devices are extremely attractive, because they can display cadence, wheel speed, distance covered, energy expenditure, and ambient temperature (just the SRM), in addition to power output, as you train and compete.

Naturally, these devices should be both accurate and reliable if they are to be truly useful to you. Let's take a moment to look at these two terms. Accuracy refers to how close the readings on the SRM or Power Tap are to your true power outputs. If, for example, you are carrying out an interval workout and your true work interval intensity is 300 Watts, it would not be good if your SRM or Power Tap provided readings of ~360 Watts. True, some experts argue that the actual number on your power meter is not so important, as long as the power device is reliable (i.e., gives readings which are highly repeatable; we'll discuss reliability in a second).  POWER

This reminds us of the "The amp goes to 11" line in the movie Spinal Tap, which referred to an ampliflier which possessed a volume scale of 1 through 11 (instead of the usual 1-10), even though it was not any louder than traditional amps. Just as the 11 on that amp was the same as 10 on other magnifiers, a 330 on a power meter might be the same as a true 300, and as long as that relationship was consistent you could still track changes in your workout quality. The problem could come when you attempted to predict performance, say in a 40-K time trial, for example. If a published scientific study revealed the relationship between power output at vV02max and 40-K performance, you would not be able to draw a valid conclusion about your own 40-K potential if your power monitor provided inaccurate readings.

The word reliability refers to the repeatability of measurements. For example, your power monitor is totally reliable (although inaccurate) if it gives a reading of 350 Watts every time you cycle at a true power output of 300 Watts. If you are always at 300 Watts but your power device gives up an array of different numbers, then the gizmo is unreliable.

To learn more about power, heart rate, & competitive cycling (the full article can be read by purchasing Vol.2 Issue 3) and many more cycling related topics. Simply enter power, in the "search archives" box, or enter any subject you wish to learn more about.  POWER